8:30 AM - 11:30 AM, Grand Canyon 6
Co-Chair: Claudia Felser, University of Mainz; Co-Chair: Guenter Reiss, University of Bielefeld
8:30 AM
AA-01. Application of magnetic Heusler alloys to CPP-GMR read sensors
Jeff Childress
Hitachi San Jose Research Center, San Jose, CA
Current magnetic recording thin-film sensors operate in the current-perpendicular-to-plane (CPP) geometry, and rely on the spin-filtering properties of MgO tunnel barriers to achieve large magnetoresistance values. Lower-resistance (and thus lower-noise) magnetic sensors are required as sensor dimensions are further reduced. All-metal CPP-GMR sensors can easily achieve low resistance values but need improved magnetoresistance to result in sufficient signal-to-noise ratios. Due to their predicted high spin polarization, a number of ferromagnetic Heusler alloys have the potential, when used as the magnetic reference and free layer in a CPP spin-valve, to significantly increase the magnetoresistance of CPP-GMR spin-valves and thus provide sufficient signal-to-noise ratio for possible application to high-density recording sensors. Nevertheless, Heusler alloys also present a number of challenges for integration due to their complex crystalline structure, composition, low magnetic damping constant, and high sensitivity to spin-torque excitations. I will discuss some of the requirements & constraints specific to recording head sensors at > 1 Tb/in2, and show some examples of materials characterization, sensor behavior, and head performance improvements resulting from the use of Heusler alloys.
9:06 AM
AA-02. Crystalline Formation of Polycrystalline Co-Based Full-Heusler Alloy Films Observed by HRTEM with in-situ Annealing
Atsufumi Hirohata1, 2, Luke R. Fleet3, Michael J. Walsh3, James Sagar3, Gleb Cheglakov1, Kenta Yoshida4, Vlado K. Lazarov3, Yoshihisa Ohba5, Edward D. Boyes1, 3 and Tadachika Nakayama5
1Department of Electronics, University of York, York, United Kingdom; 2PRESTO, Japan Science and Technology Agency, Kawaguchi, Japan; 3Department of Physics, University of York, York, United Kingdom; 4Japan Fine Ceramics Center, Nagoya, Japan; 5Department of Electrical Engineering, Nagaoka University of Technology, Nagaoka, Japan
We focus on the improvement of the interfacial magnetic and atomic ordering in a polycrystalline Co-based full-Heusler alloy films. Theoretically Heusler alloys are expected to show half-metallicity at room temperature not yet confirmed by experiment [1, 2]. We have studied crystalline formation processes of polycrystalline Co2FeSi films using high-resolution transmission electron microscopy (HRTEM). 20-nm-thick Heusler-alloy films were grown onto SiN grids using a HiTUS (high target utilisation sputtering) system [3]. The evolution of the grains was observed by double-Cs-corrected HRTEM during in-situ annealing up to 500°C. Magnetic properties were measured using a vibrating sample magnetometer (VSM). The films show a peak both in grain size and saturation magnetisation (MS) after 6 hours annealing at 500°C [4, 5]. MS differs from calculated values due to th e formation of disordered phases and Si segregation at the grain boundaries. The films form the L21 phase in a layer-by-layer mode along the [112] zone axis. This crystalline formation is energetically favoured and has the potential to maintain the half-metallicity at their interfaces and surfaces. Further details on crystalline formation processes will be discussed at the presentation.
References
[1] I. Galanakis and P. H. Dederiches (Eds.), Half-Metallic Alloys (Springer, Berlin, 2005). [2] W. Wang et al., Appl. Phys. Lett. 93, 122506 (2008). [3] M. Vopsaroiu et al., IEEE Trans. Magn. 40, 2443 (2004).
9:42 AM
AA-03. Perpendicularly Magnetized Tetragonal Heusler-like Alloy Films for Spin Torque Applications
Shigemi Mizukami1, Takahide Kubota1, Hiroshi Naganuma2, Mikihiko Oogane2, Yasuo Ando2 and Terunobu Miyazaki1
1WPI-Advanced Institute for Materials Research, Tohoku University, Sendai, Japan; 2Department of Applied Physics, Tohoku University, Sendai, Japan
Some Heusler alloys (X2YZ) have half-metallic band structure, exhibiting giant tunnel magnetoresistance (TMR) in magnetic tunnel junctions (MTJs). However, the conventional Heusler alloys, such as Co2MnSi, have chemically ordered cubic structure, so that these alloys show relatively weak magnetic anisotropy. Magnetic films with a large perpendicular magnetic anisotropy (PMA) are advantageous to spin-transfer-torque applications, such as magnetic random access memory, because PMA reduces a switching current density and increase a thermal stability of magnetization direction [1]. Thus, it is interesting to study on Heusler alloy having a large PMA. Balke et al. have predicted that Heusler-like alloy, Mn3Ga, exhibited a large spin polarization as well as a large uniaxial magnetic anisotropy [2]. The Mn3Ga has tetragonally-distorted D03 structure and its chemical composition is regarded as Mn2MnGa, like as X2YZ Heusler alloys. We have obtained Mn3-xGa (x=0.5) epitaxial films using a UHV-magnetron sputtering and reported large uniaxial magnetic anisotropy energy Ku ~ 12 Merg/cc and also low saturation magnetization ~ 250 emu/cc [3]. This alloy contains no noble and rare-earth metals which are important constituents to gain a high magnetic anisotropy, such as FePt or Nd2Fe14B. In this talk, we will present structural and magnetic properties and a relatively low Gilbert damping observed in ultrafast precessional dynamics of magnetization for the Mn3-xGa alloy films [4]. Furthermore, the TMR effects in MgO-MTJs with Mn3-xGa electrodes will also be discussed [5]. This work was partially supported by Grant-in-Aid for Scientific Research, Industrial Technology Research (NEDO), JST Strategic Japanese-German Cooperative Program (ASPIMATT), and Asahi glass foundation.
References
[1] Nakayama et al., J. Appl. Phys. 103, 07A710 (2008); Mangin et al., Nature Mater. 5, 210 (2006). [2] Balke et al., Appl. Phys. Lett. 90, 152504 (2007). [3] Wu et al., Appl. Phys. Lett. 94, 122503 (2009). [4] Mizukami et al., Phys. Rev. Lett. 106, 117201 (2011). [5] Kubota et al., Appl. Phys. Express 4, 043002 (2011).
10:18 AM
AA-04. Heusler alloys boosting the performance of TMR-biosensors
Andreas Hutten1, Camelia Albon1, Alexander Weddemann2, Alexander Auge1, Peter Hedwig1, Jan Rogge1, Dieter Akemeier1 and Niclas Teichert1
1Physics, Bielefeld University, Bielefeld, Germany; 2RLE, LEES, MIT, Cambridge, MA
Magnetoresistive Biosensors use a detection method for molecular recognition based on two recently developed devices: a combination of magnetic markers and Tunneling- Magneto Resistance (TMR) sensors [1]. Our contribution is focused on physical aspects which play an important role on the way to realize lab-on-a-chip structures [2]. Using Heusler alloyed magnetic electrodes in TMR sensors will boost their performance in terms of an accessible external field range. This is accompanied by a different noise behavior which in turn can be used so as to characterize the performance of the Heusler alloyed magnetic electrodes.
References
[1] C. Albon, A. Weddemann, A. Auge, K. Rott, A. Hütten, Appl. Phys. Lett. 95 (2009) 023101. [2] A. Auge, A. Weddemann, F. Wittbracht, A. Hütten, Appl. Phys. Lett. 94 (2009) 183507.
10:54 AM
AA-05. Tunable multifunctional topological insulators in ternary Heusler and related compounds.
Stanislav Chadov1, Xiaoliang Qi3, Jürgen Kübler2, Gerhard H. Fecher1, Claudia Felser1 and Shoucheng Zhang3
1Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universtität, Mainz, Germany; 2Institut für Festkörperphysik, Technische Universität Darmstadt, Darmstadt, Germany; 3Department of Physics, McCullough Building, Stanford University, Stanford, CA
Recently the quantum spin Hall effect was realized in quantum wells based on the binary semiconductor HgTe. Many Heusler compounds are ternary semiconductors that are structurally and electronically related to the binary semiconductors. The diversity of Heusler materials opens wide possibilities for tuning the electronic structure near the Fermi energy, i.e. the band-inversion and the gap band width, by choosing compounds with appropriate hybridization strength and magnitude of spin-orbit coupling. Many of these ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the rare-earth element Ln, which can realize additional properties ranging from superconductivity to magnetism and heavy fermion behavior.
8:30 AM - 11:30 AM, Grand Canyon 7
Chair: Tamalika Banerjee, Groningen
8:30 AM
AB-01. Chiral organic molecules as spin filter.
Ron Naaman1, Zouti Xie1, Tal Z. Markus1, Sidney R. Cohen2 and Zeev Vager3
1Chemical Physics, Weizmann Institute, Rehovot, Israel; 2Chemical Support, Weizmann Institute, Rehovot, Israel; 3Department of Particle Physics and Astrophysics, Weizmann Institute, Rehovot, Israel
Spin based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. Most of the development in spintronics is currently based on inorganic materials. Despite the fact that the magnetoresistance effect has been observed in organic materials, until now spin selectivity of organic based spintronics devices originated from an inorganic ferromagnetic electrode and was not determined by the organic molecules themselves. We found that conduction through double stranded DNA oligomers is spin selective, demonstrating a true organic spin filter. The experimental set-up is shown in Figure 1. The selectivity exceeds that of any known system at room temperature. The spin dependent resistivity indicates that the effect cannot result solely from spin-orbit coupling and must relate to a special property resulting from the chirality symmetry. The results may reflect on the importance of spin in determining electron transfer rates through biological systems. The results are related to former studies on spin dependent photoelectron transmission through monolayers of chiral molecules (1).
References
1. Göhler, B.; Hamelbeck, V.; Markus, T. Z.; Kettner, M.; Hanne, G. F.; Vager, Z.; Naaman, R.; Zacharias, H. Science 2011, 331, 894-897.
9:06 AM
AB-02. Multi-step tunneling in C60-based spin valves
Thi Lan Anh Tran, Tu Quyen Le, Johnny G. M. Sanderink, Wilfred G. van der Wiel and Michel P. de Jong
MESA+ Institute for Nanotechnology, University of Twente, Enschede, Netherlands
The observation of room-temperature magnetoresistance in hybrid devices containing thin layers of organic semiconductors1,2 provides a promising starting point for the development of practical organic spintronic devices. However, the microscopic processes that govern spin-polarized conduction remain poorly understood. In this work, we study C60-based spin valves, with structure Co(8 nm)/Al2O3(2 nm)/C60/Ni80Fe20( 15 nm), in which the C60 layer thickness was varied from 0.5 nm to 10 nm. The results are well described by a model involving a superposition of direct and multi-step tunneling via intermediate states in the C60 layer.1 The junction magnetoresistance (JMR) at room temperature decreases gradually with increasing C60 thickness, from ~16% for 0 nm to ~3% for 7 nm, while it vanishes for 10 nm, consistent with modeling of the JMR in the transition from direct, to two-step-, to multi-step tunneling. The JMR of junctions with a C60 thickness above 2 nm shows a non-monotonic temperature dependence (a maximum is found at T = 50 K), which we ascribe to a competing process: thermally activated multi-step tunneling in combination with spin relaxation and dephasing within the C60 layer due to roughness-induced stray fields in the vicinity of the ferromagnetic interfaces. The latter assertion is supported by the increasingly asymmetric bias dependence of the JMR for junctions with increasing C60 thickness. Model calculations, including a Gaussian DOS in the C60 layer, show a bias-dependent spatial distribution of active two- (or multi-) step tunneling sites, which modifies the effects of the stray fields. This is confirmed by magnetoresistance measurements (at 5 K) for a junction with 5 nm C60, which exhibit small but significant changes for different bias voltages.
References
1. J. J. H. M. Schoonus, P. G. E. Lumens, W. Wagemans, J. T. Kohlhepp, P. A. Bobbert, H. J. M. Swagten and B. Koopmans, Phys. Rev. Lett. 103 (14), 146601 (2009). 2. V. Dediu, L. E. Hueso, I. Bergenti and C. Taliani, Nature Mater. 8, 707-716 (2009).
9:18 AM
AB-03. Tunnel magnetoresistance in Self-Assembled Monolayers Based Tunnel Junctions
Sergio Tatay, Marta Galbiati, Clément Barraud, Pierre Seneor, Richard Mattana, Karim Bouzehouane, Cyrile Deranlot, Eric Jacquet, Albert Fert and Frédéric Petroff
Unité Mixte de Physique CNRS/Thales, Palaiseau, France
Molecular spintronics is an emerging research field at the frontier between organic chemistry and the spintronics concept of adding non-volatility and spin degree of freedom to electronics. Compared to traditional inorganic materials molecules are flexible and can be easily tailored by chemical synthesis. However, due to their theoretically expected very long spin lifetime opportunity, they were first only seen as the ultimate media for spintronics devices and it was only very recently that new spintronics tailoring opportunities, unachievable or unthinkable of with inorganic materials, and that could arise from the chemical versatility brought by molecules and molecular engineering were unveiled. It was shown that the molecular structure, the local geometry at the molecule-electrode interface and more importantly the ferromagnetic metal/molecule hybridization can strongly influence interfacial spin properties going from spin polarization enhancement to its sign control in spintronics devices. In this scenario, while scarcely studied, self-assembled monolayers (SAMs) are expected to become perfect toy barriers to further test these tailoring properties in molecular magnetic tunnel junctions (MTJs). Due to its very high spin polarization and air stability compared to easily oxidable ferromagnetic metals such as Co or Fe La2/3Sr1/3MnO3 (LSMO), a half-metallic manganite with perovskite-type structure, has positioned itself as the electrode of choice in most of the organic spintronics devices. We will present a missing building block for molecular spintronics tailoring: the grafting and film characterization of organic monofunctionalized long alkane chains over LSMO. We have obtained 35% of tunnel magnetoresistance in LSMO/SAMs/Co MTJs. We will discus the unusual behaviour of the bias voltage dependence of the TMR.
9:30 AM
AB-04. Observation of magnetoresistance effects at engineered ferromagnetic/organic-complex interfaces.
Alexander M. Kamerbeek1, 2, Karthik V. Raman1, Arup Mukherjee4, Swadhin K. Mandal4, Markus Münzenberg3 and Jagadeesh S. Moodera1, 5
1Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA; 2Zernike Institute of Advanced Materials, University of Groningen, Groningen, Netherlands; 3I. Physikalisches Institut, University of Göttingen, Göttingen, Germany; 4Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, India; 5Physics Department, Massachusetts Institute of Technology, Cambridge, MA
Recently, interest has gained in understanding the role of interface chemistry in spin injection properties. Charge transfer effects, adatom substitution in the organic molecule and/or hybridization of the organic and ferromagnet (FM) orbitals at the organic/FM interface are known to substantially change the interface properties. This poses a great challenge but can be turned into an advantage by engineering the interface properties, opening up a rich field of spin based interface studies, and allowing the realization of new devices. Here we report a novel interface magnetoresistance (MR) effect: the interaction of the ferromagnet and the organic radical based phenalenyl molecule fully define the device functionality. This interaction causes chemical changes in the normally diamagnetic phenalenyl based molecule resulting in magnetic properties. While the MR effect normally originates from spin-diffusive or tunneling transport, here the organic forms a spin selective injection barrier at the interface much like a spin-filter. This spin-filter like effect is explained by the weak orbital interaction of singly occupied molecular orbitals (SOMOs) causing an antiferromagnetically coupled exchange bias layer. Since the device functionality is determined by the interface it is not affected by bulk disorder. Chemical and structural tuning of the dimer manipulates the SOMO interaction strength allowing the tailoring of the interface exchange interactions. Our measurements of vertical Cobalt/Organic-complex/Permalloy sandwiches showed a unique MR profile reflecting the behavior of two separate internally exchange biased MTJs at the two interfaces. The switching of the cobalt electrode showed an MR effect of up to 50 % at 4 K and persists even above 500mV while the MR resulting from Py switching is relatively smaller. Reaching room temperature MR decreased as generally seen in standard MTJs: for example, an MR of about 22% was seen. In the above structures independent switching of Co, Py and the organic complex was clearly visible, where the organic switches at higher fields of hundreds of Oersteds. This work was supported by ONR grant N00014-09-1-0177 and NSF grant DMR 0504158.
9:42 AM
AB-05. A New Avenue towards Colossal Magnetoresistance in Organic Materials
Jian Shen
1Department of Physics, Fudan University, Shanghai, China; 2Department of Physics and Astronomy, Teh University of Tennessee, Knoxville, TN
A major challenge for the field of organic spintronics is how to achieve large magnetoresistance (MR) in a reliable manner. We have developed a new avenue that dramatically improves MR up to 80,000%. Our approach involves using magnetic quantum dots to introduce a Coulomb blockade (CB) effect in the organic spin valve. The CB effect gives rise to a colossal MR value that persists up to room temperature. The devices prepared by the conceptually new method are highly reliable as well.
10:18 AM
AB-06. Orbital hybridization and oscillatory magnetic polarization of C60/Fe(001) interfaces for spintronics
Michel de Jong1, Lan Anh Tran1, Johnny Wong1, Wilfred van der Wiel1, Yigiang Zhan2 and Mats Fahlman2
1NanoElectronics Group, MESA+ Institute for Nanotechnology, University of Twente, Enschede, Netherlands; 2Department of Physics, Chemistry, and Biology, Linköping University, Linköping, Sweden
In organic semiconductor spintronic devices, the interfacial properties play a crucial role in defining the appropriate conditions for efficient injection and detection of spin polarized charge carriers.[1,2] So far, most studies have focused on spin valve effects in vertical stacks comprising organic semiconductor thin films sandwiches between ferromagnetic electrodes. In part due to the often ill-defined hybrid interfaces in such devices, the microscopic mechanisms governing the magnetotransport behavior remain poorly understood. This understanding may be improved upon exploiting the electronic structure and magnetic properties of well-defined interfaces between ferromagnetic electrodes and organic semiconductors. In this study, we focus on C60 molecules, which are especially interesting for organic spintronics because of the absence of hydrogen nuclei and the associated spin relaxation and dephasing mechanisms by hyperfine coupling. The electronic and magnetic properties of the interface between C60 molecules and a Fe(001) surface have been studied. X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements of C60 monolayers on Fe(001) surfaces show that hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a significant magnetic polarization of C60 π*-derived orbitals. The magnitude and also the sign of this polarization were found to depend markedly on the excitation energy, leading to oscillatory behavior near EF. In parallel, the spin- and orbital moments of surface Fe atoms were found to be affected by hybridization effects as well. These observations underline the importance of tailoring the interfacial spin polarization at the Fermi level of ferromagnet/organic semiconductor interfaces for applications in organic spintronics.
References
[1] C. Barraud, P. Seneor, R. Mattana, S. Fusil, K. Bouzehouane, C. Deranlot, P. Graziosi, L. Hueso, I. Bergenti, V. Dediu, F. Petroff, A. Fert, Nature Phys. 6, 615 (2010). [2] L. Schulz, L. Nuccio, M. Willis, P. Desai, P. Shakya, T. Kreouzis, V. K. Malik, C. Bernhard, F. L. Pratt, N. A. Morley, A. Suter, G. J. Nieuwenhuys, T. Prokscha, E. Morenzoni, W. P. Gillin, A. J. Drew, Nature Mater. 10, 39 (2011).
10:30 AM
AB-07. Reduced spin injection efficiency in organic spin-valves with an interface layer of CuPc
Fengjuan Yue and Di Wu
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing, China
Giant magnetoresistance (GMR) effects were observed in spin valves with a thick organic semiconductor spacer layer, due to relatively weak spin-orbit coupling and hyperfine interaction in organic materials. Several reports showed that the interfaces between ferromagnetic (FM) electrodes and organic materials play an important role in spin transport in organic spin-valves. For instance, inserting a polar layer of LiF at the FM/organic interface can change the sign of magnetoresistance [1]. In order to understand the role of interface, we have carried out a systematic study in devices with or without a thin layer of copper phthalocyanine (CuPc) at the interface of organic spin-valve devices of La0.67Sr0.33MnO3/Alq3/Co. CuPc is one of the typical hole-transport organic semiconductors, due to its relatively high highest occupied molecular orbital (HOMO) level of 5.3 eV. The HOMO level of Alq3 is 5.7 eV. Therefore, hole-injection efficiency can be reduced by introducing a 5-nm-thick CuPc between the FM/Alq3 interfaces. Indeed, the resistance of devices with CuPc is much smaller than that of devices without CuPc. Both types of samples show clear negative GMR effect, similar to previous reports [2]. However, the magnetoresistance (MR) of devices with CuPc is only 0.5 % at 50 K, which is one order of magnitude smaller than MR of devices without CuPc. These results clearly show that the interface barrier has important impact on the spin injection into organic semiconductors.
References
1.L. Schulz, L. Nuccio, M. Willis, P. Desai, P. Shakya, T. Kreouzis, V. K. Malik, C. Bernhard, F. L. Pratt, N. A. Morley, A. Suter, G. J. Nieuwenhuys, T. Prokscha, E. Morenzoni, W. P. Gillin, and A. J. Drew, Nat. Mater. 10, 39 (2011). 2.Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Nature (London) 427, 821 (2004).
10:42 AM< /p>
AB-08. Coupled magnetic spin-valve and electric bi-stability in a single organic device
Mirko Prezioso, Alberto Riminucci, Ilaria Bergenti, Patrizio Graziosi, Rajib Rakshit and David Brunel
ISMN, CNR, Bologna, Italy
Information and communication technology (ICT) is calling for solutions enabling lower power consumption, further miniaturization and multifunctionality requiring the development of new device concepts and new materials[1]. A fertile approach to meet such demands is the introduction of the spin degree of freedom into electronics devices, an approach commonly known as spintronics[2]. This already lead to a revolution in the information storage (GMR readheads) in the last decades. Nowadays, the challenge is to bring spintronics also into devices devoted to logics. Likewise, also the electronics community is committed to follow the Moore’s law, and one of the promising approach which has been recently demonstrated is the use of arrays of crossbar memristors capable of information processing and storing (‘stateful’ logic)[3,4]. We show that an electrically controlled magnetoresistance can be achieved in organic devices combining magnetic bistability (spin-valve) and resistance switching effects and giving rise to a memristive spin-valve[5,6]. In such spin-valve the GMR effect can be turned ON and OFF by a programming bias that sets the device in low or high resistance state respectively. Moreover by applying the correct programming bias the GMR can be smoothly tuned to intermediate magnitudes, corresponding to intermediate resistive states. This kind of device merges the approaches of spintronics and electronics, extending the capabilities of memristors arrays and opening new possibilities. We demonstrate that with this approach an AND logic gate can be implemented with a single crossbar device, and show future perspective for the use of memristive spin-valve in cross bar arrays.
References
[1] International Technology Roadmap for Semiconductors, 2009 ed. www.itrs.net [2] Dediu, V. A., Hueso, L. E., Bergenti, I. & Taliani, C. Spin routes in organic semiconductors. Nat. Mater. 8, 707-716, doi:10.1038/nmat2510 (2009). [3] Borghetti, J. et al. 'Memristive' switches enable 'stateful' logic operations via material implication. Nature 464, 873-876, doi:10.1038/nature08940 (2010). [4] Snider, G. Computing with hysteretic resistor crossbars. Applied Physics a-Materials Science & Processing 80, 1165-1172, doi:10.1007/s00339-004-3149-1 (2005). [5] Hueso, L. E., Bergenti, I., Riminucci, A., Zhan, Y. Q. & Dediu, V. Multipurpose magnetic organic hybrid devices. Adv. Mater. 19, 2639-+, doi:10.1002/adma.200602748 (2007). [6] Prezioso, M. et al. Electrically Programmable Magnetoresistance in Multifunctional Organic-Based Spin Valve Devices. Adv. Mater. 23, 1371-1375, doi:10.1002/adma.201003974 (2011).
10:54 AM
AB-09. Designing molecular spintronics devices in the coherent tunneling regime
Carmen Herrmann1, Gemma C. Solomon2 and Mark A. Ratner3
1Department of Chemistry, University of Hamburg, Hamburg, Germany; 2Nano-Science Center and Department of Chemistry, University of Copenhagen, Copenhagen, Denmark; 3Department of Chemistry, Northwestern University, Evanston, IL
Charge and spin transport on the molecular scale is not only a topic of crucial importance for biological processes such as photosynthesis, but is also highly relevant for potential applications in nanotechnology, for example in molecular electronics [ 1] and spintronics [2]. Our focus is on how theory can contribute to the understanding and design of molecular spintronics devices in the coherent tunneling regime. First, we investigate for iron porphyrin molecular junctions to what extent an electron tunnels through the iron atom as opposed to through the ligand [3]. Second, we ask what makes an organic radical a good spin filter [4]. We employ analyses of local (atomic) contributions to the electronic transmission [5], and of central subsystem molecular orbitals. We use a combination of density functional theory, Green's function techniques and the Landauer-Imry-Buttiker approach. Our results may be relevant for molecular bridges in environments ranging from single-molecule junctions to electron transfer in donor-bridge-acceptor systems to polymeric structures.
References
[1] Nitzan, A., Ratner, M. A., Science 300 (2003) 1384. [2] (a) Sanvito, S., Rocha, A. R., J. Comput. Theor. Nanosci. 3 (2006) 624; (b) Emberly, E. G., Kirczenow, G., Chem. Phys. 281 (2002) 311. [3] Herrmann, C., Solomon, G. C., Ratner, M. A., J. Phys. Chem. C 114 (2010), 20813. [4] (a) Herrmann, C, Solomon, G. C., Ratner, M. A., J. Chem. Phys. 134 (2011) 224306; (b) Herrmann, C., Solomon, G. C., Ratner, M. A. J. Am. Chem. Soc. 132 (2010) 3682. [5] (a) Solomon, G. C., Herrmann, C., Hansen, T., Mujica, V., Ratner, M. A., Nat. Chem. 2 (2010) 223; (b) Todorov, T. N., J. Phys.: Condens. Matter 14 (2002) 3049; (c) Pecchia, A.; Carlo, A. D. Rep. Prog. Phys. 67 (2004) 1497.
8:30 AM - 11:30 AM, Grand Canyon 8
Chair: Oleksandr Serha, Kaiserlauten
8:30 AM
AC-01. Frequency-selective control of FMR linewidth in magnetic multilayers
Sebastian Schäfer1, Neha Pachauri1, Claudia Mewes1, Tim Mewes1, Christian Kaiser2, Qunwen Leng2 and Mahendra Pakala2
1MINT Center, University of Alabama, Tuscaloossa, AL; 2Western Digital, Fremont, CA
We report on frequency-specific, anisotropic linewidth broadening of the ferromagnetic resonance (FMR) mode of a NiFe layer within a magnetic multilayer stack. Besides the NiFe (free layer) the multilayer stack consists of a synthetic antiferromagnet (SAF) pinned by an antiferromagnet. Our broadband FMR studies reveal a significant broadening of the FMR line of the free layer at frequencies where this resonance is degenerate with FMR modes stemming from the SAF (see Fig. 1). We also show that due to the pinning of the SAF, the ferromagnetic linewidth of the free layer becomes anisotropic for a specific frequency. This is caused by the anisotropic ferromagnetic resonance of the pinned SAF, which in turn leads to an anisotropic degeneracy with the (isotropic) ferromagnetic resonance of the free layer. This effect provides a new approach to influence the magnetization dynamics in magnetic multilayers which can be used for example for spin-torque oscillators as well as spin-transfer-torque-switching based memory devices.
8:42 AM
AC-02. Damping phenomena in Co90Fe10/Ni multilayers and alloys
Justin M. Shaw, Hans T. Nembach and Thomas J. Silva
NIST, Boulder, CO
Perpendicular magnetic materials such as Co/Ni are showing promise for use in spintronics and b it patterned media. Measurements of the damping parameter, α in these materials have only been reported in a handful of works.[1-3] Among these reports, α is seen to vary from 0.015 to 0.036 without a full understanding as to why there is so much variation among studies, and what factors such as anisotropy, spin-pumping, and microstructure affect α. Recently, we used perpendicular ferromagnetic resonance to systematically measure the damping parameter in Co90Fe10/Ni multilayers over a wide range of layer thickness.[4] The magnetic anisotropy within this range varied from in-plane to out-of-plane. We measured (Co90Fe10)xNi1-x alloys of identical thickness over the same compositional range of Co90Fe10 and Ni in order to isolate the influence of the multilayer structure. α varied from 0.004 to 0.030 and depended only on the relative amounts of CoFe and Ni, and was independent of the magnetic anisotropy and layer structure. As a result, we will show that α can be calculated from the damping values obtained from pure CoFe and Ni weighted to the spin density of the multilayer or alloy constituents. Our data indicate that the damping in this system is predominately a local effect reflective of the particular damping for the constituent atomic species, subject to averaging by the strong exchange interaction.
References
[1] J.M.L. Beaujour et al. Eur. Phys. J B 59 475 (2007) [2] W.R. Rippard et al. PRB, 81, 014426 (2010) [3] J.M. Shaw et al. JAP 108, 093922 (2010) [4] J.M. Shaw et al. APL, in press (2011)
8:54 AM
AC-03. Observation of nonlinear bistability by use of ferromagnetic resonance in an array of patterned Permalloy stripes
Tom Silva, Hans Nembach and Justin Shaw
Div. 687.03, NIST, Boulder, CO
Nonlinear ferromagnetic resonance (FMR) in spintronic devices has been the subject of recent experimental and theoretical investigations [1,2,3]. Understanding such phenomena is necessary to design spintronic devices for operation at large precession amplitudes [4]. To this effect, we measured nonlinear FMR spectra in lithographically patterned 1 μm stripes of Permalloy. Spectra were obtained using a broadband vector network analyzer system operating in cw mode (i.e. constant frequency) while sweeping the dc applied field. The dc magnetic field was oriented perpendicular to the substrate plane. The microwave magnetic field was as large as 3.5 mT, which was sufficient to observe foldover, i.e. bistable response, in the FMR spectrum of the lowest order eigenmodes for the Permalloy stripes. The critical field for bistability exhibited hysteresis, whereby the critical field at which the system switches from large to small (or small to large) amplitude response depended on the direction of the swept dc field. Data were compared with theory obtained by numerically solving the equation Mssin(θ) = | χ(θ)|hmw, where Ms is the saturation magnetization, θ is the effective precession cone angle, χ is the complex susceptibility, and hmw is the amplitude of the microwave field. The susceptibility χ was derived from the Landau-Lifshitz equation for the case of an ellipsoid, where the effective ellipsoid shape parameters were determined by numerical fitting of small amplitude FMR data for the same stripe sample. In the nonlinear case, the demagnetization tensor component for the substrate normal direction was modified to explicitly depend on the precession cone angle Nzcos(θ). The data and theory for the microwave amplitude dependence of the downward swept bistability critical field agree well until the spectral peak of the susceptibility crosses the resonance field of the next order standing spin wave mode, at which point premature saturation occurs, presumably the result of magnon scattering of the nonlinear pumped mode with degenerate spin waves facilitated by finite stripe width. We conclude that avoidance of nonlinear mode degeneracy is a critical design criterion for large amplitude operation of spintronic devices.
References
[1] W. Chen, et al., Appl. Phys. Lett. 95, 172513 (2009) [2] X. Fan, et al., J. Appl. Phys. 108, 046102 (2010) [3] M. A. Hoefer, et al., Phys. Rev. B 82, 054432 (2010) [4] O. Mosendz, et al., Phys. Rev. Lett. 104 046601 (2010)
9:06 AM
AC-04. Manipulating Spin Dynamics on the Single Atom Scale
Sebastian Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler and A. J. Heinrich
IBM Research - Almaden, San Jose, CA
When magnetic structures shrink to a point where they consist of only a few atoms, their continuously variable magnetization transforms into a spin with quantized states. Then, their dynamical behavior is determined by the energetic distribution of these states and the coupling to the immediate environment. We demonstrate that low temperature scanning tunneling microscopy (STM) can access both the energetic and dynamical properties of electron spins in nanostructures on surfaces. Spins in a solid-state environment experience magneto-crystalline anisotropy due to the electric fields of the surrounding crystal. Fe atoms embedded in the surface molecular-network of a copper nitride (Cu2N) overlayer on Cu (100) possess particularly large easy-axis anisotropy and can be addressed by the STM’s probe tip [1]. The tunnel current between probe tip and magnetic atom excites the atom's spin system. Inelastic electron tunneling spectroscopy (IETS) measures the energy dependence of these spin excitations and enables quantifying the static magnetic properties of the Fe atoms. The STM gains access to the corresponding dynamical properties by replacing the continuous measurement scheme of IETS with an all-electronic pump-probe scheme: short voltage pulses create spin excitations in the magnetic atom and time-delayed weaker probe pulses follow the post-excitation dynamics of the spin system with nanosecond precision [2]. We combined these analytical tools with the STM’s ability to manipulate individual atoms [3]. This enables us to influence the dynamics of individual Fe spins by altering their immediate environment. We find that the spin relaxation time that characterizes the Fe atom’s dynamical behavior increases drastically when copper atoms are placed in close proximity. In contrast, significant reductions of the spin relaxation time can be detected when other magnetic atoms are placed as far as 2 nm away. The ability to manipulate the immediate environment of spins and measure the resulting variations in their dynamical behavior takes the first steps towards intentional engineering of spintronic devices on the atomic scale.
References
[1] C.F. Hirjibehedin, C.-Y. Lin, A.F. Otte, M. Ternes, C.P. Lutz, B.A. Jones, A.J. Heinrich, Science 317, 1199 (2007). [2] S. Loth, M. Etzkorn, C. P. L. Lutz, D. M. Eigler, A. J. Heinrich, Science 329, 1628 (2010). [3] D. M. Eigler, E. K. Schweizer, Nature 344, 524 (1990).
9:42 AM
AC-05. A Quantum-Mechanical Relaxation Model
Ralph Skomski1, Arti Kashyap2 and David J. Sellmyer1
1Physics and Astronomy, Univ Nebraska, Lincoln, NE; 2IIT, Jaipur, India
Our understanding of time-dependent magnetization processes is largely based on the Néel-Brown model and on the Landau-Lifshitz-Gilbert equation. The former model is actually a variation of the Arrhenius model, first applied to magnetism around 1930 [1] and put on a sound statistical foundation by Kramers in 1940. Both models are mesoscopic, that is, they employ parameters such as activation energies and damping constants. From a quantum-mechanical point of view, this approach is very crude, and in the 1960s it became clear that there atomic corrections such as memory functions [2]. These corrections are important for the determination of micromagnetic parameters and have a direct impact on magnetization dynamics. A key aspect is the involvement of the heat bath. If the physical details of the heat bath and the nature of the interactions with the heat bath are known, one can use numerical methods, such as Monte-Carlo simulation. There are also several rather complicated model calculations, often assuming a classical heat bath [3, 5]. Here we present a physically very transparent and analytically solvable quantum-mechanical model. The model is an extension of earlier approaches dealing with classical heat-bath degrees of freedom [3, 4] and can be solved exactly, by using a projection-operator technique. As in other heat-bath master-equation approaches, there is an energy transfer with the heat bath while the total energy remains conserved. The relaxation time is a simple function of the density of states (DOS) of the heat bath, and Zermelo's recurrence paradox is solved by assuming a continuous DOS, in agreement with earlier research on the subject. The simplicity and transparence of the model is paid by a relatively poor description of the internal degrees of freedom of the spin system. In particular, the present approximation yields a straightforward relaxation of the magnetization but ignores spin precession. In the final part of our presentation, we show how precession and spin waves naturally arise as the Hamiltonian is sophisticated. — This research is supported by NSF-MRSEC, DOE, ARPA-E, BREM, and NCMN.
References
[1] R. Becker and W. Döring, Ferromagnetismus, Springer, Berlin 1939. [2] R. Zwanzig, Phys. Rev. 124, 983 (1961). [3] R. Zwanzig, Nonequilibrium Statistical Mechanics, University Press, Oxford 2001. [4] R. Skomski, Simple Models of Magnetism, University Press, Oxford (2008). [5] A. A. Starikov, P. J. Kelly, A. Brataas, Y. Tserkovnyak, and G. E. W. Bauer, Phys. Rev. Lett. 105, 236601 (2010).
9:54 AM
AC-06. Shape dependent magnetization dynamics in single FePt nanomagnets
Rebekah Brandt1, Christoph Brombacher2, Dustin Gilbert3, Philipp Krone2, Fabian Ganss2, Tobias Senn4, Kai Liu3, Manfred Albrecht2 and Holger Schmidt1
1School of Engineering, UC Santa Cruz, Santa Cruz, CA; 2Institute of Physics, Chemnitz University of Technology, Chemnitz, Germany; 3Physics, UC Davis, Davis, CA; 4Institute of Nanometer Optics and Technology, Helmholtz Center Berlin for Materials and Energy, Berlin, Germany
In magnetic recording, fabrication of nanometer-scale features is expensive and time-consuming; therefore, using self-assembled particles for magnetic patterning is an attractive alternative. The deposition of magnetic films on spherical particles [1] results in magnetic caps that exhibit a thickness variation and a change of easy axis direction due to the curvature, so a change in the magnetization behavior compared to flat disks is expected. We report the magnetization dynamics of a 20 nm thick fcc disordered FePt alloy deposited onto 100, 160, and 300 nm SiO2 spheres. The same sizes were patterned using e-beam lithography to create corresponding flat disks. The dynamics were measured using a time-resolved MOKE microscope [2] with a probe beam spot size of 1 μm. Fig. 1 shows the measured peak frequencies of single d=100 nm (a) disks and (b) spheres. Despite their identical dimensions (diameter, height), the nanomagnets show very different dynamics. The flat dots exhibit two main modes (bulk, edge) whose frequencies increase with applied field strength, while the curved magnets show additional modes and different field dependence. Furthermore, using the first-order reversal curve (FORC) method [3], we find that the curved nanomagnets can exhibit qualitatively different reversal behavior compared to flat elements of the same size (Fig. 1(c/d)). Using micromagnetic simulation, we find that the curvature of the magnetic element creates varying demagnetizing fields across the sphere, causing additional oscillation modes. Therefore, it may be possible to design the geometry of a magnetic element such that the dynamics can be tuned.
References
[1] M. Albrecht et al., Nature Materials 4, 203 (2005). [2] A. Barman et al., Nano Lett. 6, 2939 (2006). [3] R. Dumas et al., Phys. Rev. B 75, 134405 (2007).
10:06 AM
AC-07. Intrinsic damping due to electron-magnon interactions
Shulei Zhang and Shufeng Zhang
Physics, University of Arizona, Tucson, AZ
Magnetic damping in transition-metal based ferromagnets has usually attributed to the interaction between conduction electrons and collective spin excitations or magnons[1,2,3]. The interaction generates two types of physical processes. First, an electron absorbs or emits a magnon. At low temperature, this process is operative only for disordered ferromagnet; otherwise the energy and momentum conservation would prohibit absorption or emission of magnons (zero wave-vector magnons correspond to the uniform magnetization precession mode). Another process, known as spin-conversing electron-magnon scattering, is that an electron is scattered by magnons without changing its spin[4]. We show that this latter process exists without disorders at zero temperature and we present our calculations for this intrinsic damping mechanism. By explicitly expressing the electron-magnon interaction in terms of the above two processes, we have calculated the self-energies and deduced damping parameters. We have found that the intrinsic damping in general has the same order of magnitude compared to extrinsic damping at low temperatures. We further apply our calculations to a thin magnetic film with magnetization parallel or perpendicular to the layer. The intrinsic damping parameter scales as the square of temperature. A more interesting result is the thickness dependence of the damping parameter. For an in-plane magnetized film, the damping is inversely proportional to the thickness, which agrees with experimental results[5]. However, for the perpendicularly magnetized layer, the damping increases with thickness. For example, for an ultrathin film with surface perpendicular magnetic anisotropy Ks=-0.6 erg●cm-2, we find that the damping is enhanced by 42% when the film thickness is increased from 4 monolayers to 6 monolayers. These results can be explained in terms of the scattering phases in these two geometries. This work was partially supported by DOE and NSF.
References
[1] E. Abrahams, Phys. Rev. 98, 387 (1955). [2] L. Berger, J. Phys. Chem. Solids. 38, 1321 (197 7). [3] P. Landeros, Rodrigo E. Arias and D. L. Mills, Phys. Rev. B 77, 214405 (2008). [4] A. H. Mitchell, Phys. Rev. 105, 1439 (1956). [5] W. Stoecklein, S. S. P. Parkin, and J. C. Scott, Phys. Rev. B 38, 6847 (1988).
10:18 AM
AC-08. Dynamical Modeling of Nanoparticle Fluctuations and FMR
Stephen E. Russek and Robert J. Usselman
Natl Inst of Standards & Tech, Boulder, CO
We have modeled the temperature and field dependence of ferromagnetic resonance (FMR) in magnetic nanoparticles using a stochastic LLG equation and a modified dynamical equation using stochastic jump processes. The FMR spectra is calculated by taking the averaged power spectra of the transverse magnetization and using the fluctuation-dissipation theorem to obtain the imaginary part of the susceptibility. The parameters are taken to replicate the properties of iron-oxide mineralized Listeria protein cages[1]. The nanoparticles have two magnetic components: a superparamagnetic core with a diameter of ~4 nm and a second component that appears to be small g=2 clusters with ~8μB moments. The stochastic LLG equation adequately models FMR in the superparamagnetic core (Fig. 1) showing a broad peak at low temperatures due to random anisotropy and an apparent narrowing at high temperatures when the particles begin fluctuating and the magnetizations no longer reside near their low energy positions. The LLG equation, which is valid in the regime where the thermal kicks are much smaller than the magnetic energies, cannot model the small spin clusters. Here the stochastic jump model, which does not incorporate a phenomenological damping term, is used. Combining the simulations we can adequately model the observed magnetization and FMR spectra.
References
Usselman R.J.,et al., J.Appl. Phys.107, p114703, (2010)
10:30 AM
AC-09. Origins of Damping in Ultra-Thin Ferromagnetic Films
Lei Lu1, Zihui Wang1, Griffin Mead1, Mingzhong Wu1, Christian Kaiser2, Qunwen Leng2 and Mahendra Pakala2
1Department of Physics, Colorado State University, Fort Collins, CO; 2Western Digital, Fremont, CA
Identification of physical damping processes in magnetic materials is critical to the understanding and control of magnetization dynamics in the materials. This presentation reports on the identification and quantization of magnetization relaxation processes in a multilayered film material which is very similar to the free layers in conventional tunneling magneto-resistance readers. The sample was prepared by magnetron sputtering and consists of a magnetic layer of CoFeB/NiFe and a non-magnetic capping layer of Ru/Ta/Ru. The work was done through ferromagnetic resonance (FMR) measurements and numerical analyses. The FMR measurements used shorted rectangular waveguides and waveguide cavities and were carried out for a field polar angle range from -25 degree to 95 degree, a 180 degree in-plane field rotation, and a frequency range from 8 GHz to 18 GHz. The results indicate that, when an out-of-plane magnetic field is applied, the FMR linewidth consists of a relatively large contribution from Gilbert-type damping and a very small component from film inhomogeneity-associated line broadening. The obtained Gilbert damping constant is about 0.0081. This value is slightly larger than the intrinsic damping constant generally accepted for transition metals, and this slight enha ncement is partially due to the spin pumping effect [1]. When the magnetic field is applied in a direction away from the film normal, there is also a contribution from grain-to-grain two magnon scattering [2], which, however, is smaller than the Gilbert damping contribution. The static magnetic properties determined from the frequency-dependent FMR measurements agree with those obtained from the angle-dependent FMR measurements. [3]
References
[1] B. Heinrich and G. Woltersdorf, J. Supercond. Novel Magnetism 20, 83 (2007). [2] P. Krivosik, N. Mo, S. Kalarickal, and C. E. Patton, J. Appl. Phys. 101, 083901 (2007). [3] This work was supported in part by the U. S. National Science Foundation, the U. S. National Institute of Standards and Technology, and Western Digital Technologies.
10:42 AM
AC-10. Tilt and coherent precession of magnetization induced by picosecond acoustic pulses in ferromagnetic (Ga,Mn)As
Michael Bombeck1, Alexey S. Salasyuk1, 2, Alexey V. Scherbakov2, Dmitri R. Yakovlev1, 2, Andrey V. Akimov2, 3, Xinyu Liu4, Jacek K. Furdyna4, Viktor F. Sapega2, Christian Brüggemann1 and Manfred Bayer1
1Experimentelle Physik II, TU Dortmund, Dortmund, Germany; 2Ioffe Physical-Technical Institute, Russian Academy of Sciences, St.Petersburg, Russian Federation; 3School of Physics and Astronomy, University of Nottingham, Nottingham, United Kingdom; 4Department of Physics, University of Notre Dame, Notre Dame, IN
In this work we use picosecond acoustic pulses to manipulate the magnetization of ferro-magnetic semiconductors (FMSs) in the GHz regime [1]. The direct effect of acoustic pulses on the magnetization we demonstrate is based on the strong dependence of the FMS’s magneto-crystalline anisotropy (MCA) on the strain [2]. Our results open a new approach for control of magnetization in magnetic materials. The structure studied is a 200-nm layer of the FMS GaMnAs grown by low-temperature MBE. The in-plane compressive strain of the FMS layer determines the MCA and leads to the in-plane orientation of the easy axis of magnetization [2] and thus to the orientation of the spontaneous magnetization M in the layer plane. When an external magnetic field B is applied, the balance between the MCA and B determines the direction of M. The idea of our experiment is to modulate the MCA by a picosecond strain pulse and thus to tilt M. In the pump-probe experiments performed at helium temperature, a laser pulse of 200-fs-duration excites a 100nm Al film deposited on the polished back side of the substrate. As a result, a strain pulse of ~10-4 amplitude and ~100 nm spatial sizes is injected into the sample. The strain pulse propagating in the substrate reaches the FMS layer and modulates the direction of M. We monitor the time evolution of M by means of the magneto-optical Kerr effect, measuring the Kerr rotation angle of the linearly polarized probe pulse. The variable delay between the pump and the probe pulses provides sub-ps time resolution. We observe the strain-pulse-induced tilt of M followed by the coherent magnetization precession, which accompanies the return of M to its equilibrium orientation. The maximal achieved tilt amplitude is 10-2 of the saturation magnetization. The tilt amplitude as well as the precession frequency depends on the direction and the strength of B. A simple model, which assumes a linear dependence of the MCA coefficients on the strain, perfectly describes the experimental data. The proposed approach is suitable to man ipulate magnetization and to study ultra-fast magnetic phenomena in various magnetic materials and is not restricted to FMSs or low temperatures.
References
[1] A.V. Scherbakov et al., Phys. Rev. Lett. 105, 117204 (2010) [2] M. Glunk et al., Phys. Rev. B 79, 195206 (2009)
10:54 AM
AC-11. Model of spin transfer induced processional switching in in-plane magnetized magnetic tunnel junctions with perpendicular polarizer
Bertrand Lacoste, M. Marins de Castro, R. C. Sousa, L. D. Buda-Prejbeanu and B. Dieny
Spintec UMR 8191, CEA/CNRS/UJF/G-INP, Grenoble, France
In magnetic tunnel junctions with in-plane magnetization, the influence of a perpendicular anisotropy polarizer in the spin torque (STT) induced free layer switching has been numerically studied. Without perpendicular polarizer, current-induced magnetization reversal is stochastic since in the minimum energy configurations (parallel or antiparallel), the torque exerted on the free layer magnetization to bring it out of its equilibrium position is initially zero. The addition of a perpendicular polarizer gives rise to a finite spin torque as soon as a current flows through the device even in the stable position. This hence initiates the reversal dynamics without requiring random thermal fluctuations. The coupling of a planar free layer with a perpendicular polarizer also gives rise to a magnetization precession around the normal axis, a phenomenon used to create spin torque oscillators (STO) [1]. In this study, we modeled an in-plane free layer under the combined influence of two polarizers: one perpendicularly and one longitudinally magnetized. We use a macrospin model based on the Landau-Lifshitz-Gilbert equation enhanced by the Slonczewski-like term to describe the free layer dynamics. Current pulses of different durations and amplitudes were applied to the structure. By changing the relative STT efficiency of the two polarisers we found two possible outcomes: i) when the longitudinal polarizer spin torque dominates, the magnetization reverses along the easy axis direction. A bipolar current must be used to write the parallel and antiparallel configurations. ii) when the perpendicular polarizer has a stronger influence, the magnetization precesses as in a spin oscillator. The pulse duration must be properly controlled to write the appropriate state. Between these two limiting cases, the magnetization dynamics can be rather complex. Experimental data and real-time magnetization dynamics will be compared to the model results.
References
[1] D. Houssameddine et. al, Spin-torque oscillator using a perpendicular polarizer and a planar free layer Nat. Mater. 6(6) 447-453 (2007)
11:06 AM
AC-12. Electrically detected ferromagnetic resonance measurements in Permalloy nanowires
Zheng Duan1, Carl T. Boone1, Ilya N. Krivorotov1, Nathalie Reckers2, Juergen Lindner2 and Michael Farle2
1University of California, Irvine, Irvine, CA; 2Universität Duisburg-Essen, Duisburg, Germany
We report measurements of electrically detected broadband ferromagnetic resonance (FMR) in lithographically defined Permalloy nanowires. For these measurements, the Permalloy nanowire is placed in close proximity to the short of a gold coplanar strip waveguide. The microwave power applied to the waveguide drives the magnetization pre cession in the nanowire and the four-point resistance of the nanowire is measured as a function of DC magnetic field [1]. The time-averaged resistance of the nanowire depends on the amplitude of the magnetization precession via anisotropic magnetoresistance (AMR), and thus peaks in the dependence of resistance on the bias magnetic field shown in Fig. 1(a) arise from resonant excitation of spin wave modes in the nanowire. Using this electrically detected FMR technique, we measure the frequency and linewidth of both the quasi-uniform mode of magnetization precession and the localized edge mode that exists for magnetic field applied in the plane of the sample perpendicular to the nanowire. The field dependence of the quasi-uniform and the localized modes are shown in Fig. 1(b). We will present measurements of the resonance frequency and linewidth of both modes for several values of the nanowire width and compare our results to theoretical predictions of the field dependence of the mode frequency.
References
[1] M. V. Costache et al., Appl. Phys. Lett. 89, 192506 (2006).
11:18 AM
AC-13. Anisotropy and damping in collective precessional dynamics in arrays of Ni80Fe20 nanoelements
Bivas Rana1, Dheeraj Kumar1, Saswati Barman1, Ruma Mandal1, Semanti Pal1, Satoshi Sugimoto3, Yasuhiro Fukuma2, YoshiChika Otani3, 2 and Anjan Barman1
1Department of Material Sciences, S. N. Bose National Centre for Basic Sciences, Kolkata, India; 2Advanced Science Institute, RIKEN, Wako, Japan; 3Institute for Solid State Physics, University of Tokyo, Kashiwa, Japan
Nanomagnet arrays are technologically important because of their potential applications in magnetic storage, memory, logic, sensors and magnonic crystals. The precessional dynamics occurring in the picosecond timescale can potentially increase the operation time for all these devices. The dependence of the collective precessional dynamics on the geometry of the array and the bias magnetic field is an important issue in magnetization dynamics [1-4]. Here, we report a systematic study of the precessional dynamics of arrays of Ni80Fe20 elements with 200 nm width, 20 nm thickness and edge to edge separation (S) varying from 50 nm to 400 nm. The dynamics was measured in an all-optical time-resolved magneto-optical Kerr effect microscope with an in-plane bias field. The dynamics shows three clear regimes. For S < 100 nm and at bias field = 1.2 kOe, we observe a single precessional mode but for 100 ≤ S ≤ 300 nm, we observe mode splitting and a number of non-uniform collective modes. For S > 300 nm transition to a non-collective regime occurs and we observe only the edge and centre modes of the isolated elements. We have further investigated the dependence of the collective dynamics on the relative orientation of the bias magnetic field with the symmetry of the array. In general we observe a four-fold anisotropy in the precession frequency and in the damping of the precession. In addition, we observe a deviation from the uniform collective dynamics when the angle of the in-plane bias field is varied from parallel to the edge to parallel to the diagonal of the arrays. Acknowledgments: We gratefully acknowledge the financial supports from Department of Science and Technology, Government of India under the grant numbers SR/NM/NS-09/2007 and INT/JP/JST/P-23/09 and Japan Science and Technology Agency Strategic International Cooperative Program under the grant numbers 09158876.
References
1. S. Tacchi et al. Phys. Rev. B 82, 024401 (2010). 2. J. M. Shaw et al., Phys. Rev. B 79, 184404 (2009). 3. A. Barman and S. Barman, Phys. Rev. B 79, 144415 (2009). 4. V. V. Kruglyak et al., Phys. Rev. Lett. 104, 027201 (2010).
8:30 AM - 11:30 AM, Grand Canyon 9-11
Chair: Guoxing Miao, MIT
8:30 AM
AD-01. Soliton propagation through magnetic multilayers
Dorothee C. Petit, JiHyun Lee, Amalio Fernandez-Pacheco, Rhodri Mansell, Reinoud Lavrijsen and Russell P. Cowburn
Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
Antiferromagnetically (AF) coupled multi-layered (ML) stacks with uniaxial anisotropy are promising candidates for non-volatile, ultrahigh density 3D data storage [1]. In these systems, chiral topological kink solitons formed at the interface between two out-of-phase antiparallel domains are used as data carriers (Fig 1a), which can be synchronously shifted up and down using an in-plane rotating field. Proper functioning of these devices is determined by the ability to control the interlayer coupling and the uniaxial anisotropy of the individual layers. Here we experimentally show soliton-like behaviour in a 5μm wide thermally evaporated Py(5nm)/[Cu(10nm)/Py(10nm)]x7 ML disk (Fig 1b). The magnetic anisotropy is induced by application of a magnetic field during growth and the AF coupling is due to the magnetostatic coupling between adjacent disks. Fig 1c shows the MOKE signal vs time when the field sequence shown by the top black arrows is applied. A soliton is injected at the top of the pillar during the initialisation phase, which then propagates down the stack. Because of the limited skin depth of the ML, the reversal of deeper layers in the stack as the soliton propagates downwards gives a smaller MOKE amplitude. This is the first experimental evidence of the existence of these objects.
References
[1] Patent No.: GB0820844.9, US 20100128510
8:42 AM
AD-02. Hall effect-induced acceleration of electromigration failures in spin valve multilayers under magnetic field
Dinggui Zeng1, Jing Jiang1, Kyung-Won Chung2 and Seongtae Bae1
1Electrical and Computer Engineering, Biomagnetics Laboratory, National University of Singapore, Singapore, 117576, Singapore; 2Nuri Vista Co. Ltd., Gasan-dong, Geumcheon-gu,, Seoul 153-786, Republic of Korea
Recently, research interests on electromigration (EM)-induced failures of spin valve multi-layers (SV-MLs), GMR SVs, and MTJs have been dramatically increased due to the geometrically-induced high operating current density [1-2]. However, all the research efforts made so far were focused on studying the physical mechanism responsible for the EM-induced failures under the accelerated electrical stress and temperature conditions, there has been no report on the physical effects of applied magnetic field on the EM-induced failure lifetimes and its characteristics, although most of the GMR SVs, the SV-MLs, and the MTJ based spintronics devices are operated by an externally applied magnetic field [3]. In this work, we present the EM-induced failure characteristics and physical mechanism of NiFe(3)/Co(0.5)/Cu(2)/Co(0.5)/NiFe(3 nm) SV-ML devices stressed by both magnetic and electric fields. It was found that EM-induced failures in SV-MLs were severely accelerated by an externally applied magnetic field. As shown in Fig. 1(a) and (b), the electrically stressed SV-ML device showed a typical slit void failure, while the SV-ML device stressed by both magnetic and electric fields showed a completely different failure modality that a few of amorphous regions were found at the Cu/Co interface and underneath of the Cu spacer. The theoretical and experimental results confirmed that Hall effect-induced Lorentz force applied to the perpendicular-to-the-film-plane direction is primarily responsible for the severe acceleration of the EM failures due to its dominant contribution to abruptly increasing local temperature and current density (see Fig. 1(c) and (d)).
References
[1] S. Bae, J. H. Judy, I.-F. Tsu, M. Davis, and E. S. Murdock, Appl. Phys. Lett. 79, 3657 (2001). [2] J. Ventura, J. B. Sousa, Y. Liu, Z. Zhang, and P. P. Freitas, Phys. Rev. B 72, 094432 (2005). [3] C. Chappert, A. Fert, F. N. Van Dau, Nature Materials 6 813 (2007).
8:54 AM
AD-03. Graded anisotropy and Pd polarization in pressure-varied Co/Pd multilayers
Brian J. Kirby1, Peter Greene2, Mike Fitzsimmons3 and Kai Liu2
1Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; 2Physics, University of California - Davis, Davis, CA; 3LANSCE, Los Alamos National Laboratory, Los Alamos, NM
Recent work predicts that varying the anisotropy in multilayer magnetic media can be used to strike an optimal balance between thermal stability and ease of switching.1 Sensitive to magnetic depth profiles, polarized neutron reflectometry (PNR) can probe how individual regions of differing anisotropy interact to produce the collective magnetic behavior of the multilayer as a whole.2 In this work, we use high-field PNR to characterize a pair of deposition pressure (pd) graded3 Co/Pd multilayer samples - a “lo-hi” sample with 30 repeats deposited at 5 mTorr, followed by 15 at 12 mTorr, and 15 at 20 mTorr, and a “hi-lo” with the reverse deposition order. Fig. 1a shows the lo-hi profiles in absolute units. Although the Co content is uniform, higher pd regions exhibit lower magnetization (M) - even at 10 T - indicating that disorder corresponding to increased pd has a pronounced effect on the saturation M. Additionally, the 10 T M is much larger than can be accounted for by Co alone, implying a Pd polarization of at least 0.7 μB/ Pd. The M / Msaturation profiles are shown in Fig. 1b, and clearly reveal graded anisotropy, with softer M corresponding to lower pd. Similar measurements for the hi-lo sample show a consistently lower M for a given pressure/field as compared to the lo-hi sample - indicating that the magnetic disorder induced by pd can be transmitted across nm of material. This work demonstrates the suitability of pressure grading for producing an anisotropy gradient. The roles of Pd polarization and underlayer-induced disorder in Co/Pd graded media will be discussed. We acknowledge support from NSF (DMR-1008791).
References
1D. Suess, Appl. Phys. Lett. 89, 189901 (2006). 2B. J. Kirby, et al., Phys. Rev. B 81 100405(R) (2010). 3B. J. Kirby, et al., J. Appl. Phys. 105, 07C929 (2009).
9:06 AM
AD-04. Imprinting perpendicular domains into NiFe
Yeyu Fang1, T. N. Anh Nguyen2, Randy K. Dumas1, S. M. Mohseni2 and Johan Åkerman1, 2
1Department of Physics, University of Gothenburg, Gothenburg, Sweden; 2Materials Physics, Royal Institute of Technology (KTH), Stockholm, Sweden
Due to competition between the in-plane (IP) shape anisotropy of NiFe and the strong perpendicular magnetic anisotropy (PMA) of [Co/Pd]5 multilayers(MLs), unique magnetic configurations are achievable in exchange coupled [Co/Pd]5-NiFe thin films [1]. While fundamentally interesting, this system also has potential in magnetic recording media,MRAM, and spin torque oscillators.We present a systematic magnetic force microscopy (MFM) study of the magnetic domain configurations, after out-of-plane ac demagnetization, in two different magnetron sputtered sample series,A: [Co/Pd]5/NiFe(tNiFe) and B: [Co/Pd]4Co-Pd(tPd)-NiFe(5 nm). Selected MFM images for series A are shown in Fig.1 (a)-[(i)-(iii)]. Somewhat surprisingly we find perpendicular labyrinth domains are imprinted very deeply into the top NiFe for tNiFe of up to at least 20 nm. The average domain size decreases with tNiFe, Fig. 1(a)-[iv], consistent with an increase in the effective film thickness of a PMA material.Although the expected magnetization state of a 20 nm thick NiFe would be IP, the exchange coupling to a much thinner PMA material transforms the preferred demagnetized state of NiFe to perpendicular labyrinth domains. Exchange de-coupling via a Pd layer is then investigated in series B and MFM images are shown in Fig.1 (b)-[(i)-(iii)]. The domain size is found to dramatically increase over a small tPd range, consistent with a rapid decrease in exchange coupling. Finally, for tPd=3nm, the labyrinth domains disappear as the NiFe magnetization now lies in its preferred IP orientation as it is decoupled from the Co/Pd ML.
References
[1] T. N. Anh Nguyen,et al, Appl. Phys. Lett., 98, 172502 (2011).
9:18 AM
AD-05. Magnetostatically driven domain replication induced by temperature cycling
S. Majid Mohseni1, Randy K. Dumas2 and Johan Åkerman1, 2
1Materials Physics, Royal Institute of Technology (KTH), Stockholm, Sweden; 2Department of Physics, University of Gothenburg, Gothenburg, Sweden
Spin valves and tunnel junctions with perpendicular magnetic anisotropy (PMA) are promising candidates for spin transfer torque devices [1,2]. A properly functioning device relies on understanding [3, 4] and minimizing interlayer coupling. Here, the temperature dependent interlayer coupling mechanisms in [Ni/Co]5/Cu/[Ni/Co]2 PMA pseudo spin valves are studied. Standard major loop, Fig. 1(a, inset), and first order reversal curve analyses reveal that the [Ni/Co]5 and [Ni/Co]2 layers are highly coupled, even for a thick Cu(8 nm) spacer, consistent with magnetostatically driven domain replication. However, a complete decoupling is found for T<150 K, Fig. 1(a, main panel). The coupling is further probed by temperature cycling measurements (TCMs). During a TCM the temperature is progressively increased through T=150 K to a given maximum temperature (Tmax) and then reduced to 100 K. Prior to each TCM, Figs. 1(b-d), the sample is first AC demagnetizing at room temperature, zero field cooled to 100 K, and a conditioning field (Hcond. = 100, 250, or 500 Oe) is applied and then removed to achieve the three distinct remanent states, illustrated in Fig. 1(a, main panel). For either Hcond.=100 or 500 Oe the TCM is reversible, Fig. 1(b) and (d), respectively. This is consistent with domain replication of either nearly demagnetized [Fig. 1(b)] or uniform [Fig. 1(d)] states onto another, and hence no net decrease in moment is expected. Interestingly, after Hcond.=250 Oe the TCM reveals a significant hysteresis as the remanent magnetization at T=100 K gradually decreases. Here the hard [Ni/Co]2 layer progressively imparts its demagnetized magnetization state into the previously saturated soft [Ni/Co]5 layer as the temperature is increased above the decoupling temperature.
References
[1] S. Mangin, et al. Nature Mater. 5 210 (2006). [2] D. Houssameddine, et al. Nat. Mater. 6 447 (2007). [3] T. Hauet, et al. Phys. Rev. B 77 184421 (2008). [4] V. Baltz, et al. Phys. Rev. B 75 014406 (2007).
9:30 AM
AD-06. [Co/Pd]-NiFe exchange springs with a highly tunable/uniform magnetization tilt angle
Anh Nguyen1, N. Benatmane1, V. Fallahi1, Yeyu Fang1, S. Majid Mohseni1, Randy K. Dumas2 and Johan Åkerman1, 2
1Materials Physics, KTH Royal Institute of Technology, Stockholm, Sweden; 2Department of Physics, University of Gothenburg, Gothenburg, Sweden
The utilization of a tilted spin polarizer is particularly promising for spin transfer torque applications in that both zero field operation and high output power are simultaneously achievable. Unfortunately, the tilt angle is often defined by magnetocrystalline anisotropy, which cannot be readily tuned. An alternative approach, based on exchange coupling a Co/Pd multilayer with a strong perpendicular magnetic anisotropy to a NiFe layer with in-plane anisotropy was recently proposed [1]. In this work we report on an extension to this technique in order to further increase the tunable range of tilt angles and uniformity of the magnetization within the tilted NiFe layer. This is achieved by introducing an exchange de-coupling Pd spacer layer between the Co/Pd multilayer and NiFe. Samples were deposited by magnetron sputtering, and have the following layer structure: [Co(0.5 nm)/Pd(1 nm)]4/Co(0.5 nm)/Pd(tPd nm)/NiFe(5 nm). The calculated tilt angle through the entire film thickness for various Pd spacer thicknesses (tPd) is shown in Fig. 1(a). By simply varying tPd from 0-8 nm, the angle of the magnetization within the NiFe can be tuned from 20-80○, and shows a relatively high uniformity. Furthermore, we are also able to tune α, the damping parameter of the NiFe layer, which shows a twofold increase (Fig. 1(b)).
References
[1] T.N. Anh Nguyen, et. al, APL 98, 172502 (2011)
9:42 AM
AD-07. Magnetization reversal and exchange-bias in hard/soft ferromagnetic bilayers with orthogonal anisotropies
David Navas1, 2, Jacob Torrejon3, Fanny Béron3, Carolina Redondo2, Francisco Batallan4, Boris P. Toperverg5, Anton Devishili5, Borja Sierra2, Fernando Castaño2, Kleber R. Pirota3 and C aroline A. Ross1
1Materials Science and Engineering Department, MIT, Cambridge, MA; 2Quimica-Fisica, Universidad del Pais Vasco (UPV), Leioa, Spain; 3Inst. Fis. Gleb Wataghin, UNICAMP, Campinas, Brazil; 4Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain; 5Department of Physics, Ruhr-Universitat Bochum, Bochum, Germany
The magnetization reversal processes are discussed for exchange-coupled ferromagnetic hard/soft bilayers made from Co0.66Cr0.22Pt0.12 (10 and 20 nm)/Ni (from 0 to 40 nm) films with out-of-plane and in-plane magnetic anisotropies respectively. Room temperature hysteresis loops and first-order reversal curve analysis have been performed to determine the magnetization reversal of the exchange-coupled systems as a function of hard and soft layer thicknesses and field direction. On increasing the Ni layer thicknesses, the net easy magnetization axis reorients in-plane due to the shape anisotropy contribution. When an out-of-plane magnetic field is applied, the hysteresis loops of the bilayers show the characteristic shape of magnetic thin films with perpendicular magnetic anisotropy and a multidomain structure. The different steps of the reversal magnetization process have been analyzed by FORC. “Field-dependent” exchange-bias has been observed at room temperature without the use of an antiferromagnetic layer or a field cooling protocol. A shift along the field axis (HEB) in the in-plane hysteresis loops of the Ni soft layer is observed when measured after the whole system was saturated in-plane. This effect has been associated with specific ferromagnetic domain configurations which were experimentally confirmed by Polarized Neutron Reflectivity measurements. Finally, perpendicular “field-dependent” exchange-bias effects have been also obtained from the out-of-plane hysteresis loops of the CoCrPt hard phase after the whole system was saturated out-of-plane. Residual domains, which were not fully reversed during the minor hysteresis loops, are responsible for this behaviour. The dependence of both in- and out-of-plane “field-dependent” exchange-bias effects of the Ni and CoCrPt layers have been studied as a function of the layer thicknesses and the strength of the applied magnetic field.
9:54 AM
AD-08. Impact of MgO Deposition Conditions on the Texture of Adjacent CoFeB Layers using Ion Beam Assisted deposition
Ricardo A. Ferreira1, 4, Susana C. Freitas1, 2, Paulo P. Freitas1, 2, Rumyana Petrova3, 4 and Stephen McVitie3
1Microsystems and Nanotechnologies, INESC-MN, Lisbon, Portugal; 2Dep. Physics, IST, Lisbon, Portugal; 3Department of Physics and Astronomy, Univ. Glasgow, Glasgow, United Kingdom; 4International Iberian Nanotechnology Laboratory, INL, Braga, Portugal
Ion Beam Assisted Deposition has been used [1] to produce low resistance MgO-based magnetic tunnel junctions. In this paper, the MgO deposition conditions are studied over a wide range of operating conditions, using the assist beam either with large power (concurrent etching regime) or low power (soft energy transfer regime). XRD spectra of Glass/MgO(30nm) show that MgO deposited without the assistance source is always amorphous, and the use of an assist beam has strong impact on the crystallization of both MgO and adjacent Co56Fe24B20 films, which will determine the final device performance [2]. Multilayers of Ta(3)/[CoFeB(2)/MgO(3)]xN/CoFeB(2)/Ta(3) [nm] were deposited using either regime for the MgO growth and annealed at 360C for 1 hour, for final assessmen t of crystallization coherence across the CoFe/MgO films. High resolution TEM cross-sectional images of the stacks (Figure 1) indicate that both growth conditions provide crystalline MgO with (100) texture, but only when the MgO is grown in the soft energy transfer regime can induce the crystallization of adjacent CoFeB layers upon annealing.
References
[1] “Ion Beam Assisted Deposition of MgO barriers for Magnetic Tunnel Junctions”, S.Cardoso, R.J.Macedo, R.Ferreira, A.Augusto, P.Visniowsky, and P.P.Freitas, J.Appl.Phys., vol.103, pp.07A905-07A907, April 2008. [2] “Tunnel magnetoresistance in MgO-barrier magnetic tunnel junctions”, S.Ikeda, et.al, J.Appl.Phys. 99, 08A907 (2006)
10:06 AM
AD-09. Perpendicular magnetic anisotropy in CoFeSiB/Pd multilayers
Seung Hyun Kim, Byong Sun Chun, Do Kyun Kim and Young K. Kim
Department of Materials Science and Engineering, Korea University, Seoul, Republic of Korea
We present magnetic properties of CoFeSiB/Pd multilayers which exhibit and maintain perpendicular magnetic anisotropy at as-deposited state as well as after a series of cumulative heat treatments up to 400°C. The high temperature resistance appears attractive for MRAM applications where high temperature back-end processing influences the stability of the MRAM cells. The CoFeSiB thin film [1] is an amorphous ferromagnet with relatively low Ms (560 emu/cm3). Multilayer samples of Ta 5/Pd 10/[CoFeSiB 0.3/Pd t]9/CoFeSiB 0.3/Ta 5 (in nm), where Pd thickness was varied from 0.3 to 1.5 nm, were prepared by UHV sputtering on Si wafers. Figure 1 (a) shows the magnetic properties of the multilayers as a function of Pd thickness (t). When t > 0.3 nm, a clear PMA was appeared at as-deposited state. As displayed in Fig. 1 (b), the PMA was maintained after a cumulative annealing up to 400°C. At each annealing temperature, the samples were exposed for 1 hr excluding ramping up and down times. The estimated Ku values are over 1 x 106erg/cm3. Upon analyzing microstructural data including XRR, we think the reason for high temperature endurance observed is probably due to the nature of CoFeSiB which is amorphous with no grain boundaries, effective for hindering Pd interdiffusion at elevated temperatures.
References
[1] B. S. Chun et al., Acta Materialia 58, 2836 (2010).
10:18 AM
AD-10. Real-space observation of chiral magnetic order in metallic thin films at room temperature
Yizheng Wu
Physics department, Fudan university, Shanghai, China
Magnetic domains, usually considered as the result of competition of exchange interaction, dipolar interaction and magnetic anisotropy, will not contain chiral order. However, the Dzyaloshinskii-Moriya (DM) interaction, which arises from the spin-orbit scattering of electrons in a system with broken inversion symmetry, will cause chiral magnetic order. Chiral spin structures were observed in helimagnets such as Fe0.5Co0.5Si with a non-centrosymmetric crystal structure (Uchida et. al, Science, 311, 359(2006), Yu et. al, Nature, 465, 901(2010)), and in a Mn atomic layer on a tungsten substrate with inversion symmetry broken at the interface (Bode et. al, Nature, 447,190(2007)). Chirality in nanoscale magnets may play a crucial role in spintronic devices, but this should be performed at room temperature for real applications. In this talk, we will present our results on chiral magnetic order observed directly in real space at room temperature. The experiments were performed with spin polarized low-energy electron microscopy (SPLEEM) at the Lawrence Berkeley national laboratory. An Fe/Ni bilayer grown on Cu(001) exhibits a magnetic stripe domain phase. The domain wall of the magnetic stripe is Néel-type, and the in-plane components of the neighboring domain walls are always antiparallel. These results clearly demonstrate the existence of a cycloidal chiral magnetic order when the magnetic spins rotate. The chiral order in the Fe/Ni bilayer is independent of the orientation and the width of the magnetic stripes, but will disappear for a Ni layer thicker than 7ML. The chirality can switch from the right-hand cycloid in Fe/Ni/Cu(001) to the left-hand cycloid in Ni/Fe/Cu(001), which indicates that the chirality is caused by the DM interaction mainly located at the Fe/Ni interface. A Monte-Carlo simulation can fully explain our results, and also predicts a new type of the skyrmion spin structure. Our results provide a new way to enhance the DMI at room temperature, which will benefit the application of spintronic devices. Work was done in collaboration with G. Chen, J. Zhu, A. Quesada, J. Li, A. N’Diaye, Y. Huo, T. P. Ma, Y. Chen, H. Y. Kwon, C. Won, Z. Q. Qiu, and A. K. Schmid.
10:54 AM
AD-11. Dependence of perpendicular magnetic anisotropy on the buffer layer in CoFeB-MgO based structures
Sung-Min Ahn1, Ocker Berthold2, Alessio Lamperti3, Weiwei Lin1 and Dafine Ravelosona1
1Institut d'Electronique Fondamentale, Orsay, France; 2Singulus technology AG, Kahl am Main, Germany; 3Laboratorio MDM, CNR-IMM, Agrate Brianza, Italy
A perpendicular magnetic anisotropy (PMA) is believed to originate from the presence of interface anisotropy at both the CoFeB-MgO and Ta-CoFeB interfaces after the CoFeB crystallization into the bcc phase under annealing [1,2]. In this study, we experimentally investigate the dependence of PMA of CoFeB-MgO structures on the buffer layer. Samples were deposited using rf sputtering for MgO layers and dc sputtering for metallic layers and were annealed in vacuum for 2 hours at 300 °C. They consist of buffer/CoFeB(1 nm)/MgO(2 nm)/Ta(5 nm), where we adopted four different buffer layers: Ta(5 nm)/Ru(1 nm), Ta(5nm)/Pt(3nm), Ta(5 nm)/Cu(3 nm), and Ta(5 nm). Firstly, note that only the film deposited on Ta/Pt buffer layer exhibits The PMA in the amorphous state, i.e. with no annealing. This is explained by the high interface anisotropy for the bottom Pt/CoFeB interface. As seen in Fig. 1, the PMA was generated after the annealing only when a pure Ta buffer layer is used whereas the in-plane magnetic anisotropy remains after the annealing for Ta/Ru and Ta/Cu buffer layers. It is also demonstrated by using XRD measurements that unlike a Ta layer with bcc structures, the presence of fcc or other structure such as a Cu or a Ru layer underneath the CoFeB layer is not favorable for the crystallization of the CoFeB layer into the bcc structures under the annealing. The authors acknowledge the financial support from European FP7 program through contract MAGWIRE number 257707.
References
[1] S. Yuasa, and D. D. Djayaprawira, J. Phys. D: Appl. Phys. 40 (2007) R337. [2] D. C. Worledge, et al., Appl. Phys. Lett. 98, 022501 (2011).
11:06 AM
AD-12. The concept and fabrication of Exchange Switchable Trilayer of FePtX/FeRh/FeCo with reduced switching field
T iejun Zhou, Kelvin Cher, Zhimin Yuan, Jiang Feng Hu and Bo Liu
Data Storage Institute, Singapore, Singapore
Heat-assisted magnetic recording (HAMR) is a promising approach to achieve multiple Tbit/in2 magnetic recording. Thiele et al [1] proposed the use of FePt/FeRh bilayer as the composite HAMR media for HAMR. Zhu et al [2] further developed the concept and proposed the binary anisotropy media consisting trilayer of a magnetic recording layer, a magnetic assist layer with negative anisotropy and a phase transition layer of FeRh between the recording and assist layers. With such structure, switching field can be greatly reduced based on simulation. In this work, we propose and investigate the reduction of switching field in a exchange switchable trilayer of FePt/FeRh/FeCo, with FeRh forming a very thin layer between FePt and FeCo and working as an exchange-switching layer to turn on/off the coupling between FePt and FeCo upon heating/cooling. At room temperature, FePt and FeCo are magnetically isolated by the antoferromagnetic FeRh layer. After the metamagnetic transition of FeRh layer by heating, FePt and FeCo are exchange-coupled together through ferromagnetic FeRh layer, and therefore the switching field of FePt can be greatly reduced. Here the FeCo provides a higher magnetic moment which can further reduce the switching field compared to the FePt/FeRh bi-layer. FeCo layer also functions as a soft magnetic underlayer to enhance writing. Simulation work was carried out to understand the exchange coupling strength and the FeCo thickness effects on the switching field reduction. It was found that switching field decreases with exchange coupling and but is saturated at a certain level. The switching field also decreases with FeCo thickness. Substantial reduction of switching field can be achieved with increased FeCo thickness. The trilayer was also successfully fabricated. Firstly, (002) oriented FeCo was deposited onto MgO substrate. Then (002) oriented FeRh was grown on FeCo . Last, (001) oriented FePt was deposited onto FeRh layer. XRD showed very good (001) orientation of FePt layer with switching field up to 13 kOe. Temperature dependent DCD curves show a 3 times switching field reduction upon the phase transition of FeRh. The results showed the promising for the trilayer structure for HAMR applications.
References
[1] J.-U. Thiele, S. Maat, and E.E Fullerton, Appl. Phys. Lett. 82, 2859, 2003. [2] J. G. Zhu and D. E. Laughlin, US patent, US2008/0180827
11:18 AM
AD-13. Non-collinear magnetic profile in (Rh/Fe1-xCox)2/Rh(001) bilayer probed by polarized soft x-ray resonant magnetic reflectivity
Marek Przybylski1, Jean-Marc Tonnerre2, Fikret Yildiz1, Helio Tolentino2 and Jurgen Kirschner1
1Max-Planck-Institut für Mikrostrukturphysik, Halle, Germany; 2Institut Néel, CNRS & Université J. Fourier, Grenoble, France
A uniaxial anisotropy of volume character which can keep perpendicular magnetization in relatively thick films is rare. The concept relates to the lattice distortion which modifies the electronic band structure. A model system is provided by Fe1-xCox alloy films tetragonally distorted due to their pseudomorphic growth on Rh(001) [1]. In particular, the Fe0.5Co0.5 films show perpendicular magnetic anisotropy (PMA) up to 20 monolayers at room temperature, whereas Fe layers (x=0) are magnetized in plane. The Fe0.5Co0.5 and Fe layers can be separated by a Rh non-magnetic spacer which mediates a ferro- or antiferromagnetic exchange coupling depending on spacer thickness. Due to competition between the anisotropy and the exchange coupling, the magnetization configuration in such a Fe/Rh/Fe0.5Co0.5 trilayer system can be non-orthogonal [2]. A depth resolved knowledge of the spin structure is required to achieve a complete physical understanding of the system. Soft x-ray resonant magnetic reflectivity (SXRMR) allows us to probe the magnetic profile along the growth axis with in- and out-of-plane magnetization components [3]. The element-resolved magnetic properties are determined from the magnetic asymmetry derived from angle-dependent specular intensities collected for two states of circular polarization or two opposite orientation of an external applied magnetic field. We investigated a Fe/Rh/Fe0.5Co0.5 trilayer system. By using different modes of acquisition, we show that it is possible to determine separately the magnetic profile of the in- and out-of-plane components. Measurements at the Co L3-edge enable us to investigate the magnetism of the alloy layer independently from the pure Fe one. This allows us to propose a magnetic model that describes the effect of an external magnetic field on the magnetic configuration and the interactions between both layers depending on the interlayer exchange coupling. Strong deviation from in-plane magnetization can be observed in the Fe layer and rotation of the magnetization in the FeCo layer towards the film plane is evidenced.
References
[1] F. Yildiz, M. Przybylski, X.-D. Ma, J. Kirschner, Phys. Rev. B 80, 064415 (2009) [2] F. Yildiz, M. Przybylski, J. Kirschner, Phys. Rev. Lett. 103, 147203 (2009) [3] H. L. Meyerheim, J.-M. Tonnerre, L. Sandratskii, H. Tolentino, M. Przybylski, Y. Gabi, F. Yildiz, X. L. Fu, E. Bontempi, S. Grenier, J. Kirschner, Phys. Rev. Lett. 103, 267202 (2009)
8:30 AM - 11:30 AM, Grand Canyon 2-3
Chair: Bala Balamurugan, University of Nebraska
8:30 AM
AE-01. Fe16N2 Interstitial Compound - New Candidate for Permanent Magnetic Material with Rare Earth Element Free -
Migaku Takahashi1, 2 and Tomoyuki Ogawa1
1Department of Electronic Engineering, Tohoku University, Sendai, Japan; 2Center for Nanobioengineering and Spintronics, Chungnam National University, Daejeon, Republic of Korea
To realize ideal ecological social infrastructure in various IT devices and motor application, necessity issue is to develop a new ecological system applying a highly potentialized material newly developed. For the permanent magnet type motor, not only high coercivity but also higher saturation magnetic flux density should be indispensable. In currently used Nd2Fe14B magnet, the magnetocrystalline anisotropy of this phase is still enough to show high Hc. However, we cannot expect high energy product, (BH)max,~64 MGOe due to physical limitation of saturation magnetization, Ms, 168 emu/g. While, from the view point of a mineral resources, rare earth elements such as Nd, Dy, Sm and etc. commonly used for permanent magnet exist in highly deviated regions in the world, therefore, the mineral resources problem in the world wide scale become much more serious for the rare earth elements, especially for Dy. We will focus on a α’, α”-Fe16N2 iron nitride metastable phase with b.c.t structure as a new candidat e for the futured permanent magnetic material with rare earth element free. Gram scale of single phase α” -Fe16N2 nanoparticle powder ranged from several tens to several hundreds nm in size was successfully synthesized with extra high reproducibility via our uniquely developed multi-step procedures using home made iron oxide nanoparticle powder. Thus synthesized α” -Fe16N2 nanoparticle powder shows 234 emu/g of saturation magnetization at 5 K and 9 x 106 erg/cm3 of magnetocrystalline anisotropy energy constant, whose values are superior to those of bulk pure iron and comparable to those of α’,α”-Fe16N2 thin film previously reported by the present authors. X-ray diffraction and Mössbauer spectra revealed the perfect formation of the single phase α”-Fe16N2. Furthermore, through the analysis of switching field distribution in three-dimensional randomly packed this powder, we can estimate that the present powder thus synthesized satisfied the single domain of α”-Fe16N2 single crystal. These results could open a new path of a bulk formation of non-equilibrium metastable interstitial iron nitride phase and will be utilized for the new permanent magnet material.
9:06 AM
AE-02. Magnetism of Directly Ordered Sm-Co Nanoclusters
Balamurugan Balasubramanian1, Ralph Skomski1, Bhaskar Das1, Xingzhong Li1, Shah R. Valloppilly1, George C. Hadjipanayis2 and David J. Sellmyer1
1Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE; 2Department of Physics and Astronomy, University of Delaware, Newark, DE
Bulk Sm-Co has long been valued in permanent magnetism, especially SmCo5, which crystallizes in the hexagonal CaCu5-type structure and combines a huge room-temperature magnetic anisotropy (K1 = 22 × 107 ergs/cm3) with a high Curie temperature and an appreciable saturation magnetic polarization. Research on nanoparticles of Sm-Co alloys is, however, challenged by the need to control the particle size, size-distribution and oxidation. SmCo5 and Sm2Co17 nanoparticles have been previously prepared by surfactant-assisted ball milling of bulk Sm-Co alloys, but these nanoparticles show a very low room-temperature coercivity of 100 Oe and a substantial reduction of magnetization due to the presence of surfactants, incomplete ordering, and oxidation.1 Our recent work on plasma-condensation-type cluster-deposition performed under high-vacuum conditions has been shown to reduce the rare-earth oxidation and also to produce rare-earth alloy nanoparticles with a high-degree of atomic ordering.2 In the present study, we have produced directly ordered Sm-Co nanoparticles by controlling the cluster-deposition conditions prior to deposition and investigated their structural and magnetic properties. The XRD patterns show that Sm-Co nanoparticles with desired crystalline ordering (SmCo5 and Sm2Co17 with hexagonal CaCu5-type and rhombohedral Th2Zn17 structures, respectively) can be directly produced without a high-temperature thermal annealing, which is generally required for alloy formation and crystalline ordering in the case of Sm-Co bulk magnets. Poorly crystallized Sm-Co nanoparticles exhibit only a low room-temperature coercivity of only 100 Oe, whereas crystalline SmCo5 and Sm2Co17 nanoparticles show room-temperature coercivities of 2000 and 750 Oe, respectively. The effect of cluster-deposition conditions on the s tructure and magnetic properties of Sm-Co nanoparticles will be discussed. The direct ordering of Sm-Co nanoparticles prior to deposition will permit the alignment of their easy axes of magnetization with an applied magnetic field and also the assembly of the Sm-Co nanoparticle building blocks for future permanent-magnet and other applications. This research is supported by ARPA-E, DOE, NSF-MRSEC, and NCMN.
References
1 V.M. Chakka, B. Altuncevahir, Z.Q. Jin, Y. Li, and J.P. Liu, J. Appl. Phys. 99, 08E912 (2006) 2 B. Balasubramanian, R. Skomski, X.Z. Li, S.R. Valloppilly, J.E. Shield, G.C. Hadjipanayis, and D.J. Sellmyer, Nano Lett. 11, 1747 (2011).
9:18 AM
AE-03. Aligned and Exchange-Coupled L10 (Fe,Co)Pt-Based Magnetic Films
Yi Liu, Tom A. George, Ralph Skomski and David J. Sellmyer
Physics and Astronomy and Nebraska Center for Materials and Nanoscience, Univ Nebraska-Lincoln, Lincoln, NE
Exchange-coupled magnetic nanostructures have potential as candidates for permanent magnets with higher energy product [1-2]. To maximize the energy product of exchange- coupled magnets, it is important to maximize both the magnetization of the soft phase and the alignment of the hard-phase grains. In this paper we report our recent results of synthesizing aligned L10-structure (Fe,Co)Pt films with Fe-Co soft-phase grains by sputtering Fe-Co and Pt on (001) MgO substrate with in-situ heating at 830oC. The nanostructures and magnetic properties of the films were characterized by X-ray diffraction, transmission electron microscopy and SQUID. The composition of samples are designed so that the Fe:Co atomic ratio is held at 2:1, and the Fe-Co versus Pt ratio is increased about 0.8 at% for each successive sample. In X-ray diffraction patterns three strong peaks are observed in low Fe-Co concentration samples: the L10 (Fe,Co)Pt (001), (Fe,Co)Pt (002) and MgO (002). The fourth peak is observed in high Fe-Co concentration samples and is identified to be a (002) diffraction from a fcc phase. The X-ray diffraction pattern confirmed the formation of L10 (Fe,Co)Pt and its epitaxial growth on MgO. TEM shows that isolated grain structure similar to those observed in FePt is formed. Nano-diffraction patterns confirmed the L10 structure (Fe,Co)Pt and identified the existence of L12 structure (FeCo)3Pt which can be derived from L12 structure Fe3Pt by addition of Co. Fe3Pt has cubic symmetry and is a soft phase. Hysteresis-loop measurements by SQUID show that as the Fe-Co concentration is increased from 57.3 to 62.9 at%, the Ms value increases from 1245 emu/cc to 1416 emu/cc, and the coercivity decreases from 32 kOe to 8.9 kOe. The estimated maximum energy product is 64 MGOe. This work is supported by DOE grant DE-F602-04ER46152 and NCMN.
References
1. W. Liu, Y. Liu, R. Skomski and D.J. Sellmyer, in Advanced Magnetic Materials, eds. Y. Liu, D. Shindo, and D.J. Sellmyer, Volume I, Nanostructural Effects (Springer, Berlin, 2006), p. 182. 2. J.P. Liu in Nanoscale Magnetic Materials and Applications, eds. J.P. Liu, E. Fullerton, O. Gutfleisch, D.J. Sellmyer (Springer, Berlin, 2009), p. 309.
9:30 AM
AE-04. Structural studies of Co-W clusters produced by Inert Gas Condensation
Matthew J. Kramer1, Ying Zhang1, Farhad Golkar2, R. W. McCallum1, Ralph Skomski2, Dave J. Sellmyer2 and Jeff E. Shield2
11Materials Sciences a nd Engineering, Ames Laboratory, Ames, IA; 2Mechanical and Materials Engineering, University of Nebraska, Lincoln, NE
Inert gas condensation (IGC) produces non-equilibrium structures that can vary widely with processing conditions and cluster composition. In this paper, we have investigated the structure, morphology, and composition of clusters produced under different conditions and compositions using transmission electron microscopy. The nanostructural features, especially the particle sizes and the morphology, have been shown to critically influence the magnetic behavior. Both the selected area electron diffraction pattern (SAED) and the high resolution image (HRTEM) show that the Co-W clusters with the power setting of 50W and 100W cooled with liquid nitrogen are amorphous while the Co-W clusters produced with power setting of 150W display discrete diffraction spots in addition to the amorphous rings in the SAED and HRTEM images display some lattice fringes in some clusters. The Co-W clusters with power setting of 50W, 100W and 150W but using water cooling are uniformly crystalline as indicated by both SAED and HRTEM images. Fine clusters with the diameter less than 5 nm were found on the background with the lower power of 50W and the size of the cluster tends to be larger and more uniform with the diameter around 15 nm with the increase of the power. Some hexagonal shape cluster with the size of about 20 nm are observed at the power setting of 100W with increasing proportion at 150W with water cooling. The round shape clusters with the diameter about 10 nm formed at lower power appears to be FCC structure and the larger hexagonally shape clusters are single crystal HCP. High-angle annular dark field (HADDF) STEM image shows a composition contrast in hexagonal-shape cluster with W rich in the 6-symmetry boundary area (confirmed by EDS). Compared with the clusters with water cooling, clusters with liquid nitrogen cooling are prone to be amorphous indicative that the faster cooling rate suppresses crystallization.
9:42 AM
AE-05. Anisotropic nanocrystalline MnBi with high coercivity
Yunbo Yang1, Xuegang Chen1, Xiaobai Ma1, Yingchang Yang1, Jinbo Yang1, Shuai Guo2, Aru Yan2, Qingzhen Huang3, Meimei Wu4 and Dongfeng Chen4
1School of Physics, Peking University, Beijing, China; 2Ningbo Institute of Materials Technology and Engineering,, Ningbo, China; 3National Institute of Standards and Technology, Gaithersburg, MD; 4China Institute of Atomic Energy, Beijing, China
MaThe intermetallic ferromagnetic compound MnBi have attracted much attention because of its high uniaxial magnetic anisotropy, high magneto-optical Kerr effect, and high spin polarization at room temperature. Magnetic hard nanocrystalline MnBi has been prepared by melt spinning and subsequent low temperature annealing. A coercivity of 2.5 T can be achieved at 540 K for MnBi with an average grain size of about 20-30 nm. The coercivity Hc, mainly controlled by the coherent magnetization rotation, shows a strong dependence on the time of grinding and exhibits a positive temperature coefficient from 100 up to 540 K. The unique temperature dependent behavior of the coercivity (magnetocrystalline anisotropy) has a relationship with the variations in the crystal lattice ratio of c/a with temperatures. In addition, discontinuity can not be found in the lattice parameters of a, c, and c/a ratio at the magnetostructural transition temperature. The nanocrystalline powders of MnBi has a maximum energy product (BH)max of about 7.1 MGOe at room temperature and shows anisotropic characteristics with high Mr/Ms ratio up to 560K.
References
[1]T. Chen and W. E. Stutius, IEEE Trans. Magn. MAG-10, 581-686 (1974). [2]X. Guo, X. Chen, Z. Altounian, J. O. Strom-Olsen, Phy. Rev. B 46, 14578 (1992). [3]K. Kang, L. H. Lewis and A. R. Moodenbaugh, Appl. Phys. Lett. 87, 062505(2005).
9:54 AM
AE-06. Hysteresis and Relaxation in Granular Permanent Magnets
Ralph Skomski1, Bala Balamurugan1, Tom A. George1, Mircea Chipara2, Xiao-Hui Wei1, Jeff E. Shield3 and David J. Sellmyer1
1Physics and Astronomy, Univ Nebraska, Lincoln, NE; 2Department of Physics and Geology, University of Texas-Pan American, Edinburg, TX; 3Mechanical Engineering, University of Nebraska, Lincoln, NE
Hard-magnetic nanoparticles, such as FePt [1] and YCo5 [2], can be interpreted as ensembles of interacting magnetic grains, and the same is true for granular permanent magnets. The interactions have far-reaching consequences for the magnetic hysteresis, including demagnetizing factors, and for the slow magnetization dynamics, as observed in magnetic viscosity and ZFC/FC measurements. The simplest approach is the mean-field approximation, which is also the basis for plots derived from Wohlfarth's remanence relation, such as DM plots. However, the mean-field approximation is a poor approach to coercivity and hysteresis [3]. Furthermore, it treats exchange and magnetostatic interactions on equal footing, describing them as positive and negative interactions, respectively. Here we show that magnetostatic interactions cannot be mapped onto a negative interaction field J < 0. To solve this problem and to improve the coercivity and loop-shape predictions of the mean-field approximation, we develop a local mean-field approach with restricted phase space. We consider uniaxial and non-uniaxial particles and model both the hysteresis loops and the slow dynamics of the nanoparticles, the latter by directly solving the thermodynamic master equation. The corrections have a transparent structure for weak interactions but become more complicated as the interaction strength increases. To discuss the applicability of our local approximation and the role micromagnetic correlations, we define and evaluate a micromagnetic Onsager reaction field, in partial analogy to similar atomic approaches [4]. We show that the local mean-field approach present method is fairly accurate for nonequivalent grains with a pronounced switching-field distribution, that is, in the non-cooperative limit [5]. In fact, the Onsager field is a micromagnetic analog to the self interaction in many-body quantum-mechanics and partially accounts for cooperative magnetization-reversal effects. Finally, we compare the present model with other interaction models, especially the Preisach and VFT models. — This research is supported by NSF-MRSEC, DOE (DJS), ARPA-E (BB, JES), BREM (RS, JES), and NCMN.
References
[1] C. B. Rong, D. R. Li, V. Nandwana, N. Poudyal, Y. Ding, Z. L. Wang, H. Zeng and J. P. Liu, Adv. Mater. 18, 2984 (2006). [2] B. Balasubramanian, R. Skomski, X. Z. Li, S.R. Valloppilly, J. E. Shield, G. C. Hadjipanayis, and D. J. Sellmyer, Nano Lett. 11, 1747 (2011). [3] E. Callen, Y. J. Liu, J. R. Cullen, Phys. Rev. B 16, 263 (1977). [4] P. Fulde, Electron Correlations in Molecules and Solids, Springer, Berlin 1991. [5] R. Skomski, J. Phys.: Condens. Matter 15, R841 (2003).
10:06 AM
AE-07. Separated Sm- Co hard nanoparticles by an optimization of mechanochemical processes
Liyun Zheng1, 2, Baozhi Cui1, 3, Wanfeng Li1 and George C. Hadjipanayis1
1Department of Physics and Astronomy, University of Delaware, Newark, DE; 2School of Electromechanical Engineering, Hebei Unversity of Engineering, Handan, China; 3Electron Energy Corporation, Landisville, PA
Synthesis of rare-earth-based magnetically hard nanoparticles with high anisotropy and significant magnetic hardness is highly chanllenging. Mechanochemical processing is a very promising technique for the preparation of separated Sm-Co-based hard nanoparticles. 1 In this study, different types of Sm-Co hard nanoparticles have been synthesized by a modified mechanochemical processing. The phases, microstructure, particle sizes and magnetic properties of the synthesized Sm-Co nanoparticles have been investigated by x-ray diffractometer, scanning electron microscope, transmission electron microscope, vibrating sample magnetometer. The results showed that the precursors and annealing temperatures had great effects on the structure and magnetic properties of the synthesized Sm-Co nanoparticles. It is interesting to find that either Sm2Co7, SmCo5 or Sm2Co17 hard single phases can be obtained by manipulating the amount and ratio of starting materials of Sm and Co. After the mixture of Sm2O3, Co, Ca and CaO powders was milled for 2, 4, 6, and 12 h, no Sm-Co based hard phases was formed, whereas the Sm-Co hard nanocrystallites were formed by a subsequent annealing at 650 oC for 60 min. The synthesized SmCo5 nanoparticles are single-crystals with the CaCu5-type structure, have an average grain size of 150 nm and the highest coercivity of 36 kOe. Both separated Sm2Co17 and SmCo5 nanoparticles were obtained after washing the annealed powder with a solution of acetic acid plus water and ethanol. The washing time and process had critical influence on the magnetic properties of the synthesized nanoparticles. After washing, these nanoparticles showed a core-shell nanostructure with Sm2Co17 or SmCo5 as the core and Sm2O3 as the shell. The coercivity of the washed SmCo5 nanoparticles decreased to 22 kOe by about 30%. The decrease of coercivity resulted from that some of the SmCo5 changed into Sm2Co17 phase due to the partially samarium to form the Sm2O3 nanoshell. Work supported by DOE ARPA-E
References
1. Tsuzuki T, McCormick P G, Mechanochemical synthesis of nanoparticles, J. Mater. Sci.2004, 39: 5143-5146.
10:18 AM
AE-08. Effect of film thickness on magnetic properties and structure in Cr/SmCo/Cr films
Ning Li1, Baohe Li2, Chun Feng1 and Guanghua Yu1
1Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing, China; 2Department of Physics, Beijing Technology and Business University, Beijing, China
Exchange-spring (ES) magnet, which has well exchange coupled interfaces, exhibits a single-phase behavior in the demagnetization process[1]. Nevertheless, it is a challenging work to obtain such a well coupled ES system with a single-phase behavior. Recently, a measurement of the recoil loops of Fe/SmCo bilayers suggested that decoupled behavior was present not only at the interface but also in the SmCo layer[2]. In other words, there is not a single phase in the SmCo layer. In this paper, SmCo films with different thickness are used to investigate the effect of thickness on the magnetic properties and structure. Cr/SmCo/Cr films were deposited on Si substrates by magnetron sputtering, followed by an annealing process. Experimental results shown that the SmCo film of 30 nm exhibited two-phase behavior in the demagnetization process (Fig. 1 inset). The SmCo5 single phase became obvious with the increase of SmCo layer thickness, and shoulder disappeared for the SmCo film of 70 nm (Fig. 1). Further increasing to 140 nm, the film shown two-phase behavior again. Moreover, the coercivity reduced slightly and the saturation magnetization increased obviously with the increasing thickness. XRD results indicated that the phenomenon was attributed to the formation of different SmCo phases in the different thickness, thus the films performed different magnetic behaviors.
References
[1] R. Skomski, J. M. D. Coey, Phys. Rev. B 48, 15812 (1993) [2] Y. Choi, J. S. Jiang, J. E. Pearson, S. D. Bader, Appl. Phys. Lett. 91, 022502 (2007)
10:30 AM
AE-09. Magnetic properties of Sm5Fe17/Fe composite magnets produced by spark plasma sintering method
Tetsuji Saito and Hiroya Miyoshi
Chiba Institute of Technology, Chiba, Japan
There have been a great deal of interest in nanocomposite R-Fe-B magnets due to the high remanence magnetization and maximum energy products[1]. Since the existence of the soft magnetic phase in the composite magnet weakens the coercivity, the hard magnetic phase should have coercivity as large as possible. We have recently found that Sm5Fe17 phase obtained by annealing amorphous Sm-Fe melt-spun ribbon exhibited a large coercivity[2]. In this study, we produce Sm5Fe17/Fe composite magnets by spark plasma sintering (SPS) method. Sm5Fe17 melt-spun ribbons were prepared by melt-spinning. The amorphous melt-spun ribbons were comminuted into powders and then mixed with Fe powders. The Sm5Fe17/Fe powders were consolidated into bulk materials at 773-973 K under an applied pressure of 100 MPa by the SPS method. The Sm5Fe17/Fe powders were successfully consolidated into bulk magnets regardless of their Fe content. X-ray diffraction and thermomagnetic studies revealed that these bulk materials were composite magnets of Sm5Fe17 and α-Fe phases. The Sm5Fe17/Fe composite magnet acted as a single hard magnetic phase and showed a smooth hysteresis loop. Although the coercivity of the composite magnet was smaller than the Sm5Fe17 magnets, it exhibited a higher remanence than the Sm5Fe17 magnets.
References
[1] R. Coehoorn, D. B. de Mooij, J. P. Duchateau, and K. H. J. Buschow, J. Phys., 49, C8-669 (1988). [2] T. Saito, J. Alloys Compd. 440, 315 (2007).
10:42 AM
AE-10. Huge thermal hysteresis loop in indium substituted ε-Fe2O3 nanomagnet
Shin-ichi Ohkoshi1, 2, Takenori Yorinaga1, Shunsuke Sakurai1 and Asuka Namai1, 2
1Department of Chemistry, The University of Tokyo, Tokyo, Japan; 2CREST, JST, Tokyo, Japan
Recently, we have reported single phase of ε-Fe2O3 which exhibits a lot of interesting features. For example, ε-Fe2O3 shows a large coercive field of 20 kOe at room temperature [1]. Herei n, we report that indium-substituted ε-Fe2O3 (ε-In0.04Fe1.96O3) shows a huge thermal hysteresis loop. ε-In0.04Fe1.96O3 nanoparticle was prepared by the combination method between the reverse-micelle and sol-gel methods. Barium nitrate was added to the solution as a shape control agents. With increasing the ratio of [Ba]/[Fe+In], the average volume of particles increased from 6.6×104 nm3 (sample 1; [Ba]/[Fe+In]= 0) to 88×104 nm3 (sample 4; [Ba]/[Fe+In]= 0.4) (the inset of Figure 1). As for sample 1, the magnetization versus temperature curves showed a spontaneous magnetization with a TC of 484 K. As the temperature decreased, the magnetization increased and then dropped at 160 K, whereas, as the sample was warmed, the magnetization increased around 90 K and returned to the initial value. The thermal hysteresis value (ΔT ≡ T1/2↑-T1/2↓) was 6 K. The ΔT value increased with the increase of particle volume, and the ΔT value of sample 4 was as large as 47 K (Figure 1b). It is considered that the observed thermal hysteresis was due to spin-reorientation phenomena [2], and the volume of the particle affected the thermal hysteresis.
References
[1] J. Jin, S. Ohkoshi, K. Hashimoto, Adv. Mater.,16, 48 (2004). [2] S. Sakurai, S. Kuroki, H. Tokoro, K. Hashimoto, S. Ohkoshi, Adv. Funct. Mater., 17, 2278 (2007).
10:54 AM
AE-11. Simulation studies of the coercive behaviour and the energy product for multilayers of FeCo and SmFeN
Alexander Belemuk and Siu-Tat Chui
Department of Physics and Astronomy, Univ Delaware, Newark, DE
We discuss finite temperature Monte Carlo simulation results under periodic boundary conditions for the temperature dependence of the intrinsic coercive field and the energy product of hard soft composites SmFeN/FeCo with the easy axis parallel to the layers. This is compared with our previous studies of composites of SmCo5/FeCo with easy axis perpendicular to the layers [1] and NdFeB/FeCo [2]. In these two previous studies, the energy product is much lower than that based on simple estimates. The orientation of the easy axis is now more favorable than that for [1]. In addition, the intrinsic anisotropy is much better than that for [2]. We determine the system parameters for the optimum energy product.
References
[1] "Magnetizing reversal in multilayer hard-soft composites SmCo5-FeCo" Belemuk AM, Chui ST, JOUR. APP. PHYS., 109 07A729 (2011). [2] "Coercivity and energy product in multilayer hard-soft composites Nd2Fe14B-FeCo", Belemuk AM, Chui ST JOUR. APP. PHYS. 109, 093909 (2011).
11:06 AM
AE-12. One-Step Fabrication of fct FePt Nanocubes and Rods by Cluster Beam Deposition
Ozan Akdogan1, Wanfeng Li1, George C. Hadjipanayis1, Ralph Skomski2 and David J. Sellmyer2
1Physics and Astronomy, University of Delaware, Newark, DE; 2Physics and Astronomy, University of Nebraska, Lincoln, NE
Fabrication of well separated, single crystal nanoparticles with moderate coercivity plays a key role in the development of high density recording media. In this work, single crystal fct FePt nanocubes have been succ essfully produced by a cluster beam deposition technique without the need of post annealing. Particles have been deposited by DC magnetron sputtering using high Ar pressures on both single crystal Si substrates and Au grids for the measurement of magnetic and structural properties, respectively. The nanocubes have a uniform size distribution with an average size of 5 nm. At 1 Torr, the particles have the fct structure with an order parameter of 0.5 and a RT coercivity of 2 kOe with high switching fields seen in the hysteresis loop. Further annealing increased the particle size to 20 nm and the RT coercivity to 10.2 kOe with perfect chemical ordering. In addition to these nanocubes, micron size rods with the fct structure have been observed near the cluster gun. SEM analysis showed that these rods consist of nanoparticles with 20 nm average size. Surfactant assisted high-energy ball milling has been used for separation of the nanoparticles. After one hour of milling, these particles showed a room temperature coercivity of 9 kOe with an order parameter of 0.85. These FePt nanocubes have a potential for use in the development of future high-density magnetic recording media because of their high coercivity, good shape and very narrow size distribution. Work is supported by DOE DE-FG02-04ER4612
11:18 AM
AE-13. High temperature performance of Pr10(Fe,Co,Ni)84B6 nanocomposite alloys
Maria Daniil2, 1, Lamar Minter3 and Matthew A. Willard1
1Naval Research Lab., Washington, DC; 2Physics, George Washington University, Washington, DC; 3Mechanical Engineering, Tennessee State University, Nashville, TN
Nanocomposite (Nd,Pr)2Fe14B/α-Fe magnets have attracted a lot of attention for permanent magnet applications because of their exceptional performance and lower cost. Although they are excellent for room temperature applications, their performance deteriorates rapidly at higher temperatures due to the low Curie temperature of the hard phase (~300°C). On the other hand the Sm-Co-based magnets are preferable for high temperature applications but are considerably more expensive. In this work we improved the high temperature dependence of the Pr-Fe-B-based nanocomposite magnets by simultaneous substitution of Co and Ni for Fe. We prepared a series of nanocomposite ribbons with composition Pr10(Fe1-2xCoxNix)84B6 and x=0.0, 0.05, 0.10 and 0.15 by the melt-spinning technique. X-ray diffraction showed that the melt-spun ribbons are composed of a mixture of 2:14:1 and BCC-(Fe,Co,Ni) phases. The amount of the soft phase increases with the substitution of Co and Ni. Ribbons with higher amounts of Co and Ni substitution also showed an additional minor soft phase with a Pr2Fe23B3-based crystal structure. The room temperature coercivity decreases from 7.4 kOe to 2 kOe with Co and Ni substitutions likely due to the decrease of the magnetocrystalline anisotropy of the 2:14:1 phase and the increase of the amount of the soft phases. Saturation magnetization increases slightly through the composition series while remanence shows a slight decrease for x up to 0.10 and then it drops drastically. Thermomagnetic measurements showed a significant increase of the Curie temperature of the 2:14:1 phase from 281°C for x=0 to 476°C for x=0.15. High temperature hysteresis loop measurements showed improved temperature dependence of remanence and coercivity for x=0.05.
8:30 AM - 11:30 AM, Grand Canyon 4-5
Chair: Jason Janesky, Everspin Technologies
8:30 AM
AF-01. Investigation of perpendicular interface magnetic anisotropy in CoFeB films using seed and insertion layers
David Abraham and D. C. Worledge
IBM-MagiC MRAM Alliance, IBM T.J. Watson Research Center, Yorktown Heights, NY
Recently we demonstrated perpendicular magnetic tunnel junctions with low switching voltages and switching speeds as short as 1 ns[1]. This milestone established a practical realization of spin torque switched magnetic random access memory, and was made possible by the creation of perpendicular magnetic anisotropy (PMA) due to carefully chosen seed and cap layers adjacent to the CoFeB free layer. In this talk, we discuss a strategy of creating thin film samples and measurements using VSM and polar Kerr magnetometry, allowing a systematic survey of the materials, deposition conditions, anneal and geometry that resulted in a viable PMA system. We show results from experiments using Cr, W, Ta, V and oxide and nitride seed layers as a means of inducing PMA on a CoFeB thin film, and in addition discuss the effects of insertion layers (which introduce additional anisotropy-inducing interfaces by dividing the CoFeB film into multiple layers). We will show the effect of varying sputter power used during deposition of the seed layer on the induced anisotropy of the films. Results are presented as a universal plot of perpendicular saturation field measured as a function of the moment/area of the (initially) in-plane CoFeB film. Finally, we introduce the concept of multiple insertion layers which serve to enhance the PMA of the CoFeB system and discuss use of Cr and Ta in this capacity. We show how PMA of this structure gradually diminishes as a function of anneal temperature (Fig. 1).
References
1. D.C. Worledge et al., Appl. Phys. Lett. 98, 022501 (2011)
8:42 AM
AF-02. Statistical and Time Resolved Studies of Switching in Orthogonal Spin Transfer MRAMs
Daniel Bedau1, Dirk Backes1, Huanlong Liu1, Jürgen Langer2, Pradeep Manandhar3 and Andrew D. Kent1
1New York University, New York, NY; 2Singulus Technologies AG, Kahl am Main, Germany; 3Spin Transfer Technologies, Boston, MA
Spin-transfer MRAM memory cells hold great promise as a universal memory technology, promising non-volatility, high speed and small cell sizes. However, challenges remain to achieving high speed and reliable MRAMs with in-plane or perpendicularly magnetized bit cells. As both layers are collinear in both states of the device, there is no spin torque from an applied current in these states and switching cannot be initiated by spin torque alone. Instead, switching relies on the amplification of thermal excitations to initiate the switching process, making it stochastic and leading to long switching times and wide switching distributions [1]. Alternatively to in-plane MRAM, a perpendicular polarizer can be used [2]. OST-MRAM provides near maximal initial torque and does not rely on thermal initiation. The switching process is further accelerated by the demagnetizing field it creates, making it possible to reach sub-100 ps switching times [3]. We fabricated devices based on a Co/Ni-Co/Pd multilayer as a perpendicular polarizer, an in-plane free layer and a readout element formed by a thin MgO tunnel barrier on top of the free layer and a pinned in-plane layer [4]. Above 100% MR were reached for RA = 5 Ω μm 2. The films were structured into devices with sizes from 40nm x 80nm up to 80nm x 240nm. T he devices switch reliably at 0.7V and 500 ps pulse duration. Depending on the pulse amplitude, the switching process is bipolar, as expected for a perpendicular polarizer. We present single-shot time-resolved measurements and discuss the statistics of the switching process. This work was supported by Spin Transfer Technologies.
References
[1] T. Devolder et al., Phys. Rev. Lett 100, 057206 (2008); R. H. Koch et al., Phys. Rev. Lett. 92, 088302 (2004). [2] A. D. Kent et al., Appl. Phys. Lett. 84, 3897 (2004); A. D. Kent, Nature Materials 6, 399 (2007). [3] J.-M Beaujour et al., SPIE, 7398, 73908D (2009). [4] H. Liu et al., Appl. Phys. Lett. 97, 242510 (2010);
8:54 AM
AF-03. Thermally assisted writing in magnetic tunnel junctions with perpendicular anisotropy
Sébastien Bandiera1, Ricardo C. Sousa1, Maria Marins de Castro Souza1, Clarisse Ducruet2, Céline Portemont2, Laurent Vila3, Stéphane Auffret1, Lucian Prejbeanu2 and Bernard Dieny1
1SPINTEC, Grenoble, France; 2Crocus Technology, Grenoble, France; 3CEA/SP2M/NM, Grenoble, France
Magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy (PMA) attract much interest since they allow to scale down the dimension of spintronic devices below the 45nm node, while keeping a sufficient thermal stability. In these MTJ, the magnetization can be switched either by using spin transfer torque (STT) or by field. This work shows how field writing can be used in combination with thermal assistance, as in Heat Assisted Magnetic Recording to achieve simultaneously high coercive field in standby mode (Hc>1000 Oe) and low coercive field at the write temperature. Heating is indeed a very efficient mean to decrease the PMA of the storage layer in order to reduce the field required to switch the free layer in a out-of-plane magnetized MTJ. The developed MTJ stack consists of SAF/MgO/FL, where SAF is a synthetic antiferromagnetic reference layer and FL a CoFeB/(Co/Pd)n multilayer. This free layer has been optimized in such a way that it presents high PMA at room temperature but loses its anisotropy when heated to about 175°C. The anisotropy of FL can fulfill stability requirements down to the 22nm technological node. It will be shown that if a 1.2V pulse is applied, the coercivity is decreased to about 60 Oe, making possible field switching in MRAM cells. Additional optimizations may further reduce the energy consumption. Ultimately, current pulses could also switch the free layer by combined thermal assistance and spin transfer torque.
References
P.J. Jensen, and K.H. Bennemann, Phys. Rev. B 42, 849 (1990)
9:06 AM
AF-04. Spacer layers to improve the magneto-resistance in perpendicular magnetic tunnel junctions with Co|Pd reference layers
Guohan Hu1, Teya Topuria2, Philip M. Rice2, Jean Jordan-Sweet3 and Daniel Worledge1
1IBM-MagIC MRAM Alliance, IBM T J Watson Research Center, Yorktown Heights, NY; 2IBM Almaden Research Center, San Jose, CA; 3IBM T J Watson Research Center, Yorktown Heights, NY
Spin momentum transfer (SMT) devices with perpendicular magnetic anisotropy (PMA) electrodes are expected to greatly reduce the switching voltage compared to the ones with in-plan e magnetic materials. The recently discovered Ta|CoFeB|MgO PMA system provides an ideal free layer for SMT devices because of its high spin polarization, low moment, low damping constant and reasonable anisotropy. To date, an ideal reference layer material, with high spin polarization, high anisotropy and good thermal stability, is still under search. Co|Pd multilayer is one of the most studied PMA systems which can be grown at room temperature. It has been believed that these multilayers are not structurally compatible with MgO tunnel barrier to achieve high tunneling magneto-resistance (TMR). To address this problem, usually a CoFeB like interfacial layer is inserted between the MgO and multilayers. However, various groups have reported fairly low TMR even with a relatively thick CoFeB interfacial layer (1). Our study shows that the structural mismatch between the (111) oriented Co|Pd multilayer and the (100) oriented MgO tunnel barrier may only play a small role in lowering the TMR, while Pd diffusion during the annealing process seems to be the dominating factor. Using in-plane stacks as reference, we have identified various materials as good Pd diffusion barriers during annealing. Among them, thin layers of Ta, Cr and V were successfully incorporated into our PMA stacks, where they serve as spacer layers between the CoFeB interfacial layers and the Co|Pd multilayers. A significant improvement in TMR was observed. For example, in a stack with Ta|CoFeB free layer and Co|Pd reference layer, the TMR was improved from 33% to 71% (measured by current-in-plane-tunneling method), when 2Å of Ta was inserted between 5Å Fe|10Å CoFeB and the Co|Pd multilayers. TMR up to 98% was achieved in fine tuned stacks with RA of 10Ω-µm2 when annealed at 240°C. As annealing temperature increases, TMR drops quickly as a result of more severe Pd diffusion at high temperatures. Our results suggest that a wider search for better spacer layers is one route to improve TMR and thermal stability of PMA junctions with Co|Pd reference layers.
References
1. M. Tofizur Rahman et al., J. Appl. Phys., 109, 07C709 (2011).
9:18 AM
AF-05. Characterization of Interlayer Interactions in OST-MRAM Layer Stacks using Ferromagnetic Resonance
Dirk Backes1, Daniel Bedau1, Huanglong Liu1, Juergen Langer2 and Andrew D. Kent1
1Physics, New York University, New York, NY; 2Singulus Technologies AG, Kahl am Main, Germany
Orthogonal spin-transfer magnetic random access memory (OST-MRAM) devices consist of a spin-polarizing layer with perpendicular magnetization (PP) and a magnetic tunnel junction (MTJ) with in-plane magnetization orientation of the electrodes (see inset of Fig. 1). One of the electrodes of the MTJ, the reference layer, may be pinned by a synthetic antiferromagnet (SAF). Large spin-transfer torques and fast switching have been predicted [1] and prototype devices have recently been demonstrated [2]. For layer stack characterization and optimization, static measurement techniques such as vibrating sample magnetometry (VSM) turn out to be difficult to interpret due to the contributions of each layer to the hysteresis loops. We present an alternative method based on ferromagnetic resonance (FMR) spectroscopy which is commonly applied to individual magnetic thin films [3]. We measure the exchange coupling and exchange bias fields for a MTJ with a SAF, which incorporates a Ru wedge (see Fig. 1). At a Ru thickness of 1.45 nm a change from antiferromagnetical (AFM) to ferromagnetical (FM) coupling is observed while the average exchange bias field is 1.34 kG. Furthermore, we determine the anisotropy and the damping of the free layer (FL) . The impact of these interactions on device performance will be discussed. This work was supported by Spin Transfer Technologies.
References
[1] A. D. Kent et al., Appl. Phys. Lett. 84, 3897 (2004); A. D. Kent, Nature Mater. 6, 399 (2007). [2] H. Liu et al., Appl. Phys. Lett. 97, 242510 (2010). [3] J.-M Beaujour et al., Eur. Phys. J. B 59, 475 (2007).
9:30 AM
AF-06. Design Considerations for Thermal-assistant STT-RAM through Joule Heating
Xiuyuan Bi1, Xiaobin Wang2 and Hai Li1
1Polytechnic Institute of New York University, Brooklyn, NY; 2Seagate Technology, Bloomington, MN
Due to the linear magnetization reversal mechanism, when an MTJ operates at close to Curie temperature, its switching time can be significantly reduced to sub-nanosecond with well-controlled variations [1]. Furthermore, as magnetic element dimension decreases, Curie temperature drops as a finite sizing effect. Therefore, thermal-assistant write scheme through Joule heating is promising in providing ultra-fast STT-RAM technology. Raising MTJ temperature affects the electrical parameters of both MTJ and CMOS devices. Those design issues need to be comprehensively understood and considered. For example, what is the optimal STT-RAM cell structure to raise MTJ temperature through current Joule heating? At which dimension size, Curie temperature is low enough so that the thermal-assistant write scheme through Joule heating becomes feasible? Moreover, previous experiments have shown that the higher temperature can significantly reduce MTJ resistance and degrade MOS transistor driving ability. These two factors together may result in a larger sense range in read operations. In this paper, first we propose the cell design for thermal-assistant STT-RAM with current Joule heating, and analyze the scalability of such a design. A thermal model of STT-RAM cell is built by combining the thermal behaviors of both CMOS and magnetic devices. The model can reveal the temperature relationship of MTJ and MOS transistor with the environment conditions in consideration. Combing this model with a classic sensing scheme designed at room temperature under 45nm technology, the obvious increment in both sensing error rate and sensing latency can be observed. Unless increasing the sense region by introducing more complex sense circuitry design, we propose to relax the design requirement by proving enough cooling time after writing a STT-RAM. The required cooling time and impacts of the operations of the adjacent cells in the array can be obtained by using our thermal model.
References
[1] Xiaobin Wang; , "Exploration on sub-nanosecond spin torque random access memory," GLOBECOM Workshops (GC Wkshps) IEEE, pp.1881-1885, Dec. 2010
9:42 AM
AF-07. Spin torque switching of sub 30-nm CoFeB/MgO MTJ pillars with perpendicular magnetic anisotropy.
Martin Gajek, Michael C. Gaidis, Janusz Nowak, Guohan Hu, Jonathan Z. Sun, Plillip L. Trouilloud, David D. Abraham, Stephen Brown, Yu Zhu, William J. Gallagher and Daniel C. Worledge
IBM-MagIC MRAM Alliance, Yorktown Heights, NY
MRAM cells relying on current-induced spin torque transfer switching are amongst the most promising candidates for the next generation of high-performance memories. The requirements for increased memory density and hence scalability of MRAM call for structure sizes below 40nm. I t has been shown that at small scales, perpendicular anisotropy of the free and reference layers is essential to realize low switching currents with suitable thermal stability and retention time [1,2]. Our approach employs the CoFeB/MgO/CoFeB system with Ta/CoFeB interfaces to impart sufficient perpendicular anisotropy [3,4]. We demonstrate that with such stacks patterned into sub-30nm diameter magnetic tunnel junctions, promising switching characteristics are achievable: TMR ratio over 70%, switching currents of 25μA, and thermal stability Eb~38kT. We will discuss fabrication techniques, test and characterization methodology, and comparisons between our experimental results and the predictions of a theoretical macrospin model.
References
[1] J. C. Slonczewski, U.S. Patent 5,695,864 _Dec. 9, 1997 [2] J. Z. Sun, Phys Rev B. 62, 570, (2000) [3] D. C. Worledge et al, Appl. Phys. Lett. 98, 022501 (2011) [4] S. Ikeda et al, Nature Materials, 9, 721 (2010)
9:54 AM
AF-08. Numerical investigation of damping effects on single and dual-polarizer devices in scaling down perpendicular and in-plane STT-MRAM Cells
Kwaku Eason1, Kim Piew Tan3 and Rachid Sbiaa2
1Advanced Concepts Group, Data Storage Institute, Singapore, Singapore; 2Spintronics, Media, and Interface Division, Data Storage Institute, Singapore, Singapore; 3Mechatronics and Recordings Channel Division, Data Storage Institute, Singapore, Singapore
Spin-transfer torque magnetic RAM devices are strong candidates for improved on-chip memory not only owing to nonvolatility but also attractive features such as low power consumption, fast performance and high endurance are expected. As candidates for memory, there is an interest to scale the devices down for higher capacity. But, magnetic samples have shown increased magnetic damping constants with decreasing dimensions,e.g.[1]. Since magnetic damping is known to be detrimental to STT devices, effects of damping/sensitivity to damping, and ways to control it may be important to understand for increasing capacity. To examine device performance with damping, micromagnetics modified to include adiabatic [2] and nonadiabatic [3] spin torque terms is solved and effects of damping are investigated numerically for single STT-MRAM cells with single/dual reference layers(Figure 1). Two observations are discussed: (1) adverse effects of damping are found to be significantly different between in-plane and perp anisotropy, where perp damping sensitivity is seen to be much better(Figure 2); (2) a dual-polarizer configuration, e.g. [4],[5], is considered and it is seen that further improvements, not only in reduced switching current/asymmetry, but reduced sensitivity to damping is seen with dual anti-aligned polarizers.
References
[1] J. P. Nibarger, R. Lopusnik, Z. Celinski, T.J.Silva, “Variation of magnetization and the Lande g factor with thickness in Ni-Fe Films”, Appl. Phys. Lett., 83, pp 93-95, 2003 [2] J. C. Slonszewski “Current-driven excitation of magnetic multilayers,” J. Mag. and Mag. Mat, 159, pp L1-L7,1996 [3] S. Zhang and Z. Li, “Roles of Nonequilibrium Conduction Electrons on Magnetization Dynamics of Ferromagnets,” Phys. Rev. Lett., 93, 127204, 2004 [4] R. Sbiaa, Randall Law, Ei-Leen Tan, and Thomas Liew, “Spin transfer switching enhancement in perpendicular anisotropy magnetic tunnel junctions with a canted in-plane polarizer”, J. App. Phys., 105, 013910, 2009 [5] Driskell-Smith, et al., “Non-volatile Spin-Transfer Torque RAM (STT-RAM): An analysis of chip data, thermal stability, and scalability”, Memory Works hop (IMW), 2010 IEEE International, doi:10.1109/IMW.2010.5488325
10:06 AM
AF-09. Enhanced Perpendicular Magnetic Anisotropy in thin CoFeB Films
Jimmy J. Kan1, Kangho Lee2, Jonathan J. Sapan1, Seung H. Kang2 and Eric E. Fullerton1
1Center for Magnetic Recording Research, University of California, San Diego, La Jolla, CA; 2Advanced Technology, Qualcomm Incorporated, San Diego, CA
There has been great interest in perpendicular magnetization of thin CoFeB films to fabricate perpendicular magnetic tunnel junctions (pMTJs) using CoFeB-based free layers. pMTJs with CoFeB free layers have recently been reported, demonstrating feasibility of using a simple CoFeB single free layer for pMTJ fabrications [1-2]. However, it is still questionable whether CoFeB-based pMTJs can meet MTJ performance metrics required for spin-transfer-torque magnetoresistive random access memory (STT-MRAM). The key material challenge for realizing manufacturable CoFeB-based pMTJs is to maximize the product of effective magnetic anisotropy energy (Keff) and the film thickness (t) because thermal barrier (EB) between magnetic states is proportional to Keff-t. Since perpendicular magnetic anisotropy in thin CoFeB films results from surface anisotropy (Ks), Keff-t can be described by a phenominological relationship: Keff-t= 2πMs2-t + Ks. When a MTJ cell size and target EB are provided, the required Keff-t can be determined. In order to minimize spin-torque switching current density, it is desirable to meet the target Keff-t with decreased Ms and increased film thickness. The equation implies that for STT-MRAM applications, critical film thickness for perpendicular magnetization (tc) is more relevant parameter than Ks. In this talk, we report that PMA in thin CoFeB films can be enhanced by engineering the top and bottom CoFeB interfaces, resulting in thicker CoFeB perpendicularly magnetized. JJK is supported by NSF award #1008654 and JJS is supported by NSF award #1002147.
References
[1] S. Ikeda, et al., Nat. Mater. 9, 721 (2010). [2] D. C. Worledge, et al., Appl. Phys. Lett. 98, 022501 (2011).
10:18 AM
AF-10. Development of perpendicular-MgO-MTJs with RA-product below 3 Ωμm2 prepared at room temperature
Kay Yakushiji, Hitoshi Kubota, Akio Fukushima, Shinji Yuasa and Koji Ando
Spintronics Research Center, AIST, Tsukuba, Japan
MgO-based magnetic tunnel junctions with perpendicularly magnetized electrodes (p-MgO-MTJs) have been attracting a great deal of attention as memory cells in ultra-high-density spin-transfer-torque random access memory (Spin-RAM or STT-RAM). Concerning the readout performance of Spin-RAM, a p-MgO-MTJ as a memory cell has to satisfy not only a magnetoresistance (MR) ratio higher than 50-100% for the high signal to noise ratio but also a low resistance-area (RA) product for the impedance matching [1]. With increasing the memory capacity, the smaller memory cell requires the lower RA-product, e.g., RA of 3 Ωμm2 for 10 gigabit-scale. So far, we have already attained a low RA of 5.6 Ωμm2 with a high MR ratio of 105% in a p-MgO-MTJ [2]. Here we report a low-RA below 3 Ωμm2 with an MR more than 100% which firstly achieves the 10 gigabit requirement. We fabricated p-MgO-MTJs with a [Co(0.24 nm)/Pt(0.16 nm)]7/CoFeB/CoFe bottom electrode (free layer) and a CoFe/CoFeB/TbFeCo top electrode (reference layer) using C-7100 (Canon Anelva). All of the process was carried out at room temperature (RT). Samples were post-annealed at 275 °C. Thanks to the RT synthesis of an artificial Co-Pt alloy, the better flatness of the free layer than the one using heating process yielded a low RA-product of 2.7 Ωμm2 and a high MR ratio of 105% simultaneously. We also prepared a p-MgO-MTJ with ultra-low RA-product, and obtained RA-product of 1.0 Ωμm2 with the MR ratio of 50%. This work was supported by New Energy and Industrial Technology Development Organization (NEDO).
References
[1] K. Yakushiji et al., Appl. Phys. Exp. 3 (2010) 053003. [2] K. Yakushiji et al., Digest of 55th MMM (2010) GA-03.
10:30 AM
AF-11. Towards Planar-Hall-effect magnetic random access memory with permalloy.
Yevgeniy Telepinsky, Vladislav Mor, Moty Schultz and Lior Klein
Department of Physics, Bar-Ilan University, Ramat Gan, Israel
While currently developed magnetic random access memory (MRAM) is widely based on tunneling magnetoresistance, there could be advantages (such as in simplicity and power consumption) to MRAM based on other magneto-transport phenomena such as planar Hall effect (PHE). An obstacle for the realization of PHE-based MRAM is the need for a magnetic layer with bi-axial magnetic anisotropy - a property found intrinsically in compounds which are not widely used by industry. Here we demonstrate that by nanolithography-induced shape anisotropy we can achieve stable bi-axial magnetic anisotropy in permalloy films for devices as small as 100 nm x 100 nm which may make PHE-MRAM more feasible. The top figure demonstrates the existence of two stable states. It shows PHE as a function of the angle between the current and the applied magnetic field of 100 Oe (full symbols) and with the field switched-off at each angle (empty symbols). The bottom figure demonstrates sharp switching between the two stable states. The device is prepared in a state with positive value of PHE and the PHE is measured after an increasing field is switched on and then switched off. This experiment is repeated for different field orientations and the field value at which switching occurs is consistent with bi-axial magnetic anisotropy.
10:42 AM
AF-12. Spin Transfer Torque Switching Above Room-Temperature
Hui Zhao1, Pedram K. Amiri2, Yisong Zhang1, Andrew Lyle1, Yu-Jin Chen3, Graham Rowlands3, Pramey Upadhyaya2, Zhongming Zeng4, J. A. Katine5, Juergen Langer6, Kosmas Galatsis2, Hongwen Jiang4, Ilya N. Krivorotov3 and Jian-Ping Wang1
1Electrical Engineering, University of Minnesota, Minneapolis, MN; 2Electrical Engineering, University of California, Los Angeles, Los Angeles, CA; 3Physics and Astronomy, University of California, Irvine, Irvine, CA; 4Physics and Astronomy, University of California, Los Angeles, Los Angeles, CA; 5Hitachi Global Storage Technologies, San Jose, CA; 6Singulus Technologies, Kahl/ Main, Germany
The temperature dependent spin transfer torque (STT) switching measurement is crucial for spin transfer torque random access memory (STT-RAM) application since the device wo rks in a heated environment. In this work, we investigate the magnetic tunnel junction (MTJ) performance from 20 °C to 90 °C. The particular temperature range is chosen to imitate the real working environment of STT-RAM application. The TMR ratio, coercivity, critical current and STT switching speed are studied. Figure 1(a) shows the TMR ratio and free layer coercivity as a function of temperature. The TMR ratio decreases for 12.5% at 90 °C. The thermal stability factor is estimated to be 70 from the fitting of coercivity reduction on temperature [1]. STT switching probability measurement at nanosecond region is also performed at various temperatures as shown in Figure 1(b). The temperature dependent read error and write error are investigated.
References
[1] R. H. Victora, Phys. Rev. Lett. 63, 457 (1989).
10:54 AM
AF-13. Effects of CoFe Seed layer on Structural and Magneto-transport Properties of MTJs with Natural Oxidized MgO Barrier
Chikako Yoshida, Takao Ochiai and Toshihiro Sugii
Low-power Electronics Association & Project, Tsukuba, Japan
For naturally oxidized MgO barrier MTJs, magnetotransport properties are poor due to inferior crystallinity of the MgO barrier. It is effective to insert a CoFe seed layer under the MgO barrier as a template for crystallization [1]. Here, we investigate the effects of CoFe seed layer using high resolution TEM and EELS. RA vs. MR ratio for rf-sputterd MgO MTJ(sample-A), oxidized MgO barrier MTJ(sample B), and oxidized MgO barrier MTJ with CoFe seed layer (sample-C) are shown in Fig. 1(a). The sample-B shows relatively low MR ratio. After inserting the CoFe seed layer, the MR ratio has improved about 13%(sample-C). According to the TEM images the large fluctuations in barrier layer interfaces and high intensities of boron in the barrier layer were observed in sample-B. Note that both interface fluctuation and boron content reduced drastically, by inserting the CoFe layer(sample-C). The sharp interface of MgO barrier and low boron content in the MgO barrier might be of key importance for the high magneto-transport properties. This work is supported by the Ministry of Economy, Trade and Industry (METI) and New Energy and Industrial Technology Development Organization (NEDO), both in Japan.
References
[1]Y. S. Choi et al., Jpn. J. Appl. Phys. 48, (2009) 120214.
11:06 AM
AF-14. MTJ Design Margin Exploration for Self-Reference Sensing Scheme
Zhenyu Sun1, Xiaobin Wan2 and Hai Li1
1Electrical and Computer Engineering, Polytechnic Institute of New York University, Brooklyn, NY; 2Seagate Technology, Bloomington, MN
Spin-transfer torque random access memory (STT-RAM) becomes a promising memory candidate for future computing systems for its fast access time, high density, nonvolatility, and small write current. However, like all the other nanoscale technologies, STT-RAM design suffers from process variations, which causes large distributions of the high and low resistances of memory cells. As a result, the read failure of the conventional sense scheme using dummy cells as references increases significantly. Recently, some self-reference sensing schemes [1][2] have been proposed. By distinguishing MTJ’s slope difference between high and low resistance states and eliminating reference dummy cells, the impact of bit-to-bit variations can be minim ized. As technology keeps scaling down, the process variations show more severe impacts on STT-RAM design, and hence, the self-reference scheme will become the main trend. In this paper, we will discuss the MTJ design requirements for the self-reference scheme. Unlike the conventional sensing scheme that requires MTJs with a large TMR ratio and uniform high and low resistance, the self-reference sensing scheme is more sensitive to the roll-off slope of MTJ’s R-I or R-V curve. In addition, it requires two read currents which are constrained by both circuit design and MTJ device characteristics. Because the system design is limited by many factors such as sense margin of sense amplifier and current source accuracy, the MTJ design should be optimized to obtain a better trade-off between device and circuit levels. In this work, the MTJ device physics (such as bias voltage dependent conductance, spin torque, etc.) and design matrix to facilitate the self-reference scheme will be comprehensively investigated and analyzed. The variations of MTJ resistances and roll-off slope of R-I or R-V curves are also considered.
References
[1] Y. Chen, et al., ISLPED 2010. [2] Z. Sun, et al., ICCAD 2010.
11:18 AM
AF-15. Multiscale Micromagnetism of Co-Pd Multilayers
Priyanka Manchanda1, 2, Ralph Skomski1, Pankaj K. Sahota1, 2 and Arti Kashyap2
1Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE; 2Department of Physics, The LNM Institute of Information Technology, Jaipur, India
Magnetic thin films and multilayers consisting of alternating layers of 4d/5d and 3d elements are interesting research objects, due to their intriguing electronic structure and to potential applications in permanent magnetism and other areas such as ultra high density magnetic recording and magneto-optical memory storage devices [1]. In our presentation, we use first-principle and micromagnetic model calculations to investigate multiscale effects in ComPdn multilayers (m = 2, n = 4).The first-principle calculations are employed using the Vienna ab-intio simulation package (VASP) program with a spin-polarized generalized-gradient approximation (GGA) to determine atomically resolved intrinsic parameters. The interlayer exchange coupling, mediated by the Pd, is 22.4 mJ/m2, the net anisotropy is 0.75 MJ/m3, and the average magnetization is 0.51 T. The layer-resolved atomic magnetic moments are 1.923 µB for the Co layers, 0.256 µB for the Pd layers adjacent to the Co, and 0.180 µB for the Pd atoms in the middle of the Pd layers. The anisotropy largely arises from the Co-Pd interface, although there is a uniaxial bulk correction due to the tetragonally strained Co layers. From the calculated interlayer exchange we calculate an exchange stiffness, which enters the micromagnetic equations as a parameter and is much smaller than for typical [2] room-temperature ferromagnets. We discuss two micromagnetic phenomena and the corresponding multi-scale corrections. First, we consider the long-wavelength spin-wave spectrum and show that the micromagnetic approach works very well. Second, we look at the formation of magnetic domain walls. In this case, there are substantial corrections, similar to the narrow-wall corrections originally discussed for SmCo5 [3], which can be interpreted as a natural multilayer of Sm-Co and Co layers. However, the magnetization remains nearly homogeneous in each Co layer, due to the relative weakness of the interlayer exchange. This work is s upported by DST (Indo-European DYNAMAG project and Nano Mission), NSF MRSEC, BREM, and NCMN.
References
References [1]. D. Weller and T. McDaniel, in: Advanced Magnetic 1. Nanostructures, Eds.: D. J. Sellmyer and R. Skomski, Springer, Berlin 2006, Ch. 11, p. 295. [2]. R. Skomski and J. M. D. Coey, Permanent Magnets, Institute of Physics, Bristol 1999. [3]. H. R. Hilzinger and H. Kronmüller, Phys. Lett. A 51, 59-60 (1975).
8:30 AM - 11:30 AM, Grand Canyon 12-13
Chair: Yayoi Takamura, UC Davis
8:30 AM
AG-01. Interfacial ferromagnetism and exchange bias in CaRuO3/CaMnO3 superlattices
Chunyong He1, Meng Gu2, Nigel D. Browning2, 3, Yayoi Takamura2, Brian J. Kirby4, Julie A. Borchers4, Xiaofang Zhai1, Virat V. Mehta1, 5, Franklin J. Wong1, 5 and Yuri Suzuki1, 5
1Materials Science and Engineering, University of California-Berkeley, Berkeley, CA; 2Chemical Engineering and Materials Science, University of California-Davis, Davis, CA; 3Condensed Matter and Materials Division,Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA; 4NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; 5Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
Novel functionalities from heterointerfaces not present in the constituent materials have been a central topic in complex oxides recently. In particular, ferromagnetism (FM) generated from two non-ferromagnetic (FM) materials has attracted special attention not only from the perspective of fundamental research but also from potential applications associated with spintronics architecture. Takahashi et al. discovered interfacial FM in superlattices made of G-type antiferromagnetic (AFM) insulator CaMnO3 (CMO) and paramagnetic metal CaRuO3 (CRO) [1]. The origin of FM has been attributed to electron leakage from CRO to CMO [1, 2], although interdiffusion at interfaces could not be completely dismissed [3, 4]. In order to determine the origin of FM in CMO/CRO superlattices, we have examined epitaxially grown superlattices [(CRO)3/(CMO)N]10 on (001) SrTiO3 with variable CMO thickness and fixed CRO thickness. We observe exchange bias in the system that is direct evidence of magnetic coupling between an interfacial FM layer and an adjacent AFM layer. The CMO layer thickness dependence of the exchange bias field suggests that the FM layer induced at interfaces remains constant from superlattice to superlattice. The observations made from polarized neutron reflectivity (PNR) experiments indicate that FM signal arises from interfacial layers in the superlattices. Moreover, two distinctive interfacial Mn moments of ~1.0µB/Mn and ~0.5µB/Mn are found for the even and odd N superlattices for N >3. Such a striking difference in moments indicates the possible presence of an oscillatory interlayer coupling between neighboring FM interfaces via AFM CMO layers. In conclusion, the observation of exchange bias and its CMO thickness dependence together with the PNR experiments suggest that interfacial FM does not primarily come from interdiffusion and is limited to only one CMO layer at each interface. This work is supported by the Army Research Office MURI program. VVM is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Depa rtment of Energy under Contract No. DE-AC02-05CH11231.
References
[1] K. S. Takahashi, M. Kawasaki, and Y. Tokura, Appl. Phys. Lett. 79, 1324 (2001). [3] B. R. K Nanda, S. Satpathy, and M. S. Springborg, Phys. Rev. Lett. 98, 216804 (2007). [3] A. Maignan, C. Martin, M. Hervieu, and B. Raveau, Solid State Commun. 117, 377 (2001). [4] C. He, X. Zhai, V. V.Mehta, F. J. Wong, and Y. Suzuki, J. Appl. Phys. 109, 07D729 (2011).
8:42 AM
AG-02. Evidence for High Spin Ru4+ in SrRuO3 Thin Films
Alexander Grutter1, 2, Franklin Wong1, Elke Arenholz3, Arturas Vailionis4 and Yuri Suzuki1, 2
1Materials Science and Engineering, University of California, Berkeley, Berkeley, CA; 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA; 3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA; 4Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA
Thin films of the transition metal complex oxide SrRuO3, a unique example of strong 4d itinerant ferromagnetism, have shown highly tunable magnetic properties including enhanced saturated magnetic moments and the stabilization of a high-spin Ruthenium state. [1,2] Other studies have reported the magnetic easy axis of SrRuO3 films on (100) and (110) oriented SrTiO3 to be normal to the substrate surface regardless of crystallographic orientation.[3] We have explored the relationship between the enhanced moment and magnetic anisotropy in compressively strained films on (100), (110), and (111) SrTiO3, (La,Sr)(Al,Ta)O3, LaAlO3, and tensily strained films on KTaO3. By systematically varying the magnitude and symmetry of the applied lattice distortions, we demonstrate that strain is controlling both the magnetic anisotropy and enhanced moment. We find that all compressively strained films have enhanced moments which are highly sensitive to the magnitude and symmetry of the strain. In trigonally distorted films the enhanced moment is so large that it can only be explained in terms of a transition from low-spin to high-spin Ru4+. Carrier concentrations extracted from Hall Effect measurements show a higher carrier concentration in more distorted films with higher saturated magnetic moments. At the same time, the out-of-plane easy axis exhibited by all compressively strained films is correlated with a greater out-of-plane orbital moment, as calculated from the sum rules for X-ray magnetic circular dichroism spectra. The Hall Effect and XMCD measurements can be explained by the same root cause - anisotropically reduced orbital overlap leading to an anisotropic reduction of the crystal field splitting. Such an alteration of the electronic structure may simultaneously involve the eg levels in the magnetism and enhance the spin-orbit interaction by increasing the out-of-plane orbital moment and pinning the easy direction to the substrate normal.
References
[1] A. Grutter, F. Wong, E. Arenholz, M. Liberati, A. Vailionis, and Y. Suzuki, Appl. Phys. Lett. 96, 082509 (2010) [2] M. Bohra, C. P. Wu, H. J. Yeh, Y. H. Cheng, C. C. Peng, and H. Chou, J. Appl. Phys. 109, 07D728 (2011) [3] R. Palai, H. Huhtinen, J. F. Scott, and R. S. Katiyar, Phys. Rev B 79, 104413 (2009)
8:54 AM
AG-03. Understanding Spin-Flop Coupling at Perovskite Oxide Interfaces
< i>Yayoi Takamura1, Erik Folven2, Fan Yang1, Andreas Scholl3, Anthony T. Young3, Scott T. Retterer4, Michael D. Biegalski4, Hans M. Christen4, Thomas Tybell2 and Jostein K. Grepstad2
1Chemical Engineering and Materials Science, UC Davis, Davis, CA; 2Department of Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim, Norway; 3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA; 4Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN
The interfaces of perovskite oxides have been shown to possess unique functional properties that are of interest for fundamental studies as well as device applications. These properties arise due to various structural and chemical changes as well as electronic and/or magnetic interactions which occur over nanometer length scales at the interfaces. In this work, we studied exchange interactions in all perovskite oxide bilayers and superlattices consisting of the ferromagnet (FM) La0.7Sr0.3MnO3 (LSMO) and the G-type antiferromagnet (AFM) La1-xSrxFeO3 (LSFO). In the absence of uncompensated spins in the AFM, this type of interface exhibits spin-flop coupling characterized by a perpendicular orientation between the FM moments and the AFM spin axis. Furthermore, an applied magnetic field is able to reorient the AFM spin axis while maintaining this perpendicular orientation.[1] Using soft x-ray photoemission electron microscopy, the local AFM/FM domain patterns of the LSFO/LSMO layers, respectively, were selectively imaged in blanket films as well as 2 x 2 micron features. This size lies near the transition between flux closure and multi-domain patterns. By varying the thickness of the LSFO and LSMO layers as well as the orientation of the patterned edges, we investigated the competition between various magnetic interactions in this system, including magnetocrystalline anisotropy, shape anisotropy, magnetostatic energy, and spin-flop coupling. This competition overcomes the pinning effect of structural defects that typically define the locations of AFM domains and it allows us to control factors such as the orientation of the AFM spin axis and the types of flux closure patterns formed in the AFM/FM layers. This type of control has direct implications for the use of perovskite oxide heterostructures in magnetic recording and sensor technology.
References
[1] E. Arenholz, G. van der Laan, F. Yang, N. Kemik, M. D. Biegalski, H. M. Christen and Y. Takamura, Appl. Phys. Lett. 94, 072503 (2009).
9:06 AM
AG-04. Potential of Fe-doped CoFe2O4 for Magnetic Layers in Multiferroic Heterostructures
Jarrett A. Moyer1, Carlos A. F. Vaz1, Dario A. Arena2, Matthew S. J. Marshall1, Divine Kumah1 and Victor E. Henrich1
1Applied Physics, Yale University, New Haven, CT; 2National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY
Developing complex oxides that can be combined with ferroelectrics to create devices in which the magnetic moment of the oxide is modulated via changes in the electric field of the ferroelectric is the focus of much current research. One material that has been extensively used in multiferroic ferroelectric-ferromagnetic nanostructures is the insulator CoFe2O4.[1] However, Fe-doped cobalt fe rrite (Co1-xFe2+xO4), whose magnetic moment and carrier concentration can be significantly altered by small changes in Fe doping, has yet to be incorporated into multiferroic heterostructures. In this work, thin films of Co1-xFe2+xO4, ranging from 3 - 20 nm in thickness, have been grown by MBE on MgO (001) substrates. The magnetic structure is investigated through use of SQUID magnetometry, XMLD and XMCD. The magnetic moments of the Co1-xFe2+xO4 (0.01 ≤ x ≤ 0.63) films are significantly reduced from their bulk values; however, as x increases, the magnetic moments tend toward their bulk values and increase more rapidly as x approaches 1. Changes in the magnetic moment as x is increased are linked to changes in the carrier concentration caused by Fe doping. Due to this connection between the magnetic moment and the carrier concentration, Fe-doped CoFe2O4 has the potential to be used as the magnetic layer in charge-driven magnetoelectric coupled multiferroic devices. Currently, PbZryTi1-yO3/Co1-xFe2+xO4 heterostructures are being grown in an attempt to modulate the magnetic moment of the Co1-xFe2+xO4 layer through changes in its carrier concentration, which are induced by the electric field of the PbZryTi1-yO3 layer.
References
[1] H. Zheng, J. Wang, S. E. Lofland, et al., Science 303, 661 (2004).
9:18 AM
AG-05. Modified magnetic structure in complex oxide magnetic tunnel junctions.
S.G.E. te Velthuis1, Yaohua Liu1, M. Zhernenkov2, M. R. Fitzsimmons2, J. W. Freeland1, Z. Sefrioui3, 4, C. Visani3, A. Barthélémy4 and J. Santamaria3
1Argonne National Laboratory, Argonne, IL; 2Los Alamos National Laboratory, Los Alamos, NM; 3Universidad Complutense de Madrid, Madrid, Spain; 4Unité mixte de Physique CNRS/Thales, Palaiseau, France
Newly discovered phases at the interfaces of complex oxides are driving the effort to combine these materials in ways that will lead to devices useful for spintronic applications. This category includes half metallic manganese oxides, which have the potential of producing a large tunneling magnetoresistance (TMR). To this purpose we have investigated magnetic tunnel junctions (MTJs) with ferromagnetic manganite, La0.7Ca0.3MnO3(LCMO) electrodes and an antiferromagnetic insulating cuprate, PrBa2Cu3O7(PBCO) barrier. In these MTJs the TMR initially increases with decreasing temperature [1], as expected, but then upon further cooling starts to decrease. To better understand this behavior Polarized Neutron Reflectivity (PNR) and X-Ray Magnetic Circular Dichoism (XMCD) studies were undertaken. PNR reveals differences in the temperature dependent magnetizations, reversal behavior, and anisotropy, between the thicker bottom and thinner top LCMO layers. The difference in magnetization is consistent with the observation of two different Curie temperatures (TC), which is explained by a difference in mismatch strain with the underlying layer. While the magnetization of the top LCMO rotates gradually over a relatively large higher applied field range, its magnitude also decreases, indicating domain formation. The magnetization of the bottom LCMO reverses within a short lo wer field range via domain wall nucleation and propagation. Similarly to what was observed for the YBa2Cu3O7(YBCO)/ La0.67Ca0.33MnO3 system [2], with XMCD we have found a non-zero net magnetic moment on the Cu of PBCO at low temperature. Hysteresis loops measured at the Cu L-edge show a two-step behavior of this moment that is correlated to the two separate coercive fields of the top and bottom LCMO layers, indicating the interfacial origin and the direct link to the adjacent LCMO. Unlike in the YBCO system, the Cu moment does not persist until TC of LCMO, and disappears at a temperature below the lower TC. These combined results provide a possible origin of the anomalous TMR behavior. Work supported by U.S. Department of Energy, Office of Science, Basic Energy Sciences, under contract No. DE-AC02-06CH11357 and DE-AC02NA25396, and by the Spanish MICINN.
References
[1] Z. Sefrioui, V. Cross, A. Barthelemy, V. Peña, C. Leon, J. Santamaria, M. Varela, S.J. Pennycook, Appl. Phys. Lett. 88, 022512 (2006). [2] J. Chakhalian, et al., Nature Physics 2, 244-248 (2006). Ibid. Science 318, 1114-1117 (2007).
9:30 AM
AG-06. Impact of nanostructuring on the magnetic and magnetocaloric properties of La0.25Pr0.375Ca0.375MnO3
Paula J. Lampen1, N. S. Bingham1, M. H. Phan1, H. Srikanth1, C. L. Zhang2, S. W. Cheong2, T. H. Hoang3 and H. D. Chinh3
1Physics, University of South Florida, Tampa, FL; 2Physics, Rutgers University, Piscataway, NJ; 3Chemical Engineering, Hanoi University of Technology, Hanoi, Viet Nam
Bulk manganites of the form La5/8-yPryCa3/8MnO3 (LPCMO) exhibit a complex phase diagram due to coexisting and competing charge-ordered (CO) and ferromagnetic (FM) phases.1 The behavior of the CO phase is of particular interest due to its instability under various perturbations including carrier doping, strain, and magnetic and electric field. We report systematic studies of the influence of particle size on the magnetic and magnetocaloric properties of nanocrystalline LPCMO (y=3/8) synthesized by a sol-gel method. After thermal treatments, nanocrystalline samples with mean particle sizes of 55 nm, 150 nm, and 400nm were obtained. XRD and TEM were used to confirm the phase purity, size, and crystallinity of each sample. Magnetic and magnetocaloric measurements were conducted using a Quantum Design PPMS, and the properties of the nanocrystals were compared to those of a single crystal sample. We find that the 400 nm and 150 nm samples exhibit features similar to their bulk counterpart. However, the case is very different for the 55 nm samples where only a paramagnetic (PM) to FM transition occurs. Size reduction has been found to suppress the CO phase, decrease the magnetization, and strongly modify the magnetocaloric effect (MCE) in LPCMO. The behavior of the phase-separated samples near transition temperatures was investigated in detail using MCE. At TCO, the range of fields over which the CO, FM, and PM states coexist is found to broaden from 1T in the single crystal up to 2.4T in the 150nm samples. The onset of CO melting occurs at progressively lower fields as size is reduced, ranging from 3.4T in the bulk to 1.6T in the 150nm particles. The absence of long-range CO strains increases the sensitivity of the nanocrystalline samples to small applied fields - a desirable characteristic for active magnetic refrigeration.
References< /p>
1P. A. Sharma, S. B. Kim, T. Y. Koo, S. Guha and S. W. Cheong, Phys. Rev. B 71, 224416 (2005)
9:42 AM
AG-07. Uncovering Hidden Magnetic Phases in La0.7Sr0.3MnO3 Thin Films: A Deeper Look with X-rays
Dario Arena1, Jun-Sik Lee2, Pu Yu4, R. Ramesh4, 5, Tiffany S. Santos3 and Chi-Chang Kao2
1National Synchrotron Light Source, Brookhaven National Lab, Upton, NY; 2Stanford Synchrotron Radiation Lightsource, SLAC, Menlo Park, CA; 3Center for Nanoscale Materials, Argonne National Lab, Argonne, IL; 4Dept. of Physics, Univ. of California, Berkeley, Berkeley, CA; 5Materials Science Division, Lawrence Berkeley National Lab, Berkeley, CA
Mixed valence manganites, in which a delicate interaction between electronic, orbital, magnetic and structural degrees of freedom produces rich phase diagrams reflecting the competing, nearly-degenerate ground states, have been under intense investigation for decades. We present evidence for an unusual magnetic configuration in La0.7Sr0.3MnO3 (LSMO) epitaxial films grown on SrTiO3(001) (STO) substrates. At reduced temperatures, the remanent state of the near-surface region in thick LSMO films is aligned anti-parallel to the the applied magnetic field [1]. The effect is robust and independent of sample growth method (pulsed laser deposition or molecular beam epitaxy) [1,2]. By using a combination of soft x-ray absorption spectroscopy and hard x-ray reflectivity, we find that epitaxial films of LSMO grown on STO(001) substrates exhibit an inhomogeneous 3d electron-distribution along surface normal direction, divided between an intermediate layer (enriched in Mn3+) and a nominal mixed-valence layer (Mn3+ and Mn4+) of LSMO. This unusual magnetic configuration is also correlated with an in-plane structural fluctuation, as measured by x-ray diffraction. We suggest that the unexpected magnetic ordering in these films may also be associated with an orbital reconstruction of the Mn eg orbitals.
References
[1] J.-S. Lee, D. A. Arena, P. Yu, C. S. Nelson, R. Fan, C. J. Kinane, S. Langridge, M. D. Rossell, R. Ramesh, and C.-C. Kao, Phys. Rev. Lett. 105, 257204 (2010). [2] J.-S. Lee, C.-C. Kao, T.S. Santos, E. Negusse and D. A. Arena, Journal of Physics:D 44, 245002 (2011).
10:18 AM
AG-08. Valence transition in (Pr,Ca)CoO3 cobaltites: Charge migration at the metal-insulator transition
José Luis García-Muñoz1, Carlos Frontera1, Aura J. Barón-González1, Jessica Padilla1, Javier Herrero1, Sergio Valencia2, Ralf Feyerherm2, Esther Dudzik2, Florin Radu2, Javier Blasco3, Gloria Subías3 and Radu Abrudan4
1Instituto de Ciencia de Materiales de Barcelona.ICMAB-CSIC, E-08193 Bellaterra, Spain; 2Hemholtz-Zentrum Berlin, BESSY, 12489 Berlin, Germany; 3Instituto de Ciencias de Materiales de Aragón, CSIC-Univ. de Zaragoza, 50009 Zaragoza, Spain; 4Institut für Experimentalphysik/Festkörperphysik, Ruhr-Universität Bochum, Germany
Metal-insulator transitions (MIT) in perovskite cobaltites is an important research topic due to the relevance of the spin state of Co for electron mobility in these strongly correlated oxides [1-3]. Pr0.5Ca0.5CoO3 is also being investigated as a good candidate to display photoinduced phase transition phenomena, related to induced spin state changes [3]. The carriers introduced by doping in Pr1-xCaxCoO3 can be viewed for x=0.5 as low spin (LS) Co4+ (S=1/2, t52g) species moving through the matrix of intermediate spin (IS) Co3+ (S=1, t52geg1) centers. Initially the MIT (TMI≈80 K.) in Pr0.5Ca0.5CoO3 was attributed to a sudden change of IS ions to the LS state (S=0). Our findings clearly point to an active participation of the 4f electrons of Pr atoms [4-5]. X-ray absorption spectroscopy at several Pr, Co and O edges and neutron diffraction measurements in (Pr,Ca)CoO3 cobaltites reveal an abrupt Pr valence transition and an astonishing charge migration from Pr to Co at the metal-insulator transition. The valence of praseodymium ions is stable and essentially +3 (Pr [4f2]) in the metallic state, but abruptly increases approaching the oxidation state +4 (Pr [4f1]) at the MIT transition. Electrons leaving Pr sites are used to stabilize the trivalent low-spin state of Co. The ground state at low temperature is not realized through segregation of low- or intermediate-spin Co3+ and low-spin Co4+ ionic species. Instead, the t52g(σ*)0.5 metallic state is substituted by an homogeneous Co3.5-δ state stabilized by electron transfer from praseodymium.
References
[1] K. Takada, H. Sakurai, E. Takyama-Muromachi, et al , Nature 422, 53 (2003). [2] A. Maignan, V. Caignaert, B. Raveau, et al , Phys. Rev. Lett. 93, 026401 (2004). [3] Y. Okimoto, X. Peng, M. Tamura, et al , Phys. Rev. Lett. 103, 027402 (2009). [4] A.J. Barón-González, C. Frontera, J.L. García-Muñoz, J. Blasco and C. Ritter,Phys. Rev. B 81, 054427 (2010). [5] J. L. García-Muñoz, C. Frontera, A. J. Barón-González, S. Valencia, J. Blasco, R. Feyerherm, E. Dudzik, R. Abrudan, F. Radu, Phys. Rev. B 84, 045104 (2011).
10:30 AM
AG-09. Multiphase transitions and complex phase diagram in mixed phase (La,Pr,Ca)MnO3 manganites
N. S. Bingham1, M. H. Phan1, C. L. Zhang2, S. W. Cheong2 and Hariharan Srikanth1
1Department of Physics, University of South Florida, Tampa, FL; 2Rutgers Center for Emergent Materials, Rutgers University, Piscataway, NJ
Mixed phase La5/8-yPryCa3/8MnO3 (y~3/8) manganites are ideal for studying competing magnetic phases. Of particular interest is the charge-ordered insulating (COI) phase that coexists and strongly competes with the ferromagnetic metallic (FMM) phase and is unstable under various perturbations such as carrier doping, strain, magnetic field, and electric field [1]. Strain associated with the phase coexistence has been known to stabilize a strain-glass state as well as a strain-liquid state. To fully assess the origin of phase coexistence and separation in this system, it is essential to employ experimental methods that allow detailed investigation of the temperature and magnetic field response of the different phases. Here we introduce magnetocaloric effect (MCE) and radio-frequency transverse susceptibility (TS) experiments as being ideally suited for this purpose [2]. MCE and TS measurements were performed on La5/8-yP ryCa3/8MnO3 (y=0.275 and 0.375) single crystals. MCE experiments probe multiphase transitions, with the COI phase dominant at high temperature whereas the FMM phase being the low-temperature magnetic ground state. The large MCE is observed in the “dynamic” strain liquid state, while it is relatively small in the “frozen” strain-glass state. The MCE data reveal that the sharp increase of the magnetization below the Curie temperature (TC) in the strain-liquid region is attributed to the enhancement of the FMM domain regions that are already present in the material. MCE has also proved useful in probing the subtle balance of coexisting phases in mixed phase manganites. TS experiments probe a phase conversion between the COI and FMM phases and an anomalous magnetic field-induced kinetic arrest just below TC. A new, complex phase diagram is established from MCE and TS data, providing a deeper understanding of the phase coexistence and separation in this system.
References
[1] M. Uehara, S. Mori, C. H. Chen, and S.-W. Cheong, Nature 399, 560 (1999). [2] M.H. Phan, B. Morales, N.S. Bingham, H. Srikanth, C.L. Zhang, and S.W. Cheong, Phys. Rev. B 81, 094413 (2010).
10:42 AM
AG-10. Simultaneous metal-insulator, ferrimagnetic and structural transitions at 295 K in YBaCo2O5.5
Jessica Padilla-Pantoja, Carlos Frontera, Javier Herrero-Martin and Jose Luis Garcia-Muñoz
Institute of Materials Science of Barcelona (ICMAB-CSIC), Barcelona, Spain
The layered cobaltites LnBaCo2O5.5 have attracted much attention as a spin-charge-orbital coupled system. Ln and Ba cations order in alternating planes along c-axis yielding an ordered arrangement of CoO5 pyramids and CoO6 octahedra. Research focuses on the nature of the spin-state transitions and the interplay of the spin-state with the charge, orbital and metal-insulator transitions occurring in these Co3+ compounds [1-5]. We have performed a thorough study of the electrical, magnetic and structural properties of YBaCo2 O5.5 from 4 K to 350 K. Our results evidence the existence of a strong magnetostructural coupling in this compound presenting three magnetic transitions. By means of synchrotron x-ray and neutron powder diffraction we have shown that the high temperature Pmmm orthorhombic structure transforms, on cooling below TMI ≈ 295 K, into a monoclinic (P112/a) phase concomitant to the appearance of a ferromagnetic moment and a metal to insulator transition (MIT) [6]. By further decreasing temperature, ferrimagnetic long range order vanishes at TN1 ≈ 267 K and the crystal structure can be described by the coexistence of Pmmm and P112/a phases in a ratio that varies non monotonically with temperature. Cobalt magnetic moments rearrange into an antiferromagnetic structure at TN2 ≈ 231 K. While in this phase the Pmmm phase dominates, at T < TS3 ≈ 55 K the compound recrystallizes mainly into the P112/a structure. Finally, we have carried out soft x-rays absorption spectroscopy measurements as a function of temperature to ascertain the changes in the electronic structure and the spin states of Co across the MIT.
References
[1] A. Maignan et al., J. Solid State Chem. 142, 247 (1999). [2] E. Suard et al., Phys. Rev. B 61, R11871 (2000). [3] C. Frontera et al., Phys. Rev B 74, 054406 (2006). [4] V.P. Plakhty et al., Phys. Rev. B 71, 214407 (2005). [5] M. Soda et al., J. Phys. Soc. Jpn. 72, 1729 (2003). [6] J. Padilla -Pantoja et al., Phys. Rev. B 81, 132405 (2010).
10:54 AM
AG-11. Comparison of magnetic and thermoelectric properties of (Nd,Ca)BaCo2O5.5 and (Nd,Ca)CoO3
Stanislaw Kolesnik1, Bogdan Dabrowski1, 2, Omar Chmaissem1, 2, Krzysztof Wojciechowski3 and Konrad Swierczek4
1Department of Physics, Northern Illinois University, DeKalb, IL; 2Materials Science Division, Argonne National Laboratory, Argonne, IL; 3Faculty of Materials Science and Ceramics, AGH-UST University of Science and Technology, Cracov, Poland; 4Faculty of Energy and Fuels, AGH-UST University of Science and Technology, Cracov, Poland
RBaCo2O5.5 (R = rare earth and Y) layered cobaltites demonstrate a rich magnetic and electronic phase diagram due to their crystallographic complexity involving the layer ordering and oxygen vacancy ordering and possible multiple spin states of Co3+ ions. These materials are considered to be paramagnetic and metallic at higher temperatures, but become insulating below a metal to insulator transition (MIT) at ~360 K. Below room temperature, they become ferromagnetic and then antiferromagnetic at lower temperatures. We demonstrate that the Ca doping in Nd1-xCaxBaCo2O5.5 (x≤0.2) preserves the cation- [(Nd,Ca)/Ba] ordering and oxygen vacancy ordering, as well as the MIT. While the antiferromagnetic state disappears upon doping, the Curie temperature (TC) is increasing and becomes close to MIT for x>0.12. This is the largest enhancement of TC ever observed for these cobaltites. This enhancement is associated with a decrease of the net magnetic moment (although the individual moments increase), which points to the ferrimagnetic ordering of Co3+ and Co4+ spins. These layered cobaltites show a large thermopower close to optimal doping. The thermoelectric figure of merit ZT is the highest close to room temperature and reaches values up to 0.02. Above the metal to insulator transition the thermopower, and hence ZT, collapse due to a possible spin blockade. Ca doping in Nd1-xCaxCoO3 up to x=0.2 does not introduce ferromagnetic order. The magnetic susceptibility is paramagnetic, similar as of the parent compound, with some indication of cluster-glass-like behavior at temperatures increasing to about 30 K with Ca doping up to 0.2. The increase in the effective paramagnetic moments with doping suggest a low spin state of Co3+ and a high spin state of Co4+. Maximum observed ZT reaches a value close to 0.2 for x=0.15 at 800 K, which is pretty high for perovskite oxides. Work at NIU was supported by the NSF (DMR-0706610) and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357.
11:06 AM
AG-12. Magnetic and calorimetric studies of magnetocaloric effect in La0.7-xPrxCa0.3MnO3
Vinayak Bharat Naik, Sujit Kumar Barik, Aparna Devi, Alwyn Rebello and Mahendiran Ramanathan
Physics, National university of Singapore, Singapore, Singapore
There has been a resurgence of interest in new materials which can show a large reversible magnetocaloric effect (MCE) because of their applications in magnetic refrigeration which is considered to be environmentally friendly and energy efficient than conventional refrigeration. Much attention has been paid to Gd-Si-Ge and La-Fe-Si alloys which show a f irst-order magnetic phase transition that is coupled with a structural phase transition or a volume change without symmetry change[1]. A number of studies have also focused on perovskite manganites which undergo second-order magnetic phase transition[2]. We focus attention on the compounds which show-first-order magnetic phase transition[3]. Investigation of magnetization in La0.7-xPrxCa0.3MnO3 (x = 0, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5) indicate that the paramagnetic state of these compounds is unstable with respect to an external magnetic field. Application of magnetic field applied above the Curie temperature induces a first-order paramagnetic to ferromagnetic transition (i.e., field-induced metamagnetic transition ) in all of these compounds and the temperature region over which the metamagnetic transition occurs increasing x. The field-induced metamagnetic transition occurs over a temperature range of 50-80 K in these manganites which is in sharp contrast to the occurrence of the metamagnetic transition in the vicinity of ferromagnetic transition in intermetallic alloys. The hysteresis in M versus H is smaller than in intermetallic compounds which is desirable for application. The observed magnetic entropy change estimated from isothermal magnetization studies are ΔSm = 8.15, 7.27, 6.92, 6.73, 6.41, 5.84 Jkg-1K-1 for x = 0, 0.25, 0.3, 0.35, 0.4, and 0.45 for a field change of ΔH = 5 T, respectively. In addition to the magnetic measurement, we have also carried out differential scanning calorimetry (DSC) and differential thermal analysis (DTA) under magnetic field on selected compounds. A good correlation between magnetic and calorimetric measurements has been found. The origin of the observed huge magnetic entropy has been discussed in terms of the collapse of magnetic polarons and short-range charge-orbital ordered clusters.
References
[1]K. A. Gschneidner, Jr., V. K. Pecharsky, and A. O. Tsokol, Rep. Prog. Phys. 68, 1479 (2005). [2]H. Terashita, B. Myer, and J. J. Neumeier, Phys. Rev. B 72, 132415 (2005). [3]A. Rebello and R. Mahendiran, Appl. Phys. Lett. 93, 232501 (2008).
11:18 AM
AG-13. Effect of deviation from stoichiometric composition on structural and magnetic properties of cobalt ferrite, CoxFe3-xO4 (x = 0.2 to 1.0)
Cajetan I. Nlebedim1, John E. Snyder2, Anthony J. Moses2 and David C. Jiles3
1Ames Laboratory, US Department of Energy, Iowa State University, Ames, IA; 2Wolfson Centre for Magnetics, School of Engineering, Cardiff University, Cardiff, United Kingdom; 3Electrical and Computer Engineering, Iowa State University, Ames, IA
Co-ferrite based materials have potential for magnetoelastic stress sensors/actuators[1], magnetoelectric devices [2] and for drug/gene delivery [3]. A bilayer of α-Fe2O3 and CoFe2O4 has been previously studied for spin-valve sensor applications [4]. Since CoFe2O4 and Fe3O4 both have the same crystal structure (spinel), one would presume that for all 0≤x≤1, CoxFe3-xO4 would be single spinel phase. However, this study shows that is not the case. As shown in Fig. 1a, two distinct phases were observed at x = 0.2 and 0.7, validated as CoFe2O4 (indexed) and α-Fe2O3 (labelled *). For x = 0.8, samples appear to have very low concentration of α-Fe2O3 phase. Only the spinel CoFe2O4 phase was observed at x = 1.0. Lattice parameter of the spinel phase decreased with x (Fig. 1b). The effect of changing stoichiometry was also observed to have major effects on the magnetic properties of the samples. Saturation magnetization decreased with x (Fig. 1(c)), while coercive field increased (Fig. 1(d)). Since Co-ferrites prepared by the traditional ceramic approach can deviate from the stoichiometric composition (CoFe2O4), it is important to understand how much deviation is acceptable for the formation of a single phase structure and how such deviation affects structural and magnetic properties. This study will enable selective adjustment of the concentration of each phase to precisely control and optimize properties for applications.
References
[1] I. C. Nlebedim, N. Ranvah, Y. Melikhov, P.I Williams, J. E. Snyder, A. J. Moses and D. C. Jiles, IEEE Trans. Magn., 45 (2009) 4120 [2] H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-Riba, S. R. Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig,A. Roytburd and R. Ramesh Science 303 (2004) 661 [3] E. J. Juan and M. Visbal-Onufrak “AIP Conf. Proc. 1311 (2010) 273 [4] T. Fuiji, T. Yano, M. Nakanishi and J. Takada Mat. Res. Soc. Symp. Proc. 674 (2001) T1.10.1
8:30 AM - 11:30 AM, Grand Canyon 1
Chair: Leszek Malkinski, University of New Orleans
8:30 AM
AH-01. Magnetophotonic crystal comprising electro-optical layer for controlling helicity of light
Taichi Goto1, Alexander V. Baryshev1, 2 and Mitsuteru Inoue1
1Toyohashi University of Technology, Toyohashi 441-8580, Japan; 2Ioffe Physico-Technical Institute, St. Petersburg 194021, Russian Federation
Magnetization direction of amorphous magnetic materials is shown to be controlled by circularly polarized light. This femtosecond inverse Faraday effect is expected to find applications in the optic-magnetic recording and to replace the conventional thermal-magnetic recording due to the demonstrated writing speed of 100 fs [1]. For realizing fast recording by this method, a ultra-high speed micro switching device is required for controlling the helicity of circularly polarized light. In this work, we demonstrate that the electro-optical (EO) and magneto-optical (MO) effects can be utilized for the flipping the helicity of light. Optical responses of MO-EO microcavities [2] where MO and EO layers were sandwiched between Bragg mirrors were theoretically analyzed. Upon changes of the refractive index of the EO (PLZT) layer under the applied voltage, the eigen circularly polarized modes traveling in the MO layer are affected such that the polarization state of emerging light is largely altered (Fig. 1). Here, for a fixed magnetization of the MO layer, transmittance and the Faraday ellipticity of light with a wavelength of 800 nm are plotted versus the applied voltage. Results of theoretical calculations and experiments demonstrate that effective switching of the helicity of elliptically polarized light can be realized by MO-EO microcavities.
References
[1] C. D. Stanciu, A. V. Kimel, F. Hansteen, A. Tsukamoto, A. Itoh, A. Kirilyuk, and T. Rasing, Phys. Rev. B 73, 220402 (2006). [2] T. Goto, H. Sato, H. Takagi, A. V. Baryshev, and M. Inoue, J. Appl. Phys. 109, 07B756 (2011).
8:42 AM
AH -02. Fano-shape longitudinal Kerr effect enhancement in 2D magnetoplasmonic crystals
Artem Chetvertukhin1, Alexander Baryshev2, Tatiana Dolgova1, Hironaga Uchida3, Mitsuteru Inoue2 and Andrey Fedyanin1
1Faculty of Physics, Lomonosov Moscow State University, Moscow, Russian Federation; 2Toyohashi University of Thechonlogy, Toyohashi, Japan; 3Tohoku Institute of Technology, Sendai, Japan
Significant magneto-optical longitudinal Kerr effect (LKE) enhancement in 2D magnetoplasmonic crystals is observed at the spectral vicinity of surface plasmon-polariton (SPP) resonance. Recently enhancement of transversal Kerr effect in presence of SPP resonance was shown [1]. This phenomenon is interpreted as increasing efficiency of interaction of the incident wave with the structure when an energy coupling to surface plasmon-polariton occurs. The sample of 2D magnetoplasmonic crystal consisting of an array of hexagonally packed Ni disks placed on thick (500 mkm) bulk nickel is chosen to explore the influence of SPP resonance on LKE. The disc diameter is 255 nm and the height is approximately 50 nm. The side length of elementary triangle is approximately 540 nm. Plasmonic properties of the sample are experimentally studied in reflectivity spectra. Spectral shift of the Wood’s anomaly caused by the SPP resonance is observed when angle of incidence or azimuthal angle (an angle between plane of incidence and reciprocal lattice vector of the sample) are changed. Longitudinal magneto-optical Kerr effect (LKE) is studied as magnetic field is applied along the wave vector and the incident light is P-polarized. Significant LKE enhancement with Fano-resonance-form appears at the wavelength near the Wood’s anomaly. The enhancement is correspondingly shifted with changing the azimuthal angle. Dependence of LKE on the azimuthal angle changing from 0° to 30° and on the angle of incidence tuning from 40° to 45° is measured. The LKE enhancement from 0.05° to 0.13° is observed. Spectral position of the LKE resonance is in a good agreement with the Wood’s anomaly showing the clear influence of the SPP resonance on the LKE enhancement.
References
[1] A. A. Grunin, A. G. Zhdanov, A. A. Ezhov, E. A. Ganshina, and A. A. Fedyanin, Applied Phys. Lett. 97 261908 (2010)
8:54 AM
AH-03. Study of Crystallographically Amorphous Ferrimagnetic Alloys: Comparing a Localized Atomistic Spin Model with Experiments
Thomas A. Ostler1, Richard Evans1, Roy W. Chantrell1, Unai Atxitia2, Oksana Chubykalo-Fesenko2, Ilie Radu3, 6, Radu Abrudan4, Florin Radu3, Arata Tsukamoto5, Akiyoshi Itoh5, Andrei Kirilyuk6, Theo Rasing6 and Alexey Kimel6
1Physics, University of York, York, United Kingdom; 2Instituto de Ciencia de Materiales, Madrid, Spain; 3Helmholtz-Zentrum Berlin fur Materialien und Energie, BESSY II, Berlin, Germany; 4Experimentalphysik IV, Ruhr-Universitat Bochum, Bochum, Germany; 5College of Science and Technology, Nihon University, Funabashi, Japan; 6Institute for Molecules and Materials, Radboud University, Nijmegen, Netherlands
Recent work in magneto-optics has demonstrated that carefully shaped laser pulses can be used to manipulate magnetization dynamics on the sub-picosecond timescale [1-6]. However controllable magnetization switching has only been observed in G dFeCo, stimulating a great deal of effort to attempt on many levels to explain the process [7]. The compensation point in TM-RE ferrimagnets [8,9], can be well explained in terms of a two sublattice model [8,10], however there is currently a need to study atomic level dynamics. In this paper we present an atomistic, classical Heisenberg model for crystallographically amorphous ferrimagnetic alloy based on the stochastic Landau-Lifshitz-Gilbert equation. We show that it can be tested against Mean Field predictions, which we derive especially to take into account the disordered nature of the material. In order to construct and validate the model, we compare the results for the static properties of GdFe-ferrimagnetic alloy of varying composition with X-ray Magnetic Circular Dichroism measurements. It is shown that the Langevin dynamic form of the LLG equation can predict TM and its effect on the coercivity, shown to agree qualitatively with experiment. We show the compositional dependence of the Tc and TM numerically and provide a direct comparison to experiment and a mean field model. We also show that the inter-sublattice exchange parameter can lead to an effective heating channel, allowing energy to be more rapidly transferred between the two sublattices.
References
[1] T. Gerrits et al. Nature 418, 509 (2002). [2] C. D. Stanciu et al. Phys. Rev. Lett. 047601 (2007). [3] A. V. Kimel et al. Nature 429, 850 (2004). [4] C. D. Stanciu et al. Phys. Rev. Lett. 99, 217204 (2007). [5] A. V. Kimel et al. Laser and Photonics Reviews 1, 275 (2007). [6] B. Koopmans et al. Phys. Rev. Lett. [7] K. Vahaplar et al. Phys. Rev. Lett. 102, 117201 (2009). [8] M. Mansuripur, The Physical Principles of Magneto-optical Recording (Cambridge University Press, Cambrige, UK, 1995). [9] P. Hansen et al. Appl. Phys. 66, 756 (1989). [10] L. Neel, Compt. Rend 203, 304 (1936).
9:06 AM
AH-04. Magnetic properties of liquid crystals tuned by magnetic nanoparticles
Jin He Lim1, John B. Wiley2, Leszek M. Malkinski2, Anatoliy Glushchenko2, Zbigniew Celinski1 and Yura Garbovskiy1
1Physics, UCCS, Colorado Springs, CO; 2Advanced Materials Research Institute, University of New Orleans, New Orleans, LA
Liquid crystal (LC) materials are unique due to their electrically controlled birefringence. Even small changes in an applied electric field produce large changes of the effective birefringence. This property determined their use in many devices like displays, beam steering devices, tunable lenses, and optical modulators. At the same time, LC exhibit very little sensitivity to magnetic fields. Therefore, no single application using LC, controlled by a magnetic field was reported. We report on specially designed magnetic nanoparticles which, being mixed with a liquid crystal material, leading to the appearance of a new composite system. Magnetic nanoparticles were prepared using anodized aluminum (AAO) templates. [1] Fe nanowires were grown using electrodeposition in a solution containing 240 g/L FeSO4 x 7H2O, 45 g/L H3BO4, and 1 g/L of Ascorbic acid. [2] Fe nanowires were annealed at 500 °C in pure oxygen and then at 325 °C in pure hydrogen. This annealing process led to the creation of magnetic nanorods of Fe3O4 with 60 nm diameter and 150 nm length. The shape anisotropy and the surface of these nanorods had strong interactions with LC. Using magneto-optical Faraday effect we studied the change of the effective birefringence of the composite as a function of magnetic field. Even small concentration of the nanoparticles produces significant sensitivity to external magnetic fields and the disappearance of the switching threshold. We discuss the use of these new materials in many devices utilizing various configurations of electric and magnetic fields. One of the very important applications of these materials is in fast switching beam steering devices or electrically and magnetically controllable lenses. These new materials may lead to the appearance of an entire new class of devices, opening up a significant area of research and applications.
References
[1] Jin-Hee Lim, Aurelian Rotaru, Seong-Gi Min, Leszek Malkinski, and John B. Wiley, J. Mater. Chem., 2010, 20, 9246. [2] J.-H. Lim, W.-S. Chae, H.-O. Lee, L. Malkinski, S.-G. Min, J. B. Wiley, J.-H. Jun, S.-K. Ham and J.-S. Jung, J. Appl. Phys., 2010, 107, 09A334.
9:18 AM
AH-05. Femtosecond dynamics of Faraday effect in thin magnetic films and magnetophotonic crystals.
Margaret Sharipova1, Alexander Zhdanov1, Artem Chetvertukhin1, Tatyana Shapaeva1, Alexander Shaposhnikov2, Tatyana Dolgova1 and Andrey Fedyanin1
1physics, Lomonosov Moscow State University, Moscow, Russian Federation; 2physics, Taurida National V. I. Vernadsky University, Simferopol, Ukraine
A short light pulse propagating through a magnetic medium can demonstrate a nonlinear time dependence of Faraday rotation, if the medium optical width is comparable with a pulse length. The effect arises from a non-reciprocity of the Faraday rotation and multiple interference inside the medium. In this paper, the ultrafast temporal behavior of Faraday rotation is observed in thin magnetic films and magnetophotonic crystals by using a polarization-sensitive cross-correlation technique. One of the key features of photonic-band-gap materials is a possibility to achieve a very small light group velocity at the photonic bang-gap edge and at the defect mode. This leads to a row of bright “slow-light” phenomena and promising applications such as enhancement of nonlinear-optical and magneto-optical effects. An infrared femtosecond fiber laser with 70-MHz repetition rate, average intensity of 130 mW, wavelength of 1,56 μm, pulse duration of 150 fs is used as a source of radiation in the cross-correlation scheme. A Glan prism splits an incoming laser pulse into two orthogonally polarized beamlets. One of them goes through the sample placed in magnetic field. Both pulses are then focused at the same spot into a crystal with a χ(2) nonlinearity for noncollinear second-harmonic generation. Its intensity is detected by a photodiode at a magnetic field frequency as a function of the time delay. The cross-correlation function contains information about polarization rotation time dependence. Three general cases were observed in thin films: arbitrary long pulse propagating through a thin film, very short pulse propagating through a thick film and the third case, when the medium optical width is comparable with a pulse length. The first case is equivalent to quasisteady-state with constant Faraday rotation angle. The second case describes transmitted and series of reverberated pulses, which do not overlap in time. Inside each of that pulses Faraday rotation angle is a constant and proportional to the distance travelled by light in the medium. Cross-correlation function measured for the third case shows the nonlinear timporal dependence of the Faraday rotation yielded by multibeam interference of the laser pulse in the film.
References
1. A.A. Fedyanin, O.A.Aktsipetrov, D. Kobayashi, K. Nishimura, H.Uchida, M.Inoue, JMMM 282, 256-259 (2004). 2. A. G. Zhdanov, A.A. Fedyanin, O.A.Aktsipetrov, D. Kobayashi, H.Uchida, M.Inoue JMMM 300, e2 53-e256 (2006). 3. Toshihiko Baba, Nature Phot. 2, 465 - 473 (2008).
9:30 AM
AH-06. Surface modes induced magneto-optical Kerr effect enhancement in Fe films by coverage of two-dimensional array of polystyrene spheres
Xia Zhang1, Lei Shi1, Jing Li2, Yunjie Xia3, Jian Zi1 and Shi-Ming Zhou1, 4
1Surface Physics State Laboratory and Department of Physics, Fudan University, Shanghai, China; 2Department of Optical Science and Engineering, Fudan University, Shanghai, China; 3Shandong Province Key Lab of Laser Polarization and Information, Qufu Normal University, Qufu, China; 4Physics Department, Tongji University, Shanghai, China
In this work, 2D arrays of ordered polystyrene (PS) spheres were fabricated on Fe thin films using self-organization. For Fe films covered by array of hexagonal close-packed polystyrene spheres, fine structures are observed in reflection and magneto-optical Kerr rotation spectra and become more prominent for Au/PS/Fe trilayers with Au layers on PS arrays. In both cases, the Kerr rotation peaks and the reflection minima are found to be located at the same wavelengths. With the coverage of the Au layer on the polystyrene spheres, the Kerr rotation is further enhanced. The Kerr rotation enhancement can be attributed to the excitation of the guided waves, the surface plasmons, and Fabry-Pérot modes, because the effective dielectric constant of the coverage layer on the Fe layer is modified and approaches that of the Fe layer. Furthermore, with both measured reflection spectra as the incident angle and the calculated distribution of the electric field (EF), it can be concluded that the surface modes play an important role in the MOKE enhancement.
9:42 AM
AH-07. Magneto-Optical Materials and Devices for On-chip Nonreciprocal Photonic Applications
Lei Bi
DMSE, MIT, Cambridge, MA
On-chip integration of magneto-optical nonreciprocal photonic devices is becoming increasingly urgent in recent years for silicon photonic systems. [1, 2] The material incompatibility between silicon and magneto-optical garnets created a fundamental challenge for monolithic integration of such devices on silicon, which triggered a significant amount of interests and efforts in exploring novel materials and devices in the past several decades.[1, 3-5] In this report, we summarize our study in exploring and understanding novel magneto-optical materials and devices for on-chip nonreciprocal photonic applications. Intrinsic room temperature ferromagnetism accompanied with high magneto-optical figure of merit at communication wavelengths has been demonstrated in several epitaxial magnetically doped perovskites such as Sr(Ti,Fe)O3-δ and Sr(Ti,Co)O3-δ. Extraordinary magnetic anisotropy and thermo-magnetic relationships are observed in these films, which are explained and modeled by magneto-elastic induced spin ordering theory. We also demonstrate integration of polycrystalline Ce:YIG films on silicon with high figure of merit using a two step growth method. A nonreciprocal optical racetrack resonator based on polycrystalline garnet/silicon strip-loaded waveguides was experimentally demonstrated. As the first monolithically integrated magneto-optical isolator on silicon, this device provides an isolation ratio up to 19.5 dB with 10x device footprint reduction compared to conventional devices on garnet substrates, which may serve as a fundamental structure in a variety of ultra compact nonreciprocal photonic devic es on silicon such as optical isolators, circulators, switches and modulators.
References
[1] Y. Shoji, T. Mizumoto, H. Yokoi, I. Hsieh and R. M. Osgood, Appl. Phys. Lett., 92, 071117 (2008). [2] Z. Yu and S. Fan, Nature Photonics, 3, 91 (2009). [3] T. R. Zaman, X. Guo, and R. J. Ram, J. Light. Techn., 26, 291 (2008) [4] S. Sung, X. Qi and B. J. H. Stadler, Appl. Phys. Lett., 87, 12111 (2005) [5] H. S. Kim, L. Bi, G. F. Dionne, and C. A. Ross, Appl. Phys. Lett., 93, 092506 (2008) [6] L. Bi, H. S. Kim, G. F. Dionne and C. A. Ross, New J. Physics, 12 043044 (2010)
10:18 AM
AH-08. Magneto-plasmonics and magneto-transport in Au-Co nanocomposite films
Kaida Yang1, Cesar Clavero1, Jonathan Skuza2 and Ale Lukaszew1, 2
1Department of Applied Science, College of William and Mary, Williamsburg, VA; 2Department of Physics, College of William and Mary, Williamsburg, VA
We have recently reported on the magneto-plasmonic properties of Au-Co nanocomposite thin films. We found that surface plasmon excitation can enhance the magneto-optical response for specific film composition and microstructure[1] . We now discuss correlations between magneto-plasmonics and magneto-transport in such films. Spin-polarized electrons across ferromagnetic-nonmagnetic metal interface penetrate the nonmagnetic metal by the spin diffusion length δs. To observe this effect in the magneto-resistance(MR), the spacing of nonmagnetic material needs to be smaller than δs and thus the films must be tailored accordingly. Several films were prepared on glass substrates at 300°C using DC magnetron sputtering and varying the composition of Au and Co. A typical equilibrium phase[2] with segregated microstructure is observed. Conventional four-probe techniques were used to determine the MR while the magneto-plasmonic properties were measured with custom-built system. The MR curve revealed negative MR effect (Figure 1a) with anisotropic magneto resistance (AMR) values comparable to those observed in Co-Au multilayers[3] . A considerable increase and change in the shape of the MR curve is observed for Co 20% Au 80% films, suggesting that the inter-cluster separation falls in the Au thickness range where perpendicular anisotropy has been reported in Au-Co MLs[4]. The magneto-plasmonic behavior is shown in Figure 1b. Our results illustrate the correlation between magneto-transport and magneto-plasmonic properties thus enabling new possibilities for the application of these materials.
References
[1] K. Yang, C. Clavero, J. R. Skuza, M. Varela, and R. A. Lukaszew, J. Appl. Phys. 107, 103924 (2010) [2] M. Ohring, Materials Science of Thin Films, 2002 Academic Press, Second Edition, Page 545 [3] S. Stavroyiannis, C. Christides, D. Niarchos, Th. Kehagias, Ph. Komninou, and Th. Karakostas, J. Appl. Phys. 84, 6221 (1998). [4] C. H. Lee, Hui He, F. J. Lamelas, W. Vavra, C. Uher, and R. Clarke, Phys. Rev. B 42, 1066 (1990).
10:30 AM
AH-09. Magnetoplasmonic nanostructures based on nickel opal slabs
Andrey Grunin, Nina Sapoletova, Kirill Napolskii, Andrey Eliseev and Andrey Fedyanin
Lomonosov State University, Moscow, Russian Federation
Magneto-optical effects in plane films of ferromagnetic metals such as nickel are usually not large enough for device applications. One way to enhance these effects is the use of surface plasmon ( SPP) resonances that has been recently demonstrated in nickel nanogratings [1] or in noble-metal/ferromagnetic - metal multilayers [2]. Nickel inverse opals are promising magnetoplasmonic nanomaterials because they show both plasmonic and magnetic properties and they are relative ease for manufactory. In this paper, the correlation between magneto-optical response and resonant surface plasmon excitation in two-dimensional nickel inverse opals slabs is studied. Samples of nickel inverse opal slabs were fabricated by filling the opal matrix with nickel using electrodeposition technique that allows the control of normalized slab thickness which is the ratio of the nickel layer thickness to the diameter of microspheres in the opal slab. Nickel inverse opal slabs with the 560 nm - period and normalized thickness in the range from 0.2 to 1.1 are studied. Surface plasmons can be excited in 2D structures in a narrow spectral range where phase-matching conditions are fulfilled. This is manifested itself as dip in reflection spectra of the p-polarized light. The reflectivity spectra and spectral dependence of transversal magneto-optical Kerr effect (TKE) are measured. The SPP excitation at nickel surface and the resonant TKE enhancement are observed in narrow spectral range. The peak positions in the TKE spectra are in the vicinity of dips in reflection spectra and correspond to SPP phase-matching conditions. The TKE enhancement of about 2-6 times in comparison with the plain nickel film is observed. The strongest plasmonic and magnetoplasmonic resonances are detected for nickel inverse opals slabs with normalized thickness of 0.6.
References
1. A. A. Grunin, A. G. Zhdanov, A. A. Ezhov, E. A. Ganshina, and A. A. Fedyanin, Appl.Phys.Lett. 97, 26190 (2010) 2. C. Clavero, K. Yang, J. R. Skuza, and R. A. Lukaszew, Opt.Lett. 35, 1557 (2010)
10:42 AM
AH-10. Circularly Polarized Plasmon Modes in Spheroidal Nanoshells for Application in All-Optical Magnetic Recording
Ling Hung1, Garrett Lang1, Patrick McAvoy1, Charles Krafft2 and Isaak Mayergoyz3
1Electrical and Computer Engineering, University of Maryland, College Park, MD; 2Laboratory for Physical Sciences, College Park, MD; 3Electrical and Computer Engineering, UMIACS and AppEl Center, University of Maryland College Park, College Park, MD
It has been experimentally demonstrated by the group of Th. Rasing [1] that femtosecond magnetization reversals of 100 μm spots of magnetic media can be achieved by using only circularly polarized laser pulses, i.e., without any external magnetic fields. This all-optical magnetization switching will be technologically feasible in magnetic recording only if the techniques for delivery of nanoscale-focused circularly polarized light are developed. In the paper, it is demonstrated that the excitation of circularly polarized plasmon modes in spheroidal (i.e., rotationally symmetric ellipsoidal) nanoshells may result in dramatic enhancement of circularly polarized light intensity on the nanoscale. The use of silver (or gold) spheroidal nanoshells is very attractive because by controlling the aspect ratio of these shells and their thickness, plasmon resonances can be effectively shifted into the desired wavelength range (800 nm-1200 nm) where the resonance light enhancement is most pronounced due to the favorable ratio of real to imaginary parts of dielectric permittivity. In the paper, analytical calculations of circularly polarized plasmon modes in spheroidal nanoshells are presented along with the detailed study of their coupling to incident circularly polarized laser radiation and their time dynamics of excitation and dep hasing. Numerical computations are presented which clearly reveal that spheroidal nanoshells are superior to spherical nanoshells [2] as far as the nanoscale focusing of circularly polarized light and the enhancement of its intensity are concerned (see Figure below).
References
1. C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and Th. Rasing, Phys. Rev. Lett. 99, 047601 (2007) 2. Isaak Mayergoyz, Patrick McAvoy, Garrett Lang, David Bowen, and Charles Krafft, J. Appl. Phys., 105, 07B904 (2009)
10:54 AM
AH-11. Voltage-Controlled Magnetic Data Writing using Inverse Magnetostrictive Effect.
Mohmmad T. Alam, David Carlton, Eduard Techfeld, Brian Lambson and Jeffrey Bokor
Electrical Engineering & Computer Sciences (EECS), University of California Berkeley, Berkeley, CA
The inverse magnetostrictive effect can be used in a composite multiferroic system to change the magnetic anisotropy of a ferromagnet by an applied voltage [1, 2]. However, because the stress generated by the voltage does not break the time reversal symmetry in a magnetic system, a binary bit cannot be written in a single domain magnet by this process. We propose a structure that consists of five magnets that can be used to write a binary bit in the central magnet using a voltage bias. In Figure 1, magnets 1 and 5 are fixed inputs, magnets 2 and 4 are carrier magnets, and magnet 3 is the free magnet to which the data is written. All the magnets are made of Terfenol-D and have their easy axis along the out-of-plane direction due to stress-induced anisotropy. Applying a voltage of around 100 mV to the Cu/PZT/Terfenol-D stack creates a stress on the PZT layer, which is transferred to the Terfenol-D layer to change its easy axis from, out-of-plane to in-plane via magnetostriction. Figure 1 illustrates how a binary bit can be written from the left input magnet to the central magnet, using voltage clock sequence. The operation of this structure has been verified using COMSOL and OOMMF simulations. This is a low-energy writing mechanism that can be used to generate inputs to nanomagnetic logic (NML) circuits or to change the value of a magnetic memory cell.
References
1. "Multiferroic Magnetoelectric Composites: Historical Perspective, Status and Future Directions", Ce-Wen Nan et al., Journal of Applied Physics, 103, 0031101 (2008). 2. "Multiferroic and Magnetoelectric Materials", W. Eerenstein et al., Nature, 442, August 2006.
11:06 AM
AH-12. Manipulations of Vibrating Micro Magnetic Particle Chains
Yan-Hom Li, Shih-Tsung Sheu, Jay-Min Pai and Ching-Yao Chen
Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan
We experimentally investigate the motion of a micro chain consisted of several magnetic particles whose diameters are about 4.5 micro-meters. The chain is firstly formed by a uniform directional field, and then manipulated by an additional vibrating field. The present methodology represents a simple reversible chaining process, whose particles can be re-dispersed after removal of the field. This particular configuration possesses potential applications in MEMS systems, including micro mixers, actuators and swimmers. It is known that dynamics of micro-chains are dominated by the dimensionless Mason number, which determines the ratio between magnetic forces and hydrodynamic drags. We demonstrate the chains appear different behaviors, from rigid body vibrations, bending distortions to breaking failures, under increasing values of the Mason number. In addition, because of the hydrodynamic drags, the maximum vibrating phase angles lag and differ from the magnetic fields. Detailed experiments are carried to elucidate the influences of Mason number. Demonstrated in the figures are sequential images of two typical cases showing structure bending with phase angle differences (top row) and breaking failure of a chain (bottom row). We also successfully apply the vibrating chain moving forward as a micro-swimmer by connecting particles of different sizes. The Strouhal number, which is often used to measure the efficiency of propulsion generated by vibration, is found about two orders greater than the most efficient range of a nature swimmer.
11:18 AM
AH-13. Design and Test of Magnetostrictive Actuators for Nanometer Resolution and Fast Response Applications
Tianli Zhang, Heng Zhang, Jinghua Liu, Jingmin Wang and Chengbao Jiang
School of Materials Science and Engineering, Beihang University, Beijing, China
To meet the requirements of large displacement, fast response and nanometer resolution in the fields of aerospace, precision automation, robotics and other high-tech fields. A giant magnetostrictive actuator (GMA) with a flexure pre-stress structure is designed and the work is discussion in details in this paper. GMA as a key component, especially for micro-displacement control systems, large displacement, fast response and nanometer resolution are usually difficult to be simultaneously obtained. For purposes, a suitable mechanical structure must be selected based on the requirements. Because of flexure structure has extraordinary mechanical properties as free of friction, clearance and lubrication, a pre-stress structure and a flexure hinge lever amplification structure are designed. In this case, types of this flexure pre-stress structure geometry are considered, and the finite-element method is adopted for analyzing and optimizing. And elliptical flexure hinge is chosen for its large displacement output with moderate stress concentration. A 0.5mm thick flexure hinge with 3mm thick beam is confirmed, so that large amplitude can be achieved with proper GMA size. Appropriate dimensions of magnetostrictive rod and parameters of driving coil are chosen to be parts of an efficient driving system. The system also includes a closed magnetic circuit path with diminished reluctance, a power source and a command signal generator. Static and dynamic performances of the GMA are tested. As a result, displacement output of the GMA could reach 140μm driving by 2A current (500G magnetic field). The dynamic performance indicated that the GMA achieve a steady output larger than 75μm in the range of 0Hz to 500Hz. When the GMA works at resonance frequency, the displacement output can rise to more than 800μm, but the wave shape of output is aberrant. The result also shows the GMA can get a nanometer resolution at low frequency (lower than 200Hz).
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Tianlong Wen, Carnegie Mellon University
AP-01. Magnetic Field Assisted Polyol Synthesis of Cobalt Carbide Microwires
Ahmed A. Farghaly, Zachary J. Huba and Everett E. Carpenter
Chemistry, Virginia Commonwealth University, Richmond, VA
In recent years, magnetic field assisted synthesis has become a promising approach for shape and size control of magnetic particles[1,2]. One-dimensional (1D) magnetic structures have recently attracted considerable interest because of their potential applications such as permanent magnets, high density magnetic storage media, sensors, and catalysts[3]. Herein, we report the synthesis of cobalt carbide CoxC (x = 2 or 3) microwires, using a polyol method in the presence of a high external magnetic field of 4.3 kOe. X-ray diffraction (XRD) scans indicate that the synthesized wires consist of three phases Co2C, Co3C and Co. Scanning electron microscopy (SEM) studies (Fig.1) show that the microwires form having diameters in the range of 1.4-1.8 μm with their length varying between 18-30 µm. The SEM results infer that the morphology of the growing particles was controlled by magnetic lines of forces. The applied field directs the growth of the particles into wires. The saturation magnetization and coercivity of these wires are 84 emu/g and 378.5 Oe, respectively. The high saturation magnetization may be attributed to the presence of metallic cobalt.
References
(1) Wu, M. Z.; Xiong, Y.; Jia, Y. S.; Niu, H. L.; Qi, H. P.; Ye, J.; Chen, Q. W. Chemical Physics Letters 2005, 401, 374. (2) Hong, R. Y.; Pan, T. T.; Han, Y. P.; Li, H. Z.; Ding, J.; Han, S. J. Journal of Magnetism and Magnetic Materials 2007, 310, 37. (3) Balela, M. D. L.; Yagi, S.; Matsubara, E. Electrochemical and Solid State Letters, 14, D68.
AP-02. Co-Ferrite Spinel and FeCo Alloy Core Shell Nanocomposites & Mesoporous Systems for Multifunctional Applications
Kai Zhang and A. K. Pradhan
Center for Materials Research, Norfolk State University, Norfolk, VA
Magnetic nanoparticles have been extensively studied for most biomedical applications such as magnetic resonance imaging (MRI) enhancement, magnetic separation of DNA, cells and proteins, hyperthermia and magnetic targeted drug delivery. Cobalt ferrite nanoparticles received a lot of attention due to their remarkable properties of moderate saturation magnetization, relatively large magnetic anisotropy, chemical stability and are of intensive importance in the field of science and technology. In addition, soft magnetic nanoparticles, such as FeCo, are of interest in this context due to their very high Curie temperature, low coercivity, low anisotropy, high permeability, and high saturation magnetization (Ms). However, FeCo magnetic nanoparticles are very reactive and oxidized easily in the presence of air environment and subsequently lose their magnetic properties. Moreover, these two magnetic nanoparticles are very toxic and is not biocompatible and restrict them to be used in the field of biomedical applications. We report on the fabrication of condensed and mesoporous silica coated CoFe2O4 and FeCo magnetic nanocomposites. The CoFe2O4 magnetic nanoparticles were encapsulated by well defined silica layer with a uniform thickness of 5 nm. The mesopores in the shell were fabricated as a consequence of removal of organic group of the precursor through annealing. The mesoporous silica shells lead to a larger magnetic coercivity than that of the pure CoFe2O4 magnetic nanoparticles due to the decrease of interparticle interactions and magneto-elastic anisotropy. In addition, the FeCo nanoparticles were coated with condensed and mesoporous silica. As a consequence, the condensed silica protects the reactive FeCo alloy from oxidation up to 300 oC, maintaining the high magnetization of the nanoparticles. However, saturation magnetization of silica coated FeCo nanoparticles is dramatically decreased after annealing at 400 °C due to the oxidation of the FeCo core. The silica coated magnetic nanostructure may have potential use in the field of biomedical applications.
References
[1] Moeser G D, Roach K A, Green W H and Hatton T A 2004 AIChE Journal 50 11 [2] Johannsen M, Gneveckow U, Eckelt L, Feussner A, Waldöfner N, Scholz R, Deger S, Wust P, Loening S A and Jordan A 2005 Int. J. Hyperthermia. 21 (7) 637 [3] Liu H-L, Hua M-Y, Yang H-W, Huang C-Y, Chu P-C, Wu J-S, Tseng I-C, Wang J-J, Yen T-C and. Chen P-Y, 2010 Proceedings of the National Academy of Sciences 107 34 15205 [4] Limaye M V, Singh S B, Date S K, Kothari D, Reddy V R, Gupta A, Sathe V, Choudhary R J and Kulkarni S K 2009 J. Phys. Chem. B 113 9070 [5] Su X, Zheng H, Yang Z, Zhu Y and Pan A, 2003 J. Mater. Sci. 38 4581 [6] Zhang K, Holloway T, Bahoura M, Pradhan A K, Prabakaran R, Pradhan J, Smith S, Hall J C, Ramesh G T, Sahu D R and Huang J L 2009 Proc. SPIE 7291 729104 [7] Sounderya N and Zhang Y 2008 Recent Patents on Biomedical Engineering 1 34
AP-03. Gram scale synthesis of high magnetic moment Fe100-xCox alloy nanoparticles
Chins Chinnasamy1, Jonathan Herr2, Riyanka Pai1 and JinFang Liu1
1Electron Energy Corporation, Landisville, PA; 2Chemistry, PennState University, University Park, PA
The goal of this work is to develop soft magnetic Fe100-xCox alloy nanoparticles with high magnetic moment for the fabrication of microinductors and nanocomposite permanent magnets. We have successfully synthesized gram scale (Fig 1a) Fe100-xCox nanoparticles with particle sizes below 40 nm (Fig. 1a inset) and with close-to-bulk magnetic properties using the modified polyol process in an inert atmosphere. Fe(II) chloride and cobalt acetate tetrahydrate in the required ratio were dissolved in glycol with appropriate amounts of NaOH and PVP.The solution was heated to a temperature where the glycols will transform into different intermediate phases and at the same time reduce the metal salts into nanoparticles. The structure, composition and magnetic properties were investigated using the XRD, SEM, ICP, TEM and VSM. The composition, particle size and magnetic properties were tuned by controlling the reaction parameters such as the amount of NaOH concentration, amount of glycol and metal precursor ratio. The maximum magnetic moment of about 21 kG and intrinsic coercivity of 165 Oe were observed at room temperature (Fig.1b). The nanoparticles were used to fabricate microinductors and nanocomposite permanent magnets .
AP-04. Synthesis of Fe-Co nanoparticles with high saturation magnetization by low temperature post-annealing through the growth of particle from nanoparticles cluster
Tomoyuki Ogawa, Hideaki Takano, Hiroaki Kura and Migaku Takahashi
Department of Electronic Engineering,, Graduated School of Engineering, Tohoku University, Sendai, Japan
Fe-Co alloy has the highest saturation magnetization, Ms, (240 emu/g) in the magnetic materials in the Slater-Pauling Curve. Chemically synthesized Fe-Co nanoparticles (NPs) will be one of the hopeful nanostructured material for various application fields. However, the Ms of Fe-Co NPs synthesized via chemical route was much smaller than that of bulk because of small grain size and inhomogeneous chemical composition in NPs. In this study, low temperature post-annealing in an oleylamine matrix was performed for the Fe-Co NPs synthesized by a thermal decomposition of home-made mixed precursor of Fe(CO)5 and Co2(CO)8. As-synthesized Fe-Co NPs formed NPs cluster (NC) structure consisting of many primary small NPs wit h 5 nm in diameter. The NC structure was never obtained for Fe NPs or Co NPs, thus, the synthesized Fe-Co NC structure could be unique nanostructure originating from the mixed precursor. After post-annealing at 250 °C, the NC became to one NP by diffusion and growth processes of the primary NPs. As a result, thus post-annealed Fe-Co NPs with 20 nm in diameter were obtained. XRD patterns indicate that both as-synthesized Fe-Co NPs and post-annealed Fe-Co NPs show bcc structure and their grain sizes are estimated of 1.5 nm and 11.3 nm, respectively. By the post-annealing, chemical composition may be homogenized due to diffusion process of atoms during growth process from the primary small NPs. Ms of as-synthesized Fe-Co NPs were about 140 emu/gnet within our investigated chemical composition range from 30 % to 60 %. Interestingly, Ms of post-annealed Fe-Co NPs increase through the post-annealing process, and show the maximum value of 212 emu/gnet at Fe : Co = 68 : 32. The chemical composition tendency agrees well with the Slater-Pauling Curve. As taking into account the weight of surfactant, Ms of post-annealed Fe70Co30 NPs may comparable to that of bulk Fe-Co.
AP-05. Large-scale synthesis of high moment FeCo nanoparticles using modified polyol synthesis
Mehdi Zamanpour1, Vincent G. Harris2, Laura H. Lewis1, Carmen Vittoria2 and Yajie Chen2
1Chemical Engineering, Northeastern University, Boston, MA; 2Electrical and Computer Engineering, Northeastern Univeristy, Boston, MA
Magnetic nanoparticles (NPs) possess attractive properties for applications in catalysis, biomedicine, magnetic resonance imaging, data storage and environmental remediation. Among others, binary alloys consisting of Fe and Co have the highest magnetic moments, but their affinity to form oxides act to reduce the magnetic moment of nanoparticles as their size reduces below ~30 nm. The goal of this research is to fabricate single phase, size-controlled FeCo nanoparticles having high magnetic moment. To synthesize particles with these properties, a non-aqueous method (i.e. the modified one-pot polyol synthesis method) was employed. In this process ethylene glycol serves as solvent and reducing agent as well as surfactant. Sodium hydroxide is added to the solution as the hydroxyl ion source, which serves as sites for the nucleation and growth of FeCo NPs from metal precursors in the form of iron chloride and cobalt acetate. The experiments indicate a pure-phase FeCo nanoparticles, having magnetic moments up to 220 emu/g for sizes of 20-30 nm, formed by varying the amount of salt used in processing. It turns out that when the salt/metal concentration ratio lies in a specific range (Fig.1), the particles reach their size limit and show the highest magnetic moments. However below and beyond this region the moment decreases due to the formation of superparamagnetic particles and oxide phases, respectively. Importantly, this technique is verified to have the ability to be scaled to larger batches. Here we demonstrate processing of more than 2 grams per batch.
References
Bozorth, Ferromagnetism, 1993, Wiley-IEEE Press , ISBN: 978-0780310322 D. Kodoma, K. Shinoda, K. Sato, y. Sato, B. Jeyadevan, and K. Tohij, IEEE transactions on Mangnetics, 2006, 49, 2796 V. K. LaMer and M. D. Barnes, J. Colloid Sci., 1, 71-77 (1946)
AP-06. Carbon nanotube coated silicated soft magnetic carbonyl iron microspheres and their magnetorheology
Ying Dan Liu and Hyoung Jin Choi
Department of Polymer Science and Engineering, Inha Univ, Incheon, Republic of Korea
Magnetorheological (MR) fluid with soft magnetic carbonyl iron (CI) particles dispersed in medium liquid is a smart suspension with a phase change from a liquid-like to solid-like state under magnetic field [1]. Due to its well controlled and reversible mechanical properties, the MR fluid has been widely commercialized [2]. However, settling stability due to density difference between CI particle and carrier liquid must be improved. Many efforts have been paid to solve this problem, such as additive addition and coating techniques of CI particles. In this work, we fabricated carbon nanotube (MWNT) coated CI particles via a layer-by-layer self-assembly procedure by placing the silica modified CI [3] particles in two polyelectrolytes alternately and finally in negative MWNT aqueous dispersion after the last positive layer of polyelectrolyte. As shown in Figure 1, the composite particles exhibit soft and dense MWNT nest on their surface, which results in a sequential decrease in density. Not only the lower density but also the considerably rough surface of composite particles are helpful to the settling stability of their MR fluid. In the MR tests via a rotational rheometer, similar MR effect as with silica coated CI was discovered at a fixed particle concentration.
References
[1] B. J. Park, F. F. Fang, H. J. Choi, Soft Matter 6, 5246 (2010) [2] J. D. Carlson, D. M. Catanzarite, K. A. StClair, Int. J. Mod. Phys. B 10, 2857 (1996) [3] F. F. Fang, H. J. Choi, J. Appl. Phys. 103, 07A301 (2008)
AP-07. Magnetic stability of Fe-Silica core-shell nanoparticles prepared via hydrolysis
Jianhui Zhang1, Aaron Thurber2 and Alex Apunnoose2
1Department of Physics, Nanjing University, Nanjing, China; 2Department of Physics, Boise State University, Boise, ID
Fe-silica and magnetite-silica core-shell nanoparticles were prepared with a metallic Fe or magnetite core inside a silica shell following a forced hydrolysis method. The chemical synthesis provides a hematite-silica structure which, when annealed at 500 °C under flowing hydrogen, reduces the hematite core to either magnetite or metallic Fe depending on the flow rate and hydrogen concentration. Samples were characterized by x-ray diffraction, transmission electron microscopy and vibrating sample magnetometry. The magnetization of the dried silica coated Fe particles was 15 emu/g. Long-term stability of such magnetic nanoparticles is of concern, and so we present data demonstrating that the magnetic properties of the annealed core-shell particles remain constant over time, and so the silica shell successfully protects the core from chemical changes. This could lead to advancements in allowing the core-shell particles to be stable in corrosive or reactive environments which might otherwise destroy the magnetism and thus their utility. It was found that even after 6 weeks of storage in air that the magnetization remained stable, indicating that the outer silica layer ensures protection from oxydation.
AP-08. Facile Synthesis of Superparamagnetic Iron Oxide/MCM-41 Hybrid Nanospheres for Targeted Drug Delivery
Lele Yu1 and Hong Bi1, 2
1College of Chemistry and Chemical Engineering, Anhui University, Hefei, China; 2Department of Medicine, Columbia University, New York, NY
Magnetic mesoporous silic a has been intensively studied as nanocarriers in smart drug delivery and stimuli-responsive controlled release[1,2]. Herein iron oxide/MCM-41 hybrid nanospheres with a large surface area of 1334 m2 g-1 and a uniform diameter of 85 nm were synthesized via a facile sol-gel route. TEM image(Fig.1A) shows ultra-small amorphous iron oxide nanoparticles evenly distributed in MCM-41 nanospheres. A significant difference between the low-angle XRD patterns before and after MCM-41 loading iron species(designated as MSN and MMSN, respectively) indicates iron oxide incorporation into nanopores while the ordered mesoporous structure was maintained[3]. The decreasing of Si-OH groups at 966 cm-1 in FTIR spectra(Fig.1B) reveals the adsorption of iron oxide onto the wall[4], and thus the average pore diameter decreased from 4.9 to 4.2 nm(Fig.1C). The MMSN present superparamagnetic(Fig.1D) with Ms of 1.5 emu g-1 which is much smaller than those of bulk iron oxide materials due to the low Fe/Si molar ratio of 0.15. Compared with iron oxide/SBA-15 nanocomposites[3,4], larger surface area, more uniform morphology and smaller size ensure the prepared MMSN be more favorable for higher capacity drug loading and targeted drug delivery.
References
[1] Ruiz-Hernandez E, Baeza A, Vallet-Regi M. Smart drug delivery through DNA/ magnetic nanoparticle gates. ACS NANO (2011) 5: 1259-1266. [2] J Liu, SZ Qiao, QH Hu, et al. Magnetic nanocomposites with mesoporous structures: synthesis and applications. Small (2011) 7: 425-443. [3] J. S. Li, Y. S. Lin. Facile synthesis of ordered mesoporous silica with high α-Fe2O3 loading via sol-gel process. J. Mater. Sci. (2008) 43: 6359-6365. [4] Karynne C. Souza, Nelcy D. S. Mohallem, Edésia M. B. Sousa. Mesoporous silica-magnetite nanocomposite: facile synthesis route for application in hyperthermia. J. Sol-Gel Sci. Technol. (2010) 53: 418-427.
AP-09. Magnetic Properties of Thiol Capped Gold Nanoparticles
Sungwon Yoon1, Taehoon Lee2, Kyung Hoon Han1, Byoungjin Suh1, Zeehoon Jang2, Ju Hee Kim3 and Duk-Young Jung3
1Physics, Catholic University of Korea, Bucheon, Republic of Korea; 2Physics, Kookmin University, SEOUL, Republic of Korea; 3Chemistry, Sungkyunkwan University, Suwon, Republic of Korea
We present the experimental results of TEM and magnetization measurements on thiol capped gold nanoparticles (Au-Sr NPs), synthesized in a two-phase liquid-liquid system. The size of particles ranges from 2.0 nm to 3.5 nm with an average size of 2.8 nm. Magnetic properties of the Au-Sr NPs are analyzed in terms of a mixture of ferromagnetic, paramagnetic and diamagnetic components [1]. Magnetization curves show the hysteresis behavior typical for a ferromagnet in the whole temperature range investigated (from 2 K to 300 K). On the other hand, the magnetization as a function of field M(H) is not saturated at any temperatures. At low T, the gradual increase of M(H) was observed even up to H = 7 T, implying the existence of a paramagnetic component. The negative slope of M(H) at high T demonstrates that the contribution of a diamagnetic component is also considerable. From a theoretical fit, we obtained effective values, such as a paramagnetic effective moment μeff = 5.7 μB and a diamagnetic susceptibility χ dia = -1.7 × 10-7 [emu/g Au Oe]. In addition, the temperature dependence of the ferromagnetic component Mferro(T) was extracted from the experimental M(T) data by subtracting the paramagnetic and diamagnetic contributions calculated using the fitting parameters described above an d shown in the figure. The characteristics of the extracted Mferro(T) is explained in the light of a mean field model.
References
[1] P. Crespo, R. Litrán, T.C. Rojas, M. Multigner, J.M. de la Fuente, J. C. Sánchez-López, M. A. García, A. Hernando, S. Penadés and A. Fernández, Phys. Rev. Lett. 93, 087204 (2004).
AP-10. Concentration dependence of magnetic moment in Ce1-xFexO2
Geoffrey L. Beausoleil, Aaron Thurber, Alex Punnoose and Srinivasa Singamaneni
Physics, Boise State University, Boise, ID
In this study ceria (Ce1-xFexO2) with x = 0-20% were produced in triplicate through a coprecipitation in a forced hydrolysis synthesis that yielded consistently sized nanocrystals. Samples were prepared using three different methods/precursors labeled as Ce-Cl, Ce-AmNit and Ce-Nit and the magnetic moment/Fe ion varies with synthesis method (see bottom inset). X-ray diffraction shows the average crystallite size to be 4.4 +/- 1.2 nm, which agreed well with TEM. The moment per Fe ion found was largest at the lowest dopant concentrations giving 0.078 and 0.055 µB/Fe ion for 0.1 and 0.5% Fe, respectively, and quickly decreased as the concentration was increased. This indicate that the dopant ions are not important in producing the observed ferromagnetism and changes in the electronic structure of ceria due to Fe doping might be playing major role. The XPS data collected shows Fe to be in the 3+ oxidation state and confirms the nominal dopant concentration. Interestingly, there was a consistent estimated Ce3+/Ce4+ ratio of about 0.2-0.3 in Fe doped samples which further support the role of changes in the electronic structure being the key player in the observed magnetism at low x values.
AP-11. Magnetic and optical properties of monosized Eu-doped ZnO nanocrystals from nanoemulsion
Ha Young Yoon1, Jun H. Wu2, Ji H. Min1, Ji Sung Lee1 and Young K. Kim1
1Department of Materials Science and Engineering, Korea University, Seoul, Republic of Korea; 2Pioneer Research Center for Biomedical Nanocrystals, Korea University, Seoul, Republic of Korea
ZnO is an interesting wide direct band-gap semiconductor and has many remarkable properties [1]. After proper doping, it can be turned into dilute magnetic semiconductors (DMS) for spintronics applications [2]. In this study, we investigate monosized Eu-doped ZnO nanocrystals as a function of the doping ratio. High-quality, highly crystalline nanocrystals with controllable size and tight size distributions were synthesized by the nanoemulsion method [3]. In the experiments, the synthesis of nanocrystals was carried out by thermal decomposition of europium and zinc precursors in the presence of the PEO-PPO-PEO polymer surfactant, 1,2-hexadecanediol and octyl ether. The crystal structural, size and shape of the resultant nanocrystals were analyzed by XRD and TEM, whereas PL and PPMS/VSM reveal the optical and magnetic properties. Fig. 1(a) and (b) show the representative TEM images of the 5% Eu-doping (S1) and 10% Eu-doping (S2) nanocrystals, which are plainly spherical in shape and have a monosized distribution with an averaged particle size of ~10 nm in diameter. Fig. 1(c) gives the photoluminescence spectra of the nanocrystals under the excitation wavelength of 330 nm. As the Eu content increases, the emissions at 590 nm (5D0→7F1 transition) and 613 nm (5D0→7F2 transition) become stronger, signaling the existence of Eu in the nanocrystals. Fig 1(d) compares the magnetic responses of different nanocrystals at 5K. Obviously, the magnetic behavior is increasingly enhanced with the doping ratio.
References
[1] U. Ozgur et al., J. Appl. Phys. 98 041301 (2005). [2] Y. Liu et al., J. Phys. Chem. C 112, 686 (2008). [3] H. L. Liu et al., Nanotechnol. 22, 055701 (2011).
AP-12. Magnetic Properties and microstructures of defect free high crystalline NiO Nanoparticals and Nanorods
Dapeng Chen1, Xiaolin Wang1, Yi Du1, Song Ni2 and Xiaozhou Liao2
1Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW, Australia; 2School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney, Sydney, NSW, Australia
Nanosized transition-metal oxides have been extensively investigated due to their significant importance in both fundamental research and applications [1]. Among various oxides, NiO has been particularly studied as a promising candidate for the next generation of resistive switching memory devices [2-4]. The simple structure and high temperature antiferromagnetism (TN = 523 K) of NiO also made it as a promising exchange-bias material. The regularly shaped NiO nanocrystals were prepared by using a novel physical evaporation method. The NiO nanoparticles with perfect cubic shape and lattices and with average size of 8 nm and tetragonal nanorods with width of 6 nm and aspect ratio of 2-6 can be obtained by tuning evaporation conditions. The samples are highly crystallized throughout the entire nanocrystal with atomic-scale smooth surface. Both samples present a superparamagnetic behaviour at room temperature and show weak ferromagnetic state at low temperature due to uncompensated spins at the surfaces. Comparisons and discussions on the microstructures, dielectric and magnetic properties will be presented for the NiO nano-particles fabricated by both chemical method and our novel physical vapour deposition. Using the first-principles calculation, we will show that the degree atomic vacancies on either Ni or O sites play an important role in ruling the transport and magnetic states of the NiO nano-particles.
References
1. Y. Tokura and N. Nagaosa Science 288, 462 (2000) 2. H. Tang, F. Li, and J. Shinar, Appl. Phys. Lett. 71, 2560 (1997) 3. H.-D. Lee, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B 81, 193202 (2010) 4. K. Oka,T. Yanagida,,K. Nagashima,T. Kawai,,J.-S. Kim,and B.-H. Park, J. Am. Chem. Soc. 132, 6634 (2010)
AP-13. The effect of ball size on morphology and magnetic properties of anisotropic SmCo5 nanoflakes prepared by surfactant-assisted ball milling
Junwu Nie1, Xianghua Han1, Jingjing Liu1, Wei Li1, 2, Aru Yan1 and Juan Du1
1Ningbo Institute of Material Technology & Engineering,CAS, Ningbo, China; 2Division of Functional Materials, Central Iron and Steel Research Institute, Beijing, China
Surfactant-assisted high energy milling was proved to be an effective process to synthesize anisotropic magnetic SmCo5 nanoflakes. Those anisotropic nanoflakes are good precursors to make anisotropic nanocomposites such as a-Fe/SmCo5 or Fe- Co/SmCo5 magnets, aiming to enhance remanence (Br) by exchange coupling between soft and hard phase. Uniform size of anisotropic nanoflakes was important to be coated by soft phase by chemical method. The magnetic properties of the nanoflakes were influenced by its morphology, which was depended on ball milling parameters. In order to get good size and morphology of nanoflakes, we used SmCo5 cast alloy as starting powder to make a series of flakes with various sizes. Different shaped nanoparticles have been obtained by varying the ball size (12.7mm, 9.5mm, 6.35mm and 4mm) and size ratio of big/small balls during the milling process and then separated by sonication. The obtained nanoflakes had anisotropic nanocrystallines and coercivity values up to 15 kOe. The analyses of the micrograph by scanning electron microscopy (SEM), x-ray diffraction (XRD) patterns, and magnetic measurements demonstrated the effect of ball size and its ratio on the magnetic properties. The mechanism responsible for special formation of 5~10 μm submicron flakes with anisotropic nanocrystalline grains is discussed.
AP-14. Efficiently Recyclable Magnetic Core-Shell Photocatalyst for Photocatalytic Oxidation of Chlorophenol in Water
Kyong-Hoon Choi1, Seung-Lim Oh2, Jong-Hyung Jung3 and Jin-Seung Jung3, 2
1Material R&D Division, H & Glbal Co., Gwangmyeong, Republic of Korea; 2Gangneung Center, Korea Basic Science Institute, Gangneung, Republic of Korea; 3Chemistry, Gangneung-Wonju National University, Gangneung, Republic of Korea
Chlorophenols are well known as environmental pollutant and have been top listed pollutant because of their toxicity, persistence and bio-accumulation in aquatic organism[1-3]. In recent years, various approaches have been proposed for degradation of chlorophenols in natural and waste water systems[4-8]. Especially, separation and recycling of the catalysts are considered a main objective in the area of the pollutant[9]. We are reporting a novel photofunctional magnetic core-shell particle that is strategically designed and prepared by simple modified coating process. The size of core magnetic particle can be rationally tuned by using different amounts of ethylene glycol (EG) and diethylene glycol (DEG). As characterized by field emission scanning electron microscopy (FESEM), the as-synthesized Fe3O4@TiO2 particles exhibit a narrow size distribution with a typical size of 200 ± 14.5 nm and 10nm shell thickness. Magnetic measurement indicates that all the as-synthesized Fe3O4@TiO2 core-shell particles are superparamagnetic at room temperature, and their saturation magnetization (Ms) values can be controlled by modulating the core size and grain size. The multifunctional Fe3O4@TiO2 core-shell particles exhibit excellent catalytic properties for the oxidation reaction of 2,4,6-trichlorophenol (2,4,6-TCP) in aqueous solution. These novel mutifunctional composites may apply in treatment for dye water, degradation of other organic pollutants and environmental cleaning etc.
References
[1]. A. Sorokin, J. L. Seris and B. Meunier, Science,268, 1163(1995) [2]. S. Sen Gupta, M. Stadler, C. A. Noser, A. Ghosh, B. Steinhoff, D. Lenoir, C. P. Horwitz, K. W. Schramm and T. J. Collins, Science, 296, 326(2002) [3]. T. J. Collins, Acc. Chem. Res., 35, 782(2002) [4]. Jain, S.; Jayaram, R. V. Sep. Sci. Technol.,42, 2019(2007) [5]. Das, S.; Banthia, A. K.; Adhikari, B. Chem. Eng. J., 138, 215(2008) [6]. Chan, W. C.; Fu, T. P. J. Appl. Polym. Sci., 67, 1085(1998) [7]. Polcaro, A. M.; Palmas, S. Ind. Eng. Chem. Res., 36, 1791(1997) [8]. Kennedy, L. J.; Lu, J.; Mohn, W. W. Water Res., 26, 1085(1992) [9]. Review: D. J. Cole-Hamilton, Science, 299, 1702(2003)< /p>
AP-15. Preparation and magnetic behavior of self-assembled nanocrystalline CuFeS2 chalcopyrite
Chun-Rong Lin1, Yu-Jhan Siao2, Igor S. Lyubutin3, Mei-Li Chen4, Tai-Chun Han5, Geng-Hao Jhang1, Geng-Ben Chen1, Chung-Chun Wu1 and Xiaoding Qi2
1Institute of Nanotechnology and Department of Mechanical Engineering, Southern Taiwan University, Tainan, Taiwan; 2Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan; 3Shubnikov Institute of Crystallography, Russian Academy of Sciences, Moscow, Russian Federation; 4Department of Electro-optical Engineering, Southern Taiwan University, Tainan, Taiwan; 5Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan
A thermal pyrolysis route was developed to synthesize CuFeS2 nanocrystals with tetragonal cell. Depending on reaction temperatures, sample with mean crystallite size of 4.7 nm and 37 nm can be obtained. High resolution transmission electron microscopy (HRTEM) shows that the synthesized samples composed of brick-like nanocrystals with widths of 3-6 nm and lengths of 15-20 nm. Magnetic measurements display that the magnetization increases at low temperature for both samples, sample of 4.7 nm especially. This indicates that the magnetic moment configurations of nanocrystals are antiferromagnetic below their bulk Neel temperature and undergo an antiferromagnetic to weak-ferromagnetic transition due to spin canting at disorder surfaces. Room temperature 57Fe-Mossbauer spectra indicate that a part of iron ions is in the magnetically ordered state (about 43 % of total iron in both 4.7 and 37 nm size samples), and another part is in the paramagnetic state (about 57 %). In the 37nm-sample, the line broadening of the magnetic component is much higher than in the 4.7nm-sample, and the value of hyperfine magnetic fields Hhf is spread from 33 to 28 Tesla. This shows a magnetic non-homogeneity of the sample.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Masato Ohnuma, NIMS
AQ-01. Continuous Annealing Method for Producing a Flexible and Curved Soft Magnetic Amorphous Alloy Ribbon
Bruno Francoeur and Pierre Couture
IREQ, Hydro-Quebec, Varennes, QC, Canada
A method was developed for continuous annealing of an amorphous alloy ribbon moving forward at several meters per second, giving a curved shape to the ribbon which remains flexible afterwards and can be easily wound into a toroidal core with excellent soft magnetic properties. A heat pulse was applied by a compact system on a Metglas 2605HB1 ribbon moving forward at 5 m/sec to initiate a thermal treatment at 460 °C, near the crystallization onset. The treatment duration was less than 0.1 sec and the heating and cooling rates were above 10 000 °C/sec, which helped preserve most of the alloy as-cast ductility state. Such high temperature rates were achieved by forcing a static contact between the moving ribbon and a temperature-controlled roller. A tensile stress and a series of bending configurations were applied on the moving ribbon during the treatment to induce the development of a magnetic anisotropy and to obtain the desired natural curvature radius. The core losses at 60 Hz of toroidal cores wound with the resulting ribbon are lower than the specific values reported by the alloy manufacturer. This method allows a low cost, ready-to-use, handleable and cuttable ribbon to be supplied directly from the casting plant for mass production of toroidal cores for distribution transformer kernels (core & coil only), pulse power cores, etc...
References
Francoeur, B., and Couture, P., "System and Method for Treating an Amorphous Alloy Ribbon," Patent pending, International Patent Application WO 2011/060546, May 26, 2011 - Francoeur, B., and Couture, P., "Electrical Transformer Assembly," Patent pending, International Patent Application WO 2011/060547, May 26, 2011
AQ-02. Si addition effect on soft magnetic properties in FeBCCu alloy system
Fan Xingdu1, 2, Men He1, Ma Aibin2 and Shen Baolong1
1Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, China; 2College of Mechanics and Materials, Hohai University, Nanjing, China
Nanocrystalline soft magnetic alloys have been found to exhibit excellent soft magnetic properties such as high magnetic flux density (Bs), high effect permeability (μe), low coercivity (Hc) and low core loss over the past two decades. Of these alloys, FeSiBNbCu (FINEMET) has been widely used for magnetic devices due to its high μe and low core loss. However, this alloy has a rather low Bs of 1.24 T and contains expensive metal element Nb which causes the increase of the production cost. Recently, nanocrystalline alloy FeSiBPCu has been developed with high Bs of 1.9 T and low core loss of 0.12 W/kg at 1.0 T and 50 Hz. More recently, with the aim of synthesizing a new Fe-based nanocrystalline soft magnetic alloy, we developed FeBCCu nanocrystalline alloy with high Bs of 1.78 T, low Hc of 5.1 A/m. However, the annealing temperature range for this alloy is narrow owing to the precipitation of Fe3B, leading to the difficulty of further improvement in the soft magnetic properties. In this study, the soft magnetic properties and crystallization behavior of Fe83B10C6-xSixCu1 (x=0-4) alloys prepared by annealing the melt-spun ribbons have been investigated. Si element was added to this system with the aim of improving the thermal stability and soft magnetic properties because Si addition takes important influence on the precipitation of Fe-B compounds thus widens the crystallization temperature range. It is found that in Fe83B10C6-xSixCu1 alloy system, Hc decreases with the addition of Si content and exhibits a minimum at x=2, while Bs shows a slightly decreasing tendency. And when x=2, the alloy exhibits excellent magnetic properties after appropriate heat treatment with a high Bs of 1.78 T, low Hc of 4.3 A/m and low core losses P10/50 of 0.27 W/kg, P10/400 of 3.6 W/kg and P10/1k of 11.3 W/kg, respectively.
References
[1]. Y. Yoshizawa, S. Oguma, and L. Yamauchi, J. Appl. Phys. 64, 6044 (1988). [2]. K. Suzuki, A. Makino, N. Kataoka, A. Inoue, and T. Masumoto, Mater. Trans., JIM 32, 93 (1991). [3]. M. A. Willard, M. Q. Huang, D. E. Laughlin, M. E. McHenry, J. O. Cross, V. G. Harris, and C. Franchetti, J. Appl. Phys. 85, 4421 (1999). [4]. Y. Yoshizawa, K. Yamauchi, T. Yamane, and H. Sugihara, J. Appl. Phys. 64, 6047 (1988). [5]. A. Makino, H. Men, T. Kubota, K. Yubuta, and A. Inoue, Mater. Trans. 50, 204 (2009). [6]. A. Makino, H. Men, T. Kubota, K. Yubuta, and A. Inoue, IEEE Trans. Magn. 45, 4302 (2009). [7]. X. D. Fan, A. B. Ma, H. Men, G. Q. Xie, B. L. Shen, A. Makino, and A. Inoue, J. Appl. Phys. 109, 07A314 (2011).
AQ-03. Giant Magneto-Impedance in Co63Fe4B22.4Si5.6Nb5 alloy ribbons
Huaijun Sun1, 2, Qikui Man1, Yaqiang Dong1 and Baolong Shen1
1Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 2College of Physics, Mathematics and Information engineering, Zhejiang Normal University, Jinhua, China
The giant magnetoimpedance (GMI) effect in amorphous and nanostructured soft ferromagnetic materials has generated growing interest because of its promising applications in magnetic sensors. The magnetoimpedance (MI) effect consists in a strong dependence of the electrical impedance, Z, of a ferromagnetic conductor on an external DC magnetic field when an ac current flows through it. Its origin lies in the changes in the skin effect penetration depth arising from the modification of the permeability. The GMI effect is turned out to be very sensitive to composition, sample shape, annealing conditions and quenched in internal stresses. The annealing conditions can influence the easy direction, the magnetic anisotropy and thus the domain structures. These latter can have a marked influence on the permeability, and can result in an enhanced GMI. However, the glass-forming ability (GFA) of traditional Co-based alloys is still weak, which makes amorphous alloy prepared with more of the internal defects, poor soft magnetic properties, and GMI effect generally weak. Recently, to further increase the GFA of the Co-based alloys, we increased the B and Si contents and investigated the effect of Nb addition on the thermal stability of the supercooled liquid and the cooling behavior of the alloys. As a result, Co-based bulk glassy alloys (BGAs) Co63Fe4B22.4Si5.6Nb5with diameters in the range up to 4 mm were synthesized, and showed excellent soft magnetic properties and the significant GMI effect. In this paper, Co63Fe4B22.4Si5.6Nb5 amorphous ribbons with thickness of 20 μm and width of 0.6 mm were prepared by the single copper roller melt-spinning method. The amorphous ribbons were annealed at different temperatures, and the GMI effect of the ribbons were investigated under different driving frequency. The results show that the alloy has a very excellent GMI effect, and appropriate annealing treatment can improve the ratio of magneto-impedance. The maximum GMI ratio of the ribbons is about 2400%, and the largest sensitivity is about 9200%/Oe. The Co63Fe4B22.4Si5.6Nb5 ferromagnetic glassy alloy is expected to be suitable for magnetic sensor application of new magnetic materials.
References
[1] K. Mohri, K. Kawashiwa, H.Yoshida, etc. IEEE Trans. Magn. 28, 3150 (1992). [2] K. Indaa, K. Mohri, K. Inuzuka. IEEE Trans. Magn. 30, 4623 (1994). [3] K. Mohri, K. Kawashima, K. Kohzawa, IEEE Trans. Magn. 19, 3168 (1993). [4] M. H. Phan, H. X. Peng. Prog. Mater. Sci. 53, 323 (2008). [5] H. J. Sun, Q.K. Man, Y.Q. Dong, B. L. Shen, etc. J. Alloys Compd. 504, S31, (2010). [6] H. J. Sun, Q. K. Man, Y. Q. Dong, B. L. Shen, etc. J. Appl. Phy. 107, 09A319, (2010).
AQ-04. Study on the soft magnetic properties and high frequency characteristics of Co-M (M=Ti, Zr, and Hf) thin films
Huang-Wei Chang1, Ya-Hsun Huang2, Chih-Chieh Hsieh2, Chih-Wei Shih2, Wen-Cheng Chang2 and De-Sheng Xue3
1Tunghai University, Taichung, Taiwan; 2National Chung Cheng University, Chia -Yi, Taiwan; 3Lanzhou University, Lanzhou, China
Recently, much attention has been focused on the study of soft magnetic thin films with high ferromagnetic-resonance frequency (fFMR) for above giga hertz (GHz) range applications, such as data transmission, spintronic devices, and micro-inductors.1-2 In order to increase fFMR of the films, a strong in-plane uniaxial magnetic anisotropy field (Hk) is requisite in addition to their high saturation magnetization (4πMs) and high permeability (μ), and low coercivity (Hc).3 Many previous studies focused on the FeCoB-based alloys, however, very limited papers have reported on the high-frequency characteristics of the Co-based films. 4 In this study, the Co100-xMx (M = Ti, Zr, and Hf) thin films with the thickness of 110 nm were prepared by using the oblique deposition method under an external magnetic field of 1200 Oe, and the magnetic properties and high frequency characteristics of them were investigated. The experimental results show that all studied films may exhibit strong induced in-plane uniaxial magnetic anisotropy. With the increase of the doping elements content x, 4πMs and coercivity along the hard and easy axes (Hch and Hce) are decreased, while Hk reaches the maximum value for the films having the composition to become the fully amorphous phase, as determined by x-ray diffraction. In addition to the relation between the magnetic properties and the alloy composition, the glass forming ability (GFA) of those Co-M films is also investigated. Among those three refractory elements, less Hf and Zr, i.e. x=7, are sufficient in forming the Co100-xMx amorphous, in contrast, more Ti, i.e. x=15, should be needed. The optimal magnetic properties of 4πMs = 11.0 kG, Hce = 5.4 Oe, Hch = 4.0 Oe, Hk = 320 Oe and fFMR = 5.4 GHz for Co93Hf7 films and 4πMs = 11.7 kG, Hce = 10.1 Oe, Hch = 1.6 Oe, Hk = 266 Oe and fFMR = 4.9 GHz for Co93Zr7 films are obtained, reflecting the benefit of Co93Hf7 and Co93Zr7 films for above GHz applications.
References
1H. Kronmuller and S. Parkin, Handbook of Magn. and Adv. Magn. Mat. vol. 5, 2571 (2007). 2M. Yamagguchi et al., J. Appl. Phys. 85, 7919 (1999). 3V. Korenivski, J. Magn. Magn. Mater. 215-216, 800 (2000). 4C. Jiang et al., J. Appl. Phys. 106, 103910 (2009).
AQ-05. Magnetism of BaB6 thin films produced by pulsed laser deposition
Karl Ackland, M. Venkatesan and J. M. Coey
School of Physics and CRANN, Trinity College, Dublin 2, Ireland
Room-temperature ferromagnetism has been reported in the alkaline earth hexaborides MB6 (M = Ca, Sr) which contain no 3d, 4d or 4f electrons [1]. Here BaB6 powders were synthesized from high purity barium and boron powders by solid state reaction at 1000 °C in evacuated sealed quartz tubes. The BaB6 powder was diamagnetic at room temperature. Thin films were deposited by pulsed laser deposition on Al2O3 (0001) substrates at different substrate temperatures using a pressed, vacuum-sintered BaB6 target. The films are amorphous; they show no Bragg peaks in X-ray diffraction. Magnetization curves (Fig. 1) show ferromagnetic-like signals for substrate temperatures in the range 450 °C to 550 °C only. The magnetization is virtually anhysteretic, isotropic and independent of temperature down to 4 K. Only a small volume fraction of the thin films can be ferromagnetic. For example, the saturation magnetization Ms of the film deposited using a substrate temperature of 500 °C is 15 kA m-1. However the field H0 obtained by extrapolating the initial susceptibility to saturation is 80 kA m-1. This is related to the magnetisation M0 of the ferromagnetic regions by the effective demagnetising factor H0= NM0 [2]. The ferromagnetic volume fraction of the sample f may be calculated from f = Ms N/H0. If we assume N = 1/3, only 6 % of the volume of the thin film is really magnetically ordered. The results are consistent with a model where the magnetism is associated with interfaces or grain boundaries.
References
[1] L.S. Dorneles, M. Venkatesan, M. Moliner, J.G. Lunney and J.M.D. Coey, “Magnetism in thin films of CaB6 and SrB6”, Appl. Phys. Lett., vol. 85, No. 26, p. 6377, 2004. [2] J. M. D. Coey, J. T. Mlack, M. Venkatesan, and P. Stamenov, “Magnetization process in dilute magnetic oxides”, IEEE Transactions on Magnetics, vol. 46, p. 6, 2010.
AQ-06. Measurement of Volume Exchange in Soft FeCo Films of High Magnetization
Christoph Mathieu1, Hau-Jian Liu2, Kristen S. Buchanan2 and Venkateswara R. Inturi1
1Seagate Technology, Bloomington, MN; 2Physics, Colorado State University, Fort Collins, CO
Soft FeCo films of high magnetization are a well known integral part of today’s magnetic recording heads. One important magnetic parameter is the so-called volume exchange since it determines exchange length, domain wall width and energy. Volume exchange, however, is difficult to measure and therefore approximate values based on known exchange constants for similar materials are often assumed even though the volume exchange can vary significantly as a function of composition. This paper presents results for volume exchange on Fe65Co35 determined by means of a detailed Brillouin light scattering study, one of a few methods that permit the measurement of the magnetic exchange constant. A set of Fe65Co35 films of various thicknesses was examined, and the Damon-Eshbach surface mode and up to seven standing spin wave modes were detected. Fig. 1 shows the experimental data and a fit of all observed modes, which yielded a volume exchange of about 3.5×10-6 erg/cm. The fits show that this result is relatively insensitive to surface anisotropy contributions. This project was supported in part by NIST 60NANB10D011 and NSF 0907706.
AQ-07. Tuning of Magnetization Dynamics in Sputtered CoFeB Thin Film by Gas Pressure
Feng Xu1, 2, Qijun Huang1, Zhiqin Liao1, Chongkim Ong3 and Shandong Li4
1Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, China; 2Laboratory of Solid State Microstructures, Nanjing University, Nanjing, China; 3Department of Physics, National University of Singapore, Singapore 11754, Singapore; 4Physics Department, Fujian Normal University, Fuzhou, China
Our previous work has shown that FeCoSiN thin film is sensitive to the sputtering gas pressure [1]. The key issue of the influence of gas pressure is ascribed to the inner stress of film [2]. Re search also shows that the different stress will bring about different microwave characteristics in FeCoHf thin films [3]. CoFeB thin films were deposited by rf sputtering under various sputtering gas pressures. The permeability spectra were measured with the shorted microstrip line method, and fitted with Landau-Lifshitz-Gilbert (LLG) equation. The fitting parameters are plotted in Fig. 1. Results show that with the increase of pressure, the anisotropy field, resonance frequency and initial permeability have their minimums and maximum, respectively. The damping factor has a similar relation with pressure as coercivities. The monotonous decrease of damping factor with increasing pressure shows a high tunability. This study shows that the magnetization dynamics of CoFeB thin film can be effectively tuned by sputtering gas pressure. All these dependences are suggested to be related to the inner stress of sputtered films.
References
References: [1] F. Xu et al., J. Appl. Phys., 104, 093903 (2008). [2] P. Zou et al., IEEE Trans. Magn. 38, 3501 (2002). [3] S. D. Li et al., Appl. Phys. Lett. 92, 092501 (2008).
AQ-08. High stability of magnetic parameters in Fe 25 at.% Al nanocomposite
Snehal Jani, Jagdish Nehra, Sudheesh Damodaran, Lakshmi Nambakkat and Venugopalan Kanippoth
Physics, Mohanlal Sukhadia University, Udaipur, India
Fe nanoparticles fulfill the basic requirement of being economical yet having excellent magnetic properties, in particular for use in wireless communication devices in the future. However, a major concern is the stability against oxidation, temperature swings and exposure to magnetic fields. We report the formation of a highly stable Fe-Al nanocomposite which meets these criteria through formation of diffusion hindering Fe-Al phase at grain boundaries. HRTEM micrograph of our sample of Fe 25at.%Al subjected to high energy ball for 15h milling shows formation of distinct Fe-Al portions with Al at grain boundaries thus evidencing formation of Fe-Al nano-composite type of structure rather than alloying. At 8nm size, the saturation magnetization (Ms) is 117 emu/g. Low temperature, in-field Mössbauer measurements show that the magnetic moments of pure Fe and Fe with 2nn Al are coupled ferromagnetically, but are not perfectly collinear. The Curie temperature (TC) 1053 K and matches the value of pure bcc α-Fe. The magnetic moment also remains almost constant up to the transition point indicating that the intergranular exchange coupling is strong. Even though in this system pure Fe are separated out by grain boundaries of Al, the grain size of this sample is smaller than the ferromagnetic exchange length for Fe (~20nm) and thus results in a strongly exchange coupled system which does not allow randomization of spins until high thermal energies are available and so gives the Curie temperature of bulk Fe. M-H curves recorded after M-T studies are the same as before showing extreme stability of this system against high temperatures. delta-M plot obtained from IRM-DCD measurements confirm presence of interparticle interactions and are demagnetizing kind or dipolar in nature.
AQ-09. Development of a composite material with high magnetic permeability and low loss factor for high frequency application
Debangsu Roy and P S Anil Kumar
Department of Physics, Indian Institute of Science, Bangalore, India
Ferrites are considered to be one of the potential candidates among the high frequency electromagnetic devices. [1] The initial permeability and the loss factor of the ferrites are important fac tors that determine the application of these ferrites in different areas. But the application of these ferrites at higher frequency has been restricted by the eddy current loss. This eddy current loss can be reduced by enhancing the resistivity of the sample. It has been realized that the combination of high permeability and high resistivity leads to the enhanced performance, in the high frequency domain. In this work, a flexible composite suitable for MHz frequency application has been developed by combining Fe3O4 and Polyvinyl Alcohol. The loss factor and the permeability have been evaluated in the frequency range 1 MHz-90 MHz. At an optimum weight percentage of Fe3O4 in the PVA matrix, the frequency at which the loss factor gives a minimum shifts to the 34 MHz. The loss factor has been found to be lower by one order of magnitude at 70 MHz compared to the presently used Nickel Zinc Ferrite. The Henkel plot has been obtained for the understanding of the nature of the inter-particle interaction. The findings from the Henkel plot have further been correlated to the permeability dispersion of the composite. The relaxation mechanism for the composite has been understood from the Cole-Cole measurement. All these studies reveal our successful attempt in designing a flexible composite material by tuning the corresponding weight ratio of Fe3O4 and PVA which shows superior performance characteristics where the frequency of operation is few tens of MHz with very low loss factor.
References
[1] T Tsutaoka , J. Appl. Phys., 93 2789 (2003).
AQ-10. Magnetic behaviour of Ni0.4Zn0.6Co0.1Fe1.9O4 spinel nano-ferrite
Atul Thakur1, Preeti Thakur2 and Jen-Hwa Hsu1
1Physics, National Taiwan University, Taipei, Taiwan; 2Physics, Himachal Pradesh University, Shima, India
In the present study, nanoparticles of the spinel ferrite Ni0.4Zn0.6Co0.1Fe1.9O4, synthesized by co-precipitation method have been reported [1]. The X-ray diffraction pattern of the particles confirms single-phase cubic spinel structure with lattice parameter of 0.8465 nm. The Langevin function fitting on M-H data at 300K gives a log-normal particle size distribution with median diameter of 59.6 nm and standard deviation of 0.6. The isothermal dc magnetization studies have been performed using SQUID and vibrating sample magnetometer in the temperature range of 5-300K. These measurements show that the sample is superparamagnetic above the blocking temperature TB ~ 253K. The reduction in saturation magnetization in the case of nanoparticles has been explained on the basis that the magnetic moments in the surface layers outside the core are in the state of frozen disorder. A doublet observed in Mössbauer study (Fig.1) indicates the superparamagnetic behavior and nanocrystalline formation. Possible mechanisms contributing to these processes have been discussed.
References
[1] Atul Thakur, Preeti Thakur, Jen-Hwa Hsu, “Novel magnetodielectric nano-materials with matching permeability and permittivity for the very high frequency applications,” Scipita Materillia, 64, 205 (2011).
AQ-11. Effect of Cu and Nb additives on the μi-T curves in FeSiB alloys
Yun-yun Jia, Zhi Wang, Rui-min Shi and Jia Wang
tianjin university, Tianjin, China
Finemet nanocrystalline alloy has been extensively investigated due to excellent soft magnetic properties, since Cu and Nb were simultaneously injected into FeSiB alloy [1-3].The effect of Cu and Nb on the crystallization or the effective magnetic anisotropy in FeSiB alloys was also studied [4-6].Recently,the novel FeCuSiB alloys have benn reported to obtain high saturation magnetic induction and low coercivity[7-8]. However,the comprehensive studies concerning initial permeability μi of FeCuSiB, FeNbSiB and FeCuNbSiB alloys at high temperatures are still fewer.
In this paper, we focus on the evolution of μi for the as-quenched and annealed samples with nominal compositions Fe76.5Cu1Si13.5B9,Fe74.5Nb3Si13.5B9 and Fe73.5Cu1Nb3Si9B13.5 alloys as a function of temperature T(μi-T curves). The Curie temperature of amorphous phase, Tcam, for Fe76.5Cu1Si13.5B9, Fe74.5Nb3Si13.5B9 and Fe73.5Cu1Nb3Si9B13.5 alloys is about 445, 325 and 315°C. Above Tcam, μi of Fe76.5Cu1Si13.5B9 and Fe74.5Nb3Si13.5B9 alloys increases no longer, whereas Fe73.5Cu1Nb3Si9B13.5 alloy exhibits a sharper and stronger crystalline peak at 600°C due to the precipitation of large amounts of soft magnetic grains.
The optimal annealing temperature for Fe76.5 Cu1 Si13.5 B9 alloy is 490°C, above which the room-temperature μi decreases significantly. μi-T curves of all the samples annealed at 550°C for 0.5h are measured. For Fe73.5Cu1Nb3Si9B13.5 sample, μi exhibits relatively broad thermal stability,which is attributed to the two-phase structure composed of nanocrystalline phase embedded in the residual amorphous phase. A sharp Hopkinson peak is still evident in Fe74.5Nb3Si13.5B9 sample, identified to be amorphous phase, indicating that Nb retards the formation of soft magnetic crystalline phase. However, μi of Fe76.5Cu1Si13.5B9 sample is lower at room temperature but increases rapidly to 4000 at 255°C and then decreases. To clarify these behaviors further, the microstructure is analyzed from XRD patterns and the reason is also systematically interpreted.
References
[1] Y. Yoshizawa, S. Oguma, and K. Yamauchi, J. Appl. Phys. 64, 6044(1988). [2] G.Herzer, IEEE Trans. Magn.25,3327(1989). [3] G.Herzer, IEEE Trans. Magn.26,1397(1990). [4] P. García Tello, J. M. Blanco, J. González and J. M. González, J. Appl. Phys. 81(8), 4646 (1997). [5] S.Grognet, H. Atmani, R. Zuberek, K. Zellama and J. Teillet, J. Alloys Compd. 282,236 (1999). [6] M. Ohnuma, K. Hono, S. Linderoth, J. S. Pedersen, Y. Yoshizawa, and H. Onodera, Acta Mater. 48,4783(2000). [7] M. Ohta and Y.Yoshizawa, Appl. Phys. Lett. 91,062517(2007). [8] M. Ohta and Y. Yoshizawa, J. Appl. Phys. 103,07E722(2008).
AQ-12. Effect of P on soft magnetic properties of nanocrystalline Fe-Si-B-P-Cu alloys with high Bs
Akiri Urata1, Makoto Yamaki1, Masahiko Takahashi1, Koichi Okamoto1, Hiroyuki Matsumoto1, Shigeyoshi Yoshida1 and Akihiro Makin o2
1NEC TOKIN Corporation, Sendai, Japan; 2Institute for Materials Research, Tohoku University, Sendai, Japan
New soft magnetic materials with low core loss are strongly required in order to solve global environmental, energy and natural resource problems. Recently, nanocrystalline Fe-Si-B-P-Cu [1-3] alloys exhibit high saturation magnetic flux density (Bs) of 1.8 T and low core loss (W) due to high Fe content and without the use of expensive metal elements such as Nb or Zr. Additionally, these alloys show a homogeneous nanostructure composed of α-Fe grains because of an unusual effect of the simultaneous addition of P and Cu. In this study, the effect of P content on the magnetic properties of nanocrystalline Fe83.3Si8-XB8PXCu0.7(X=2, 4, 6, 8) alloys have been investigated aiming for enhancement of magnetic softness. Fe-Si-B-P-Cu ingots were prepared by induction melting using the mixture of industrial materials of Fe, Cu, Si, Fe-B and Fe-P in an Ar atmosphere. A single-roller melt-spinning method in air was used to produce the rapidly solidified ribbons about 25 μm in thickness. The initial permeability (μi) at 1 kHz of the nanocrystalline Fe83.3Si8-XB8PXCu0.7 (X=2, 4, 6, 8) alloy ribbons annealed at 723 K are enhanced with increasing P content (Fig.1) and the μi and Bs of the Fe83.3B8P8Cu0.7 alloy ribbon are 14800 and 1.7 T, respectively. The W at 1 kHz of the nanocrystalline Fe83.3Si8-XB8PXCu0.7 (X=2, 4, 6, 8) alloy ribbons decrease with increasing P content, and especially the Fe83.3B8P8Cu0.7 alloy is much lower W than Fe-amorphous alloy (Fig.2). Therefore, it can be concluded that the soft magnetic properties of Fe-Si-B-P-Cu alloys are strongly affected by P content and the Fe83.3B8P8Cu0.7 alloy with high Bs of 1.7 T and low W of 1.5W/kg compared with the Fe-amorphous alloy is suitable for a core material in electronic devices such as transformers and motors.
References
[1]A. Makino, H. Men, T. Kubota, K. Yubuta and A. Inoue, Mater. Trans. 50, 204 (2009). [2]A. Makino, H. Men, T. Kubota, K. Yubuta and A. Inoue, J. Appl. Phys. 105, 07A308 (2009). [3]A. Makino, H. Men, T. Kubota, K. Yubuta and A. Inoue, IEEE Trans. Mag. 45, 4302 (2009).
AQ-13. Electrodeposition and Magnetic Properties of FeCo Alloy Films
Dong Zhou1, 2, Mingge Zhou1, Minggang Zhu1, Zhaohui Guo1, Xu Yang2 and Fashen Li2
1Division of Functional Materials, Central Iron & Steel Research Institute, Beijing, China; 2Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou, China
Electrodeposition is one of the simplest, most flexible, and cheapest processes available for the fabrication of single component and multilayered thin films.[1] Fig.1 shows the SEM images of FeCo films fabricated by constant voltage mode(CVM) at -1.3 V for 15s with Ag coated glass as working electrode. The grain size of the sample fabricated by pulsed electrodeposition mode (PEM) is smaller than that prepared by CVM. The coercivity of the FeCo films reduced with the increase of anodic current density and cycle index number. Fig.2 shows the comp lex permeability for FeCo films fabricated by PEM, from which we can find that at high frequency, the FeCo films fabricated by PEM have excellent wave absorption property which can be used for absorbing mobile phone radiation. The experimental data are well in agreement with theoretical data and the permeability spectra can be fitted based on natural resonance.
References
[1] T. Osaka, M. Takai, K. Hayashi, K. Ohashi, M. Saito, and K. Yamada. Nature, 392, 796 (1998).
AQ-14. The Effect of Distributed Exchange Parameters on Magnetocaloric Refrigeration Capacity in Amorphous and Nanocomposite Materials
Nicholas J. Jones, Huseyin Ucar, Jhon J. Ipus, Michael E. McHenry and David E. Laughlin
Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
The temperature dependence of the magnetization of nanocomposite alloys has been fit using a modified Handrich-Kobe equation with an asymmetric exchange fluctuation parameter [1] combined with the Arrot-Noakes equation. The two equations of state are combined to calculate the entropy change in the magnetocaloric effect associated with the ferromagnetic to paramagnetic phase transformation. The complete fit for M(T) of (Fe70Ni30)88Zr7B4Cu nanocomposite powder is only accomplished by uniting the two theories. We investigate the broadening of the second-order transition arising from asymmetric exchange parameters; the broadening results from the fluctuations of interatomic spacing found in an amorphous matrix and the asymmetric dependence of exchange energy on interatomic spacing, as shown by the Bethe-Slater curve. The magnetic entropy curve revealed extra broadening with a Refrigeration Capacity (RC) value of 148 J/kg at 5 Tesla which is comparable to (Fe76Cr8-xMoxCu1B15) ribbons [2] which have an RC value of 180 J/kg at the same applied field. According to Wood and Potter’s [3] definition of RC, broadening of the magnetic entropy can lead to larger RC values and the possibility of a wider working temperature range, making nanocomposite alloys promising candidates for magnetocaloric applications.
References
[1] K.A. Gallagher et al., Journal of Applied Physics 85, No. 8, 5130 (1999). [2] V. Franco et al., Applied Physics Letters 90, 052509 (2007). [3] M.E. Wood and W.H. Potter, Cryogenics 25, 667 (1985).
AQ-15. Room-temperature deposition of nanocrystalline ferrite thin films for photomagnetic functionality
Urusa S. Alaan1, Franklin J. Wong1, Alexander J. Grutter1, 2, Jodi M. Iwata1, Virat V. Mehta1, 2, John L. Watts1 and Yuri Suzuki1, 2
1Department of Materials Science and Engineering, University of California, Berkeley, CA; 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
Nanocrystalline spinel ferrite thin films have shown promise in tuning various functional properties of materials, such as photomagnetism. We report on the sensitivity of such functional properties of nanocrystalline spinel soft ferrite thin films due to structural differences that in turn depend on deposition conditions. We have grown manganese zinc ferrite films by pulsed laser deposition at room temperature in vacuum on both silicon and lattice-matched MgAl2O4 substrates and compared them to epitaxia lly grown (Mn,Zn,Fe)3O4 on MgAl2O4 substrates at 400°C in a 99%N2/1%O2 environment. Atomic force microscopy indicates that we are able to grow remarkably smooth films with rms roughness under 1 nanometer, even for samples with thicknesses of over 300nm that were deposited on non-lattice matched silicon substrates at room temperature. The films grown on silicon were textured with very broad, low intensity (ll0) family of peaks and grain sizes on the order of 10nm. The samples on MgAl2O4 that were deposited at room temperature and elevated temperatures exhibit good epitaxy as demonstrated by their rocking curves, though growth at lower temperatures does introduce some mosaic spread. SQUID magnetometry was used to investigate changes in magnetic response as a function of microstructure. We have shown that there exists a hysteretic temperature dependence of the nanocrystalline films. Using a fiberoptic sample holder, SQUID magnetometry measurements were also conducted under incident light. Promising results for photomagnetic behavior are seen in the nanocrystalline films, while there is no such effect observable in epitaxial films. We postulate that these differences arise from a weakly coupled magnetic domain structure that can be controlled through precise modifications of the grain size.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Michael Loewenhaupt, TU Dresden
AR-01. Temperature dependent magnetic structure of lithium delithiated LixFeSO4F (x=0, 1) by Mössbauer spectroscopy
In Kyu Lee, Sung Wook Hyun, Taejoon Kouh, In-Bo Shim and Chul Sung Kim
Department of Physics, Kookmin University, Seoul, Republic of Korea
Recently, LiFeSO4F compound was found to be an attractive positive electrode material in lithium ion batteries [1]. In this paper, we have studied the crystal structure and temperature dependent magnetic structure of LiFeSO4F and it’s fully delithiated FeSO4F compounds with x-ray diffractometer (XRD) and Mössbauer spectroscopy. Pristine LiFeSO4F polycrystalline powders were synthesized by the ionothermal process with reaction of FeSO4.H2O and LiF in ionic liquid (EMI-TFSI). Fully delithiated FeSO4F samples obtained with chemical redox process using the NOBF4 as an oxidation agent in acetonitrile solution. From the refined XRD pattern, the LiFeSO4F crystallized as a triclinic structure (space group: P-1) with the lattice parameter a0=5.180, b0=5.495, c0=7.219 Å, α=106.594, β=107.171 and γ=97.745°. In case of FeSO4F, the change in lattice parameter is caused by delithiation as a0=5.084, b0=5.098, c0=7.349 Å, α=110.971, β=111.199 and γ=88.144°. The room temperature Mössbauer spectrum of the LiFeSO4F composed of two doublets which are indicating the existence of two Fe2+ sites with the values of δA=1.19 and δB=1.15 mm/s. The fitted spectrum for fully delithiated FeSO4F shows the one Fe3+ (δ=0.36 mm/s) doublet due to the valence transition by lithium ion delithiation. Mössbauer spectra of LiFeSO4F/FeSO4F at 4.2 K showed a two set of eight/six line lorentzian with the measured value of Hhf,A= 286 kOe, ΔEQ,A= 3.30 mm/s, δA= 1.52 mm/s; H hf,B= 292 kOe, ΔEQ,B= 2.65 mm/s, δB= 1.54 mm/s for LiFeSO4F and Hhf,A= 532 kOe, ΔEQ,A= 0.12 mm/s, δA= 0.43 mm/s; Hhf,B= 543 kOe, ΔEQ,B= 0.14 mm/s, δB= 0.45 mm/s for FeSO4F, respectively. The magnetic Nèel temperatures of the LiFeSO4F and FeSO4F were determined to be 20K and 99K from the temperature dependent Mössbauer spectra. From these results, we can be explain the magnetic phase change in LixFeSO4F by crystalline and Fe2+/3+ valence transition which is coming from lithium delithiation.
References
[1] N. Recham, J-N. Chotard, L. Dupont, C. Delacourt, W. Walker, M. Armand and J-M. Tarascon, Nature Mater. 9, 68 (2010)
AR-02. The strong one-dimensional antiferromagnetism in a charge-transfer insulator: AgSO4
xiao-Li Zhang1, Ting Jia1, Ting Liu1, Zhi Zeng1 and Hai-Qing Lin2
1Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Acadamy of Sciences, Hefei, China; 2Department of Physics and Institute of Theoretical Physics, The Chinese University of Hong Kong, Hong Kong, China
The strong one dimensional antiferromagnetism and the electronic structure of AgSO4 are investigated by performing the first-principles density functional calculations. The results show that the strong one dimensional antiferromagnetic coupling in AgSO4 is along the diagonal of the unit cell, and the obtained intra-chain exchange constant is in good agreement with experimentally observed results. In the planar rectangular cystal field, the Ag2+ 4d9 x2-y2 orbital is higher in energy (see Figure 1), therefore, the super-exchange interaction between the x2-y2 orbitals gives rise to an unexpected strong one-dimensional antiferromagnetic behavior. We also found that AgSO4 is a charge-transfer insulator.
AR-03. First-principles investigation of magnetic and elastic properties of Fe-Si
Won Seok Yun1, Jee Yong Lee1, In Gee Kim1, Soon Cheol Hong2 and Jae Il Lee3
1Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea; 2Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea; 3Department of Physics, Inha University, Incheon 402-751, Republic of Korea
Silicon steels, especially with 3 wt% Si, are widely used as an energy converting material, e.g., transformers, motors, etc. Silicon steel with 6.5 wt% Si may be a promising future electrical steel, due to its zero magnetostriction, reduced magnetocrystalline anisotropy, higher permeability, and lower coercivity [1, 2]. However, the compounds with Si content higher than 4 wt% become significantly brittle and consequently its rolling process is very difficult to be achieved. Therefore, there is a need for a systematic first-principles study on its magneto-mechanical properties with different concentration of Si. In this study, we investigated fundamental magnetic and elastic properties of ferrites with the various Si concentrations (Fe100-xSix, x = 3.7, 6.25, and 12.5 at%) and its D03 structure using the highly precise all-electron full-potential linealized augmented plane wave (FLAPW) [3] method within the generalized gradient approximat ion (GGA) [4]. The lattice constant shrinks as the concentration of Si increases. In all the considered systems, the ferromagnetic state is more stable than the nonmagnetic one by big energy difference of about 473, 453, 394, and 351 meV/Fe-atom for x = 3.7, 6.25, and 12.5 at%, and the D03 structure, respectively. The magnetic moments of the Fe atoms nearest neighboring to the Si atoms decreases monotonically from 2.092 µB for x = 3.7 at% to 1.312 µB for the D03 structure. This feature is considered as the result of the stronger p-d hybridization for higher Si concentration. The mechanical properties of Fe100-xSix were also investigated in terms of elastic moduli, i.e., C11, C12, and C44, which are crucial for determining ductile-brittle criterion [5] as well as magnetostriction [6]. As x increases, the bulk modulus (B) decreases from 196 GPa to 149 GPa. However, it is high (213 GPa) for D03 structure, which is considered as the result of the ordering. Detailed discussion on the correlation between magnetic and elastic properties of Fe100-xSix compounds will be given.
References
[1] T. Ros, Y. Houbaert, O. Fischer, and J. Schneider, IEEE Trans. Magn. 37, 2321 (2001). [2] Y. Tanaka, Y. Takada, M. Abe, and S. Masuda, J. Magn. Magn. Mater. 83, 375 (1990). [3] E. Wimmer, H. Krakauer, M. Weinert, and A. J. Freeman, Phys. Rev. B 24, 864 (1981), and references therein; M. Weinert, E. Wimmer, and A. J. Freeman, ibid. 26, 4571 (1982). [4] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3856 (1996); ibid. 78, 1396(E) (1997). [5] D. G. Pettifor, Mater. Sci. Technol. 8, 345 (1992). [6] D. Odkhuu, W. S. Yun, and S. C. Hong, unpublished.
AR-04. Large positive magnetroresistance ( ~ 100%) at very low temperature (< 10 K) observed in Bi2Te3/C
Yun Zhang
Institute of superconducting and electronic materials, Wollongong, NSW, Australia
Sing-wall carbon nano-tube (SWCNT) doped n-type Bi2Te3 bulk samples have been prepared via solid state reaction method with the doping level varied from 0.5 wt% to 5 wt%. The phase analysis, crystal structures, and morphologies of doped and un-doped samples were characterized via XRD, FE-SEM, TEM and Raman spectroscopy measurements. The temperature-dependent thermoelectric transport measurements showed that the transport properties of doped samples were remarkably varied. SWCNT doped samples showed a highly reduced thermal conductivity compared to the un-doped sample. Seebeck coefficients measurements indicated that the system could be turned from N-type to P-type, and P-type to N-type depending on doping levels and temperatures. A mechanism is proposed to explain this thermal conductivity reduction as well as N-type to P-type variety of Bi2Te3 by grain boundary scattering. Magnetoresistance (MR) has also been studied over a wide range temperatures and magnetic fields. Large positive MR was observed at low temperatures and high magnetic fields.
AR-05. Antiferromagnetism in the 2D Limit and Interface Superconductivity in Metal-Insulator La(2-x)Sr(x)CuO(4) Superlattices
Andreas Suter1, Elvezio Morenzoni1, Thomas Prokscha1, Bastian M. Wojek2, 1, Hubertus Luetkens1, Adrian Gozar3, Gennady Logvenov4, 3 and Ivan Bozovic3
1Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, Villigen PSI, Switzerland; 2Physics Institute, University of Zurich, Zurich, Switzerland; 3Brookhaven National Laboratory, Upton, NY; 4Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany
By reducing the dimensionality of a solid, its electronic states and physical properties can be drastically modified, but these changes are not easy to predict for strongly correlated electron materials. For example, in thin interfacial layers inside oxide heterostructures a host of electronic states were discovered experimentally - a high-mobility 2D electron gas [1], magnetism [2], quantum Hall effect [3], and interface superconductivity between insulators [4] or insulators and metals [5]. The real ground state of a spin-1/2, 2D Heisenberg antiferromagnet on a square lattice (HAF) is still a matter of debate. A model system which is believed to be close to the HAF is La(2)CuO(4) (LCO) which orders in bulk at T_N ≈ 315 K, caused by the residual out-of-plane interactions in the system. In order to approach the real 2D limit, we investigated superlattices (SLs), synthesized digitally by molecular beam epitaxy, with alternating thicknesses of LCO and non-superconducting metallic La(1.55)Sr(0.45)CuO(4) (LSCO). Counting in 1/2-unit-cell (UC) increments, each of which contains a single CuO(2) plane, the investigated SLs have the repeat structure [3LSCO+6LCO], [3LSCO+9LCO], and [3LSCO+12LCO], respectively. We show, by means of low-energy muon spin rotation measurements, that few-unit-cells thick LCO layers are antiferromagnetically ordered. Below a thickness of about 5 CuO(2) layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. At the same time superconductivity is found in all these SLs with T_c ≈ 25 K. Superconductivity in these SLs is sheet like, with the superconducting sheets found at the interface between LCO and LSCO. The magnetic state can exist in close proximity (few Å) to superconducting layers, without transmitting supercurrents.
References
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [2] A. Brinkman, et al., Nature Materials 6, 493 (2007). [3] A. Tsukazaki, et al., Science 315, 1388 (2007). [4] N. Reyren, et al., Science 317, 1196 (2007). [5] A. Gozar, et al., Nature 455, 782 (2008).
AR-06. The correlation and hybridization effects between 4f and other electrons on the ARPES and hybridization gap in CeCoGe2: DFT+DMFT study
Hongchul Choi1, Ji Hoon Shim2 and Byung Il Min1
1Physics, POSTECH, Pohang, Republic of Korea; 2Chemistry, POSTECH, Pohang, Republic of Korea
We have investigated the formation of Kondo resonance states in the heavy fermion compound CeCoGe2 using the combined approach of the density functional theory (DFT) and the dynamical mean field theory (DMFT). This compound is experimentally verified as a j=5/2 heavy fermion Coqblin-Schrieffer (CS) compound.[1] The well-defined dispersive KR states were observed in Ce 4d-4f resonant ARPES experiment of CeCoGe1.2Si0.8.[2] The calculated temperature (T)-dependent spectral function shows the slow crossover in the formation of dispersive Kondo resonance peaks in momentum space, which is well consistent with the experiments at low T. The estimated hybridization gap size as a function of T is also examined. We have found a drastic change of the dispersion (kink) in the spectral function near the Fermi level, corresponding to the evolution of the dispersive bands at high T to the flat KR bands at low T. This kink is induced by the hybridization between 4f and other electrons, which shows the different origin from the known kink physics due to the quasiparticle-excitation. A scenario of how the kink phenomena evolve into the hybridization gap is suggested.
References
[1] E. D . Mun, B. K. Lee, Y. S. Kwon, and M. H. Jung, Phys. Rev. B 69, 085113 (2004) [2] H. J. Im, T. Ito, H-D. Kim, S. Kimura, K. E. Lee, J. B. Hong, Y. S. Kwon, A. Yasui, and H. Yamagami, Phys. Rev. Lett. bf 100, 176402 (2008)
AR-07. Crossing point phenomena (T* = 2.7 K) in Specific heat curves of Superconducting Ferromagnets RuSr2Gd1.4Ce0.6Cu2O10-δ
Anuj Kumar1, 2, Ram Pal Tandon2 and Veer Pal Singh Awana1
1Quantum Phenomena and Application, National Physical Laboratory, New Delhi, India; 2Physics and Astrophysics, University of Delhi, New Delhi, India
Crossing point phenomena is one of the interesting and still puzzling effects in strongly correlated electron systems [1]. We have synthesized RuSr2Gd1.4Ce0.6Cu2O10-δ (GdRu1222) and measured its magnetization, AC susceptibility and specific heat. The compound crystallized in tetragonal structure with space group I4/mmm. GdRu1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperature around 110 K and superconductivity sets in Cu-O2 planes below around 40 K. Specific heat (Cp) was investigated in temperature range 1.9-250 K with a magnetic field up to 11 Tesla. Unfortunately there were no superconducting and magnetic transitions observed in specific heat curve while a schottky type anomaly was observed below 20 K. It was also observed that curves Cp (T) taken for different magnetic field, have the same crossing point (at T* ≈ 2.7 K), up to 7 Tesla. This low temperature anomaly can be attributed to splitting of the ground state term 8S7/2 of paramagnetic Gd3+ ions by internal and external magnetic fields. A quantitative explanation of this phenomenon, based on the form and temperature dependence of the associated generalized heat capacity (Cp), is presented. So this effect supports the crossing point phenomena, which is inherent for strongly correlated systems.
References
[1] Vollhardt D Phys. Rev. Lett. 78, 1307 (1997)
AR-08. Withdrawn
AR-09. Interplay between Magnetism and Charge Transport in Antiperovskite Manganese Nitrides: Extremely Low Temperature Coefficient of Resistance due to Strong Magnetic Scattering
Motoyoshi Hadano1, A. Ozawa1, K. Takenaka1, N. Kaneko2, T. Oe2 and C. Urano2
1Department of Crystalline Materials Science, Nagoya University, Nagoya, Japan; 2National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
Antiperovskite manganese nitrides Mn3AN (A: metal or semiconducting element) have various physical properties related to their magnetism such as negative thermal expansion, magnetostriction and magnetocaloric effects. We focused the low temperature coefficient of resistance (low TCR) in the paramagnetic phase, which was suggested for some antiperovskite manganese nitrides [1]. Here we report a systematic study on the electrical resistivity of Mn3AgN over a wide temperature T range up to 800 K using high density samples prepared by a hot-pressing method. Mn3AgN undergoes a magnetic transition from the high-T paramagnetic to the low-T weak ferromagnetic phase at 280 K. We discovered that Mn3AgN exhibits a broad maximum in the temperature-resistivity curve at 340 K and the TCR is even negative at higher temperatures despite its overall metallic character [2]. The resistivity-peak temperature is controllable by partial substitution for Ag. Mn3(Ag0.68Cu0.32)N shows a resistivity peak at 299 K and around the peak temperature TCR is estimated to be less than 6 ppm/K at T=280-322 K, especially less than 1 ppm/K at T=294-304 K. These characteristics brighten the prospects of this material for the practical uses as stable precision resistor elements and standard resistors. The resistivity-peak temperature seems to be related to the magnetic transition temperature, suggesting this peculiar phenomenon is magnetic in origin. We also discuss the interplay between transport properties and the magnetism in terms of magnetoresistance as well as Hall effects. This work was partly supported by MEXT, Japan and by NEDO, Japan.
References
[1] E. O. Chi, W. S. Kim, and N. H. Hur, Solid State Commun. 120, 307 (2001). [2] K. Takenaka et al., Appl. Phys. Lett. 98, 022103 (2011)
AR-10. Magnetic and rectifying properties in La0.8Ca0.2MnO3/SrTiO3/GaAs heterostructures
Zhenping Wu, Lin Wang and Ju Gao
Physics, The University of Hong Kong, Hong Kong, Hong Kong
The integration of colossal magnetoresistance (MR) manganites with GaAs will open the way to the development of advanced devices with novel functionalities. Compared to Silicon, GaAs has been widely used in the microwave and optoelectronic industry for its higher electron mobility, direct band gap and excellent optical characteristics. However, it is a great challenge to deposit epitaxial manganites films on GaAs single crystal substrates due to the lack of high quality, thermodynamically stable insulators that passivate the interface. In this paper, La0.8Ca0.2MnO3 (LCMO) films were grown on GaAs substrates by introducing an ultrathin SrTiO3 film (~3 nm) as a buffer layer. The crystalline quality and growth mode were investigated using X-ray diffraction and reflective high energy electron diffraction (RHEED), respectively. The results indicate that LCMO films are c-axis oriented growth with a highly epitaxial feature, as typically shown in Fig. 1(a). The structural properties of thin films were found to depend on the fabrication conditions, including deposition rate, growth temperatures and laser energy. Application of magnetic fields leads to a remarkable reduction in resistivity of the grown films, demonstrating a strong MR effect. Good rectifying characteristics were observed in LCMO/STO/GaAs heterostructures in a wide temperature range from 50 to 300 K (Fig. 1(b)). It was also found that other transport properties of these heterostructures could be considerably influenced by external magnetic fields.
AR-11. Tuning Magnetic Phase Transition by A-site Quenched Disorder in Half-Doped Manganite Pr0.5Ba0.5MnO3
Dah-Chin Ling, Ping-Cheng Hsu and Chia-Ling Lee
Department of Physics, Tamkang University, Tamsui, Taiwan
The interplay between A-site quenched disorder and magnetic phase transition in half-doped manganite Pr0.5Ba0.5MnO3 (PBMO) was extensively investigated. The H/M versus M2 isotherms show that the A-site ordered PBMO undergoes a second-order magnetic phase transition from paramagnetism to ferromagnetism, whereas the A-site disordered PBMO exhibits a fluctuation-driven first-order transition arising from a competing order phase possibly existing in the paramagnetic state. The successive step-like transition in magnetization and resistivity was observed in the A-site partially orde red PBMO at 2 K, indicating that the metamagnetic transition is likely associated with a competition between randomly distributed short-range ferromagnetic and antiferromagnetic phases. These findings provide evidence that the A-site quenched disorder not only suppresses A-type antiferromagnetism also moderately weakens long-range ferromagnetism in the A-site ordered PBMO.
AR-12. Structural and magnetic study of SmTAl single crystals (T=Pd and Ni)
Jiri Prchal, Martin Rusnak and Jiri Pospisil
Department of Condensed Matter Physics, Charles University in Prague, Prague 2, Czech Republic
SmNiAl and SmPdAl belong to a large group of compounds crystallizing in the hexagonal ZrNiAl-type of structure. Within recent years, physics of the discontinuity in the temperature or composition dependence of the lattice parameters (a and c) observed in several compounds of this family of intermetallics has been of particular interest. These materials have in common a specific “forbidden” value of the c/a ratio. This conclusion has been corroborated by ab initio calculations [1]. Although the dramatic structure change is hardly observable in the temperature dependence of the specific heat, it is accompanied with a clear change of the effective magnetic moment, change of the crystal field energy spectra [2] and namely with increasing amount of mechanical defects in the sample. The most representative example of such behavior, TbNiAl, exhibits a first-order structural transition at low temperatures around 100K [3]. We have studied the lattice-constants discontinuity in the SmNiAl and SmPdAl compounds - both in the polycrystal and single crystal form - at high temperatures around 500K, where a second order transition in the c/a vs. T dependence could be observed. The second order type of transition is caused by enhanced thermal movement at high temperatures. This is in agreement with previously revealed findings that the character of the transition changes from the first-order type to a higher-order one with moving to higher temperatures. We will present results of X-ray diffraction, specific-heat, resistivity and magnetization measurements performed for the SmPdAl and SmNiAl single crystals and polycrystals in connection with the structure discontinuity. Also low-temperature magnetization and specific heat data collected on the first-time prepared SmPdAl and SmNiAl single crystals will be presented and discussed in the context of the specific magnetism of the Sm3+ ion.
References
[1] J. Prchal et al., Phys. Rev. B 77 (2008) 134106. [2] P. Javorský et al., J Magn. Magn. Mat. 317 (2007) e400. [3] J. Prchal et al., Physica B 378-380 (2006) 1102.
AR-13. Magnetism in CeIr(SixGe1-x)3 compounds
Jan Prokleska, Jiri Pospisil, Marie Kratochvilova and Vladimir Sechovsky
Dept. of Condensed Matter Physics, Charles University,, Prague, Czech Republic
The CeMX3 intermetallics adopting the noncentrosymmetric tetragonal BaNiSn3-type crystal structure recently attracted much attention mainly owing to unconventional superconductivity observed in some of them [1]. CeIrSi3 and CeIrGe3 were reported to exhibit antiferromagnetic ordering with TN = 5.0 K and 8.5 K, respectively. CeIrGe3 also undergoes an order-order transition at T1 = 4.7 K. When applying hydrostatic pressure TN gradually decreases and a superconducting dome appears in the vicinity of QCP for the pressure pc = 2.2 and 23 GPa, respectively [1, 2]. Our work has been motivated by the desire to know how the magnetism and superconductivity evolve with the composition of the SiGe sublattice. For this purpose we have prepared polycrystalline samples of selected compositions of the SiGe sublattice by arc melting, annealed and characterized by EDX analysis and X-ray diffraction and measured magnetization and specific heat in a wide temperature range and in various magnetic fields. The CeIr(Ge1-xSix)3 solid solutions keep the crystal symmetry of the parent compounds and the lattice parameters follow Vegard’s law. The TN first decreases with increasing x, but the TN vs. x dependence is not entirely monotonous; a minimum TN is observed for x between 0.75 and 1. The magnetic phase transition at T1 in CeIrGe3 was confirmed to be of the first order type and monotonously increases with increasing x up to 0.75. No indication of this transition can be found for higher x. To study magnetism in more detail, especially the anisotropy, single crystals have been grown for x = 0.0, 0.1, 0.2. Measurements of the magnetization, electrical resistivity and specific heat as functions of temperature, magnetic field and hydrostatic pressure are in progress and the results will be presented and discussed in context of physics of the CeMX3 family.
References
[1] Ch. Pfleiderer, Rev. Mod. Phys. 81, 1551 (2009) and ref. therein [2] F. Honda et al., PRB 81, 140507 (2010)
AR-14. Relationships between crystal structure and magnetic properties in type-A hetero-epitaxial MnAs thin films
JongHyun Song1, Yongjie Cui2 and John B. Ketterson2
1Physics, Chungnam Natl Univ, Daejeon, Republic of Korea; 2Physics and Astronomy, Northwestern University, Evanston, IL
Heterostructure of ferromagnet and semiconductor, such as MnAs/GaAs and MnAs/Si, are promising candidates for future spin-based electronics applications [1,2]. In the present work, we report on the epitaxial growth of type-A MnAs thin films on GaAs(001) and Si(001) substrates using molecular-beam epitaxy and the relationships between their crystal structure and magnetic properties. The crystal structure was analyzed in detail by High-Resolution X-ray Diffraction (HRXRD) using a four circle diffractometer. The epitaxial relationships of all samples studied are the so-called type-A [3]. It should be noted that the type-A MnAs thin film grown on Si(001) substrate has not been reported in previous whereas the reports on the type-B MnAs/Si(001) and type-A MnAs/GaAs(001) thin films are abundant. The MnAs/GaAs(001) thin film has a two-fold symmetric domain while four-fold symmetric double domains are observed for the MnAs/Si(001). For the origin of the observed domain structures, we suggest the respective III-V and IV atomic configurations at the surface of GaAs(001) and Si(001) substrates. For the MnAs/GaAs(001) thin film, structural characterization indicates that the ferromagnetic α-MnAs phase has a NiAs-type hexagonal crystal structure. However, the MnAs/Si(001) thin film exhibits a strongly distorted hexagonal crystal structure; the hexagon is stretched and compressed along the in-plane and out-of-plane directions, respectively. The electrical-transport measurement shows around three-times higher resistivity than that of MnAs/GaAs(001) in the entire temperature range studied. In particular, MnAs/Si(001) thin film exhibits a weak ferromagnetism with a curie temperature exceeding 400 K. These observations of electrical-transport and magnetic properties which are strongly dependent on the crystal structure provides important clues on the origin of ferromagnetism in MnAs.
References
[1]. A. K. Das, C. Pampuch, A. Ney, T. Hesjedal, L. Daweritz, R. Koch, and K. H. Ploog, Phys. Rev. Lett. 91, 087203 (2003). [2]. M. Ramsteiner, H. Y. Hao, A. Kawaharazuka, H. J. Zhu, M. Kastner, R. Hey, L. Daweritz, H. T. Grahn, and K. H. Ploog, Phys. Rev. B 66, 081304(R) (2002). [3]. M. Tanaka, J. P. Harbison, and G. M. Rothberg, J. of Cryst. Growth 150, 1132 (1995).
AR-15. Anomalous low temperature magnetic and magneto-transport properties in Ru deficient SrRuO3
Chanchal Sow1, D. Samal1, 2 and P. S. Anil Kumar1
1Department of Physics, Indian Institute of Science, Bangalore, India; 2Presently at Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, Netherlands
SrRuO3 is a well known Itinerant Electron Ferromagnet with many intriguing characteristics1-3, rarely observed in other conventional itinerant magnetic systems. The Ru deficiency in this system is believed to play a pivotal role to influence many of its magnetic and transport properties. The present study involves the magnetic and magneto-transport properties of Ru deficient samples(SrRu0.93O3) to gain more insights into the unusual low temperature behavior. The ac susceptibility study reveals a sharp ferromagnetic transition at 150 K followed by a hump at Th~50K, which has anomalous frequency dependence. Besides, the Th shifts to lower temperature with increase in superposed dc biasing field and adheres to H2 dependence, in accordance with GT line4 for the Heisenberg spin glass systems. We also observe the memory effect, a typical characteristic of glassy behavior, below Th. Besides the remnant magnetization relaxation (RMR) at 20 K shows an increase of the magnetization on removal of the magnetic field initially and then it saturates. On the other hand at 100K the RMR behaves like a conventional FM material. Moreover, the temperature dependent magneto-resistance exhibits enhanced value in the vicinity of Th, in addition to the usual maximum observed, around the ferromagnetic transition temperature. All the interesting findings rolled together, unambiguously, demonstrate the existence of an additional low temperature cryptic magnetic phase. In this presentation we will be compiling all this anomalous behavior observed at low temperature in this compound.
References
[1] M. S. Laad, I. Bradaric, and F. V. Kusmartsev, Phys. Rev. Lett. 100, 096402 (2008). [2] L. Klein et al., Phys. Rev. Lett. 84, 6090(2000). [3] L. Capogna et al., Phys. Rev. Lett. 88, 076602 (2002). [4] R. Palai et al., Phys. Rev. B 79, 104413 (2009).
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Kaizhong Gao, Seagate Technology
AS-01. Spin torque transfer effects in CPP differential dual spin valve
Hao Meng and Guchang Han
Data storage institute, Singapore, Singapore
Differential dual spin valve (DDSV) design provides a method to increase linear density without physically reducing shield to shield spacing (SSS) [1]. However, spin torque transfer (STT) effect [2,3] in CPP spin valve reading sensor is still a concern for high density magnetic recording, especially when the width of reading sensor keeps shrinking [4]. In this work, we studied STT effects in sub 100nm DDSV devices. Inset of Figure 1 shows a resistance-field (R-H) loop of a DDSV device, where the applied field was along the direction of easy axis and the setting direction of the pinning field. The asymmetrical curve indicates the different magnetic response of the two free layers. Figure 1 shows the R-H loop with transverse measuring field, where the pinning field was set perpendicular to the easy axis. The differential effect was captured in positive field region but became worse in negative field region. Spin torque transfer study showed that there was a resistance jump under a wide range of applied bias field, indicating that one of the magnetic layers was switched by spin current. Such STT induced magnetization switching has to be considered for future DDSV design. Detailed discussions will be presented.
References
[1]G. C. Han, et al. J. Appl. Phys. 109, 07B707(2011) [2]J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1-L7 (1996) [3]L. Berger, Phys. Rev. B 54, 9353 (1996) [4]J. R. Childress, et al. Physique 6, 997 (2005).
AS-02. Enhancement of current-perpendicular-to-plane giant magnetoresistance by insertion Fe(001) layers at alternate monatomic [Fe/Co]n superlattices /Ag interface
JinWon Jung1, Yohei Shiokawa1, Zhenhu Jin1, Masaaki Doi2 and Masashi Sahashi1
1Electonic Engineering, Tohoku University, Sendai, Japan; 2Electonic Engineering, Tohoku Gakuin University, Tagajyo, Japan
We have reported that Fe50Co50 alloy crystallizes in an artificially ordered B2 state, which shows larger bulk spin asymmetry coefficient (βF=0.80) related to the spin polarization in conductance of the ferromagnetic (F) material. We fabricated artificially ordered B2 structure of Fe50Co50 on MgO (001) substrate by alternate monatomic layered(AML[Fe/Co]n) deposition, in which monolayers (MLs) of Fe and Co were deposited alternately (1 monolayer 1.4Å). We investigated the effect of insertion of Fe(001) layer between an AML[Fe/Co]n layer and an Ag spacer layer on the MR properties of exchange-biased spin-valves (EBSVs). The reason for the choice of Ag as a spacer material was that the large interfacial spin asymmetry coefficient(γF/N)and interfacial specific resistance (AR*) at Fe/Ag interface was theoretically calculated by Stiles[1]. The layer structure of MgO(001)/ seed layer / bottom electrode / AML[Fe/Co]21MLs~37MLs/insertion layer/ Ag 5nm/ insertion layer / AML[Fe/Co]21MLs~37MLs / IrMn 7nm /Cap was deposited. In CPP-GMR, ΔRA can be analyzed using Valet-Fert’s two-current model[2]. According their model, βF, γF/N were estimated to be 0.80±0.02, 0.80±0.007, respectively. In addition, AR*F/N of the interfacial specific resistance was found to be 1.23mΩ μm2, which is 2 times higher than that of EBSVs without insertion Fe layer. As indicated, enhancement of the interfacial spin scattering and interfacial specific resistance via insertion of thin Fe layer. Also, a considerably large ΔRA, when the thickness of F layer of 5nm, was obtained to be more than 4.2mΩ μm2, which is larger value than that of ever reported values in half-metallic Co-based Heusler alloys[3] at room temperature.
References
[1] M. D. Stiles, and D. R. Penn, Phys. Rev. B. 61, 3200 (2000) [2] T. Valet, and A. Fert, Phys. Rev. B. 48, 7099 (1993) [3] N. Hase, T. M. Nakatani, S. Kasai, Y. K. Takahashi, and K. Hono, J. Appl. Phys 109, 07E112 (2011)
AS-03. Spin torque noise properties in exchange biased spin-valve and trilayer CPP-GMR devices using Co2Fe(Al0.5Si0.5) Heusler alloy layers
Tomoya M. Nakatani, Masamitsu Hayashi, Takao Furubayashi and Kazuhiro Hono
National Institute for Materials Science, Tsukuba, Japan
We studied the spin torque noise properties in exchange biased spin-valve (EBSV) and trilayer CPP-GMR devices with Co2Fe(Al0.5Si0.5) (CFAS) Heusler alloy layers and a Ag spacer layer. The magnetic layer thicknesses are 5 nm for the EBSV and 4 nm for the trilayer device. The values of the resistance change-area product (ΔRA) at room temperature were ∼6 mΩμm2 and the MR ratios were 18-23% in a four-probe geometry with a small bias current density of J = 1×107A/cm2. The EBSV showed a reduction of ΔR by the spin torque at J higher than 3×107A/cm2, which is defined as the critical current density Jc. The value of Jc in the EBSV is comparable to those with the CoFe magnetic layers [1]. The power spectra of the spin torque noise at the high resistance (antiparallel magnetization) state increased significantly for J > 5×107A/cm2 with a 1/f like behavior at below 5 GHz as reported previously [2]. Two types of the trilayer devices were studied, i.e. the CFAS/Ag/CFAS trilayers with and without the antiferromagnetic interlayer exchange coupling (AFM-IEC)) dependent on the Ag spacer layer thickness. By applying an external magnetic field along the hard axis of the shape anisotropy of the pillar, the scissoring-like magnetization rotation of the two free layers was obtained regardless of the presence of the AFM-IEC as shown in Fig. (a). The effect of the spin torque noise was more significant in the trilayer with the AFM-IEC, showing a larger and broader noise spectrum in the low frequency region than that in the devices without the AFM-IEC (Fig. (b)). The results are discussed with micromagnetic simulations.
References
[1] Maat et al. Appl. Phys. Lett. 93, 103506 (2008). [2] Covington et al. Phys. Rev. B 69, 184406 (2004).
AS-04. Initial Magnetic Damage in Tunneling Magnetoresistance Head due to Temperature Increase Caused by Electrostatic Discharge Models
Chayada Surawanitkun1, Arkom Kaewrawang1, Tim Mewes2, Claudia K. Mewes2 and Apirat Siritaratiwat1
1KKU-Seagate Cooperation Research Laboratory, Department of Electrical Engineering, Khon Kaen University, Khon Kaen, Thailand; 2Physics & Astronomy, University of Alabama, Tuscaloosa, AL
Electrostatic discharge (ESD) is the main cause of damage in tunneling magnetoresistance (TMR) recording heads.1 The past studies indicated that the resistance is condensed before the breakdown and the magnetic damage is arisen before the physical damage.2 Further, as the size of the TMR head will be further reduced to increase the density of recording it is expected that damage in these smaller size devices will be more easily caused by ESD.3 Therefore, we studied the initial magnetic damage in TMR head from the thermal increas e generated using three ESD models: human body model (HBM), machine model (MM) and charge device model (CDM). A 3D finite element method is used for estimating the temperature, T, during the discharge. We report on investigations of devices consisting of Ta(1:1)/IrMn(2:6)/CoFe(3:1.8)/Ru(4:0.8)/CoFeB(5:1.6)/CoFe(6:0.5)/MgO(7:0.8)/CoFe(8:0.3)/CoFeB(9:1.5)/Ta(10:0.25)/NiFe(11:3.5)/Ta(12:5) (layer number: thickness in nm).4 Magnetic damage is considered when the T of the materials exceeds the Curie temperature, TC. Fig. 1(a) shows the highest T occurred in the MgO barrier and the CDM and MM effect on this head is more severe than the HBM effect at the same voltage, V. The results of MM in Fig. 1(b) present the magnetic damage beginning to occur in layer 2, 11, 6, 8, 5, 9 and 3 respectively. A similar sequence of the damage occurred in HBM and CDM. This report discusses the initial layer damaged from ESD. Although the highest T occurs in MgO layer, the initial damage likely arises in the IrMn layer due to its low Neel temperature and protection from damage caused by MM and CDM is important for developing future TMR heads. This work is funded by RGJ Ph.D Program.
References
1M. Miyatake, F. Kugiya and N. Kodama, IEEE Trans. Device Mater. Rel. 10, 476 (2010). 2T. W. Chen, A. Wallash and R. W. Dutton, EOS/ESD Symp., 258 (2008). 3Y. Yang, S. Shojaeizadeh, J. A. Bain, J. G. Zhu and M. Asheghi, J. Appl. Phys. 95, 6780 (2004). 4T. Yang, M. Otagiri, H. Kanai and Y. Uehara, J. Magn. Magn. Mater. 322, L53 (2010).
AS-05. Magnetic nanocontact MR with high MR ratio and low RA
Hitoshi Iwasaki1, Susumu Hashimoto1, Hiromi N. Fuke1, Masayuki Takagishi1 and Masashi Sahashi2
1Corporate Research & Development Center, Toshiba Corporation, Kawasaki, Japan; 2Department of Electronic Engineering, Tohoku University, Sendai, Japan
It is well known that both high dR/R and low RA are required for a reader to realize higher areal density HDD. A dR/R of more than 30 % at RA~0.2 Ωμm2 is roughly required for over 2 Tb/in2 [1]. It is very challenging to reduce RA for tunneling MR to less than 0.5 Ωμm2, while dR/R of CPP-GMR using Heusler alloy or current-confined-pass Cu spacer remains in 20~25 % for realistic RA of ~0.04 Ωμm2 or more to head application. We have already reported that FeCo nanocontact MR (NCMR) with CPP spin-valve structure shows an MR ratio of over 30 % at RA>0.4 Ωμm2 by CIPT technique [2]. In this paper, we will report MR performance for a low RA of less than 0.4 Ωμm2. The CPP-SV films with FeCo NC were deposited by the same method reported in previous paper [2]. MR performance was measured by fabricating test chip with 0.3~0.6 μm element size. Post annealing temperature was changed from 290 to 440 degree C. The metal part of Al and Si was confirmed by high-sensitive XPS using SPring 8 X-ray. The additions of Al and Si were effective to increase dR/R. This may suggest the formation of ordered structure with higher spin-dependent-scattering by post annealing, in addition to reduction of oxide impurity in NC. Test chips with a lead part RA of ~0.08 Ωμm2 showed the maximum dR/R of 45 % at RA~1 Ωμm2 and ~30% at RA~0.2 Ωμm2. The relation between dR/R and RA is well explained by current-confined-pass model simulation [3], which implies dR/R~37% at RA~0.2 Ωμm2 if we use a read part RA of ~0.04 Ωμm2 reported in CPP-GMR heads with metal spacer (100% density of NC). This work was partly supported by a NEDO program.
References
[1] M.Takagishi et al, IEEE Trans Magn., 46 (2010) 2086 [2] H. Iwasaki et al, Intermag Conference 2011, EE-06 [3] H. Fukuzawa et al, IEEE Trans Magn., 40 (2004) 2236
AS-06. Ferromagnetic resonance line widths of metastable Co single crystal thin films
Masato Sakamoto1, Haruhisa Ohashi1, Mitsuru Ohtake2, Masaaki Futamoto2 and Nobuyuki Inaba1
1Department of Electrical Engineering, Yamagata University, Yonezawa, Japan; 2Chuo University, Bunkyoku, Japan
Fast dynamic response of magnetic recording heads is an important factor to develop high frequency magnetic recording system, as well as that of magnetic recording layers in media. The dynamics strongly depends on the Gilbert's damping constant α, which has been evaluated by ferromagnetic resonance (FMR) line widths in thin films. Co/MgO/Co magnetic tunnel junctions with metastable bcc Co(001) electrodes were reported to exhibited giant magetoresistance ratios up to 410%. In this work, we investigated the FMR line widths for bcc and fcc Co single crystal thin films by employing a Q-band(35GHz) FMR analysis in terms of the crystal structure. The bcc Co(110) and the fcc Co(001) single crystal thin films were epitaxially deposited on GaAs(110) and MgO(001) single crystal substrates, respectively. As shown in Fig. 1, with changing the field direction from φ = 0 to 180 deg, Hr has the minimum and the maximum values at φ = 0, 180 deg and φ = 55, 125 deg, which are probably due to the magnetic easy and hard axes directions, respectively. ΔH has the maximum values at φ = 55, 125 deg (or magnetic hard axes directions). The maximum α values are calculated to be 0.06 from the resonance peak width ΔH of 1400 Oe. As shown in Fig. 2, the Hr of fcc Co specimen was four-fold symmetrically varied, since the magnetization hard axes in the fcc-Co(001) plane are the [100] and the [010] directions. While the FMR peak width does not depend on the external field direction. These results are probably caused by the reason why the magnetocrysatlline anisotropy constant of fcc-Co specimen is smaller than that of the bcc-Co specimen.
AS-07. Effect of interlayer coupling on the reversal process of the Differential Dual Spin Valves
Chandrasekhar Murapaka1, 2, Chenchen Wang2, Guchang Han2 and Wen Siang Lew1
1Division of Physics and Applied Physics, Nanyang Technological University, Singapore, Singapore; 2Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Singapore, Singapore
Differential dual spin valve (DDSV) is a potential read sensor for ultrahigh density magnetic recording [1]. A DDSV consists of two mirror-imaged SVs separated by a conductive gap layer.Recently, it has been found that self-biased DDSVs in which the two free layers (FLs) align in flux closure configuration could potentially remove the hard bias. This feature enables the implementation of side shields to diminish the inter-track interference. In this work, we study the effect of interlayer coupling on the reversal process of a self-biased DDSV through micromagnetic simulations. We have investigated the uniform and differential field response of a DDSV. It was found that the competition among magnetostatic coupling, interlayer coupling and the shape anisotropy of the FLs gives rise to distinctive reversal behaviours which are useful for the optimization of the DDSV.Shown in fig 1(a) is the MR response of the DDSV w ith zero interlayer coupling in a uniform field. The GMR response of each SV shows a linear behaviour at low field range and saturated at 1.2KOe. As the interlayer coupling is introduced, the saturation field decreases linearly as the interlayer coupling constant(σ) increases from negative to 0.04 erg/cm2, and the field saturates at 400 Oe after that, as shown in the insert of 1(a). The interlayer exchange energy profile for various coupling strengths is shown in fig 1(b), and it is evident that the reversal process is heavily dependent on σ. For the differential field response, the net output of the DDSV follows the individual SV with twice the magnitude. An opposite trend is observed for the saturation field variation, as we observe a constant plateau for the saturation field for σ<0.04 erg/cm2, and a linear increase for σ>0.04 erg/cm2. We also look into the detailed magnetization states of the DDSV, and an interesting evolution of states as a function of J was extracted to elucidate competition of the energy terms.
References
[1] G. C. Han, J. J. Qiu, C. C. Wang, V. Ko and Z. B. Guo, Appl. Phys. Lett. 96, 212506 (2010).
AS-08. Effect of exchange stiffness of shield material on the sensitivity profile of read heads
Yoshio Suzuki
Electric and Electronic Engineering, Nihon University, Koriyama-shi, Japan
The effect of stiffness A of the shield material is often ignored in simulations of magnetic recording such as finite element method calculation of electromagnetic field. However, as the size of crucial parts of heads becomes smaller the effect of stiffness becomes more pronounced. In our previous study [1] we have evaluated the effect of the stiffness of the write head shield, and showed that it may cause significant degradation in the recording performance. In this report we focus on the effect of stiffness of the shield material on the read head sensitivity profile. To evaluate it we used micromagnetics models and analytical models applied on simple structures. Numerical analysis revealed that exchange stiffness in the shield material may cause large degradation in the efficiency of the shields in short wavelength modes, affecting read head performances such as resolution and side reading properties. It was also found that simple analytical parameters can be introduced to evaluate the relative significance of the effects of various mechanisms on the behavior of shields. We can define susceptibility due to stiffness, that due to anisotropy, and that due to demagnetizing field. Any phenomenon is controlled by the mechanism with smallest susceptibility among them. This analytical approach was also found to have great similarity to the analytical equations introduced to deal with the readback responses involving multilayered recording media which include the stiffness in some of the layers in the media[2]. It was found that as long as the stiffness A is proportional to square of the magnetization Ms the effect of the stiffness remain almost the same whether the magnetization is increased or reduced. The stiffness effect can only be reduced by intentionally cutting the exchange by using lamination with non-magnetic layer.
References
1. Y.Suzuki and Y.Uesaka, "Effect of exchange stiffness on efficiency of shields in magnetic recording heads", IEEE Trans. Mag., to be published. 2. D.Wilton and R.Wood, "Readback Responses for Complex Recording Media Configuration" IEEE Trans. Mag. vol. 39, No.6, pp.1-17.
AS-09. Thermal Response Characteristics and Model for Head/Disk Interaction in TMR Heads
Po rnchai Supnithi1, Piya Kovintavewat2 and Chittiporn Pupaichitkul3
1Faculty of Engineering and College of Data Storage Innovation, King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand; 2Data Storage Technology Research Unit, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, Muang, Thailand; 3Seagate Technology (Korat), Nakorn Ratchaseema, Thailand
As the areal density increases, the actual flying clearance of the TMR heads with MgO barrier above the media surface is reduced to less than 10 nm. The head/disk interaction under such conditions results in a thermal response (TR), which causes a shift in the baseline of the readback signal. This magneto-striction (MS) causes the TR to look different from the classical thermal response [1] that could distort the signal to the extent of causing a sector read failure. We performed some experiments on 200 HDD samples in the laboratory. The spacing between the read gap of the head and the medium surface is controlled by adjusting the pole-tip protrusion. The result of the slap of protrusion can be seen by a corruption the readback signal caused by TR as depicted in Fig. 1(a). The shape of the signal affected by TR remains the same for the OD, MD and ID zones as seen in Fig. 1(b). On the effects of temperature, the shape of the corrupted readback signal is still the same from 0C to 55C . We also propose the TR model in Fig. 2(a) with four regions, namely, a linear positive MS rise-time, a linear negative MS fall-time, a linear TR rise-time, and an exponential TR decay-time and can be expressed as in Fig. 2(b), where A0 and A1 are the positive and negative peak of MS, respectively, A2 is the peak amplitude of TR signal, Tp and Tn are the positive and negative time of MS, Tr and Td are the rise time and decay constant, and Tf = Tr + 3Td.
References
[1] S. E. Stupp, et al, “Thermal asperity trends,” IEEE Trans. Magn., vol. 35, no. 2, pp. 752-757, Mar 1999. [2] Z. Gao, B. W. Karr, S. Xue, E. L. Granstrom, K. T. Tran, Y. X. Li, “Magnetic sensing device including a sense enhancing layer,” U.S Patent no. 0139827A1, June 2007. [3] S. Chikazumi, “Physics of Magnetism”, John Wiley & Sons, 1964.
AS-10. Measuring and Understanding Write Width and Off-Track as a function of Linear Density in Perpendicular Recording
Juan Fernandez-de-Castro, Gene Sandler, Michael Hurben, Pu-Ling Lu and Nathan Curland
Seagate Technology, Bloomington, MN
Multiple techniques have been developed and used to characterize the width of a written track in perpendicular recording. This work investigates written tracks using the WPE (write plus erase) test based on writing the main track at a density and trimming multiple times both sides of the track with another density. The edge of the original track, after each trimming step, is characterized by a narrow band measurement of the read-back signal as a function of cross-track position. The use of a narrow band filter improves the test SNR. This technique helps separate a track into 2 sections: A good region (track center) and a degraded region (track edge). Test and micro-magnetic analysis were done under multiple linear densities for the main track and for the trimming tracks. The results indicated that the WPE value decreased as the density increased between 200kfci and 1500kfci. For densities smaller than 200kfci and larger than 1500kfci, the WPE value did not changed significantly. When the main track was fixed at 200kfci, increasing the trimming density resulted in a higher WPE. Using a micro-magnetic model and spin-stand tests, it was demonstrate d that the apparent increase in WPE was caused by a region with opposite polarity at the edge of the main track (in the media) produced by the magneto-static (demagnetization) field generated by the main track. The section with opposite polarity is found to be coherent with the main track. During the read-back process, the signal from the opposite polarity region in the media is integrated by the reader (read sensitivity function) and subtracted from the main track signal. The result is a loss in signal at the track edge that yields a larger WPE value. This phenomenon has a direct impact on Shingled recording. Modeling and experimental results will be shown.
AS-11. Time resolved scanning Kerr microscopy of the vector magnetization within thin film write head structures
Prim Gangmei1, Paul S. Keatley1, Wei Yu1, Robert J. Hicken1, Mark A. Gubbins2, Peter J. Czoschke3 and Radek Lopusnik3
1Physics, University of Exeter, Exeter, United Kingdom; 2Research & Development, Seagate Technology, 1 Disc Drive,Springtown Industrial Estate, Derry, United Kingdom; 3Recording Heads Operation, Seagate Technology, 7801 Computer Avenue South, Bloomington, MN
Stroboscopic time resolved scanning Kerr microscopy (TRSKM) has been used to make wafer level measurements of magnetization dynamics within the CoFe yoke and pole piece of partially built inductive perpendicular write head structures. All three Cartesian components of the vector magnetization were recorded simultaneously using a polarimeter consisting of a beam-splitting polarizer and two quadrant photodiodes. A static bias field of 200 Oe was applied in the plane of the substrate and orthogonal to the axis of symmetry of the device. After comparing the effect of driving the writer coil with pulses of different shape and amplitude, detailed measurements were performed with pulses of 1.6 ns duration and 11.2 V amplitude. Laser pulses of 100 fs duration, 800nm wavelength, and 1MHz repetition rate were focused to a 600 nm spot size and used to record the time dependent magneto-optical response. The rise time, phase and amplitude of each component of the magnetization was found to reflect the static orientation of the magnetization and the orientation and amplitude of the field generated by the coil at the measurement position. For maximum write field the amplitude of the in-plane magnetization component orthogonal to the bias field should be maximized within the pole piece. Dynamic images revealed “flux beaming” [1] in which this magnetization component was largest, and decayed more slowly than the other components, along the symmetry axis of the yoke, its spatial distribution becoming more irregular as the amplitude of the driving field was reduced. The results show that TRSKM may be used to quantify the effect of yoke composition and geometry upon the magnetization within the writer pole piece.
References
[1] M. Mallary, Chapter 11 “Recording Head Design” in "The Physics of Ultra-High-Density Magnetic Recording" ed. M. Plumer, J. van Ek and D. Weller, Springer-Verlag, Berlin Heidelberg (2001).
AS-12. Characterization method of magnetic properties in ion-beam-etched main pole using magnetoresistance measurements
Yuichi Ohsawa1, 2, Kiyoshi Yamakawa2 and Hiroaki Muraoka2
1CR&D center, Toshiba corp, Kawasaki, Japan; 2RIEC, Tohoku Univ., Sendai, Japan
A new method to estimate magnetic prope rties in a track portion in main poles (MPs) using in situ magnetoresistance (MR) measurements, whose method would be feasible for trackwidths in nanometer scale, is reported. An Fe-Co film was patterned into a MP configuration (inset in Fig. 1). An nonmagnetic electrode was connected to the track portion for current supply. The MR output in the track portion is set to occupy most of the output signals by the conductance design. The sidewall of the MP was ion-beam (IB) etched to decrease in the trackwidth. IB etching and MR measurements were performed alternately for the single sample in vacuum. MR responses in normalized conductance (Gn) of 1.0, 0.7 and 0.4 were shown in Fig. 1. Those curves were a typical anisotropic MR response. In the cases of Gn=1.0 and 0.7, resistances hit the bottom at about 4-7 kA/m, then returned to high resistance state quickly. Whereas in the Gn=0.4 case, low resistance state were kept longer to about 17 kA/m. This would be understood that saturation field near the track portion increased due to trackwidth reduction. This magnetic characterization method for IB etched MP would be useful to assess etching conditions, and feasible for nanometer scale in trackwidth. This work was supported in part by Research and Development for Next-Generation Information Technology of MEXT.
AS-13. Head Field Measurement by Anomalous Hall Effect of Recording Layer with Soft Under Layer
Kiyoshi Yamakawa1, 2, Yuichi Ohsawa1, 3, Takanori Kiya2, Kazuyuki Ise2 and Hiroaki Muraoka1
1RIEC, Tohoku University, Sendai, Japan; 2Akita Industrial Technology Center, Akita, Japan; 3R & D Center, Toshiba Corp., Kawasaki, Japan
Head field of a single-pole head is significantly improved by introduction of media soft under layer (SUL). However, field measurement method so far reported, such as MFM and electron holography, could not evaluate head field in the presence of the SUL. In this paper, we proposed a novel method to measure the practical head field with both SUL and recording layer as an anomalous Hall effect (AHE) sensor. A recording head with planar pole structure [1] was fabricated onto the recording layer of the medium that worked as an AHE sensor. An extremely narrow spacing was possible without any HDI problems for Tera-bit recording tests. The head pole was excited not with a coil but with an external magnet as shown in Fig. 1. In the planar head, the field excited with a coil was quite similar distribution to that produced with an external field. AHE sensors made with recording media composed of Co-Zr-Nb SUL / SiO2 or Al2O3 15 - 80 nm / Co-Pt recording layer showed high AHE voltages comparable to that of media without SUL. Therefore, the AHE will be a useful tool for evaluating head recording performance aside from spin stands. The results of AHE measurement of the united media and heads will be presented at the conference. This work was supported in part by the Research and Development for Next-Generation Information Technology program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
References
[1] K. Yamakawa, H. Muraoka, K. Fudano, S. J. Greaves, Y. Ohsawa, K. Ise, and Y.Nakamura, “High Field-Gradient Design of Single-Pole Write-Head With Planar Pole Structure,” IEEE Trans. Magn., vol. 46, pp. 730-737, MARCH 2010.
AS-14. Write Field Asymmetry in Perpendicular Magnetic Recording
Zhanjie Li, Daniel Bai, Ed Lin and Sining Mao
Western Digital, Fremont, CA
< p height="0">We present a systematic study of the magnetization dynamic behavior by using micromagnetic modeling for a perpendicular magnetic recording (PMR) writer with Full-Wrap-Around-Shielded (FWAS) structures. In particular, we focus on the write field asymmetry between the two opposite polarities. Modeling shows that it is a phenomenon related to the magnetization dynamics in the head. Parameters investigated include initial magnetization condition, write current amplitude, write current frequency, and initial write current polarity. It is found that the write current amplitude and frequency (date rate) are the dominant factors that impact the field asymmetry. Lower write current amplitude and higher write current frequency will deteriorate the write field asymmetry, causing recording performance (such as bit error rate) degradation. To simulate the effect of the reader hard bias initialization field, an external field (up to 1.0 T) was applied to the whole writer structures and then removed, and the remanent states are taken as the initial magnetization condition for the modeling. Results show that the effect of initial magnetization patterns from the initialization field direction (forward vs. reverse) on field asymmetry is negligible. Furthermore, we have studied the main pole material damping constant effect on the magnetization dynamics, where an optimal range of damping constant was found for minimizing the field asymmetry. In addition, the main pole shape effect was also studied
AS-15. Structural and magnetic characterization of epitaxial Fe16N2 thin films with giant saturation magnetization
Nian Ji1, Valeria Lauter2, Lawrence F. Allard2, Cecilia Sanchez-Hanke3, Hailemariam Ambaye2, Edgar Lara-Curzio2, Frank Groot4 and Jian-Ping Wang1
1U of Minnesota, Minneapolis, MN; 2Oak Ridge National Laboratory, Oak Ridge, TN; 3Brookhaven National Laboratory, Upton, NY; 4Utrecht University, Utrecht, Netherlands
Fe16N2 was regarded as one of the most “mysterious” material in the magnetic research society for the last 40 years owing to the controversial reports on its saturation magnetization (Ms) from different research groups[1]. Even in samples that appear to be quite similar, reported Ms can vary widely. Therefore, to extract the most accurate information from any magnetic data, it is vital to perform a rigorous structural characterization on the samples in question. A combination of X-ray diffraction and high resolution transmission electron microscopy analysis can provide excellent insight into the material structure down to the atomic level. In this paper, we present a detailed structure analysis on epitaxial Fe16N2 thin films fabricated by a facing target sputtering process. Systematic magnetic characterization including x-ray magnetic circular dichroism and polarized neutron reflectivity show a correlation between N site ordering [2] and formation of giant saturation magnetization.
References
[1]R. M. Metzger, et.al., J. Appl. Phys. 76, 6626 (1994) and references therein [2]N. Ji, et. al, Appl. Phys. Lett. 98, 092506 (2011)
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Simon Greaves, Tohoku University
AT-01. A new kind of Non-linear Distortion in Perpendicular Magetic Recording Systems.
Mark Nichols and Nenad Miladinovic
SISA, San Jose, CA
For some time non-linear distortion (NLD) in magnetic recording systems has been attributed to demagnetization driven transition shift in the write process. The negative effects of these timing shifts can be mitigated using preemptive timing correction. This paper details a new form of non-linear distortion in the read-back process which can not be successfully mitigated using write precompensation. These read-back non-linearities appear to be due to non-linear field vs. amplitude "MR transfer curves" visualized using quasi-static tester (QST), as shown in figure one below. This paper will show both real world transfer curves from failed drives, as well as a model using synthesized waveforms with & without distortion input into a soft channel.
References
[1] Non-linear Transition Shift Measurements in Perpendicular Magnetic Recording, H.Muraoka, R.Wood, Y.Nakamura, IEEE Trans.Mag. v32(5) pp.3926~3928.
AT-02. Understanding and Improving a Micro-Track Test in Perpendicular Recording
Juan Fernandez-de-Castro, Gene Sandler, Grace Le and Pavol Krivosik
Seagate Technology, Bloomington, MN
Micro-track test is a valuable tool used to characterize the electrical reader width in perpendicular recording. The test is based on trimming both sides of a track written with a single tone and followed by a cross-track scan of the remaining track. The reader cross-track profile or read sensitivity function is characterized by a narrow band measurement of the read-back signal as a function of cross-track position. With increasing track density, the signal amplitude decreases due to incomplete media saturation and to an increase in the off-track noise due to percolation and opposite polarity media saturation. These effects cause a lump at the edge of the micro-track profile that changes the width of the profile (MT10 & MT50). Micro-magnetic analysis indicates that the off-track demagnetization field creates a track-edge section in the media with opposite polarity relative to the originally written track. This section is coherent with the main track and decreases the read-back signal during the scan of the micro-track. And, consequently, the shape of the cross-track read sensitivity function becomes distorted, causing a reduction in MT10 and a side lump. The solution to the problem is to increase the amplitude of the micro-track read-back signal by reducing trimming and by the use of a higher density (KFCI) in the main track that minimizes the off-track demagnetization field. These test improvements are shown to preserve the integrity of the micro-track profile and to enable an accurate measurement of MT10 & MT50.
AT-03. The Effects of Writer Widths and Shingling Percentages in Shingled Write Recording
Pornchai Supnithi1 and Selvan Chandrasekaran2
1Telecommunications Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand; 2Western Digital, Bang Pa-in, Thailand
Shingled write recording (SWR) appears as one of the promising candidates to extend the areal density beyond 1Tb/in2. In [1-3], SNR and Reverse overwrite (ROW) were studied at various track and linear densities. In this paper, we investigate the feasibility of the SWR using the TuMR head samples selected from the manufacturing process at Ayuthaya plant. The magnetic write widths are from 3.5-8 μn, while the TuMr reader widths are from 1.1-1.5 μin. The perpendicular medium has the reference SUL thickness of around 2 μin and the recording layer coercivity of 4500 to 5000 Oe. For SWR, the shingled overlapping has a various track pitches. The MD zone is selected for the testing. The preconditioning is done with the AC band erase. The shingling percentage (α) is calculated as α = TP/MWW * 100, where TP is the shingling track pitch. Shingling TP is the overlapping track pitches from 1.5 to 3.5 μin. From Fig. 1, the BER values ranges from -1 to -5.5 for various samples. The x-axis denotes the shingling percentage and the y-axis denotes the BER performance. From the graph, the value 100% means no shingling that are tested using the same write width values as the track pitch. The lower the percentage of shingling, the more the BER tends to drop. There are two groups from the plot. The writer width values of 3.5 to 4.5 μin seems to separate from the rest. The α below 50% exhibits large variations on the BER compared to the α above 50%. Fig. 2 shows the ROW performances against α for various write width samples. The ROW values ranges from 30 dB to 53 dB from the experiment. We observe that the ROW drops for the lower and higher α. The ROW has a roll off at α ~ 45%-50%. Note: This work is partially supported by DSTAR, KMTIL and NECTEC, NSTDA, Thailand.
References
[1] Li, S. P., et al. IEEE Trans. Magn., 46, no. 6, pp. 2497-2500, 2010. [2] K. Miura, E. Yamamoto, and H. Muraoka, IEEE Trans. Magn., 45, no. 10, 2009. [3] Y. Kanai, et al., IEEE Trans. Magn., 46, pp. 715-720, 2010.
AT-04. Optimization and Design of Shingle Magnetic Recording Systems
Kheong Sann Chan1, Rachid Elidrissi Moulay1, Kim Keng Teo1 and Yasushi Kanai2
1MRC, Data Storage Institute, Singapore, Singapore; 2Department of Information and Electronics Engineering, Niigata Institute of Technology, Kashiwazaki,, Japan
Shingled Magnetic Recording (SMR) [1] is a technology to extend the life of conventional recording, while the Grain Flipping Probability (GFP) model was developed to study and optimize the behavior of magnetic recording systems. Much work has been done in developing and using the GFP model in testing and prediction for SMR and Two Dimensional Magnetic Recording [2,3]. The previous work has focused mainly on the effect of the read/write head and medium parameters on the final error rates of the channel. In the current work, we analyze the achievable track densities for shingled writing as compared to conventional writing and use the GFP channel model to examine the performance of SMR 747 curves. We also perform 747 analysis for SMR on the spin-stand as a validation that the GFP channel model is adequately predicting single-sided (SMR) and double-sided (conventional recording) squeeze plots. Based on the results we identify the key parameters to optimize the gains in an SMR recording scenario. Figure 1 (left) shows a single-side squeeze 747 plot obtained through GFP simulation. The write head was derived through FEM simulations for a triangular/trapezoidal head geometry at densities commensurate with ~500Gbpsi recording. For the analysis, several key head parameters such as MWW, MRW and EBW are needed. Figure 1 (right) shows the the probability footprint contours obtained from the GFP probability LUT. The 0.02, 0.05, 0.95 and 0.98 probability contours are plotted. The numbers indicate the contours for probabilities of grains flipping at the given location. MWW and EBW can be estimated from these GFP contour plots and are tailored to match the numbers used in the spin-stand tests. The analysis shows that while conventional recording densities were dominated by MWW, SMR densities are determined primarily by MRW and EBW.
References
[1]R. Wood et al, “The Feasibility of Magnetic Recording at 10 Terabits per Square Inch on Conventional Media”, IEEE Trans. Magn. Vol. 45, No. 2, pp917-923, Feb 2009. [2] E.M. Rachid, et al. “Modeling of Two-Dimensional Magnetic Recording and a comparison of Data Detection Schemes”, IEEE Trans. Magn. accepted for publication. [3] K.S. Chan, et al. “Comparison of One and Two Dimensional Detectors on Simulated and Spinstand Readback Waveforms”, J.Magn. Magn. Mater, doi:10.1016/j.jmmm.2010.12.022
AT-05. Atomistic Simulation Method in Head-Disk Interface of Magnetic Data Storage System
Robert L. Smith1, Pil Seung Chung1 and Myung S. Jhon1, 2
1Chemical Engineering and Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 2School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Republic of Korea
The conventional modeling paradigm of head-disk interface (HDI) in magnetic data storage system was based on meso/macro scale modeling. Our analysis will provide novel HDI simulation methodology for the first time introducing atomistic architectural design criteria for lubricants through scaling or coarse-graining, and computation of inter-molecular force field parameters will provide the foundation for accurate simulation of static and dynamic properties of perfluoropolyether (PFPE) lubricants, lubricant/overcoat interaction obtained from the molecular and mesoscale modeling of HDI [1]. Here, we calculated intra-molecular force field parameters for functional PFPEs based on model molecular structures from quantum mechanical calculations in order to link electronic structure to classical molecular dynamics. We further explored how the flexibility of various intra-molecular degrees of freedom depends upon the polarity and size of the covalently bonded atoms [2]. We also investigate inter-molecular interaction energies for a set of model PFPE dimers and elucidate the importance of hydrogen bonding between the hydroxylated functional endgroups in interaction strength. We found that for these model dimers representing PFPEs, the DDPA-DDPA (non-hydroxylated) dimer demonstrates the least stable interaction as shown in Fig. 1(a). We further investigated binary lubricant blends of hydroxylated and non-hydroxylated model PFPE and observed diminished interaction strength as compared to pure hydroxylated dimers (see Figs. 1(b) & (c)). Our atomistic interaction energy study will provide insight for tuning of lubricant film physiochemical properties by controlling blending ratios to obtain desired lubricant performance at the HDI.
References
[1] P.S. Chung, H. Park, and M.S. Jhon, IEEE Trans. Magn., 45, 3644 (2009). [2] R. Smith, P.S. Chung, J.A. Steckel, M.S. Jhon, and L.T. Biegler, J. Appl. Phys., 109, 7B728 (2011).
AT-06. Novel Head-disk Interface Design in Magnetic Data Storage
Robert L. Smith1, Pil Seung Chung1 and Myung S. Jhon1, 2
1Chemical Engineering and Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 2School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Republic of Korea
Superior electrical conductivity and thermo-mechanical response of graphene [1] can significantly improve areal density in magnetic data storage. We show that current head media spacing (HMS) in hard disk drives (HDD) is 5 nm (Fig. 1(a)), and replacing conventional carbon overcoat (COC) with graphene will drastically reduce HMS, increasing areal density by eight times its present value [2]. A paradigm shift in HDD systems can be achieved via selection of a novel combination of new lubricants and unconventional architecture of COC systems (Fig. 1 (b)). When evaluating the feasibility of graphene overcoat (GOC), it becomes important to understand GOC-lubricant interactions. We further introduce new alternative head-disk interface (HDI) designs consisting of buffer / lubricant layers (i.e., graphene / carbon nanotube (CNT) or fullerene / perfluropolyether (PFPE)) (Fig. 1(c)). These novel hybrids could further enhance tribological performance including the reduction of wear and friction while drastically increasing areal density. We examined atomistic / molecular interaction of various layers between magnetic and air bearing surface (ABS) via meso / macro scale coarse-graining procedures. Molecular interaction between a variety of PFPEs, CNT, and fullerene with graphene is examined to find novel head disk interface (HDI) designs and verify feasible mechanisms. Our study here will lead to vigorous investigation of the novel HDI in magnetic data storage, including heat-assisted magnetic recording (HAMR), with tuned atomistic design criteria.
References
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science, 306, 666 (2004). [2] B. Machron and T. Olson, IEEE Trans. Magn., 45, 10 (2009).
AT-07. Novel Graphene Overcoat in Magnetic Data Storage Technology
Sesha Hari Vemuri1, Robert Smith1, Young In Jhon2, Pil Seung Chung1 and Myung S. Jhon1, 2
1Chemical Engineering and Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 2School of Advanced Materials Science and Engineering,, Sungkyunkwan University, Suwon, Republic of Korea
The emergence of state-of-the-art recording technologies such as heat assisted magnetic recording (HAMR) place stringent operational limits while demanding high performance and reliability of various components in head-disk interface (HDI). In our work, graphene is introduced for the first time, as a novel material in designing HDI (Fig. 1 (a)) to shed light on its viability due to its thermal and mechanical durability, especially in HAMR applications where periodic high temperature stresses are common. We first examine the interaction between various lubricants and a graphene sheet in the atomistic scale (Fig.1a) by extending our previous approach [1] (Fig. 1 (b)), and investigating the mechanical feasibility of large graphene sheets (in manufacturing steps). We further molecularly characterize the influence of grain boundaries of graphene to observe the mechanical behavior under elongation and compression (Fig. 1(c)&(d)). The atomistic interaction parameters will be implemented into molecular dynamics simulations to accurately investigate the thermo-mechanical properties for larger graphene sheets. Our study will open a new paradigm in various magnetic data storage systems.
References
[1] R. Smith, P.S. Chung, J.A. Steckel, M.S. Jhon, and L.T. Biegler, “Force field parameter estimation of functional perfluoropolyether lubricants,” J. Appl. Phys., 109, 7B728 (2011).
AT-08. Multiscale Modeling in Head/Disk Interface of Magnetic Data Storage System
Pil Seung Chung1, Robert Smith1, Sesha Hari Vemuri1 and Myung S. Jhon1, 2
1Chemical Engineering and Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 2School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Republic of Korea
The head-disk interface (HDI) in the hard disk drive (HDD) system, which includes the hierarchy of highly interactive layers (magnetic layer, carbon overcoats (COCs), lubricants, and air bearing system (ABS)), has been examined via an integrated systems approach with optimization to resolve technical barriers and enhance reliability [1]. Heat-assisted magnetic recording (HAMR), which is our benchmark example, especially requires modeling of thermo- mechanical properties with the enormous combinatorial complexity of materials and multiscale / multiphysical phenomena. In this paper, we provide novel methodology for the HDI, which will holistically integrate several sub-systems of HDI in non-isothermal conditions (Fig. 1) from atomistic / molecular / mesoscale and continuum in an optimized manner. To establish the integration between sub-component systems, the lubricant layer is coarse-grained from quantum mechanics / molecular dynamics to obtain input for mesoscale simulation parameters descriptive for the hotspot having cyclic thermal stresses [2]. The multiscale / multiphenomena simulation tool we developed here can equally be applied to other magnetic data storage systems and will provide the methodology for the investigation and the selection criteria of novel HDI design configuration and materials. Robust lubricants with desirable pairs of multiple functional groups as well as a new overcoat such as graphene in various magnetic data storage will be investigated.
References
[1] D. Kim, P.S. Chung, P. Jain, S.H. Vemuri, and M.S. Jhon, IEEE Trans. Magn., 46, 2401 (2010). [2] S.S. Ghai, W.T. Kim, C.H. Amon, and Myung S. Jhon, J. Appl. Phys., 99, 08F906 (2006). [3] Y. Ma and B. Liu, IEEE Trans. Magn., 44, 3691 (2008).
AT-09. High Speed Magnetisation Reversal In Heat Assisted Recording On Continuous Media
Simon Greaves1, Hiroaki Muraoka1 and Yasushi Kanai2
1RIEC, Tohoku University, Sendai, Japan; 2IEE, Niigata Institute of Technology, Kashiwazaki, Japan
Heat assisted magnetic recording (HAMR) is of much interest due to its potential as a future data storage technology. At high areal recording densities the data rate of written bits will also be very high and the time available to write each bit will be short: possibly 1 ns, or less. Therefore, it is important to evaluate the magnetic reversal process over short time scales and at elevated temperatures. In this work a micromagnetic model based on the Landau-Lifshitz-Bloch (LLB) equation was used to simulate switching times under the influence of write head fields and uniform fields at various temperatures. For a grain with a Curie temperature (Tc) of 545 K the switching time increased as the temperature increased, peaking at around 400 K, followed by a decrease towards zero somewhere above Tc. For a damping constant of 0.01 the average switching time of a typical ferromagnetic grain at room temperature was about 0.5 ns. At a temperature 50 K below Tc the switching time was typically half the room temperature value. A similar trend, albeit with different switching times, was observed for other damping constants. The reduced switching times at high temperature should enable HAMR to support higher data rates than conventional recording at room temperature. Simulations of switching under the influence of head field and heat pulses were also carried out for portions of recording media. The results of multiple tests were averaged to produce a probability map of the regions most likely to switch after a pulse. For extremely short pulse durations of 0.1 ns and a heat spot radius (half-width) of 20 nm an annulus of reversed magnetisation formed outside of the region where the temperature exceeded Tc. For pulses of 0.2 ns duration a central spot also reversed in addition to the annulus. As the pulse duration was lengthened the magnetisation switching probability became more uniform over the heated region. These results show that for very short pulse durations the magnetisation switching probability is strongly influenced by the temperature distribution and is somewhat less affected by the head field distribution. The probability maps were used to determine transition widths and track widths for areal density estimates.
AT-10. Magnetic origin of further coercivity reduction in FePt/Fe ECC media by forming magnetic graded interface
Lisen Huang1, 2, Jiang Feng Hu2 and Jing Sheng Chen1
1Materials Science and Engineering, National University of Singapore, Singapore, Singapore; 2Data storage Institute, Singapore, Singapore
New generation of magnetic recording media requires large magneto-anisotropy Ku to maintain thermal stability but relatively small coercivity Hc for writing consideration. Exchange-coupled composite (ECC) media, multilayer exchange spring media and graded media were proposed to reduce the switching field of hard magnetic media such as L10-FePt and Sm5Co. It also reported that a magnetically graded interface helped in reducing the Hc in FePt-based ECC media [1-3]. However, no one has reported the separate contribution of atomic interface diffusion and exchange coupling on the Hc reduction. We designed an experiment to separate the effect of composition change and exchange coupling in graded interface on the Hc of FePt/Fe ECC media. Two sets of samples with granular structure were fabricated: MgO substrate/FePt(8 nm)/Fe(3 nm)/C and MgO substrate/FePt(8 nm)/Pt(3 nm)/C. In-situ annealing was used to promote interface atomic diffusion and form graded interface. The magnetic gradation at interface was tuned by annealing temperature. FeaPt(1-a) and Fe(1-a)Pta have similar Ku, hence the Ku change caused by FePt/Fe atomic interface diffusion can be studied from FePt/Pt graded interface. Figure 1 shows that Ku of FePt/Pt reduces with annealing temperature, which means atomic diffusion at interface degrades Ku. At room temperature, FePt/Fe has lower effective Ku than FePt/Pt because of the hard/soft exchange coupling effect. Ku eff of FePt/Fe reduces faster than FePt/Pt especially at high annealing temperature, which implies that the exchange coupling effect is enhanced by forming magnetic graded interface. Therefore, Hc is further reduced by the magnetic graded interface with more gradient.
References
[1] D. Goll, A. Breitling, L. Gu, P. A. van Aken, and W. Single, J. Appl. Phys. 104, 083903 (2008) [2] F. Wang, X. H. Xu, Y. Liang, J. Zhang, and H. S. Wu, Appl. Phys. Lett. 95, 022516 (2009) [3] J. L. Tsai, H. T. Tzeng, and B. F. Liu, J. Appl. Phys. 107, 113923 (2010)
AT-11. Granular L10 FePt:X (X = B, B-SiO2 and C-SiO2) (001) Thin Films for Heat Assisted Magnetic Recording
Steven D. Granz1, 2, Katayun Barmak2, 3 and Mark H. Kryder1, 2
1Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA; 2Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 3Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
A comparison was made of 10 nm thick thin films of L10 FePt:X(X =B, B-SiO2 and C-SiO2) having different vol% of boron and carbon which were RF sputtered with in-situ heating from 425oC to 575oC onto Si substrates coated with a 20 nm thick MgO(200) texture layer[1]. It was found that introducing boron or carbon into the FePt-SiO2, reduced the grain size and promoted columnar growth, but also made ordering more difficult. We observed that the FePt grain size reduced significantly from 30 nm to 10 nm when the boron vol% increased from 10% to 40% at a deposition temperature of 575oC. The boron in the FePtB diffused to the grain boundaries. The FePtB had significantly smaller grains than FePtC[2] at lower vol% as seen in Figs. 1a and 1b. The boron promoted layer-by-layer growth in the FePt even at high temperature (575oC). The FePtB films remained columnar beyond 10 nm thickness unlike the FePtC films which typically exhibited a secondary growth layer[3]. In order to further reduce the grain size, FePt-SiO2(25 vol%) was sputtered while adding small quantities(0-20 vol%) of boron and carbon into the films. A reduction in the grain size from 5.8 nm at 0% to 4.7 nm at 20% boron vol% and 4.5 nm at 20% carbon vol% and the promotion of columnar microstructure was observed as seen in Figs. 1c-f. Although the reduced grain size and columnar microstructure are favorable for application in HAMR, the reduced ordering and perpendicular coercivity are unfavorable.
References
[1] S. Granz, M. Kryder, JMMM In Press, 2011. [2] A. Perumal, Y. Takahashi, K. Hono, J. Appl. Phys. 105 (2009) 07B732. [3] J. Chen, J. Hu, B. Lim, Y. Lim, B. Lui, G. Chow, G. Ju, J. Appl. Phys. 103 (2008) 07F517.
AT-12. A Study on Modeling of Writing Process and Two-Dimensional Neural Network Equalization for Two-Dimensional Magnetic Recording
Masato Yamashita1, Yoshihiro Okamoto1, Yasuaki Nakamura1, Hisashi Osawa1, Kenji Miura2, Simon J. Greaves2, Hajime Aoi2, Yasushi Kanai3 and Hiroaki Muraoka2
1Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan; 2RIEC, Tohoku University, Sendai, Japan; 3Niigata Institute of Technology, Kashiwazaki, Japan
A simple writing process considering magnetic clusters due to exchange coupling between grains is studied for two-dimensional magnetic recording. The bit error rate (BER) performance of two-dimensional neural network equalizer (2D-NNE) [1] which can diminish the influences of jitter-noise and inter-track interference is compared with that of two-dimensional linear equalizer (2D-LE) [1] based on the writing process. The medium model [1] and the slop model [2] are assumed. The exchange coupling field Hex,ij of grain i with grain j is proportional to γij=Sij exp(1-aij/a0) / Vi, where Sij is the area of their facing surfaces, Vi is the volume of grain i, aij is the separation between their facing surfaces, and a0 is the atomic spacing which is assumed to be 0.2nm [3]. It is assumed that the magnetic clusters are formed by combinations (i, j), where γij exceeds a threshold value lgf. Figure 1 shows an example of magnetization on the granular medium obtained by our model, where lgf=5.5×10-3 nm-1 and standard deviation of jitter is 1.2nm. The other conditions are the same as those appeared in [1] and [2] for the case of 4Tb/in2. As can be seen in the figure, the influence of jitter-noise due to magnetic clusters is extremely large. The BER performance of generalized partial response system using low-density parity-check coding with the 2D-NNE and 2D-LE designed by genetic algorithm and Levinson-Durbin algorithm [4] is obtained based on the above-mentioned writing process. It is clarified that the gain of the 2D-NNE over 2D-LE is about 1.7 dB at no errors.
References
[1] M. Yamashita et al., IEEE Trans. Magn. (INTERMAG2011 issue), (accpeted). [2] M. Yamashita et al., The 35th Annual Conference on Magnetics in Japan, no.29pA-8, Sept. 2011. [3] S. Greaves, J. Magn. Magn. Mat., vol.321, no.6, pp.477-484, March 2009. [4] H. Osawa et al., IEEE Trans. Magn., vol.44, no.11, pp.3777-3780, Nov. 2008.
AT-13. Microwave assisted magnetic recording simulation on ECC medium
Ayumu Kato1, Yoshitoki Furomoto1, Terumitsu Tanaka1, Anis Faridah Md Nor2, Yasushi Kanai3 and Kimihide Matsuyama1
1Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan; 2Department of Physics, Malaya University, Kuala Lumpur, Malaysia; 3Department of Information and Electronics Engineering, Niigata Institute of Technology, Kashiwazaki, Japan
Microwave assisted magnetic recording (MAMR) is one of the candidates for realizing ultra-high density recording over Tbits/inch2[1]. In order to realize ultra-high density recording, magnetic materials with a strong perpendicular magnetic anisotropy are required. These materials, however, need extremely high frequency microwave for magnetization reversal, which is an issue of writability for MAMR. Li et al. reported the combination of MAMR with exchange coupled composite (ECC) structured medium for solving the writability problem taking single ECC grain for example[2]. Meanwhile, signal recording in MAMR with ECC medium is not reported in simulations or experiments so far. In this study, signal recording with MAMR on ECC medium and reproducing processes are numerically simulated, and SNR is estimated concerning soft-hard thickness ratio of the ECC grain. Fig. 1(a) and (b) show ECC granular medium model and recorded bits pattern with 846 kbpi of linear density, respectively. Fig. 1(c) shows SNRs as a parameter of soft layer thickness, ls. SNRs increase and the frequency maximum SNR obtained decrease with increase in ls below 7 nm. The increase of SNR attributes to the reduction of reversal field due to reverse domain assist in soft layer [3]. The decrease of the optimal frequency comes from reduction of FMR frequency because of reduced effective field in the top end of the soft layer. In this models, the highest SNR of 16 dB is obtained at 846 kbpi with ls = 6 nm and the microwave frequency f = 10 GHz which is significantly lower than that of homogeneous medium.
References
[1] J. G. Zhu et al., IEEE Trans. Magn., 44, 125(2008). [2] S. Li, et al., Appl. Phys. Lett., 94, 202509(2009). [3] A. Yu. Dobin et al., J. Appl. Phys., 101, 09K108(2007).
AT-14. Micromagnetic Studies on Exchange Coupled Composite Recording Media
Hailong Xie1, Hongjia Li2, Ying Wang1, Kaiming Zhang2, Yi Wang2, Zhenghua Li1, Jianmin Bai1, Fulin Wei1 and Dan Wei2
1Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou, China; 2Materials Science and Engineering, Tsinghua University, Beijing, China
Exchange coupled composite (ECC) have been applied in current hard disk system [1-5]. In this work, a micromagnetic model based on microstructures is built for Co-TiO2 (4nm)/Pt(δ=0-2.8nm)/CoPt-TiO2 (16nm) ECC media to investigate the intrinsic magnetic parameters such as exchange coupling between the soft/hard layers.
The micromagnetic cell is 2×2×2nm3 and the averaged grain size is 7nm. In soft layers, the saturation Ms=1200emu/cc, the uniaxial anisotropy field Hk=1.2T; while Ms=619emu/cc, Hk=1.6T for hard layers. The orientation distribution coefficient αθ=1.0. The exchange coupling constant A*1=0.2×10−6erg/cm in a grain A*2=0.1×10−7erg/cm across the grain boundary, while A*3=(0-0.2)×10−6erg/cm was chosen between soft and hard layers. The in-plane magneto-elastic field Hms=2.0T along x-axis is applied.
The microstructures have a great influence on simulating the M-H loops. Fig.1(a) illustrate the experimental and simulated M-H loops of CoPt-TiO2(16nm) single film. In Fig.1(b)-(e), the effects of the Pt interlayer thickness δ are shown respectively: simulations (line) can fit with experiments (dots) only when both A*3 and δ are adjusted simultaneously: A*3 is equal to A*1 when δ is very small (≤1.5nm), and A*3 decreases from A*2 to 0 when δ increases from 2.2nm to 2.8nm. This results mean the soft and hard layer still cohere when δ≤1.5nm, and are segregated when δ≥2.2nm.The simulation results agree well with the experimental results.
References
1. R.H.Victora and X.Shen, IEEE Trans. Magn. 41, 537(2005).
2. M.Kapoor, X.Shen and R.H.Victora, J. Appl. Phys. 99, 08Q902 (2006).
3. J.P.Wang, W.K.Shen, J.M.Bai, R.H.Victora, J.H.Judy and W.L.Song, Appl. Phys. Lett. 86, 142504 (2005).
4. J.P.Wang, W.K.Shen and J.M.Bai, IEEE Trans. Magn. 41, 3181(2005).
5. Y.Wang, J.Ariake, T.Wang, S.Watanabe, N.Honda, F.S.Li, K.Ouchi and S. Ishio, J. Appl. Phys. 107, 103925(2010)
AT-15. Demonstration of read-write on Co/Pd bit patterned media fabricated by direct deposition method
Naganivetha Thiyagarajah1, Siang Huei Leong2, Huigao Duan3, Mohamed Asbahi3, Joel K.W. Yang3 and Vivian Ng1
1Department of Electrical and Computer Engineering, National Univ Singapore, Singapore, Singapore; 2Data Storage Institue, Singapore, Singapore; 3Institute of Materials Research and Engineering, Singapore, Singapore
Bit pattern media (BPM) is a candidate for next generation recording media. We previously demonstrated BPM fabrication method using direct deposition of [Co/Pd]n magnetic material onto stable electron beam resist posts up to 1.5 Tbit/in2 [1]. In this work, we fabricated 50nm pitch BPM over 1mm2 in order to demonstrate read-write. Fig 1 shows AFM/MFM pictures of the fabricated BPM with bright and dark s pots corresponding to 1 and 0 from the individual bits. Note that magnetic film exists between two neighboring bits due to the direct deposition method. Fig 2 shows the write and read-back signals measured using a drag tester. It can be seen from Fig 2(b) that bits of various lengths from 1μm (20 × pitch) to 100nm (2 × pitch) can be successfully written and read-back using a standard head on our BPM structures. In addition as shown in Fig 2(a) different bit patterns (“110” in this case) could be written and read back several times. We plan to extend this result to BPM with smaller pitches that our group have fabricated and demonstrate compatibility with current commercial heads.
References
[1] YJ Chen, TL Huang, S Leong, V Ng, JKW Yang, MMM 2010, CF-4
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: José I Arnaudas, Univerisdad de Zaragoza
AU-01. Optimization of sputter deposition parameters for magnetostrictive Fe62Co19Ga19/Si(100) films
S.U. Jen1 and Tsung-Lin Tsai1, 2
1Academia Sinica, Institute of Physics, Taipei, Taiwan; 2Dept. of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan
A good magnetostrictive thin-film actuator, transducer, or sensor should acquire following characteristics, large saturation magnetostriction (λS) and low saturation (or anisotropy) field (HS), so that its magnetostriction susceptibility (SH/HS= Δλ/HS= [λLS - λTS]/HS, where λLS and λTS are the longitudinal and transverse magnetostriction at HS) can be as large as possible. In this study, we have made Fe62Co19Ga19/Si(100) nano-crystalline films by using the magnetron sputtering technique under various fabrication conditions: Ar working gas pressure (pAr) varied from 1 to 15 mTorr, sputtering power (Pw) from 10 to 120 watt, deposition temperature (TS) from room temperature (RT) to 3000C, and film thickness fixed at 175 nm. Following experiments were performed on those films: [1] the atomic force and magnetic force microscope (AFM and MFM), [2] the magnetic easy- and hard-axis hysteresis-loop, [3] the longitudinal and transverse magnetostriction, and [4] the electrical resistivity (ρ) measurements. Each magnetic domain looks like a long leaf, with its long-axis being about 12 to 15 μm and short-axis about 1.5 μm. The size of the magnetic domain is much larger than the size of the nano-crystalline grain, which is about 7 to 30 nm.1 The optimal magnetic and electrical properties are collected from the Fe62Co19Ga19 film made with the following sputter deposition parameters, such as pAr= 5 mTorr, Pw= 80 watt, and TS = RT. Those optimal properties include λS = 80 ppm, HS = 19.8 Oe, SH = 6.1 ppm/Oe, and ρ = 57.0 μΩcm. Note that SH for the conventional magnetostrictive Terfenol-D film is, in general, equal to 1.5 ppm/Oe only.2
References
1 Q. Xing and T. A. Lograsso, Appl. Phys. Lett. 93, 182501 (2008). 2 H. Uchida, Y. Matsumura, H. Uchida, and H. Kaneko, J. Magn. Magn. Mater. 239, 540 (2002).
AU-02. Giant Magnetostriction in Tetragonally-Distorted Antiperovskite Manganese Nitrides
Takeshi Shimizu, T. Shibayama, K. Asano and K. Takenaka
Department of Crystalline Materials Science, Nagoya university, Nagoya, Japan
The peculiar relationship between magnetism and the crystal lattice in antiperovskite manganese nitrides Mn3AN (A: Zn, Ga, Ag, etc) is now attracting significant attention because of the unique properties such as negative thermal expansion, magnetocaloric effects, and magnetoresistance. The recent discovery of huge magnetostriction of up to 2000 ppm in tetragonally-distorted antiperovskite Mn3CuN [1] is an important milestone in this field of study. This magnetostriction is interpreted to be due to ferromagnetic shape memory effect [2]. Tetragonal distortion in antiperovskites is now a new paradigm for ferromagnetic shape memory effects. Here, we present a possible new family of ferromagnetic shape memory alloys (FSMAs) with the tetragonally-distorted antiperovskite structure, Mn3SbN. It undergoes the first-order transition from a high-temperature paramagnetic to a low-temperature ferromagnetic phase at the Curie temperature TC = 360 K, accompanied by cubic-to-tetragonal structural deformation. In the tetragonally distorted ferromagnetic phase, Mn3SbN exhibits a magnetostriction of 450 ppm. The partial replacement of Sb with other transition metals increases magnetostriction to 1000 ppm. The upper limit of the operating temperature (360 K) is comparable with the highest operating temperatures of all known FSMAs. The findings presented here will strongly affect future research on magnetostriction. We discuss the effects of partial substitution for the constituent elements, not only for Sb but also for Mn and N, as well as the effects of nitrogen deficiency on the magnetostrictive behavior such as the magnitude of magnetostriction, the operating temperature, and the operating magnetic field. This work was partly supported by MEXT, Japan and by NEDO, Japan.
References
[1] K. Asano, K. Koyama, and K. Takenaka, Appl. Phys. Lett. 92, 161909 (2008). [2] R. C. O’Handley, J. Appl. Phys. 83, 3263 (1998).
AU-03. Effect of the Mn substitution for Fe on magnetic and magnetostrictive properties of SmFe2 compound
Y. Wang, W. J. Ren, Z. H. Wang, Y. Q. Zhang, J. Li and Z. D. Zhang
Shenyang national laboratory for materials science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
The structural, magnetic and magnetostrictive properties of Sm(Fe1-xMnx)2 (0 ≤ x ≤ 0.20) alloys have been investigated. The alloys are almost single Sm(Fe, Mn)2 Laves phase with cubic MgCu2-type structure. When Mn content x is over 0.15, impurity phases appear. The difficulty for the formation of Laves phase with a high Mn content may be ascribed to the larger atomic radius of Mn than that of Fe. The lattice parameter and the Curie temperature of the Sm(Fe1-xMnx)2 compounds increases and decreases linearly with increasing Mn content, respectively. Temperature dependence of magnetization was measured under a magnetic field of 50 Oe. The spin reorientation temperature for [110] to [111] of SmFe2 was determined to be 200 K. It decreases to 140 K for x = 0.15 with increasing Mn content. The magnetization at 295 K of the Sm(Fe1-xMnx)2 alloys decreases from 58 emu/g for x = 0 to 35 emu/g for x = 0.20 with the substitution of Mn for Fe. The atomic moment diffe rence between iron and manganese (μFe -μMn) was deduced to be 1.34 μB. The magnetostriction coefficient λ111 of the Sm(Fe1-xMnx)2 compounds was determined by step scanning the (440) reflection of the Laves phase. All the (440) reflections are double splitted, suggesting the spontaneous magnetostriction. λ111 maintains larger than 1900 ppm when 0 ≤ x ≤ 0.15. The field-dependent linear anisotropic magnetostriction λa = λ∥ - λ⊥ of all the Sm(Fe1-xMnx)2 alloys studied is not saturated at the maximum applied magnetic field of 12 kOe. λa decreases with increasing Mn content: one aspect is ascribed to the increasing anisotropy, that is, the decreasing spin-reorientation temperature; and the other aspect is the increasing impurity quantity when x ≥ 0.15.
References
H. Samata, N. Fujiwara, Y. Nagata, T. Uchida, and M. D. Lan, J. Magn. Magn. Mater. 195, 376 (1999). A. E. Clark, J. P. Teter, and M. Wun-Fogle, J. Appl. Phys. 69, 5771 (1991).
AU-04. Withdrawn
AU-05. Structural, magnetic and magnetoelastic effects in Sr(Ti1-xMx)O3 (M=Fe, Co, or Cr) epitaxial thin films
Dong Hun Kim, Lei Bi, Peng Jiang, Gerald F. Dionne and Caroline A. Ross
MIT, Cambridge, MA
SrTi1-xMxO3 films in which M = Fe, Co or Cr (STF, STC and STCr respectively) and x = 0.04-0.5 have been grown in vacuum epitaxially on CeO2-buffered Si, SrTiO3, and LaAlO3 substrates. The films grow in the perovskite structure with oxygen deficiency and mixed valence Fe, Cr or Co ions are present. Films of SRF and STC typically showed a strong uniaxial magnetic anisotropy in the out-of-plane direction, a saturation moment on the order of 0.5µB/Fe or Co, and a magnetism that persists to temperatures of order 1000 K. In contrast, films containing Cr showed no evidence of a spontaneous magnetic moment. The films are typically in a high in-plane compressive strain state due to epitaxial growth. These results are attributed to magnetoelastic anisotropy, which can explain the presence of significant anisotropy even at low concentrations of the transition metal, a high ‘Curie temperature’, and a relation between magnetic moment and temperature which does not follow the Brillouin function [1]. These characteristics differ qualitatively from the behavior expected in systems dominated by exchange or by metal clusters. The magnetization vs. temperature behavior of STF and SRC is well fitted using a magnetoelastic model, [2]. The results are discussed in terms of the magnetoelastic effects from specific ionic valence states. In STF and STC, Fe2+ and 4+ and Co2+, 3+ and 4+ in the octahedral sites are magnetoelastic, while in STCr, the majority Cr3+ ions are not magnetoelastic because they stabilize ground-state singlets. An understanding of magnetoelastic effects is key to the development of transition-metal oxides for magnetic, spintronic or multiferroic devices, where high temperature magnetization is required, or for strain-control of properties.
References
[1]D.H. Kim et al., Phys. Rev. B (2011) [2]Dionne, J. Appl. Phys., 101, 09C509 (2007)
AU-06. Effects of Ni addition on the magnetostriction and microstructures of Fe70-xPd30Nix high-temperature ferromagnetic shape memory alloys
Yin-Chih Lin1, Chien-Feng Lin2 and Jin-Bin Yang2
1Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan; 2Department of Mechanical and Automatic Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan
This study investigated the effects of adding a third alloying element, Ni, to create Fe70-xPd30Nix(x=2, 4, 6, 8 at%Ni) ferromagnetic shape memory alloys(FSMAs). The Ni replaced a portion of the Fe. The Fe70-xPd30Nix alloys were homogenized through hot and cold forging to a ~38% reduction in thickness, solution-treated (ST) with annealing recrystallization at 1100 °C for 8 h and quenched in ice brine, and then aged at 500 °C for 100 h. The magnetostrictive strains were investigated by the P3 strain indicator and recorder combined with a strain gauge and magnetostrictive-meter setup. Simultaneously, the microstructures were studied by XRD, SEM, and TEM. Investigation of the magnetostriction and microstructures indicated that the greater Ni amount in the Fe70-xPd30Nix alloys caused the less saturation magnetostriction at RT, it was observed that it was more difficult to generate an annealed recrystallization. However, greater Ni addition into the Fe70-xPd30Nix(x=6, 8 at%Ni) alloys can suppress the L10+L1m phase decomposition into stoichiometric L10+L1m+αbct structures, improving the Fe70-xPd30Nix alloys and maintaining high magnetostriction after the alloys were aged at 500 °C for 100 h. This magnetic property of the Fe70-xPd30Nix(x=6, 8 at%Ni) alloys is suitable for application in a high temperature (T>500 °C) and high frequency environment.
References
[1] P.K. Kumar, D.C. Lagoudas, Acta Materialia. 58 (2010) 1618. [2] Y. Xin, Y. Li, Z. Liu, Scripta Materialia. 63 (2010) 35. [3] Y. Li, Y. Xin, L. Chai, Y. Ma, H. Xu, Acta Materialia. 58 (2010) 3655. [4] Y.C. Lin, C.F. Lin, J.B. Yang, H.T. Lee, J. Appl. Phys. 109 (7) (2011) A912(1). [5] Y.C. Lin, H.T. Lee, J. Magn. Magn. Mater. 322 (2010) 197. [6] C.F. Lin, J.B. Yang, IEEE Trans. on Magn. 45 (6) (2009) 2499. [7] K.C. Atli, I. Karaman, R.D. Noebec, H.J. Maierd, Scripta Materialia 64 (2011) 315.
AU-07. Effect of Orthogonal Fields on Magnetostrictive Power Harvesting
Amr Adly1, Daniele Davino2, Alessandro Giustiniani3 and Ciro Visone2
1Elect. Power & Machines Dept., Cairo University, Giza, Egypt; 2Dept. of Engineering, University of Sannio, Benevento, Italy; 3DIIIE, University of Salerno, Fisciano, Italy
Recently, the inverse magnetostriction effect (Villari effect) has been utilized for energy conversion purposes. Within this context, numerous efforts have been carried out in the past few years towards the design, modeling and development of magnetostrictive power harvesting devices [1,2]. It has been shown that the performance of these devices is affected by several parameters such as longitudinal bias magnetic field, mechanical load conditions (pre-stress) and electrical load. In order to further check their basic behavior and performances, a 3X3X18 mm sample has been subjected to a general field having both longitudinal and transverse components. In particular, the sample experienced different values of axial and transverse bias field, during harvesting operations, i.e. with time-varying applied mechanical load. The preliminary results sketched in Fig. 1(a) shows a dependence of the induced voltage on the transve rse field, which could be exploited to increase the amount of converted harvested power. In order to describe the sample behavior under vector input fields, which represent the real working condition, a vector Preisach-type model was used to investigate the dependence of power harvesting from a magnetostrictive rod subject to an axial bias magnetic field and time-varying stress in addition to a transverse magnetic field [3,5]. Fig. 1(b) seems to confirm such dependence and can be considered as a valid motivation to deeply discuss such phenomena in the full paper.
References
[1] D. Davino, A. Giustiniani and C. Visone J. Appl. Phys., vol. 105, p. 07A939, 2009. [2] D. Davino, A. Giustiniani and C. Visone, IEEE Transactions on Ind. Electronics, 58 - 6 pp. 2556 - 2564 , 2011 [3] A.A. Adly and I.D. Mayergoyz IEEE Trans. Magn., vol. 32, pp. 4773-4775, 1996. [4] A.A. Adly and S.K. Abd-El-Hafiz, Physica-B: Condensed Matter, vol. 403, pp. 425-427, 2008.
AU-08. Magnetovolume effect in Ho2Fe17-xMnx compounds
Jianli Wang1, 2, Andrew J. Studer1, Shane J. Kennedy2, Rong Zeng1, Shi X. Dou1 and Stewart J. Campbell3
1Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, NSW, Australia; 2Bragg Institute,, ANSTO, Sydney, NSW, Australia; 3School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, The Australian Defence Force Academy, Canberra, ACT, Australia
We have investigated the structural and magnetic properties of Ho2Fe17-xMnx compounds (x=0-5) by different experimental techniques. Similar to other ferrimagnetic R2Fe17-xMnx systems [1-3], the unit cell volume at room temperature in Ho2Fe17-xMnx generally increases with Mn content other than for low Mn values where a slight maximum is detected around x ≈ 0.5. This unusual composition dependence of the unit cell volume can be ascribed to a strong magnetovolume effect around Curie temperature. Magnetovolume effects in Ho2Fe17-xMnx compounds have been investigated in details using the high intensity powder netron diffractometer Wombat (OPAL, Australia) over the temperature range 10-450 K (see Fig. 1 for a typical example). A pronounced positive spontaneous volume magnetostriction has been observed below the Curie temperature. The nature of magnetic transition has been discussed in terms of the classical model for itinerant ferromagnets.
References
[1] P. C. Ezekwenna, G. K. Marasinghe, W. J. James, O. A. Pringle, Gary J. Long, H. Luo, Z. Hu, W. B. Yelon, and Ph. l’Heritier, J. Appl. Phys. 81, 4533 (1997). [2] Y. M. Hao, Y. Gao, B. W. Wang, J. P. Qu, Y. X. Li, J. F. Hu, and J. C. Deng, Appl. Phys. Lett. 78, 3277 (2001) [3] J. L. Wang, S. J. Campbell, O. Tegus, C. Marquina, and M. R. Ibarra, Phys. Rev. B 75, 174423 (2007).
AU-09. Structure and bias exchange of Ni 50Mn37Sn13 ribbons
Y.b. Yang, J.z. Wei, X.b. Ma, C.s. Wang, Y.c. Yang and J.b. Yang
School of Physics, Peking university, Beijing, China
Recently, some ferromagnetic shape memory alloys have been found in the Ni-Mn-X (X=Sn, In, and Sb). They are of considerable interest because of their exceptional magnetoelastic properties, and large exchange bias effect in these alloys which has been observed under zero-Field cooling from an unmagnetized state.[1] Up to now, the investigated alloys are usually bulk materials obtained by arc or induction melting followed by a high temperature annealing, or single crystals grown by Czochralski method. Here Ni 50Mn37Sn13 alloys are produced by melt spinning.[2] The magnetization and high resolution neutron powder diffraction measurements on the Ni 50Mn37Sn13 have confirmed that it is ferromagnetic below 325 K and undergoes a structural phase transition of TM=250 K on cooling and 240 K on warming. The high temperature phase (austenitic phase) has the cubic L21 structure, with the excess manganese atoms occupying the 4(b) sites. The low temperature phase (martensitic phase) stable below TcM has an orthorhombic structure with space group Pmma related to the cubic phase through a Bain transformation.[3] Martensitic transformation temperatures (TcM) decrease while ferromagnetic-paramagnetic phase transformation temperatures (TcA) increase with increasing magnetic field in zero cooling field curves due to the magnetic field induced antiferromagnetic-ferromagntic transfaormation. Exchange bias effect is observed below TN. The exchange bias field HE reaches a maximum of 290 Oe when the cooling field increases to 250 Oe, which is attributed to a ferromagnetic unidirectional anisotropy formed at the interface between different magnetic phases.
References
[1].B. M. Wang, Y. Liu, P. Ren, B. Xia, K. B. Ruan, J. B. Yi, J. Ding, X. G. Li and L. Wang, Phys. Rev. Lett. 106 (7), 077203 (2011). [2].J. D. Santos, T. Sanchez, P. Alvarez, M. L. Sanchez, J. L. S. Llamazares, B. Hernando, L. Escoda, J. J. Sunol and R. Varga, J. Appl. Phys. 103 (7), - (2008). [3].P. J. Brown, A. P. Gandy, K. Ishida, R. Kainuma, T. Kanomata, K. U. Neumann, K. Oikawa, B. Ouladdiaf and K. R. A. Ziebeck, J. Phys-Condens. Mat. 18 (7), 2249-2259 (2006).
AU-10. Dynamic Magnetoelastic Properties of Epoxy-Bonded Sm0.88Nd0.12Fe1.93 Pseudo-1-3 Magnetostrictive Particulate Composites
Fang Yang1, 2, Siu Wing Or1, Wei Liu2, Xiangke Lv2 and Zhidong Zhang2
1Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China; 2Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
Pseudobinary RR’Fe2 Laves alloys (R, R’ ≡ rare earths, where R ≠ R’) have received considerable research interest in recent years for magnetostrictive transducer and actuator applications. Among the available RR’Fe2 Laves alloys, Tb0.3Dy0.7Fe1.92 (Terfenol-D) and Sm0.88Dy0.12Fe1.93 (Samfenol-D) have been widely investigated because of their giant positive and negative magnetostrictions, respectively, at room temperature. The inclusion of large amounts of scanty and expensive heavy rare earths Tb and Dy in Terfenol-D have led to great research efforts to develop light rare earth-substituted positive magnetostrictive alloy systems in the past decade. However, research on the equally important negative magnetostrictive alloy systems remains relatively limited. Following our recent success in synthesizing light rare-earth-based Sm1-xNdxFe1.93 (0 ≤ x ≤ 0.56) Laves alloys, we report, in this work, the dynamic magnetoelastic properties of epoxy-bonded Sm0.88Nd0.12Fe1.93 pseudo-1-3 magnetostrictive particulate composites consi sting of 0.5 volume-fraction light rare earth-based Sm0.88Nd0.12Fe1.93 particles with a size distribution of 10-300 µm embedded and aligned in a passive epoxy matrix. The dynamic relative permeability μr33 exhibits a flat response with frequency, expect for the resonance range. The two elastic moduli, EH3 (at constant magnetic field strength) and EB3 (at constant magnetic flux density), show negative -ΔE near HBias = 120 kA/m. The piezomagnetic coefficient (d33) displays a large maximum of -2 nm/A at 100 kA/m as a result of the maximum motion of non-180° domain walls.
AU-11. Phase identification and magnetostrictive property of Fe-19at.% Ga single crystal
Xiaoxi Zhu, Jinghua Liu and Chengbao Jiang
Materials science and engineering School, Beijing University of Aeronautics & Astronautics, Beijing, China
This work presents detailed information of phase identification of a single-crystalline Fe-19at.% Ga (hereafter Fe81Ga19) by transmission electron microscopy. The single crystal was grown in a floating zone melting furnace by using a seed crystal. Magnetostrictive property along the axis of the crystal λ∥ was measured for the <001> oriented single crystals, and the highest magnetostriction λ∥up to 327ppm was achieved under the pre-stress of 60MPa. Initial magnetization curves were measured in single crystals along [100], [110] and [111] axis, respectively. From the magnetization curves, magnetocrystalline anisotropy constants of Fe81Ga19 alloys were calculated, and the values of K1 and K2 were 1.3×104 J/m3 and -2.6×104 J/m3, respectively. The diffraction pattern on [001] zone axis showed the existed of superlattice. Presence of the dislocation microstructure was obviously shown in the high-resolution electron macrograph along [001] zone axis. The [001] Ga-Ga pair defects were found to exist, which enhance the magnetostriction of Fe81Ga19 single crystal by inducing large strains in the surrounding Ga-Fe bonds and the magnetocrystalline anisotropy.
AU-12. Direct measurement of magnetocaloric effect in Co-doped Mn-rich Ni2MnGa alloys
Jiri Kastil1, Jiri Kamarad2, Simone Fabbrici3, Franca Albertini3, Antonio Paoluzi3 and Zdenek Arnold2
1Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Praha 2, Czech Republic; 2Institute of Physics ASCR, Prague 8, Czech Republic; 3IMEM CNR, Parma, Italy
The structural and magnetic properties of the Heusler Ni2MnGa alloys are very sensitive to an off-stoichiometry and a doping by another transition metal. The giant magnetoelastic and magnetocaloric effects was found as a result of magneto-structural transition between the low symmetry martensite (M) and cubic austenite (A) phases. We have studied magnetic and magnetocaloric properties of the polycrystalline Co-doped Mn-rich Ni50-xCoxMn25+yGa25-y alloys. Positive magnetization jump (ΔM) is observed at the M-A transitions in all compounds with x ≥ 5. The inverse MCE was expected and verified in these alloys by the positive isothermal magnetic entropy changes ΔS that were calculated using measured magnetization isotherms and the Maxwell relation [1]. As magneto-structural transitions in these alloys are of the first order, the direct measurements are of particular importance for the characterization of MCE [2]. Our direc t measurements of ΔTad have provided a large collection of the experimental data on the polycrystalline Ni50-xCoxMn25+yGa25-y alloys where x=5, 7 for y = 6 and x = 9 for y = 7. The data allow to compare results of the direct and the indirect (based on ΔS) MCE measurements and to discuss physical peculiarities of the inverse MCE in the studied alloys with the hysteretic first order magneto-structural transitions. The direct measurement confirmed the inverse MCE around the M-A transition, however, the magnitude of the temperature change was rather small of about 1 K for a field change of 4.7 T. On the other hand, the temperature change associated with the paramagnetic-ferromagnetic transition of the austenite phase reached almost 2 K for the same field change. We also investigated MCE around the magnetic ordering temperature of the martensite phase, TCM, that was observed in the studied alloys with x=7 and 9. The direct measurement has shown almost no magnetocaloric effect in corresponding temperature. This is an important contribution into the discussion of character of magnetic state in the martensite phase at temperature region from TCM to M-A transition.
References
1 S. Fabbrici et al.: Acta Mater. 59 (2011) 412
2 J. Kamarad et al: Acta Physica Polonica A 118 (2010) 1000
AU-13. Grain Interactions During the Phase Transition in Ni-Mn-Ga
Ryan A. Booth, Shiu F. Li, Robert M. Suter and Sara A. Majetich
Physics, Carnegie Mellon University, Pittsburgh, PA
We have investigated the interaction between grains in a polycrystalline sample of the magnetic shape memory alloy (MSMA) Ni-Mn-Ga using a combination of SQUID magnetometry, optical microscopy, magnet-optic microscopy, and high energy x-ray diffraction microscopy (HEDM). The composition was determined to be Ni54Mn20Ga26 using x-ray fluorescence. At this composition the Curie temperature is 80 °C. The martensitic transformation temperatures were determined by differential scanning calorimetry (DSC) to be 70 °C on heating and 52 °C while cooling. Because the sample is polycrystalline and a large volume change takes place during the phase transition, interactions between grains cannot be ignored. Figure 1 shows is an optical micrograph showing the surface relief contrast of the sample in a) the martensite phase at room temperature, b) the austenite phase at 100 °C, c) the martensite phase after cooling back to room temperature. Figure 1c shows clearly that upon cooling the strain between neighboring grains was sufficient to fracture the sample. HEDM results describing the structural change and strain through the phase transition in a macroscopic, 3D sample will be discussed. We will emphasize the importance of understanding the interaction between grains of MSMAs if polycrystals (or even imperfect single crystals) of this class of materials are used for applications such as actuators or magnetocaloric effect (MCE) materials.
AU-14. A cheaper NiMnGa based Heusler alloy for magnetic refrigeration
Catalina Salazar Mejia1, Angelo M. Gomes1 and Luiz Augusto S de Oliveira2
1Instituto Fisica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil; 2Instituto Fisica "Gleb Wataghin", Universidade Estadual de Campinas, Campinas, Brazil
Ni-Mn-Ga Heusler alloys have been attracting considerable interest for presenting attractive properties as magnetocaloric effe ct [1] and shape memory effect [2]. Recently, the partial substitution of Ga for Al have been study as a way to improve the ductility an cost of Ni-Mn-Ga alloys [3,4]. On the other hand, it has been shown that the magnetic (TC) and martensitic (TM) transitions can be tuned through atomic substitution of Cu on Mn site, leading to a giant magnetocaloric effect (GMCE) in Ni(Mn,Cu)Ga alloy at room temperature[5]. In the present work we have studied the effect of Cu substitution in the magnetic and structural properties of Ni2Mn1-xCuxGa0.9Al0.1 with x=0.0, 0.2 and 0.3. Magnetization measurements as a function of temperature performed from 10 to 400 K, with an applied field of 0.02 T showed a ferromagnetic state, with critical temperature Tc =295 and 300 K for the samples with Cu, x=0.2 and 0.3 respectively. For the sample without Cu a complex behavior is observed with TC= 370 K and TM=220 K and a pre-martensitic transition is also present at TPM= 250 K, as previously reported [3]. X-rays measurements were perform at room temperature and shows a L21 structure for x=0.0 and a mixture of L21 and martensitic for the other samples. The aim of our study is to produce cheaper magnetocaloric material based on Ni-Mn-Ga Heusler alloys through partial substitution of Ga by Al. Our results indicate that a martensitic and structural phase transition occurs at the same temperature for the sample with x=0.2, which is the same mechanism responsible for the GMCE in the Ni(Mn,Cu)Ga. Heat capacity measurements were performed in order to calculate magnetocaloric effect in the samples and the results confirms that the news alloys can be a cheaper magnetic refrigerant at ambient temperatures.
References
[1] A. Planes, et al., J. Mag. Mag. Mat. 310 (2007) 2767-2769. [2] A. N. Vasil'ev, et al., Physics-Uspekhi 46 (2003) 559 - 588. [3] A. C. Abhyankar, et al., Intermetallics 18 (2010) 2090-2095. [4] H. Ishikawa, et al., Acta Materialia 56 (2008) 4789-4797. [5] S. Stadler, et al., Appl. Phys. Lett. 88, (2006) 192511.
AU-15. Refrigerant capacity of austenite in as-quenched and annealed Ni51.1Mn31.2In17.7 melt spun ribbons
Jose L. Sanchez Llamazares1, H. Flores-Zuñiga1, C. F. Sanchez-Valdes2 and Carlos Garcia3
1División de Materiales Avanzados, Instituto Potosino de Investigación Científica y Tecnológica (IPICyT), San Luis Potosí, Mexico; 2Institut de Ciencia de Materials de Barcelona (C.S.I.C.), Bellaterra, Spain; 3Physics, Bogazici University, Istanbul, Turkey
Melt spinning has proven to be an excellent technique to produce homogeneous and single-phase Heusler alloys in the Ni-Mn-In system [1]. In this contribution, we focus on the thermal dependence of the magnetic entropy change ΔSM(T) and refrigerant capacity RC of as-quenched (aq) and their change upon a stress and structural relaxation annealing at 1073 K (during 10 minutes and 2 hours). The alloy ribbons studied show the average chemical composition Ni51.1Mn31.2In17.7 and were produced by melt spinning technique in Ar environment. Samples crystallized into a single-phase austenite with the L21-type crystal structure (a=0.5989(3) nm) and a Curie temperature TC, close to room temperature, of 275 K. The second order nature of the magnetic transition of austenite in the studied samples was confirmed by the Arrott plots method. ΔSM(T), and its peak value ΔSMpeak, have been determined from a set of isothermal magnetization curves measured up to μoHmax= 5.0 T using the Maxwell relation; RC was estimated, in a first approach, as the product of ΔSMpeak, and the full width at half maximum temperature range δTFMHW of the ΔSM(T) curve. For aq ribbons the ΔSM(T) curve shows a broad peak at TC (ΔSMpeak= -3.0 Jkg-1K-1, RC= 345 Jkg-1, and δTFMHW= 113 K). Upon annealing, the crystal structure of austenite is preserved and the main changes observed are a slight reduction of its cell parameter, and an increase in both, TC (up to 281 K) and in the saturation magnetization. The stress and structural relaxation upon annealing lead to a faster drop in magnetization and narrower ΔSM(T) curves with the consequent increase and reduction in ΔSMpeak, δTFMHW and RC, respectively (ΔSMpeak= -4.0 Jkg-1K-1, RC= 268 Jkg-1, and δTFMHW= 65 K after a 10 min. of annealing). Thus, aq samples exhibit a better efficiency than the annealed ones for a refrigerant cycle. In both, aq and annealed ribbons ΔSMpeak is linearly dependent on (μoH)2/3.
References
[1] J.L. Sánchez Llamazares, B. Hernando, C. García, J. González, Ll. Escoda, J.J. Suñol, J. Phys. D: Appl. Phys., 42 (2009) 045002.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Steven Turner, National Physical Laboratory, UK
AV-01. Magnetic Property Modeling of Silicon Steel Sheets Under DC-Biasing Magnetization
Zhigang Zhao1, Fugui Liu1, Pengxiang Ren2, Youhua Wang1, Luzhen Zhao1, Dandan Li1 and Weili Yan1
1Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Tianjin, China; 2School of electronic ,information and electrical engineering, Shanghai Jiao Tong University, Shanghai, China
An efficient method for modeling the global magnetic properties of the silicon steel under dc-biased condition was proposed. The measurement was carried out based on a laminated core model of product level which was designed and made by the authors. However, the DC bias magnetic flux can not be measured directly in the experiment, and then a simulated method was presented to calculate the dc magnetic flux in the iron core. The calculation program was developed based on MATLAB, thus the hysteresis loops of the iron core under biasing magnetization were obtained. In addtion,the experimental study on the iron loss of silicon steel was carried out.The different iron loss curves under various dc-biased magnetic field strength were determined. They are necessary for the analysis of dc bias magnetic field and electromagnetic design of products.
References
1. Z.Cheng, N.Takahashi, B.Forghani, et al. Analysis and measurements of iron loss and flux inside silicon steel laminations [J]. IEEE Trans. on Magnetics,2009, 45(3):1222-1225. 2. D. Miyagi, T. Yoshida, M. Nakano, et al. Development of measuring equipment of dc-biased magnetic properties using open-type single-sheet tester [J]. IEEE Trans. on Magnetics, vol. 42, no. 10, pp.2846-2848, Oct. 2006.
AV-02. Solution to the Problem of E-Cored Coil above a Layered Half-Space Using the Meth od of Truncated Region Eigenfunction Expansion
Fahimeh Sakkaki and Hossein Bayani
Physics, K.N.T. University of Technology, Tehran, Islamic Republic of Iran
A model of an axis symmetric E-cored coil in the presence of a layered conducting half space is analyzed. The boundary value problem is formulated in terms of the magnetic vector potential, which is expanded in a series of appropriate eigenfunctions. The unknown coefficients of the series are computed by solving a matrix system which is formed by applying the usual interface conditions. Closed-form expressions are presented for the induced eddy current density, the components of magnetic flux density and frequency variation of normalized impedance change. The results of the calculations are in very good agreement with results from COMSOL.The results represent larger signal of the E-cored coil (due to the concentration of the field), so this is their superiority over air-cored probes in specific applications.
References
[1]Theodoros P. Theodoulidis, Jour. Appl. Physics,93,5,(2003) [2]Hossein Bayani,Theodoros P. Theodoulidis,Ichiro Sasada, Electro. Nondestr. Eval.(X), IOP Press, p. 57-64, 2007
AV-03. Electrostatically tunable inductor with improved operational frequency and quality factor
Jing Lou1, Zhijuan Su1, Ming Liu2, Massimo Pasquale3 and Nian Sun1
1Electrical and Computer Engineering, Northeastern University, Boston, MA; 2Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL; 3Divisione Elettromagnetismo, INRIM, Torino, Italy
Inductors find widespread use in various applications. Inductors with large tunability would lead to new paradigm on circuit design, new electronic systems, etc. Traditional tunable inductors however, have the problems of small tunable range and high power consumption. Multiferroic materials with strong magnetoelectric (ME) coupling have led to many novel devices. Most recently, we have demonstrated that by using a ME composite of Metglas/PZT as the inductive core, it is possible to construct an electrostatically tunable inductor. However, due to the large thickness of the Metgals ribbons (~25um), excessive eddy current loss limits the operational frequency range in 100 kHz range, and suppresses the quality factor as well. In this paper, we investigated the effect of Metglas thickness on the performance of the tunable inductors. Different methods were used to reduce the thickness of the Metglas ribbons down to about 8um, which dramatically reduces the eddy current loss at higher frequencies. With zero electric field, the inductance is about 2uH at 1MHz, which reduced to 1uH with an electric field of 15 kV/cm. Compared to our previous results, the new tunable inductor show much flatter frequency response that extends its applicable range into MHz range. Meanwhile, the quality factor of the tunable inductor shows great improvement. This is a much better result compared to the previous ones, which makes such tunable inductor applicable in MHz applications.
References
1. C. W. Nan, M. I. Bichurin, S. X. Dong, D. Viehland and G. Srinivasan, J. Appl. Phys. 103, 031101 (2008). 2. J. Zhai, Z. Xing, S. X. Dong, J. F. Li, and D. Viehland, Appl. Phys. Lett. 88, 062510 (2006). 3. J. Lou, M. Liu, D. Reed, Y. Ren, and N. X. Sun, Adv. Mater., 21, 4711 (2009). 4. J. Lou, D. Reed, M. Liu, N. X. Sun, Appl. Phys. Lett. 94, 112508 (2009).
AV-04. A study on the characteristic of motor according to using slot wedge in Induction motor
Do-Jin Kim, Su-Jin Lee, Jae-Woo Jung and Jung-Pyo Hong
Hanyang University, Seoul, Republic of Korea
1. Introduction In order to install stator winding, the induction motors with large open winding slots are used in industry. Due to the large open winding slots, the motors tend to produce higher harmonic components in the air gap flux density. Because of this phenomenon, the core loss and acoustic noise are increased. The stator slot wedge is installed to reduce the core loss and acoustic noise. 2. Main subject The characteristics of motor are analyzed by changing dimension and relative permeability of wedge. Core loss is decreased when the higher harmonic components which are caused from slot of stator are decreased. However, the leakage flux is increased by installing wedge. To solve this problem, the dimension and permeability of wedge are determined by using the optimum design in Fig. 1. In order to analyze slot harmonic components, the electromagnetic exciting force which is calculated from air gap flux density is analyzed according to time and space. The components are reduced by installing stator wedge, and acoustic noise is also effected by this stator wedge. 3. Conclusion When magnetic wedge are applied in stator slot open, the air gap tends to decrease because change of the permeance are reduced. Accordingly, the dimension and material of stator wedge is determined by using the 2D FEM analysis and optimum design while considering characteristic of motor and reduction of acoustic noise of motor.
AV-05. Prediction of Iron Losses in Doubly Salient Permanent Magnet Machine with Rectangular Current Waveform
Jianzhong Zhang, Ming Cheng and Minxi Wang
Southeast University, Nanjing, China
The thermal dispersion of the doubly salient permanent magnet (DSPM) machine is favored as the stationary permanent magnets and armature winding may be cooled at the same time [1-2]. However, the iron losses in DSPM machine are difficult to predict, as the flux waveforms are complex and DC bias existed [3]. This paper uses loss separation method to predict the iron losses, where the factor of hysteresis loss, eddy-current loss and excess loss are calibrated from measured losses at no load in different rotor speed. Fig. 1 shows the prototype machine where two magnets can be pulled out and then the stray loss and mechanical loss may be measured in advance. The fitting results of iron losses are shown in Fig. 2 and parameters Kh=0.0089, ke=0.89e-5, kexc=1.18e-3, a=1.314 are determined. The iron losses at rated load are predicted by rectangular currents exert on the armature windings, as shown in Fig. 2. The results show most of the iron losses are located on the stator-side and rotor-side iron losses are not so high due to the pulse magnetic field on DSPM machine. By considering the good thermal dissipation for salient rotor structure, the over-heat would not happen if appropriate electrical load is selected.
References
[1] F. Blaabjerg, L. Christensen, P. O. Rasmussen, L. Oestergaard, P. Pedersen, “New advanced control methods for doubly salient permanent magnet motor,” IEEE IAS Annual Meeting, 1995, pp. 222-230. [2] M. Cheng, K.T. Chau and C.C. Chan, “Design and analysis of a new doubly salient permanent magnet motor,” IEEE Trans. Magn., vol. 37, no. 4, pp. 3012-3020, 2001. [3] J.T. Charton, J. Corda, A. Hughes, J.M. Stephenson and M.L. McClelland, “Modelling and prediction of iron loss with complex flux waveforms,” IEE Proc.-Electr. Power Appl., vol.152, no.4, 2005.
AV-06. Harmonic Analysis and Design of an Advanced Permanent Magnet Vernier Machine
Junhua Wang1, Jiangui Li2, Siu Lau Ho1, K.t. Chau2 and Weinong Fu1
1Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong; 2Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, Hong Kong
This paper utilized an analytical technique to predict the magnetic field of a new permanent magnet vernier (PMV) machine with outer rotor topologies and then the harmonic components are analyzed. The technique is based on two-dimensional models in polar coordinates and solves the governing Laplacian/quasi-Poissonian field equations in the airgap regions. Both of the no-load EMF and the electromagnetic torque performances obtained by analytical approach agree well with those predicted by FEM simulation and the experiments. The TS-FEM is applied to simulate the field distribution of the PMV machine under no-load and at two extreme positions, namely at 0° and 90°. It is noted that the magnetic field rotates a large angle when the rotor rotates only 3.75° mechanical degree. The corresponding airgap flux density waveforms and their harmonic spectra are shown in Figs for the radial and tangential airgap flux density of the proposed machine. The 3rd, 24th, 48th, 72th, 120th harmonics have prominent values due to the slot effect. The small tangential component is beneficial for tangential force, torque ripple, acoustic noise, etc., while the radial component is for the effective torque. The averaged peak values of the axial and tangential flux density are 0.42 T and 0.04 T. The averaged peak values of the axial and tangential flux density in the outer airgap of the magnetic-geared machine are 0.86 T and 0.31 T. Compared with the magnetic-geared machine, the performance of the proposed PMV machine improves greatly by integrating the FMPs together with the stator. Fig. 9 shows the no-load EMF waveforms of the proposed PMV machine and the magnetic-geared machine. The root mean square (RMS) values of the proposed machine and the magnetic-geared machine are 71.7 V and 60.4 V. Fig. 10 shows the electromagnetic torque waveforms of both machines under the locked-rotor operation. The peak value of steady-state torque of the proposed machine and the magnetic-geared machine are 52.7 Nm and 46 Nm, respectively. Fig. 11 shows the measured no-load EMF waveforms of the proposed PMV machine and the steady-state current and torque. The figures will be presented in full paper.
References
[1] S. Taibi, A. Tounzi, and F. Piriou, “Study of a stator current excited vernier reluctance machine,” IEEE Trans. Energy Convers., vol. 21, no. 4, pp. 823-831, 2006. [2] J. G. Li, K.T. Chau, J. Z. Jiang, C. H. Liu and W. L. Li, “A new efficient permanent-magnet vernier machine for wind power generation,” IEEE Trans. Magn., vol. 45, no. 6, 2010, pp. 1475-1478. [3] A. Toba, and T. A. Lipo, “Generic torque-maximizing design methodology of surface permanent-magnet vernier machine,” IEEE Trans. on Industry Appl., vol. 36, no. 6, 2000, pp. 1539-1546. [4] Z. P. Xia, Z. Q. Zhu, and D. Howe, “Analytical magnetic field analysis of halbach magnetized permanent-magnet machines,” IEEE Trans. Magn., vol. 40, no. 4, pp. 1864-1872, Jul. 2004.
AV-07. Improved Thrust Calculations of Active Magnetic Bearings considering Fringing Flux
Seok-Myeong Jang1, Kwan-ho Kim1, Kyong-Jin Ko1, Ji-Hwan Choi1 and Sung-Ho Lee2
1Chungnam Nati onal uuiversity, Daejeon, Republic of Korea; 2Korea Institute of Industrial Technology Gwangju Reserch Center, Gwangju, Republic of Korea
Magnetic bearings (MB) can suspend rotor shafts by electromagnetic force without mechanical contacts and lubrication [1]. This paper deals with improved thrust calculations of Active Thrust Magnetic Bearings (ATMB). Fig. 1(a) shows the analysis model of ATMB. There are two methods how to analyze static characteristic of MBs: one is an Equivalent Magnetic Circuit (EMC) method, and the other is a Finite Element Method (FEM). In case of analyzing static characteristics, the EMC method is more beneficial than FEM in terms of analysis time, but it is more disadvantageous than FEM in terms of accurate calculation of magnetic field distribution. Therefore, this paper suggests an equation considering fringing effect at air-gap to overcome the defect of EMC. As described in Fig. 1(b), fringing effect is the phenomena which extending flux line at air-gap toward outside. As a result, the magnetic force calculated by EMC has higher value when it does not consider fringing effect. To verify the consideration, FEM and experiment were performed and those results were compared. Fig. 1(c) shows equipment which measures magnetic force follows interval change between air-gap and input current of ATMB for verifying analytical result. Fig. 2 shows the analytical result of static characteristic obtained from EMC, FEM and experiment. Analytical result of conventional EMC has about 20% of err span when it compared with FEM, while it calculated with fringing effect, the result err span is diminished about 2%. From these results, it could be proved that EMC suggested in this paper can content both fast analysis time and high reliability when it is applied to static characteristic analysis of ATMB.
References
[1]A. Chiba, T. Fukao, O. Ichikawa, M. Takemoto, and D. G. Dorrell, Magnetic Bearings and Bearingless Drives. U.K.: Newnos, 2005, pp. 1-15.
AV-08. A Model of Linear Synchronous Motor Based on Distribution Theory
Marco Trapanese
Dipartimento di Ingegneria Elettrica, Elettronica e delle Telecomunicazioni, Università di Palermo, Palermo, Italy
The fundamental idea of this paper is to use the distribution theory to analyse linear machines in order to include in the mathematical model both ideal a non ideal features. The fundamental difference between rotating and linear machines is that the former have periodic structures and the latter do not. However, despite to the fact that linear machines do not have a periodic structure, the mathematical tools generally used to describe and design them are based on Fourier analysis, see for example [1]. End and side effects are generally included in the theory, but especially in long machines this approach introduces a sort of periodic boundary condition in the description of the machine that limits in somehow the analysis: as an example in this kind of analysis the role of the displacement of the armature coils from their ideal position as well as the error in the position of the field coils cannot be taken into account straightforwardly. On the contrary, an approach based on stochastic analysis can allow to obtain a deeper analysis. This paper shows how the use of distribution theory can be used in order to establish a mathematical model able to describe both the ordinary working condition of a Linear Synchronous Motor (LSM) as well the role of the unavoidable irregularities and non-ideal features. The model developed allows to compute the thrust provided by the motor. The model include both side and end effects as well as the effect of errors in the placement of the armature coils. The model is v alidated against a FEM analysis.
References
[1] G. Stumberger, D. Zarko, M.T. Aydemir, T.A. Lipo, “Design and Comparison of Linear Synchronous Motor and Linear Induction Motor for Electromagnetic Aircraft launch System”, Research Report 2003-14, Wisconsin Electric Machines & power Electronics, Consortium, 2003. [2] M.Trapanese,, “The influence of the stochastic features of the energy source on the design of an electromagnetic generator”, Journal of Applied Physics, Volume 105, Issue 7, 2009, Article number 07F120.
AV-09. Design Verification of Electromagnet for Magnetic Levitation Systems through Static and Dynamic Analyses
Jang-Young Choi, Hyeon-Jae Shin and Seok-Myeong Jang
Chungnam National University, Dae-jeon, Republic of Korea
The magnetic levitation (maglev) train is one of the best candidates for a new-generation transportation system [1-2]. Therefore, a study on the maglev train system has been conducted. In particular, even more attention is paid to the design and control of the levitation systems to improve its reliability. In this paper, we focus on the design verification of the electromagnet for the magnetic levitation system through static and dynamic analyses. First, on the basis of the results obtained from the equivalent magnetic circuit (EMC) method (see Fig. 1(a)) and the 3D finite element (FE) analyses (see Fig. 1(b)), the initial and detailed designs of the electromagnet are created. The actual electromagnet manufactured by using the designs is shown in Fig. 1(c). Next, the block diagram for dynamic analysis, which includes a PID controller, is obtained by using the voltage and motion equations; it is shown in Fig. 2(a). In particular, by estimating the exact control parameters which considering the delay, an exact air-gap can be maintained under a rated current without considering the increase in the winding temperature and the saturation of electromagnets, as shown in Fig. 2(b). More detailed analysis results and discussions will be presented in the final paper.
References
[1] H. W. Lee, K. C. Kim and J. Lee, “Review of maglev train technologies,” IEEE Trans. Magn. vol. 42, no. 7, pp.1917-1925, 2006. [2] H. Weh and M. Shalaby, “Magnetic levitation with controlled permanentic excitation,” IEEE Trans. Magn., vol. 13, no. 5, pp.1409-1411, 1977.
AV-10. New Linear Fault-Tolerant Permanent-Magnet Motor for Levitation Applications
Wenxiang Zhao1, Ming Cheng2, K. Chau3 and Ruiwu Cao2
1School of Electrical and Information Engineering, Jiangsu University, Zhenjiang, China; 2School of Electrical Engineering, Southeast University, Nanjing, China; 3Department of Electrical and Electronic Engineering, University of Hong Kong, Hong Kon, China
Magnet technology has played a quite important role in the magnetic levitation (Maglev) train, which is one of the best candidates for new generation transportation system [1]. Induction motors have been developed for linear transportation applications [2], but it still suffers from relatively low power density and efficiency. The conventional permanent-magnet (PM) brushless motors with linear topology have gained more and more attention [3]. However, due to the fact that the PMs or the windings should be located in the long stator, they suffer from the significant high cost. Recently, a new class of rotary motors with PMs located in the stator, so-called the stator-PM motors, has been proposed [4]. These stator-PM motors can offer high power density, strong mechanical integrity and good immunity from the thermal problem. On the other hand, the continual operation of motor drives is extremely important for the high-speed maglev train [5]. A new primary-PM motor was developed by employing the flux barriers in the primary, but it reduced the power density of the motor drive [6]. The purpose of this paper is to propose a new linear fault-tolerant primary-PM motor, termed as fault-tolerant primary-PM (FTPPM) motor, for the high reliability maglev transportation system. The key is the new linear motor topology which introduces the concept of fault-tolerant teeth into the primary-PM motors in order to provide the desired decoupling among phases. By using the finite-element method (FEM), the electromagnetic performances of the proposed FTPPM motor are analyzed, including the flux linkages, back-EMFs, inductances and torques. Also, the co-simulation technique is employed to assess the time-domain performance of the whole drive system, including the proposed motor, power inverter and mechanical load. The analysis results show that the proposed FTPPM motor incorporates the merits of high power density, strong mechanical integrity and low cost. Finally, the experimental results confirm the validity of the proposed motor drive. This work was supported in part by National Natural Science Foundation of China (Project No 50907031).
References
[1] L. Yan, ‘The Linear Motor Powered Transportation Development and Application in China’, Proceedings of the IEEE, vol. 97, no. 11, pp. 1872-1880, November 2009. [2] A. Z. Bazghaleh, M. R. Naghashan, and M. R. Meshkatoddini, “Optimum Design of Single-Sided Linear Induction Motors for Improved Motor Performance,” IEEE Transactions on Magnetics, vol. 46, no. 11, pp. 3939-3947, May 2010. [3] J. Wang, W. Wang, and K. Atallah, “A Linear Permanent-Magnet Motor for Active Vehicle Suspension,” IEEE Transactions on Magnetics, vol. 60, no. 1, pp. 55-63, May 2011. [4] W. Hua, M. Cheng, Z. Q. Zhu, W. Zhao and X. Kong, “Comparison of electromagnetic performance of brushless motors having magnets in stator and rotor,” Journal of Applied Physics, vol. 103, no. 7, 07F124, pp. 1-3, March 2008. [5] W. Zhao, M. Cheng, W. Hua, H. Jia and R. Cao, “Back-EMF harmonic analysis and fault-tolerant control of flux-switching permanent-magnet machine with redundancy,” IEEE Transactions on Industrial Electronics, vol. 58, no. 5, pp. 1926-1936, May 2011. [6] M. Jin, C. Wang, J. Shen, and B. Xia, “A Modular Permanent-Magnet Flux-Switching Linear Machine with Fault-Tolerant Capability,” IEEE Transactions on Magnetics, vol. 45, no. 8, pp. 3179-3186, August 2009.
AV-11. Characteristic and Magnetic Field Analysis of a HTS Axial-Flux Coreless Induction Maglev Motor
Qin Wei, Fan Yu, Li Guo Guo, Fang Jin and Lv Gang
Electrical Engineering, Beijing Jiaotong University, Beijing, China
In this paper, a new High Temperature Superconductor Axial-Flux Coreless Maglev Motor(HTS AFIM)is proposed, of which the primary windings are made of HTS tapes and the secondary is Non-magnetic conductor. Main works of this paper are the magnetic-field computation and characteristics analysis of HTS AFIM. For the first one, In order to analyze the mechanical characteristics, 2-D electro-magnetic model of this new device is established. In particular, the reduction of magnetic fields near outer and inner radius of the HTS AFIM is solved by introducing sub-loop electro-magnetic model along radial position. For the second one, According to the characteristic of the HTS coils, we propose a new structure of the primary windings, of which the AC losses and critical current are calculated under th e different structural parameters. The relationships between device’s characteristics and device parameters are presented, the results indicate that under certain frequency and current levitation device can output enough lift force, however, the secondary loss is inevitable. The conclusions are verified by finite element calculations. The predictions are shown in good agreement with those obtained from 3-D finite element analyses (FEA).
AV-12. Modeling and Analysis of a Magnetically Levitated Synchronous Permanent Magnet Planar Motor
Baoquan Kou, Lu Zhang and Liyi Li
Harbin Institute of Technology, Harbin, China
With the challenge of the shrinking dimensions in the fabrication process, demands are growing for higher precision and more powerful positioning device to be used in integrated circuits. As a result, magnetically levitated synchronous permanent magnet planar motors(SPMPM) are developed to instead of conventional two-dimensional (2-D) positioning device with cumbersome stacked linear motors. The magnetically levitated synchronous permanent magnet planar motors have many advantages such as direct driving, low friction, no backlash, high accuracy, and the most important is that they can operate in vacuum, for example in extreme-UV lithography equipment. This paper introduces a new magnetically levitated synchronous permanent magnet planar motor which is driven by composite currents. The mover of the planar motor consists of a coil array and the stator consists of 18 magnets. The coil pitch τt and permanent magnet pole pitch τp satisfy the following relationship 3nτt=(3n±1)τp. In this research, we obtain an analytical model of the planar motor, the flux density distribution of a two-dimensional magnet array for the SPMPM was obtained by solving the equations of the scalar magnetic potential. And the expression of the maglev force induced by magnetic field and composite current was derived. To verify the model and the maglev force calculation method, the finite element method (FEM) is used for calculating the electromagnetic forces of an example SPMPM. And the results from FEM are in good agreement with the results from the analytical equations. This indicates that the model can be used for calculating the value of the maglev force in a magnetically levitated synchronous permanent magnet planar motor.
AV-13. Stabilization of Input Impedance for Wireless Power Supply Circuit
Takahiro Misawa1, Tadakuni Sato2, Tetsuya Takura1, Fumihiro Sato1 and Hidetoshi Matsuki2
1Graduate of engineerring, Tohoku University, Sendai, Japan; 2Graduate of biomedical engineering, Tohoku University, Sendai, Japan
There are some types of wireless power supply technique; electromagnetic induction, electromagnetic radiation, electrical conduction, and, etc. In these methods, we have researched electromagnetic induction type. Now, many engineers are developing battery chargers employed this technique for consumer products. We assume that a load fluctuates during battery charge because the battery voltage fluctuates at that time. Therefore, it is preferable that an input impedance of transmission circuit don’t fluctuate under the influence of the load fluctuation for stabilization of an electrical power supply. We suggest a transmission circuit which has this characteristic. Composing resonant circuit make it possible to achieve efficient power transmission. We research the series-parallel resonant circuit that has less input impedance fluctuation when the load fluctuates. This circuit is shown in figure 1. Adjustment of series and parallel capacitors according to certain load enable us to gain max transmission efficiency. To research the effectiveness of series-parallel resonant circuit for the load fluctuation, we actually calculated and measured the input impedance of the circuit. At this time, a coupling coefficient between a primary coil and a secondary coil is constant value. The result is shown in figure 2. This result shows that the input impedance is constant value when the value of optimum load is 4.7 ohm. It means we are able to design the circuit which has constant value of input impedance under the influence of load fluctuation and achieve stabilization of the electrical power supply.
AV-14. A New Dual Output Phase-shift Distribution Transformer- Input Harmonic Current Mitigating Performance
Jian Guo1, Ping Jin2 and Shuhua Fang2
1School of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China; 2School of Electrical Engineering, Southeast University, Nanjing, China
Electrical systems with conventional transformers are generally not suited to handle the harmonics generated by non-linear loads and have many power quality problems. One of the problems is that transformer losses increase due to the higher frequency harmonic currents [1]. The dual output phase-shift transformer (DOPST) is specifically designed to treat the harmonics generated by non-linear loads. The purpose of this paper is to identify the turns selection, connection mode and structure design of DOPST windings, and analyze the harmonic mitigating preformance under different non-linear loads. Fig. 1(a) shows the secondary windings connection diagram of DOPST. The zigzag connection is used to weaken the zero sequence flux. Fig. 1(b) is the turns match diagram. The winding turns meet W11=W12=1.732N2 in one output and W21=2W22=2W22=2N2/3 in the other, where N2 is the equivalent turns of the secondary winding. As a result, there exists a 30 degree phase difference between the output voltages of the two secondary as shown in the Fig. 1(c). It is noted that the 5th, 7th, 17th, and 19th harmonics MMF of the two output windings are opposite. Combining zero sequence flux cancellation with phase shifting treats the 5th, 7th, 17th, 19th, and triple harmonics within its secondary windings and reduces the primary windings loads. The harmonic mitigating performance of a designed 20kVA DOPST is analyzed by an improvement of the field-circuit coupled method proposed in [2]. The typical non-linear load harmonic spectrums and waveforms are obtained which show that DOPST can provide extremely low primary current distortion even under severe non-linear loading conditions.
References
[1] MIRUS Harmony™ Series Technical Guide , http://www.mirusinternational.com. [2] Guo Jian , Lin Heyun. Calculation and Analysis of Branch Currents of Single Spiral Winding Transformer Based on Field Circuit Coupled Method [J]. Transactions Of China Electrotechnical Society, 2010, 25(4): 65-70.
AV-15. Transient Analysis and Control of Bias Magnetic State in the Transformer of On-Line PWM Switching Full Bridge DC-DC Converter
Jiaxin Chen1, Youguang Guo2, Jianguo Zhu2 and Zhi Wei Lin2
1College of Electromechanical Engineering, Donghua University, Shanghai, China; 2University of Technology Sydney, Sydney, NSW, Australia
Compared with other pulse-width-modulation (PWM) switching DC-DC converters, the PWM switching full bridge converter has the highest power density and is often adopted as the main topology circuit of power electronic device. However, the bias magnetic state of the full bridge converter must be measured and controlled in time to ensure that the transformer would not be driven to saturation at any operation condition [1]. Some efforts [2, 3] have been made to handle this problem, but much improvement is needed on accuracy, efficiency, reliability and flexibility. This paper presents the transient analysis and control of the bias magnetic state in the transformer of an on-line pulse-width-modulation (PWM) switching full bridge DC-DC converter, which has a rated output power of 1.5 kW, input DC voltage of 360-400 V, output DC voltage of 20-80 V, and maximal output current of 30 A. Firstly, a general method based on both 2-dimensional finite element analysis and dynamic modeling technique is introduced to predict the transient bias magnetic state in the converter transformer. Then a novel transformer model which can address not only its basic input-output characteristic, but also the nonlinear magnetizing inductance, is proposed. Both the asymmetric characteristic and the variable laws of the current flowing through the two secondary windings during the period of PWM switching-off state are derived, and they are verified by both theoretical analysis and simulation results. Finally, the peak magnetizing current controlled method based on the on-line magnetizing current computation is introduced. The most valuable part of the proposed method is that it can address the magnetic saturation at winding ends, and hence many previously difficult processes, such as the start up process and asymmetry of power electronics, can now be controlled easily. The complexity of system realization might be simplified and the reliability of the converter is enhanced. The detailed analysis and experimental verification will be given in the full paper.
References
[1] Q. Li, F.C Lee, and M.M. Jovanovic, “Design considerations of transformer DC bias of forward converter with active-clamp reset”, in Proceedings of 14th Annual Conf. on Power Electronics Conference and Exposition, 1999, pp. 553-559. [2] L. Zeng, Z. Zhu, B. Bai, and Y. Song, “Research on influence of DC magnetic bias on a converter transformer”, in Proceedings of Int. Conf. on Electrical Machines and Systems, 2007, pp. 1346-1349. [3] D. Jia, T. Xu, Z. Ju, and Q. Niu, “Strategy on eliminating transformer bias magnet in push-pull forward”, in Proceedings of the Asia-Pacific Power and Energy Engineering Conference, 2009, pp. 1-4.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Hermann Stoll, Max Planck Institute for Intelligent Systems
AW-01. Electrical detection of antivortex wall dynamics
Mahdi Jamali, Jae Hyun Kwon, Kulothungasagaran Narayanapillai and Hyunsoo Yang
Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore
We have studied for the first time the antivortex dynamics using the electrical measurement techniques.We have fabricated a ferromagnetic nanostructure made of Permalloy using e-beam lithography as shown in Fig. 1(a). It is found that above 127 mT perpendicular field, the antivortex nucleated in the structure as confirmed by the MFM image as well as by micromagnetic simulations in Fig. 1(c) & (d). For the measurements of the antivortex resonance frequency, we have used the concept of the domain wall diode effect. Fig. 1(b) shows the measurement of the domain wall dynamics during of the antivortex nucle ation. The spectrum of the output signal suddenly changes and certain peaks appear that indicate the different modes of the antivortex wall. We also report the time domain response of an antivortex wall with a short pulse excitation.
AW-02. Dimensional transition of current driven domain-wall dynamics
Kab-Jin Kim1, 2, Jae-Chul Lee1, 3, Sang-Jun Yun1, Gi-Hong Gim1, Kyung-Ho Shin3 and Sug-Bong Choe1
1Department of Physics, Seoul National University, Seoul, Republic of Korea; 2Institute for Chemical Research, Kyoto University, Kyoto, Japan; 3Center for Spintronics Research, Korea Institute of Science and Technology, Seoul, Republic of Korea
The magnetic domain wall (DW), due to its simple geometrical structure, has been regarded as model systems of driven interfaces which have abundant physical properties. Especially when the driving force is much weaker than its critical value, the dynamics of DW exhibits scaling behaviors which only depend on symmetry and dimensions. In our previous works [1], we demonstrated that the DW dynamics changes from 2-dimensional (2D) to 1-dimensional (1D) behavior as the wire width decreases. It was done with the driving force induced by a magnetic field. It is thus natural to examine whether the dimensional transition subsists when the driving force is the current. Here, we report an experimental demonstration of dimensional transition in current driven DW dynamics. To do this, purely current-driven DW motion is achieved in Pt/Co/Pt nanowires with perpendicular magnetic anisotropy. From the current-driven DW motion, we observed that the dynamics also changes from 2D creep to 1D hopping behavior [Fig.1]. It demonstrates that the DW motions driven by either electric current or magnetic field have same physical core dynamics. It also implies that current and magnetic field belong to the same universality class in metallic Pt/Co/Pt ferromagnet [2].
References
[1] Kab-Jin Kim, et al. Nature, 458, 740 (2009) [2] Jae-Chul Lee, et al. arXiv. 1006.1216 (2010)
AW-03. Dynamics of Interlayer Coupled Magnetic Vortex Pairs
Sebastian Wintz1, Christopher Bunce1, Michael Körner1, Thomas Strache1, Jörg Raabe2, Christoph Quitmann2, Jeffrey McCord3, Artur Erbe1 and Jürgen Fassbender1
1Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 2Paul Scherrer Institut, Villigen, Switzerland; 3Christian-Albrechts Universität zu Kiel, Kiel, Germany
A magnetic vortex structure consists of a planar magnetization curl with a perpendicularly magnetized nanoscopic core in its center. As a consequence of the different combinations possible for the curl’s rotation sense (circulation: c ∈ {+1,-1}) and the orientation the core (polarity: p ∈ {+1,-1}), magnetic vortices occur with opposite handednesses. When excited by magnetic fields or spin polarized currents, magnetic vortices exhibit different kinds of eigen modes of which the gyrotropic core mode is most prominent. It corresponds to an orbiting of the core around its equilibrium position, where the sense of motion is determined by p as a topological charge only[1,2]. The static and dynamic properties of single layer vortices have been the subject of numerous fundamental investigations during the past decades[e.g.1,2,3], which also led to the proposal of their application for memory devi ces[4] and spin-torque oscillators[5]. On the technological context as well as from a basic perspective, the coupling between spatially confined vortices is a key issue. Here we report on the magnetization dynamics of coupled vortex pairs, separated by a nonmagnetic spacer in a stacked geometry. Besides magnetodipolar interaction, interlayer exchange coupling (IEC) can be present in such a system, depending on the spacer material and thickness. We have experimentally identified two nongenerate configurations for both, IEC and purely dipolar coupled vortex pairs regarding their relative circulation orientations by means of scanning transmission x-ray microscopy (STXM)[6]. Time-resolved STXM furthermore allows to probe the layer-specific response of coupled vortex pairs to magnetic excitations. By this, we have observed a principally different core gyration behavior for the different circulation configurations. Also, a collective reorientation of the core polarities has been detected for sufficiently strong excitations.
References
[1] D. L. Huber et al., J. Appl Phys. 53, 1899 (1982).
[2] S.-B. Choe et al., Science 304, 420 (2004).
[3] B. Van Waeyenberge et al., Nature 444, 461 (2006).
[4] N. Kickuchi et al., IEEE Trans. Magn. 37, 2082 (2001).
[5] A. Ruotolo et al., Nat. Nanotechnol. 4, 528 (2009).
[6] S. Wintz et al., Appl. Phys. Lett. 98, 232511 (2011).
AW-04. Domain Wall Motion and Interactions in Multi-Nanowire Systems
Andrew Kunz, Rebecca D. McAuliffe, Daniel V. Olson and Kyle F. Kimminau
Physics, Marquette University, Milwaukee, WI
The motion of a domain wall through a single ferromagnetic nanowire has been investigated for potential use in magnetic storage and sensing systems. Whether driven by currents or external fields, in many of these devices a domain wall is going to be necessarily moved in the presence of other wires which may contain domain walls near the region of motion. Micromagnetic simulation has been used to investigate the interaction of a moving domain wall in the presence of neighboring ferromagnetic wires. In general the relative domain orientation with respect to that of the neighboring wire(s) can significantly impact domain wall speed and the ability to travel the wires length. When a domain wall is driven near a neighboring wire with uniform magnetization the wall speed can be enhanced or diminished depending on their relative magnetic structures. The speed differences can be amplified more neighboring wires. There is no change in the Walker breakdown field which implies that the neighboring wires are not just supplying additional field to account for the speed change. If the neighboring wire(s) contain domain walls then we find that the relative orientation of the interacting domain wall structures in each wire is critical. In a long, thin wires there are only four possible primary transverse wall orientations and we have investigated the interaction of each of the 16 possible interactions as a function of wire separation. In each case a domain wall in one wire is trapped and the wall in the other wire is free. In dynamic simulations the free wall is driven by an external field toward the trapped wall at which point it may pass, or become trapped. The simulations show different behaviors for the different orientations and wire spacing ranging from strong attraction to strong repulsion with many systems exhibiting both attractive and repulsive behaviors. A detailed look at the ferromagnetic domain structures during the interaction can be used to understand the behavior which impacts the necessary wire and domain wall geometries necessary to move, and pin domain walls reliably. This work is supported by the National Science Foundations (DMR-1006947) and the NASA Wis consin Space Grant Consortium.
AW-05. Stepwise behavior of gyrovector in magnetic vortex dynamics under AC magnetic field
Je-Ho Shim1, Hong-Guang Piao1, 2, Sang-Hyuk Lee1, Suhk Kun Oh1, Seong-Cho Yu1, Seung Kee Han1 and Dong-Hyun Kim1
1BK-21 Program and Department of Physics, Chungbuk National University, Cheongju, Republic of Korea; 2College of Science, Huaihai Institute of Technology, Lianyunguang, China
Numerous studies have been devoted to understanding dynamic properties of the magnetic vortex particularly related to the dynamic excitation with a gyrotropic motion with a relatively small amplitude of oscillations[1]. In this work, we have investigated a magnetic vortex core motion in a ferromagnetic disk under an AC external magnetic field by means of micromagnetic simulation[2], where we have found that there is a strong correlation between the gyrovector of the Thiele’s equation[3] and the vortex core radial position from the disk center. The exchange coefficient, saturation magnetization and damping constant of the simulation are 1.3 x 10-11 J/m, 8.6 x 105 A/m, and 0.01, respectively.. The AC field strength was varied from 1 to 2.5 mT and the frequency was varied from 50 MHz to 1 GHz. It has been observed that the assumption of the rigid magnetic vortex structure becomes less valid for the large amplitude excitational gyrotropic motion of the vortex core around the resonance frequency. To better describe the vortex core motion with a slight modification from the rigid structure, we use the dynamic correction of the gyrovector of the Thiele’s equation. Simple correlation between the gyrovector and the vortex core radial position is demonstrated in Fig. 1(a). Very interesting point is that the vortex core radial position from the disk center and thus, the dynamically corrected gyrovector exhibits a stepwise behavior during the excitational gyrotropic motion for most of AC field frequencies, as shown in Fig. 1(b). The stepwise behavior seems to be originated from the interaction between the vortex structure and the spin wave modes in the disk structure.
References
[1] S.-H. Jun, J.-H. Shim, S. K. Oh, S.-C. Yu, D.-H. Kim, B. Mesler, and P. Fischer, Appl. Phys. Lett. 95, 142509 (2009). [2] M. J. Donahue and D. G. Porter, OOMMF User's Guide, from http://math.nist.gov/oommf (2002). [3] A. A. Thiele, Phys. Rev. Lett. 30, 230 (1973).
AW-06. Magnetization switching via surface acoustic waves in Co stripes
Brian B. Maranville1, Shireen Adenwalla2, Davis K. Sam2 and Julie A. Borchers1
1NIST Center for Neutron Research, Natl Inst of Standards & Tech, Gaithersburg, MD; 2Physics, University of Nebraska - Lincoln, Lincoln, NE
We have previously demonstrated magnetization switching in rectangular micron sized ferromagnetic structures using the magneto-elastic (ME) effect, the coupling between strain and magnetization direction[1]. High frequency (~100 MHz) strain is created using a surface acoustic wave (SAW) device, resulting in magnetization switching between the long easy axis to the short hard axis as a function of applied strain. Here we present neutron reflectivity and MOKE data on a similar sample albeit with much higher shape anisotropy, with an array of Co elements (stripes) that are 25000 X 10 μm2 periodically patterned between two interdigital transducers. Polarized neutron reflectivity (PNR) is an ideal tool for this experiment, enabling us to measure the contribution of domain motion vs uniform rotation in these systems. Although PNR will see only the dc (non-zero) average of the fast magnetization response, this average is predictably dependent on the applied strain if we assume coherent rotation. We present spin-flip (SF) and non-spin-flip (NSF) PNR data, in which the sample is magnetized along the easy axis (long axis of stripes, in-plane) and then neutron reflectivity is measured with the neutron polarization along the in-plane hard axis (across width of stripes). In this configuration, uniform rotation of the magnetization results in spin-splitting of the NSF channels, while a non-uniform response, in which different sub-regions of the sample rotate independently, would decrease the spin-flip scattering while not affecting the spin-splitting. Because the neutron beam is parallel to the stripes, the data must be analyzed as an incoherent sum of the reflectivity of the bare substrate between the stripes, and the stripes themselves. In addition, only a fraction of the stripe material may be rotating with the strain induced by the SAW. Results show an increase in the spin-splitting when the amplitude of the SAW is increased, but still well below the splitting for the saturated film, consistent with a coherent rotation of the magnetization but of only a small part of the stripes. Together with Kerr effect measurements these show that coherent rotation is the dominant mechanism.
References
[1] Magnetization Dynamics triggered by Surface Acoustic Waves, S. Davis, A. Baruth and S. Adenwalla, Applied Physics Letters, 97, 232507 (2010).
AW-07. Condition of the Ratchet Effect of a Magnetic Domain Wall Motion under an Asymmetric Potential Energy
Hong-Guang Piao1, Hyeok-cheol Choi2, Dong-Hyun Kim3 and Chun-Yeol You2
1College of Science, Huaihai Institute of Technology, Lianyungang, China; 2Physics, Inha University, Incheon, Republic of Korea; 3Physics, Chungbuk National University, Cheongju, Republic of Korea
Recently, possibility of magnetic diode based on ratchet effect of DW motion has been numerically and experimentally proposed for ferromagnetic wires with asymmetric artificial defects[1,2]. Fabrication of modulated wires with uniformly sized artificial defect array is a challenging issue, since DW structure is affected by wire geometry during DW propagation. In this work, we numerically investigated a consecutive operation process of DW diode for a structure-stable DW in straight CoFe wire under AC field with asymmetric potential array. The asymmetric potentials are introduced with asymmetric distribution of magnetostatic stray field by non-contact trapezoidal stubs(Fig.1a). We found that the consecutive operation process is only conditionally achieved(Fig.1b), which might be ascribed to the fact that there exists a correlation between spatial period of the potentials and the AC field frequency/strength. To find the best ratchet effect condition, we systematically examined the diode-like DW behaviors in this scheme with changing the AC field frequency/strength.
References
[1]H.-G.Piao,et al.,IEEE Trans.Magn.46,1844(2010). [2]A.Himeno,et al.,J.Appl.Phys.103,07E703(2008).
AW-08. Magnetic bubble nucleation and dynamics driven by spin-transfer torque
Giovanni Finocchio1, Alessandro Prattella1, Luis Torres2, Stavros Komineas3, Ozhan Ozatay4 and Bruno Azzerboni1
1Fisica della Materia e Ingegneria Elettronica, University of Messina, Messina, Italy; 2Universidad de Salamanca, Salamanca, Spain; 3University of Crete, Heraklion, Greece; 4Bogazici University, Instambul, Turkey
We demonstrate the possibility to nucleate a magnetic dynamical bubble[1] in a confined structure (nano-point contact geometries with circular cross sectional area (250 nm of diameter)) by means of the local injection of a spin-polarized current [2]. Those configurations are related to the presence of a limit cycle in the energy landscape. In presence of an in-plane external field large enough, it is possible to achieve microwave (in the range on 2-3 GHz) giant-magnetoresistive (GMR) signal from persistent magnetization precession (we studied in detail the dynamical properties (oscillation frequency and power) of those non-uniform mode). Our results indicate that the magnetization dynamics is characterized by a rotation of the in-plane magnetization component near the dynamical bubble boundary and from expansion/compression of the dynamical bubble itself. Similarly to what theoretically observed in point-contacts [4] we also find hysteresis, in particular the dynamical bubble is nucleated for a J_ON current and remains stable up to a J_OFF smaller than J_ON. For some range of thickness of the layer containing the dynamical bubble, it is possible to have a static bubble configuration also at zero current.
References
[1] C. Moutafis, S. Komineas, and J. A. C. Bland, Phys. Rev. B 79, 224479 (2009). [2] G. Finocchio, O. Ozatay, L. Torres, R. A. Buhrman, D.C. Ralph, B. Azzerboni, Phys. Rev. B 78, 174408 (2008). [3] S. Girod, M. Gottwald, S. Andrieu, S. Mangin, J. McCord, Eric E. Fullerton, J.-M. L. Beaujour, B. J. Krishnatreya, and A. D. Kent, App. Phys. Lett. 94, 262504 (2009). [4] M. A. Hoefer, T. J. Silva, and M. W. Keller, Phys. Rev. B 82, 054432 (2010).
AW-09. Generation of Domain Walls in Permalloy Nanowires by Local Magnetic Fields
Falk-Ulrich Stein, Lars Bocklage, Michael Martens, Toru Matsuyama and Guido Meier
Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg, Hamburg, Germany
The need for ever faster non-volatile memory has been motivating research on magnetic materials over decades. In recent years the attention focussed on the investigation of domain walls in nanowires for an application in a shift register, the so called racetrack memory [1]. The field and current-driven propagation of domain walls along nanowires is the core theme in most investigations, at which the domain walls were mostly created by a global external magnetic field [2,3]. For devices such external fields are improper because of their low efficiency, long switching times and overall impact on the magnetic structure. The nucleation of domain walls by the Oersted field of a strip line crossing the nanowire has been used instead [4]. While this method generates domain walls with high probability, the magnetization dynamics during the nucleation have not further been studied. We investigate the creation of domain walls by local field pulses of a few nanoseconds. The field pulses can create or annihilate domain walls in the permalloy nanowire. Quasistatic measurements with different pulses and external fields are compared to micromagnetic simulations to gain insight into the magnetization dynamics during the nucleation. We present measurements of stochastic field-pulse induced domain-wall formation and annihilation. From those results the strength and duration of the local fields are gained, which are required for the creation. The reliable creation and annihilation wit h pulses of different polarity is achieved without the need of an external field. The domain wall nucleation itself is investigated by micromagnetic simulations. A good understanding of the domain wall dynamics and especially an efficient switching with low current densities will be crucial for efficient memory devices. Financial support by the Deutsche Forschungsgemeinschaft via the SFB 668 and the GrK 1286 as well as the Forschungs- und Wissenschaftsstiftung Hamburg via the Exzellenzcluster “Nano-Spintronik” is gratefully acknowledged.
References
[1] S.S.P. Parkin, M. Hayshi, and L. Thomas, Science 320, 190 (2008) [2] G. Meier, M. Bolte, R. Eiselt, B. Krüger, D.-H. Kim, and P. Fischer, Phys. Rev. Lett. 98, 187202 (2007) [3] L. Bocklage, B. Krüger, T. Matsuyama, M. Bolte, U. Merkt, D. Pfannkuche, and G. Meier, Phys. Rev. Lett. 103, 197204 (2009) [4] M. Hayashi, L. Thomas, C. Rettner, R. Moriya and S.S.P. Parkin, Nature Phys. 3, 21 (2007)
AW-10. Direct Imaging of Precessional Domain Wall Propagation in Ferromagnetic Rings Induced by Circular Magnetic Fields
Andre Bisig1, 2, Martin Stärk1, 3, Christoforos Moutafis1, 3, Jan Rhensius3, 4, Jakoba Heidler1, Michael Curcic2, Edward Amaladass2, Matthias Noske2, Markus Weigand2, Tolek Tyliszczak5, Bartel Van Waeyenberge6, Laura J. Heyderman4, Hermann Stoll2, Gisela Schütz2 and Mathias Kläui1, 7
1SwissFEL, Paul Scherrer Institut, Villigen PSI, Switzerland; 2Moderne Magnetische Materialien, Max-Planck-Institut für Intelligente Systeme, Stuttgart, Germany; 3Fachbereich Physik, Universität Konstanz, Konstanz, Germany; 4Labor für Mikro- und Nanotechnologie, Paul Scherrer Institut, Villigen, Switzerland; 5Advanced Light Source, LBNL, Berkeley, CA; 6Department of Solid State Sciences, Ghent University, Ghent, Belgium; 7Institut für Physik, Johannes Gutenberg-Universität Mainz, Mainz, Germany
The controlled displacement of magnetic domain walls (DW) along ferromagnetic nanostructures is a key requisite to memory storage or DW logic devices. Depending on the strength of the driving force (magnetic field or spin-polarized currents), the propagation of DW changes from simple translation to more complex precessional modes [1], i.e. periodic transformations of vortex DWs into transverse DWs during propagation [2]. We present the first direct experimental visualization of the precessional motion of vortex DWs in permalloy nanorings controlled by circular fields. Employing scanning transmission x-ray microscopy (STXM) we image the propagation of a pair of vortex DWs in a stroboscopic measurement scheme. We find that the DW velocity strongly varies during the transformation processes and that the propagation and DW spin structures are highly reproducible indicating the direct observation of the Walker breakdown.
References
[1] M. Hayashi, et al., Nat. Phys. 3, 21 - 25 (2007). [2] L. Heyne et al., Phys. Rev. Lett. 100, 66603 (2008).
AW-11. Describing Spin Currents in Sharp Dynamical Magnetic Textures
Fatih Dogan2, Nathaniel Collier1, Victor M. Calo1 and Aurelien Manchon2
1Mathematical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia; 2Physical Sciences and Engineering, King Ab dullah University of Science and Technology, Thuwal, Saudi Arabia
Current-driven magnetic domain wall (DW) motion is currently one of the most productive research areas in spintronics and the nature of the spin torque in magnetic textures together with the magnetic DW dynamics is under intense investigation [1]. In such devices, micromagnetism has been extremely successful in describing magnetization dynamics and its interaction with spin-polarized current [2] by artificially introducing effective in- and out-of-plane torques in Landau-Lifshitz-Gilbert (LLG) equation. We have developed an extension of the LLG formalism that considers the local microscopic description of the spin current. The spin current dynamics is modeled through the spin continuity equation expressed within the drift-diffusion formalism. This approach allows for readily introducing additional effects such as spin diffusion, spin-motive force-induced damping and to study the influence of spin Hall effects on the spin texture. The influence of a magnetic texture on a spin current, and vice-versa can be accurately described. The theory we developed self-consistently evolves the two components; local magnetization and itinerant spin density, and evaluates how they affect each other. The effect on the spin current can be turned off for comparison and weighing the importance of the self-consistency. While magnetic layer is modeled by slow changing localized sites, spin current can be modeled by a density distribution that moves order of magnitude faster than the magnetic layer it interacts with. The two systems are built with separate finite difference models, are coupled via interaction terms. Long-range interactions are calculated using multipole method. In principle, this model can describe spin currents and magnetization dynamics in spin-valves, lateral structures and magnetic DW. We apply this model to the investigation of current-driven sharp DW motion. For sharp spin textures, the influence of spin diffusion becomes important and significantly influences the DW velocity. We focus on effect of having 2D versus 3D structure, different size of transverse DW and vortices. We analyze how both spin current and local magnetic layer is modified through the interaction.
References
[1] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004); A. Thiaville, Y. Nakatani, J. Miltat, and Y. Suzuki, Europhys. Lett. 69, 990 (2005). [2] Berkov and Miltat, Journal of Magnetism and Magnetic Materials 320 (2008) 1238-1259
AW-12. Magnetic Switching Behaviors of Ferromagnetic Rolled-up Nanotubes
Jehyun Lee1, 3, Denys Makarov2, Dieter Suess1, Josef Fidler1, Oliver G. Schmidt2 and Sang-Koog Kim3
1Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria; 2Institute for Integrative Nanosciences, IFW Dresden, Dresden, Germany; 3National Creative Research Center for Spin Dynamics & Spin-Wave Devices and Nanospinics Laboratory, Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea
Interest in the static and dynamic properties of ferromagnetic nanotubes has grown very considerably in the past decade, due particularly to their shape- and dimensions-tunable magnetic properties [1]. Recently, rolled-up nanotech [2] has been applied to the fabrication of ferromagnetic tube structures [3,4] and, especially, to rolled-up permalloy tubes (RUPT) [5]. The RUPT architecture has some intriguing features: (1) dipolar coupling between the overlapped ferromagnetic layers, resulting from the windings; (2) contribution of the edges to the magnetization dynamics al ong the length of the tubes. In order to study the influence of the unique features of RUPT, we used finite element models consisting of rolled-up 400 x 400 x 5 nm3 platelet film, as shown in Fig.1. The models had various winding angles (θ) and edge statuses. Hysteresis loops obtained from the easy axis (z axis) were nearly the same in all of the cases; however, the loops from the hard axis (y axis) showed significant differences among the models. The spin configurations in their remanent states explained the origin. In the models for which the edges were not adjoined (Fig.1(a,c)), the spins did not rotate around the circumference, owing to the presence of the edges. Rather, the spins tended to be aligned according to the dipolar interactions. For example, in the case of θ=360o, the magnetic moments were aligned along the easy axis (Fig.1(a)) rather than rotating (Fig.1(b)). In the cases of overlapped magnetic layers (θ=720o), the inside and outside magnetic moments were headed towards opposite edges of the tube (Fig.1(c)); that is, they did not form domains separated by a 180o Neel wall and including anti-vortices (Fig.1(d)).
References
[1] J. Escrig et al., J. Magn. Magn. Mater. 308 (2007) 233. [2] O. G. Schmidt et al., Nature 410, 168 (2001). [3] C. Müller et al., Appl. Phys. Lett. 94, (2009) 102510. [4] E. Bermúdez-Ureña et al., J. Phys. D: Appl. Phys. 42, (2009) 055001. [5] F. Balhorn et al., Phys. Rev. Lett., 104 (2010) 037205
AW-13. Domain wall motion in magnetically frustrated nanorings
Marko V. Lubarda, Shaojing Li, Ruinan Chang, Eric E. Fullerton and Vitalliy Lomakin
CMRR, UCSD, La Jolla, CA
We present a magnetically frustrated nanoring (MFNR) structure. MFNR is formed by introducing an antiferromagnetic coupling at an interface orthogonal to the ring’s circumferential direction. MFNRs have several unique properties. In particular, only one itinerant domain wall (DW) can exist in MFNR. This DW does not need to be nucleated or injected into the structure and can never escape. Using micromagnetic simulations we show that the DW can be driven consecutively around the MFNR with a prescribed cyclicity (Fig. 1). The frequency of revolutions can be controlled by the applied field (Fig. 1). The corresponding energy landscapes can be controlled as well. They can be made flat allowing for low-field of operation or to have a high barrier for thermal stability. Potential logic and memory applications of MFNRs are considered and discussed
AW-14. Direct Observation of Stochastic Domain-Wall Behavior in Numerous Magnetic Nanowires
Kai He and John Cumings
Department of Materials Science and Engineering, University of Maryland, College Park, MD
Domain-wall (DW) motion in ferromagnetic nanowires has been actively studied due to its scientific importance in fundamental physics and prospective applications in spintronic devices, such as race-track memory and DW logic [1, 2]. Controllable and reproducible DW motion is essential for achieving these goals, but the stochastic nature of DW behavior remains a challenging issue, producing statistically varying responses even under optimal conditions of uniformly controlled lithography. DW pinning and depinning at notches within individual nanowires have been investigated by microscopic observations and measurements of magnetization and magnetoresistance [3, 4], but studying the collective DW behavior of numerous identical nanowires has not yet been reported due to difficulties in simultaneous detection of multiple DWs in a large region of interest. In thi s study, we use transmission electron microscope (TEM) operated in Lorentz mode to characterize collective DW motion driven by in-situ magnetic fields in an array of twenty permalloy (Py) nanowires. Notched nanowires were fabricated using electron beam lithography and lift-off patterning of Py thin films 20 nm thick. Direct TEM observations can locate every DW position across the entire array, and in-plane field can be applied with accuracy of 1 Oe. Statistical results show that the stochastic variations from run-to-run are more dependent on width of the wire than the wire-to-wire variations caused by fabrication inhomogeneity. In 400 nm nanowires, the pinning and depinning fields are 109±11 and 137±20 Oe, respectively, but as the wire width decreases, the pinning field increases, the depinning field remains roughly the same, and stochastic variations of both increase. Thus, for nanowires in width of 200 nm, the pinning and depinning fields were commonly overlapped, resulting in weak pinning potential where >70% of DWs show no pinning behavior. Further understanding such stochastic behavior would be useful for engineering desirable DW pinning profiles.
References
[1] S. S. P. Parkin, et al., Science 320, 190 (2008). [2] D. A. Allwood, et al., Science 309, 1688 (2005). [3] M. Y. Im, et al., Phys. Rev. Lett. 102, 147204 (2009). [4] J. Akerman, et al., Phys. Rev. B 82, 064426 (2010).
AW-15. Domain growth and dipolar bias in magnetic thin films strongly coupled to a periodic pinning potential
Rafael L. Novak4, 1, Peter J. Metaxas6, 2, Stanislas Rohart1, Raphael Weil1, Jean-Pierre Jamet1, Alexandra Mougin1, Jacques Ferre1, Robert L. Stamps5, 6, Vincent Baltz3 and Bernard Rodmacq3
1Laboratoire de Physique des Solides, Université Paris-Sud/CNRS, UMR 8502, Orsay, France; 2Unite Mixte de Recherche CNRS/Thales, UMR 137, Palaiseau, France; 3SPINTEC, URA CNRS/CEA 2512, CEA-Grenoble, Grenoble, France; 4Lab. de Physique de la Matiere Condensee, Ecole Polytechnique/CNRS, UMR 7643, Palaiseau, France; 5SUPA-School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom; 6School of Physics, M013, University of Western Australia, Crawley, WA, Australia
Ferromagnetic nanostructured films are likely to play a major role in novel ultra-high density data storage devices and in spintronics. In these systems, domain walls play an increasingly important role as they can be easily generated and manipulated by means of magnetic field pulses and spin-polarized currents. However, domain walls in thin films display complex field-driven dynamics that may be intimately connected to the geometry of the system. In this work we studied the influence of arrays of magnetic nanodots on the dynamics of domain walls in an underlying soft Co/Pt multilayer stack. The nanodot arrays are separated from the Co/Pt multilayer by a thin Pt spacer leading to a predominantly dipolar coupling between the arrays and the Co/Pt stack. In the absence of nanodots, the domain wall dynamics is dominated by thermally activated creep arising from weak disorder intrinsic to the Co/Pt films. When domain walls are driven underneath nanodot arrays, significantly shifted hysteresis loops with enhanced coercivities are observed, in analogy with exchange bias observed in F/AF sandwiches. This asymmetric magnetization reversal is related to strong deviations from creep dynamics arising from the asymmetric dipolar potential landscapes associated to nanodot arrays in different magnetization configurations. Strikingly different domain wall morphologies are observed in these cases, with dendr itic domains growing from nucleation centers when the applied field is antiparallel to the magnetization of the nanodot array and faceted domains appearing when the applied field is parallel to the magnetization of the nanodots. A comparison with previous studies of similar systems will be presented. Finally, we show that the average domain wall mobility of a magnetic thin film can be controlled by an external parameter, in this case the magnetization state of an array of magnetic nanodots, opening the possibility of controlling domain wall propagation paths in thin magnetic films.
1:30 PM - 4:30 PM, Grand Canyon 6
Chair: Gerrit Bauer, T. U. Delft
1:30 PM
BA-01. Spin current generation using heat and magnetic dynamics
Eiji Saitoh
1Institute for Materials Research, Tohoku University, Sendai, Japan; 2JAEA, Tokai, Japan
Utilization of a spin current, a flow of electrons’ spins in a solid, is the key technology in spintronics that will allow the achievement of efficient magnetic memories and computing devices. In this technology, generation and detection of spin currents are necessary. Here, we review inverse spin-Hall effect and spin-current-generation phenomena recently discovered both in metals and insulators: inverse spin-Hall effect, spin pumping, and spin Seebeck effect. 1. Spin pumping and spin torque in a Mott insulator system We found that spin pumping and spin torque effects appear also at an interface between Pt and an insulator YIG.. This means that we can connect a spin current carried by conduction electrons and a spin-wave spin current flowing in insulators. We demonstrate electric signal transmission by using these effects and interconversion of the spin currents [1]. 2. Spin Seebeck effect We have observed, by using the inverse spin-Hall effect [2,5], spin voltage generation from a heat current in a NiFe, named the spin-Seebeck effect [3,6]. Surprisingly, spin-Seebeck effect was found to appear even in insulators [4], a situation completely different from conventional charge Seebeck effect. The result implies an important role of elementary excitation in solids beside charge in the spin Seebeck effect. This research is collaboration with K. Ando, K. Uchida, Y. Kajiwara, S. Maekawa, G. E. W. Bauer, B. Hillebrands, S. Takahashi, H. Adachi, and J. Ieda.
References
[1] Y. Kajiwara & E. Saitoh et al. Nature 464 (2010) 262. [2] E. Saitoh et al., Appl. Phys. Lett. 88 (2006) 182509. [3] K. Uchida & E. Saitoh et al., Nature 455 (2008)778. [4] K. Uchida & E. Saitoh et al., Nature materials 9 (2010) 894 - 897. [5] A. Ando & E. Saitoh et al., Nature materials 10 (2011) in press. [6] K. Uchida & E. Saitoh et al., Nature materials 9 (2011) in press.
2:06 PM
BA-02. Spin Seebeck effect due to spin excitations in ferromagnetic insulators
Jiang Xiao
Department of Physics, Fudan University, Shanghai, China
A local magnon-phonon temperature difference induced by the thermal gradient in ferromagnetic insulators pumps a spin current into normal metal contacts and manifests itself as spin Seebeck effect. The basic mechanism will be discussed, with emphasis on the important role of surface spin waves.
2:42 PM
BA-03. Magnons, Phonons, and Spin Seebeck Effect
Sadamichi Maekawa
Advanced Science Research Center, Japan Atomic Energy Agency, Ibaraki, Japan
When a temperature gradient is applied to a ferromagnet, non-equilibrium magnons are excited and spin currents are induced. Therefore, when the spin currents are injected to a metal film attached to the ferromagnet, which may be either metallic or insulating, the electric voltage is obtained due to the inverse spin Hall effect [1]. This phenomenon termed Spin Seebeck effect (SSE) is formulated based on the fluctuation-dissipation theorem [2,3]. The similarity and dissimilarity of charge Seebeck effect and SSE are discussed with special attention paid to the energy conversion among phonons, magnons and electrons, i.e., the phonon-drag effect contribution to SSE.
References
[1] "Concepts in Spin-Eelectronics" ed. S. Maekawa (Oxford University Press, 2006). [2] H. Adachi et al.: APL 97, 252506 (2010) [3] H. Adachi et al: Phys. Rev. B 83, 094410 (2011)
3:18 PM
BA-04. Spin Seebeck and spin Peltier effects in nanoscale non-local spin valves.
Bart van Wees
Zernike Institute of Advanced Materials, Groningen, Netherlands
I will present recent experimental results on the interaction between thermoelectric effects and spin transport in metallic nanoscale spin valve devices. In these devices the interplay of conventional (charge related) Peltier and Seebeck effects has been observed [1]. In addition we have shown that by local heating of a ferroelectric electrode spins can be injected into a non-magnetic metal, which can be detected in a non-local spin valve geometry. This effect arises from the spin dependence of the Seebeck coefficient of the ferromagnet [2]. I will also discuss recent experiments which are aimed at observing the reciprocal effect, the spin Peltier effect, where the injection of a pure spin current can lead to local heating or cooling [3].
References
[1] F.L. Bakker, A. Slachter, J.P. Adam, and B.J. van Wees, Phys. Rev. Lett. 105, 136601 (2010) [2] A. Slachter, F.L. Bakker, J.P. Adam and B.J. van Wees, Nat. Phys. 6, 879 (2010) [3] F.L. Bakker et al., in preparation
3:54 PM
BA-05. Spin-Seebeck, phonon-drag and phonon transport in GaMnAs
Joseph P. Heremans
Dept. of Mechanical and Aeronautical Engineering, and Dept. of Physics, Ohio State University, Columbus, OH
The spin-Seebeck effect, a spin-polarization induced by temperature gradients, has been discovered in ferromagnetic metals, semiconductors, and insulators. We review its observation in the ferromagnetic semiconductor GaMnAs grown by MBE on insulating GaAs substrates, and follow the effect as a function of magnetization along easy and hard axes [1]. The parasitic effects of classical and longitudinal thermomagnetic properties are sorted out. The magnitude of the spin-Seebeck effect correlates well with phonon conduction in the substrate, and also with the phonon-drag enhancement of the classical thermopower (“charge-Seebeck” effect) [2]. This suggests that, in the temperature range studied, its origin lies in a two-step process. The first step involves phonon-magnon drag dri ven by an imbalance in temperature between phonons in the substrate and magnons in the ferromagnet; the second a magnon-electron exchange. This work was supported by the NSF and the ONR. In collaboration with C. M. Jaworski, J. Yang, S. Mack, D. D. Awschalom, and R. C. Myers.
References
[1] C. M. Jaworski et al., Nature Materials 9, 898 (2010). [2] C. M. Jaworski et al., Phys. Rev. Lett. 106, 186601 (2011).
1:30 PM - 4:30 PM, Grand Canyon 7
Chair: Xiufeng Han, Inst. of Physics, CAS
1:30 PM
BB-01. Novel Ultra-Thin Dual MTJ for STT-RAM
Dmytro Apalkov, Vladimir Nikitin, Steven Watts, Xueti Tang, Daniel Lottis, Alexey Khvalkovskiy, Kiseok Moon, Raymond Kawakami, Eugene Chen, Adrian Ong, Alexander Driskill-Smith and Mohamad Krounbi
Grandis Inc, Milpitas, CA
Spin transfer torque random access memory (STT-RAM) is a promising memory technology which is conventionally built using magnetic tunneling junctions (MTJ) with a free layer (FL) and at least one pinned layer (PL). The latter is typically a synthetic antiferromagnet (SAF) structure such as CoFe/Ru/CoFeB to cancel stray field, with an adjacent antiferromagnet such as PtMn to strongly pin the magnetic orientation. Dual-MTJ (DMTJ) structures have smaller switching currents and better switching symmetry than single-MTJ cells [1], however they require two PLs and thus suffer from large stack height (few tens nm) limiting cell scalability at smaller nodes. Here we describe a novel ultra-thin DMTJ (UT-DMTJ) design in which PLs are replaced with single thin magnetic layers (Fig. 1c.). The layers are engineered so as to strongly couple to each other antiferromagnetically via dipolar stray field. We demonstrate that this design provides great stability to meet memory operation requirements and cancel stray field on the FL. The UT-DMTJ design provides all the benefits of DMTJ structures (low Jc, switching symmetry) with a stack height of about 10 nm or less—smaller than the thickness of conventional single-MTJ structures. We have experimentally demonstrated that UT-DMTJ provides high TMR (up to 165 %) [3], symmetric switching current down to 1 MA/cm2, and good thermal stability. This work was partially supported by DARPA MTO and NSF SBIR-Phase II grants.
References
[1]. Z. Diao et al., Appl. Phys. Lett., 90, 132508, (2007). [2]. D. Apalkov et al., MMM conference, HB-04, (2010). [3]. A. Driskill-Smith et al., IMW Tech. Digest, (2011).
1:42 PM
BB-02. Error rates and stability in a novel ultra-thin dual MTJ for STT-RAM
Steven M. Watts, Dmytro Apalkov, Vladimir Nikitin, Xueti Tang, Daniel Lottis, Alexey Khvalkovskiy, Kiseok Moon, Eugene Chen, Alexander Driskill-Smith and Mohamad Krounbi
Grandis, Inc., Milpitas, CA
Deep soft error rates (SER) have been measured for our novel, ultra-thin dual MTJ structure in order to assess the reliability and stability for operation in STT-RAM. The ultra-thin dual structure, described in detail elsewhere [1], does not have conventional antiferromagnetically pinned reference layers but instead relies on dipolar coupling to strongly pin the top and bottom reference layers antiparallel. This configuration substantially reduces the stack thickness and remo ves materials such as PtMn and Ru from the stack, resulting in much higher thermal budget and improved manufacturability. The dipolar coupling of the pinned layers makes it more difficult to assess device stability by conventional applied field-based measurements such as coercivity or astroid curve measurements. Here we present read disturb measurement (low-voltage write error rates) in which the thermal stability factor Δ is obtained from the logarithmic slope of the read disturb rate data as a function of voltage [2]. Figure 1A shows read disturb data for a number of devices of different sizes. We find that Δ scales between 40 and 60 over the range of sizes studied, consistent with the properties of the stack free layer. Figure 1B shows the high-voltage error rate data for the same devices, with no observation of anomalous high voltage behavior. These results demonstrate the viability of the new dual MTJ design with similar performance to conventional fully-pinned STT-RAM structures. The simplified, ultra-thin MTJ stack that can be annealed at much higher temperatures paves the way for dual MTJ utilization in advanced-node STT-RAM designs. This work has been supported in part by a SBIR-Phase II grant from the NSF.
References
[1] A. Driskill-Smith et al., IMW Tech. Digest, (2011). [2] S. M. Watts et al., presentation HC-12 at MMM Atlanta 2010.
1:54 PM
BB-03. Spin Transfer Torque Driven Switching Probability Study of Magnetic Tunnel Junctions by Single Shot Time Domain Analysis
Yisong Zhang1, Hui Zhao1, Andrew Lyle1, Paul Crowell2 and Jian-Ping Wang1
1Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN; 2Department of Physics, University of Minnesota, Minneapolis, MN
Switching probability analysis1 has been suggested to statically study the spin transfer torque (STT) switching for the STT-RAM application, and provides a quasi-time resolved switching event. Single shot time domain measurement was introduced to study the switching behavior more precisely2-5. In this work, switching of the MgO based magnetic tunnel junction (MTJ), shown in Fig. 1(a), is statistically studied by using the single shot time domain analysis under different bias voltages and fields. The bias voltages are applied through 10 ns pulses and the pulse responses of the MTJ are collected by the storage scope (50Gs/s) as waveforms. By analyzing the switching evens of each waveform, the switching probabilities (Fig. 1(b)) and the switching time distributions (Fig. 1(c)) were determined. Furthermore the real time switching waveforms, shown in Fig.1 (d) as scatters, suggest fast transactions for the thermal assisted region compared with the switching probabilities (lines in Fig.1 (d)) as statistical analysis.
References
1. F. J. Albert, N. C. Emley, E. B. Myers, D. C. Ralph and R. A. Buhrman, Physical Review Letters 89, 226802 (2002). 3 T. Devolder, J. Hayakawa, K. Ito, H. Takahashi, S. Ikeda, P. Crozat, N. Zerounian, J.-V. Kim, C. Chappert, and H. Ohno, Physical Review Letters 100, 057206 (2008). 4. H. Tomita, K. Konishi, T. Nozaki, H. Kubota, A. Fukushima, K.Yakushiji, S. Yuasa, Y. Nakatani, T. Shinjo, M. Shiraishi, and Y.Suzuki, Applied Physics Express 1, 061303 (2008). 2. T. Aoki, Y. Ando, D. Watanabe, M. Oogane, and T. Miyazaki, Journal of Applided Physics 103, 103911 (2008). 5. Y.-T. Cui, G. Finocchio, C. Wang, J. A. Katine, R. A. Buhrman and D. C. Ralph, Physical Review Letters 104, 097201 (2010).
2:06 PM
BB-04. Switching field distributions for nanopillar spin-valves with perpendicular anisotropy driven by spin-transfer and thermal fluctuations
Daniel B. Gopman1, Daniel Bedau1, C. H. Lambert2, Stephane Mangin2, Eric E. Fullerton3, Jordan A. Katine4 and Andrew D. Kent1
1Physics, New York University, New York, NY; 2Physics, Institute Jean Lamour, UMR CNRS 7198, Nancy Université, Vandoeuvre, France; 3CMRR, University of California at San Diego, La Jolla, CA; 4San Jose Research Center, Hitachi-GST, San Jose, CA
Spin transfer driven magnetization reversal is of great fundamental interest and has a direct impact on magnetic information storage technologies. The probability that at finite temperature a nanomagnet’s direction of magnetization switches in an applied magnetic field is expected to follow a simple model of thermal activation over an energy barrier [1]. A widely used model predicts that spin-transfer torques lead to a spin-current-dependent energy barrier [2]. Here we test this basic model by conducting studies of the statistics of switching in a model system, all perpendicular spin-valve nanojunction. We present results on the magnetization switching characteristics of Co|Ni free layers (FL) in spin-valve (SV) nanojunctions with a perpendicularly magnetized polarizing layer (PL). In previous work, these devices were shown to exhibit thermally stable magnetic states at room temperature and low critical currents (~100 µA) [3]. Here we measure the distribution of switching fields for >=5000 events under 100 mT/s field sweeps and at various currents (Fig 1a). We present the dependence of the energy barrier for switching, U0/kBT, on applied dc current (Fig 1b). The energy barrier is different for anti-parallel (AP) to parallel (P) and P to AP switching, which we associate with the inhomogeneous fringe field from the PL acting on the FL. We will present results on several SV devices patterned into sub-300 nm circles and ellipses and devices with synthetic AFM PL.
References
[1] M. L. Néel, Ann. Geophys. 5, 99 (1949); W. F. Brown, Phys. Rev. B 130, 1677 (1963). [2] Z. Li and S. Zhang, Phys. Rev. B 69, 134416 (2004); D. M. Apalkov, D. M. and P. B. Visscher, Phys. Rev. B 72.180405 (2005). [3] S. Mangin, Y. Henry, D. Ravelosona, J. A. Katine, and Eric E. Fullerton, Appl. Phys. Lett. 94, 012502 (2009)
2:18 PM
BB-05. Effect of Temperature and Spin Torque on Stoner-Wohlfarth Astroid of a Nanomagnet
Yu-Jin Chen1, Jordan A. Katine2, Juergen Langer3, Mark Lewis4, Graham E. Rowlands1, Jian Zhu1, Pedram Khalili Amiri4, Kang L. Wang4 and Ilya N. Krivorotov1
1Department of Physics and Astronomy, University of California, Irvine, CA; 2Hitachi Global Storage Technologies, San Jose, CA; 3Singulus Technologies, Kahl am Main, Germany; 4Department of Electrical Engineering, University of California, Los Angeles, CA
We report measurements of the Stoner-Wohlfarth switching astroid curves [1] of the free layer nanomagnet in CoFeB/ MgO/ CoFeB/ Ru/ CoFe/ PtMn elliptical nanoscale magnetic tunnel junctions made as a function of applied voltage and temperature. Measurements of the astroid area as a function of temperature in the 4 - 300 K temperature range shown in Fig. 1(a) allow us to determine the magnetic anisotropy energy barrier of the free layer a nd thereby quantify its thermal stability - an important performance parameter of spin torque nonvolatile magnetic memory. Measurements of the free layer astroid as a function of voltage (V) applied to the junction at the bath temperature of 4 K reveal significant voltage-induced deformations of the astroid curve [2] shown in Fig. 1(b). We observe a decrease of the hard-axis length of the astroid, which is nearly independent of the applied voltage polarity and arises from ohmic heating of the junction. Comparison of the hard-axis astroid length measured at T = 4 K and |V| > 0 to the hard-axis astroid length measured at T > 4 K and V = 0 allows us to quantify ohmic heating of nanoscale tunnel junctions by the applied voltage. The applied voltage reduces the easy-axis length of the astroid as well, but the reduction is asymmetric for positive and negative easy-axis field directions as shown in Fig. 1(b). We find that this easy-axis astroid asymmetry reverses upon the applied voltage sign reversal and thus its origin can be attributed to spin transfer torque. Analysis of the voltage-induced deformation of the astroid along its easy-axis gives information on the magnitudes of in-plane spin transfer torque and field-like torque in nanoscale magnetic tunnel junctions.
References
[1] J. Z. Sun et al., Appl. Phys. Lett. 78, 4004 (2001) [2] Y. Henry et al., Phys. Rev. B 79, 214422 (2009)
2:30 PM
BB-06. Perpendicular Magnetic Tunnel Junctions based on Thin CoFeB Free Layer and Co-based Multilayer SAF Pinned Layers
Anusha Natarajarathinam1, 2, Ru Zhu1, 4, Amritpal Singh1, 4, Hao Su1, 3, Pieter B. Visscher1, 3 and Su Gupta1, 3
1Center for Materials for Information Technology (MINT), The University of Alabama, Tuscaloosa, AL; 2Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, AL; 3Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL; 4Department of Physics, The University of Alabama, Tuscaloosa, AL
We have previously reported on fully perpendicular Co/Pd multilayers (ML)-based CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ’s)[1,2]. However, Co/Pd ML-based MTJ’s have rarely exhibited TMR ratios greater than about 10%. This has been attributed to the inability to pull a sufficiently thick CoFeB layer perpendicular on top of MgO, as well as the incomplete bcc templating of CoFeB from MgO owing to the adjacent fcc Co/Pd ML’s3. Recent results[3-7] have generated great interest in MTJ’s with pinned perpendicular synthetic antiferromagnets (SAF), of the form AP1/Ru/AP2 where AP1 and AP2 are Co-based multilayers, for instance, Co/Ni or Co/Pd. We report on fully perpendicular MTJ’s with a thin CoFeB free layer and a Co/Pd(Pt) ML-based SAF pinned layer. For Co/Pd ML SAF’s, strong antiferromagnetic coupling was seen at tRu of 1.1nm, with a coupling strength of 0.017 mJ/m2. For Co/Pt ML SAF’s the optimum antiferromagnetic coupling was found at slightly higher Ru thickness of 1.3 nm, with a coupling strength of 0.013 mJ/m2. Improved MTJ properties are expected from using a thin Ta-seeded CoFeB bottom free layer, along with a thin, amorphous Ta layer used to transition from bcc CoFeB to fcc Co/Pd(Pt) for the top pinned layer6. CIPT measurements indicated TMR values as high as 40% for stacks annealed at 200C for 2 hours. Our simulation assumed an out-of-plane anisotropy field of +1 kOe for the ML and -1 kOe for the coupled CoFeB, resulting in a net out-of-plane anisotropy for the pinned layer. Magnetometry showed symmetric extended plateau at zero field of constant magnetization which offers a large dynamic range over which the magnetic configuration remains stable[7]. The stacks were patterned into MTJ’s, annealed at 240C in an in-plane field of 0.5 T, and characterized magnetically and electrically. Results using MgO seed layers to template bcc (001) orientation in Co/Pd ML's will be reported as well[7]. Acknowledgements: This work is partially supported by a U.S. Department of Defense DARPA-MTO STT-RAM Universal Memory contract, and Grandis Technology, Milpitas. Dr. David Abraham of IBM and Dr. Robert Shull of NIST are gratefully acknowledged for the CIPT measurements.
References
1. Z. R. Tadisina et al., J. Vac. Sci. Technol. A 28, 973 (2010). 2. Z. R. Tadisina et al., J. Appl. Phys. 107, 09C703 (2010). 3. K. Mizunuma et al., Appl. Phys. Lett. 95, 232516 (2009). 4. H.He et al., IEEE Trans. Magn. 46, 1327 (2010). 5. D. C. Worledge et al., _Proc. Int’l. Electron. Dev. Mtg. 10-296, (2010). 6. D. C. Worledge et al., Appl. Phys. Lett. 98, 022501 (2011). 7. Appl. Phys. Express 4 (2011) 023002
2:42 PM
BB-07. Dependence of spin-transfer switching characteristics in MTJs with synthetic free layers on the coupling strength.
Masayuki Nishimura1, Mikihiko Oogane1, Hiroshi Naganuma1, Nobuhito Inami1, Tadashi Morita2 and Yasuo Ando1
1Department of Applied Physics, Tohoku University, Sendai, Japan; 2ULVAC, Inc., Shizuoka, Japan
Synthetic ferri/ferromagnetic (Sy-Ferri/Ferro) structures have attracted much attention as free layers of STTRAM (spin transfer torque random access memory), because they can increase thermal stability factor (Δ0) while keeping intrinsic switching current density (Jc0) as low as that of a conventional single free layer [1, 2]. However, the reason of that effect is not investigated sufficiently. In this work, we have investigated spin-transfer switching characteristics of MTJs using synthetic free layers with various coupling strengths (Jex). Thin films were deposited by magnetron sputtering. We measured magnetic properties of synthetic structures using vibrating sample magnetometer (VSM). The MTJs were pseudo-spin-valve type with Sy-Ferri/Ferro free layers of Co90Fe10(2 nm)/Ru(tRu)/Co40Fe40B20(2 nm) and fabricated in approximately 250×100 nm2 ellipse-shaped pillars by electron-beam lithography process with Ar ion milling. Spin-transfer switching characteristics were evaluated by measuring probability of switching due to applied current pulses under various magnetic fields. Δ0 and Jc0 were estimated using Sharrock and Slonczewski’s equations, respectively [3, 4]. Table. 1 shows Jex , Δ0 , Jc0 and Ic0/Δ0 of the MTJs with Sy-Ferri free layers with tRu=0.4, 1.0 nm, Sy-Ferro free layer with tRu=1.4 nm and single free layer of Co40Fe40B20(2 nm). The MTJ of tRu=1.0 nm exhibited larger Δ0 and lower Jc0 than that of single free layer. Meanwhile the MTJs of tRu=0.4, 1.4 nm with strong ferri or ferrocoupling exhibited very large Δ0 more than twice the value of that of the other groups even allowing for the slightly large cell size [1, 2]. When comparing Ic0/Δ0 , the MTJ with rather weak ferricoupling is suited the best for the STTRAM application. This work was supported by FIRST program.
References
[1] J. Hayakawa et al., Jpn. J. Appl. Phys., 45, L1057 (2006). [2] S. Yakata et al., Appl. Phys. Lett., 95, 242504 (2009). [3] M. P. Sharrock, IEEE Trans. Magn., MAG-20, 754 (1984). [4] J. C. Slonczewski, J. Magn. Magn. Mater., 159, L1 (1996).
2:54 PM
BB-08. Theoretical study on dependence of thermal switching time of synthetic free layer on coupling field
Tomohiro Taniguchi and Hiroshi Imamura
Nanosystem Research Institute, AIST, Tsukuba, Japan
Owing to its high thermal stability and low spin torque switching current, ferromagnetically coupled synthetic free layer in MgO based Magnetic Tunnel Junctions (MTJs) [1,2] is one of the attractive research targets for future spin-electronics devices such as Spin Random Access Memory (Spin RAM). A typical synthetic free layer consists of two CoFeB layers separated by a thin Ru spacer layer, where spin torque from the pinned layer acts on the magnetization of the first ferromagnetic layer connected to MgO barrier. The coupling field between two ferromagnetic layers, HJ, which can be systematically varied by changing the spacer thickness, plays an important role on the thermal stability and thermal switching time. In the weak coupling limit, the thermal switching of the synthetic free layer consists of two processes: one is the switching of the first layer due to spin torque, and the other is the switching of the second layer due to the coupling. By increasing the coupling field, the switching time of the first (second) layer increases (decreases). Then, it is physically interesting to study the dependence of the total switching time on the coupling field. We studied the dependence of the thermal switching time of a ferromagnetically coupled synthetic free layer on the coupling field based on the Fokker-Planck equation. The double exponential dependence of the thermal switching probability on the coupling field shows a wide distribution of the switching time. A fast thermal switching is achieved when the switching rates of two ferromagnetic layers are identical. We found that the condition to minimize the switching time is given by HJ =Hs/(2α), where Hs and α are the amplitude of spin torque and Gilbert damping constant, respectively.
References
[1] S. Yakata, H. Kubota, T. Sugano, T. Seki, K. Yakushiji, A. Fukushima, S. Yuasa, and K. Ando, Appl. Phys. Lett. 95, 242504 (2009). [2] S. Yakata, H. Kubota, T. Seki, K. Yakushiji, A. Fukushima, S. Yuasa, and K. Ando, IEEE Trans. Magn. 46, 2232 (2010).
3:06 PM
BB-09. Reduction of switching current density in perpendicular magnetic tunnel junctions by tuning the anisotropy direction of the CoFeB free layer
Md. Tofizur Rahman1, Andrew Lyle1, Pedram K. Amiri2, Brian Glass1, Jonathan Harms1, Hui Zhao1, Zhongming Zheng3, G. Rowlands3, Y. J. Chen4, H. W. Jiang3, J. Katine5, J. Langer6, I. N. Krivorotov4, K. L. Wang2 and Jianping Wang1
1Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN; 2Electrical Engineering, University of California, Los Angeles, CA; 3Physics and Astronomy, University of California, Los Angeles, CA; 4Physics and Astronomy, University of California, Irvine, CA; 5HGST, San Jose, CA; 6Singulus Technologies, Kahl am Main, Germany
We fabricated Si/ SiO2/BE/Ta (5 nm)/CoFeB(t nm)/MgO(0.8 nm)/CoFeB(t nm)/Ru(0.3 nm)/(Co/Pd)10/TE perpendicular MTJ structures by engineering the exchange coupling between CoFeB spin polarizing layer and (Co/Pd) MLs.The effect of CoFeB free layer and CoFeB spin polarizing layer thickness on the perpendicular anisotropy of both free and fixed layers was investigated first. We then fabricated circular devices with different sizes ranging from 40 nm to 120 nm in diameter and investigated the effect of CoFeB free layer and CoFeB spin polarizing layer thickness on perpendicular MR ratio and switching current density after annealing at 200oC for 2 hr. The MR ratio increased almost linearly with increase in CoFeB spin polarizing layer due to the enhancement of (001) texture in this layer adjacent to MgO. A linear increase of perpendicular MR ratio with decreasing CoFeB free layer thickness from 1.22 nm to 1.0 nm was observed, which can be attributed to the enhancement of the perpendicular anisotropy in this layer. The switching current density decreases with the increase of CoFeB free layer thickness as evidenced in Figure.1. The minimum switching current of 1.87 MA/cm2 has been achieved for a device with 70 nm diameter. This decrease of switching current is associated with the reduced perpendicular anisotropy and the tilting of the magnetization direction in the free layer that has been proven by magnetometry testing of a Ta/CoFeB/MgO/Ta sample.
3:18 PM
BB-10. Novel STT Device Design with Circuit Scheme to Enable All Metallic Logic Circuits
Jian-Gang Zhu, David Bromberg, Daniel H. Morris and Larry Pileggi
Electrical and Computer Engineering, Carnegie Mellon Univ, Pittsburgh, PA
Spin transfer torque (STT) magnetic tunnel junction (MTJ) devices represent a promising nanoscale memory technology. In this talk we present a novel STT device design -- referred to as an m-Cell -- that can be used to perform logic operations. The m-Cell uses an MTJ for its magnetic logic state evaluation and the state is switched by STT-driven domain wall motion (Fig. 1a). The state-evaluation path (read path) and state-switching path (write path) are electrically insulated from each other so that unwanted leakage paths are avoided. A novel circuit scheme connects the mCells to form logic gate circuits (Fig. 1b) that can drive multiple fanout gates with sufficient current gain. The inherent non-volatile memory of the logic states and the nature of the current-driven state switching allow the power and clocking to be delivered as one combined signal. This new logic family -- referred to as mLogic -- is exclusively based on mCells without requiring any integrated semiconductors or transistors to implement large-scale logic circuits. Micromagnetic modeling of the STT devices and their incorporation in circuit simulations demonstrate that the logic circuits can operate with supply voltages as low as 20mV with current waveform signals that make power dissipation due to interconnect capacitance negligible. Perpendicularly magnetized write path allows optimization of the thermal stability while achieving minimum threshold current for state-switching by optimizing perpendicular magnetocrystalline anisotropy.
References
[1]T. Kishi, et al, IEDM 2008, p.309-312, Paper 12.6 (2008). [2]S. Fukami, et al, 2009 Symposium on VLSI Technology,Digest of Technical Papers, p.230, (2009)
3:54 PM
BB-11. Reducing soft write error and write current overdrive in STT-RAM
Eugene Chen, Dmytro Apalkov, Steven Watts, Xueti Tang, Alexey Khvalkovskiy, Kiseok Moon, Daniel Lottis, Vladimir Nikitin, Alexander Driskill-Smith and Mohamad Krounbi
Grandis Inc, Milpitas, CA
STT-RAM (spin transfer torque random access memory) based on MgO tunnel barrier MTJ is a scalable, nonvolatile memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM, the non-volatility of Flash, and unlimited endurance. It is one of a small set of candidate memory technologies that have the unlimited endurance and fast speed operations required for replacing future SRAM and DRAM memories as a high-speed embedded memory in microprocessor and microcontroller systems on chip (SOC), as well standalone memories in computing, mobile and server systems [1-3]. To be used for these applications, STT-RAM must meet stringent soft error rate criteria [1, 2]. Although it is common in literature to use STT critical switching current Ic0 or current density Jc0 as criteria for chip requirement, we found that write current overdrives more than Ic0 or Jc0 are needed to guarantee desired write error rates such as 1E-9 at write speeds faster than 50ns. The required write current overdrive increases as the write current pulse decreases [2, 4]. By setting an offset angle between the MTJ pinned layer and the free layer does not reduce the required overdrive, since new stagnation point for zero initial spin torque will develop due to random thermal effect. In this paper, the effects of thermal stability, damping, Hk, Ms, out-of-plane anisotropy, offset angle in the pinned or free layers and external fields on write error rate (WER) will be discussed, as well as a new scheme of MTJ free layer structure that will reduce the required writing current overdrive for 5ns and WER<1E-9 switching by a factor of 5 to 10 compared to conventional in-plane or perpendicularly magnetized free layers. Acknowledgment This work was partially supported by DARPA Microsystems Technology Office.
References
1. E. Chen, et al., IEEE Trans. Magn. 46, 1873 (2010). 2. A. Driskill-Smith, et al., VLSI Tech. Digest, 51 (2010). 3. S.-O. Chung, et al., IEDM Tech. Digest 10-304 (2010). 4. R. Heindl, et al., Phys. Rev. B 83, 054430 (2011).
4:06 PM
BB-12. Reduction of switching current of current-perpendicular-to-plane giant magnetoresistance devices with perpendicular Gd1-xFex free layer for light modulator application
Ken-ichi Aoshima1, Yusuke Hashimoto1, Nobuhiko Funabashi1, Kenji Machida1, Kiyoshi Kuga1, Hiroshi Kikuchi1, Takayuki Ishibashi2 and Naoki Shimidzu1
1Science & Technology Research Laboratories, Japan Broadcasting Corp., Tokyo, Japan; 2Department of Materials Science and Technology, Nagaoka Institute of Technology, Nagaoka, Japan
A magneto-optical Spatial Light Modulator driven by Spin Transfer Switching (STS-MOSLM) has been investigated for holographic 3-dimensional display [1]. Since STS-MOSLM requires relatively thick free layer and large pixel size for better light modulation properties, the switching current tend to become large compared to MRAM. Consequently, reducing switching current maintaining light modulation properties is extremely important. In this report, we show how the composition of Gd-Fe free layer affects the STS properties of Current-Perpendicular-to-Plane Giant-Magnetoresistance (CPP-GMR) device with relatively thick Gd-Fe free layer. GMR stacks of Ru(3 nm)/Tb-Fe-Co(10 nm)/Co-Fe(1 nm)/Ag(6 nm)/ Gd1-xFex(8.9 nm)/Ru(3 nm) (x; 72.5, 75.6, 78.3, 80.3 at%) were deposited by DC magnetron sputtering. CPP-GMR devices were fabricated by photo and EB lithography and ion etching technique with size of 100×100nm2. STS properties and thermal stability were evaluated by pulse current injection with pulse duration from 10μs to 10ms [2]. MR ratio of the devices increases from 0.13% to 0.16% when Fe composition increases from 72.5 at% to 80.3 at%. We speculate that this is because the spin dependent scattering in Gd-Fe is mostly from Fe [3]. Intrinsic switching current density (Jc0) decreases remarkably with increasing Fe composition. Jc0 of the CPP-GMR with Gd19.7Fe80.3 was about 2×107 A/cm2 which was about 8 times smaller than that with Gd27.5Fe72.5. This reduction may be attributed to the decrease in effective anisotropy of the free layer [4] accompanied by increasing Fe composition. As a matter of fact, thermal stability (E/kBT) of the devices decreases with increasing Fe composition. And the device with Gd19.7Fe80.3 free layer shows E/kBT of 29.1, which corresponds to a thermal stability of about an hour. This time length is not enough for memory application but sufficient for SLM since a display normally refreshes all the pixels very frequently. This research is supported by National Institute of Communications Technology (NICT) in Japan.
References
[1] K. Aoshima, et al., J. Display Ttechnology, 6, p. 374, 2010 [2] R.H. Koch, et. al., Phys. Rev. Lett., 92, p.088302, 2004., K. Yagami, et al, Appl. Phys. Lett. 85, p.5634, 2004 [3] B. Dieny, J. Magn. Magn. Mater. 136, p.335, 1994 [4] S. Mangin, et al., Nature Mater., 5, p. 210, 2006
4:18 PM
BB-13. Decrease in intrinsic critical current density under magnetic field along hard in-plane axis of free layer in magnetic tunnel junctions with in-plane anisotropy
Katsuya Miura1, 2, Ryoko Sugano1, 5, Masahiko Ichimura1, 5, Jun Hayakawa1, Shoji Ikeda2, 3, Hideo Ohno2, 3 and Sadamichi Maekawa4, 5
1Central Research Laboratory, Hitachi, Ltd., Kokubunji, Japan; 2Center for Spintronics Integrated Systems, Tohoku University, Sendai, Japan; 3Research Institute of Electrical Communication, Tohoku University, Sendai, Japan; 4Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Japan; 5JST, CREST, Chiyoda-ku, Japan
To realize low power consumption spintronics devices, it is widely investigated to decrease the intrinsic critical current density Jc0 in magnetic tunnel junctions (MTJs) for current-induced magnetization switching (CIMS) [1]. The Jc0 in MTJs with the in-plane anisotropy can be decreased by applying a magnetic field along the hard in-plane axis Hhard [2]. However, the reason of the decrease has not been fully understood. Because applying Hhard causes change on the mutual angle θH between two magnetizations of the free and reference layers, we investigated Jc0 and thermal stability factor E/kBT as functions of θH. We prepared the stack structure of Ta(5)/Ru(10)/Ta(5)/NiFe(5)/MnIr(8)/CoFe(4)/Ru(0.8)/Co40Fe40B20(5)/MgO(0.8)/Co40Fe40B20(2)/Ta(5)/Ru(5) (in nm). They were processed into MTJs with the dimensions of 100 x 200 nm2. From the experimental results, Jc0 decreases by more than 20% on increasing θH up to 10 degrees whereas E/kBT is independent of θH. The magnitude of decrease in Jc0 exceeds the contribution of the increase in the Slonczewski's g-factor [3,4]. Then, we performed numerical simulations and a simple stability analysis. The numerical simulations reproduce these experimental results. From the analysis, applying Hhard causes a tilt of the effective magnetic field in the free layer, which leads to the imbalance between the damping and in-plane spin-transfer torques during the CIMS. In this condition, the azimuth angle φ of the magnetization in the free layer is restricted, resulting in the suppression of the effective demagnetization field Hd*. Therefore, the dominant factor of the decrease in Jc0 can be concluded to be the suppression of Hd*. This approach enables us to decrease Jc0 in the in-plane MTJs without degrading E/kBT. This work was partly supported by Research and Development for Next-Generation Information Technology Program from MEXT, Next-Generation Supercomputing Project/Nanoscience Program from MEXT, and the FIRST Program of JSPS.
References
[1] J. Hayakawa, S. Ikeda, Y. M. Lee, R. Sasaki, T. Meguro, F. Matsukura, H. Takahashi , and H. Ohno, Jpn. J. Appl. Phys. 45, L1057 (2006). [2] T. Inokuchi, H. Sugiyama, Y. Saito, and K. Inomata, Appl. Phys. Lett. 89, 102502 (2006). [3] J. C. Slonczewski, Phys. Rev. B 71, 024411 (2005). [4] J. C. Slonczewski, and J. Z. Sun, J. Magn. Magn. Mater. 10, 169 (2007).
1:30 PM - 4:30 PM, Grand Canyon 8
Chair: Robert Stamps, University of Glasgow
1:30 PM
BC-01. Spin Liquids and Magnetic Monopoles in Rare Earth Cubic Pyrochlores
Jeffrey W. Lynn1, Hiroaki Kadowaki2, Hiroshi Takatsu2, Taku J. Sato3 and Yoshikazu Tabata4
1NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; 2Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan; 3Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106, Japan; 4Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
Cubic pyrochlores such as Dy2Ti2O7 have a structure where the rare earth sites form a corner-sharing tetrahedral network, which is the prototype geometry to frustrate the magnetic interactions that can lead to novel magnetic ground states that are fundamentally different than conventional long range magnetic order. The magnetic anisotropy of the Dy3+ spins requires them to point along a local <111> axis, such that on each tetrahedron the moment can only point into the center, or in the opposite direction. The ground state then turns out to be where two spins on each tetrahedron point inward, and two point outward. But you don’t know which two are in and which two are out, giving rise to six equivalent configurations for any particular tetrahedron, and a macroscopic degeneracy and finite entropy for the ground state. This “two-in two-out” description of the spin system is identical to the proton configurational disorder for hexagonal ice discussed by Pauling [1], inspiring the name “spin ice” [2]. Recently it was realized that the magnetic excitations of spin ice can then be describ ed as magnetic monopoles [3], and we have observed such excitations experimentally [4]. We will describe the nature of magnetic frustration, spin-ice, and how magnetic monopoles emerge from the ground state of this very special magnetic material. We will also describe recent measurements on Tb2Ti2O7, which which does not show any magnetic order down to 50 mK, but exhibits condensation behavior below 0.4 K from a thermally fluctuating paramagnetic state to a spin-liquid ground-state with quantum spin fluctuations [5].
References
[1] L. Pauling, J. Am. Chem. Soc. 57, 2680 (1935). [2] J. S. Gardner, M. J. P. Gingras, and J. E. Greedan, Rev. Mod. Phys. 82, 53 (2010). [3] C. Castelnovo, R. Moessner, & S. L. Sondhi, Nature 451, 42 (2008). [4] H. Kadowaki, N. Doi, Y. Aoki, Y. Tabata, T.J. Sato, J. W. Lynn, K. Matsuhira, and Z. Hiroi, J. Phys. Soc. Japan 78, 103706 (2009). [5] Hiroshi Takatsu, Hiroaki Kadowaki, Taku J. Sato, Yoshikazu Tabata, and Jeffrey W. Lynn (preprint).
2:06 PM
BC-02. Magnetic induction and reversal studies of nanoscale artificial spin ice lattices
Charudatta Phatak1, Amanda Petford-Long1, Mihaela Tanase1, 4, Mengchun Pan3, Olle Heinonen1 and Marc De Graef2
1Materials Science Division, Argonne National Laboratory, Argonne, IL; 2Carnegie Mellon University, Pittsburgh, PA; 3Northwestern University, Evanston, IL; 4National Institute of Standards andTechnology, Gaithersburg, MD
Artificially frustrated magnetic spin ice lattices present an opportunity to study the physics of magnetic spin frustration at the macroscopic scale [1]. Recent work on these patterned lattices has shown the existence of magnetic monopole type defects which are a direct result of frustration [2]. They are being studied intensively because their behavior can be accessed easily at room temperature as opposed to bulk spin-ice compounds (e.g. DyTi2O7) where it occurs only at ultra-low temperatures. In this work, we fabricated lithographically patterned square lattices comprised of 200 nm permalloy islands with different inter-island spacings (Fig. 1). Using aberration-corrected Lorentz transmission electron microscopy (LTEM) we were able to directly observe the magnetic interactions occurring between neighboring islands, and for the first time, to visualize the magnetic induction associated with the lattice frustration at monopole defect sites; this was made possible by the high spatial resolution attainable with aberration correction [3]. Magnetic force microscopy was used to observe the reversal behavior of the lattices. Correlation with Monte Carlo and micromagnetic simulations enabled us to identify two different mechanisms of magnetization reversal as a function of lattice spacing. We will present the analysis of the reversal mechanisms as well as variation in the node statistics during reversal.
References
[1] R. F. Wang et. al. Nature 439, 303-6 (2006). [2] D. Morris et. al., Science, 326, 411-4 (2009). [3] C. Phatak et. al. Phys. Rev. B 83, 1-5 (2011).
2:18 PM
BC-03. Diversity opens dynamical pathways: disorder and energy landscape exploration in artificial spin ice.
Zoe Budrikis1, 2, Paolo Politi2 and Robert Stamps1, 3
1School of Physics, The University of Western Australia, Crawley, WA, Australia; 2Ist ituto dei Sistemi Complessi CNR, Sesto Fiorentino (Florence), Italy; 3SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
Artificial spin ice [1] has a complex energy landscape with frustation causing many states to be nearly degenerate. Furthermore, the system is athermal and dynamics must be driven by a field [2]. These globally-driven dynamics are highly constrained and the exploration of the energy landscape is non-trivial. Many states are inaccessible, regardless of their energy. We have studied this problem of constrained dynamics using complex networks to describe the space of spin configurations. By mapping dyanmics onto networks we can take advantage of the power of network theoretic tools [3, 4]. Our key result is that disorder in the spins' responses to the external field opens new dynamical pathways for the system to follow, even though the energy landscape is unchanged. This has implications for the design of field protocols to take the system to low energy states. We also make a systematic study of the effects of disorder strength and field amplitude. These network results are confirmed by numerical simulations of spin ice arrays.
References
[1] R. F. Wang et al., Artificial `spin ice' in a geometrically frustrated lattice of nanoscale ferromagnetic islands. Nature 439, 303 (2006). [2] X. Ke et al., Energy Minimization and ac Demagnetization in a Nanomagnet Array. Phys. Rev. Lett. 101, 037205 (2008). [3] Y. Han, Phase-space networks of geometrically frustrated systems. Phys. Rev. E 80, 051102 (2009). [4] R. Albert and A.-L. Barabasi, Stastistical mechanics of complex networks. Rev. Mod. Phys. 74, 47 (2002).
2:30 PM
BC-04. Measuring Disorder in Artificial Kagome Ice
Stephen Daunheimer1, Olga Petrova2, Oleg Tchernyshyov2 and John Cumings1
1Materials Science & Engineering, University of Maryland, College Park, MD; 2Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD
Artificial spin ice has become a popular tool for understanding magnetic interactions at the nanoscale. The strength in the approach lies in the ability of a synthetic material, fabricated from macroscopic artificial ``atoms'', to mimic real materials, where geometric frustration leads to stochastic disorder and residual entropy at low-T. These artificial atoms are nanometer-scale islands, patterned from magnetic alloys, and their arrangement produces frustration at the vertices, where the islands interact through dipolar fields [1-6]. This allows the study of frustration in systems where crystalline imperfections can be completely removed by design, or introduced in a controlled way. Unfortunately, roughness at edges and interfaces creates inadvertent static disorder that diminishes the ability to compare with studies of spin ice oxides, where interacting magnetic atoms are presumed to be identical. Prior studies have shown there can be substantial variation among the magnetic “atoms” of artificial spin ice in relevant quantities such as coercive field, with deviations up to 16%. By considering carefully the magnetic reversal process, we demonstrate that this variability can be greatly reduced by migrating to a system with connected magnetic islands. We will present a procedure for measuring this disorder by studying magnetic reversals by Lorentz mode transmission electron microscopy. When the direction of the reversal field is carefully selected, the reversals proceed as separate sublattices, where the width of the reversal measures the disorder within a given sublattice. In a connected structure of art ificial kagome ice, we will show that the disorder in coercive field can be reduced to 3.3%. Such small disorder will increase the ability to compare directly with behavior in canonical spin ice materials, such as Dy2Ti2O7 and Ho2Ti2O7.
References
[1]. R. Wang, et al., Nature 439, 303 (2006). [2]. J. P. Morgan, et al., Nature Phys. 7, 75 (2011). [3]. S. Ladak, et al., Nature Phys. 6, 359 (2010). [4]. E. Mengotti, et al., Nature Phys. 7, 68 (2011). [5]. M. Tanaka, et al., Phys. Rev. B 73, 052411 (2006). [6]. Y. Qi, et al., Phys. Rev. B 77, 094418 (2008).
2:42 PM
BC-05. Magnetic Reversal of an Artificial Square Ice - Dipolar Correlation and Charge Ordering
Jason P. Morgan1, Aaron Stein2, Sean Langridge3 and Christopher H. Marrows1
1School of Physics and Astronomy, University of Leeds, Leeds, United Kingdom; 2Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY; 3ISIS, STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
Artificial spin ices are lithographically patterned arrays of single domain nanomagnets [1, 2, 3]. The elongated elements form a 2D system of interlinked vertices at which Ising-like dipole moments meet with incompatible interactions. They are directly analogous to 3D bulk spin ice materials [4, 5], in which magnetic moments map robustly on to the proton ordering of water ice, the classic geometrically frustrated material. Consequently, spin ice can be considered a “magnetolyte” [5], supporting currents of interacting fractionalised “magnetic monopole” charge defects. Naturally, 2D analogs have become a source of intensive interest [2, 3], with the artificial systems allowing for vertex interactions to be uniquely controlled, and for their direct imaging via magnetic microscopy. Indirect evidence exists for monopole interactions playing a role in thermal equilibration [3]. The artificial systems studied to date are inherently athermal, with elemental reversal barriers orders of manitude greater than the thermal energy. Applied magnetic fields are required to promote dynamics, a number of recent reports addressing dc-field reversal processes, e.g. Ref. [2], where kagome ice reversal mediates via moment flip cascades, nucleating and propagating monopole configurations throughout the pattern. No evidence has yet been identified, however, for fractionalised monopole-monopole interactions during reversal. In this work, we address experimental magnetic reversal of a square ice pattern, in a dc field applied along an off-symmetry direction. A regime is accessed in which the system’s sublattices are effectively decoupled. Independent sublattice reversal occurs via bulk nucleation and correlated extension of flip-chains, acting to propagate charges along adjacent channels and suppress charge populations during midstages of reversal. Further to this, we identify interactions between adjacent flip-chains in the form of weak anticorrelation and show how this manifests as attractive/repulsive pinning of opposite/like charged vertices propagating along adjacent channels. Enhancement of the observed phenomena may be possible by reducing intrinsic patterning disorder.
References
[1] R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi & P. Schiffer, Nature (2006), 439, 303-306 [2] E. Mengotti, L. J. Heyderman, A. F. Rodríguez, F. Nolting, R. V. Hügli & H.-B. Braun, Nature Physics (2011), 7, 68-74 [3] J. P. Morgan, A. Stein, S. Langridge, & C. H. Marrows, Nature Physics (2011), 7, 75-79 [4] M. J. Harris, S. T. Bramwell, D. F. McMorrow, T. Zeiske & K. W. Godfrey, Physical Review Letters (1997), 79, 2554-2557 [5] S. R. Giblin, S. T. Bramwell, P. C. W. Holdsworth, D. Prabhakaran, & I. Terry, Nature Physics, 7, 252-258
2:54 PM
BC-06. Going up and down the energy landscape of a frustrated mixed-spin oxide
Michalis Charilaou, Kisor K. Sahu, Jörg F. Löffler and Andreas U. Gehring
ETH Zurich, Zurich, Switzerland
Magnetic structures of frustrated systems, such as spin-glasses and spin-liquids, suffer from competing interactions resulting in complex energy landscapes with meta-stable local minima and degenerate global minima. It can be beneficial, from a scientific and engineering point of view, to be able to control the energy landscape, and to drive the system onto chosen energy minima thus controlling the macroscopic magnetic thermodynamics. In this study we show how this can be done with a classic mixed-spin oxide solid solution FeTiO3 - Fe2O3, which exhibits frustration on the sublattice-level across the layers of the R-3 crystal structure at temperatures below 100 K. The magnetic thermodynamics of the system are manipulated by selecting the relative layer orientation, which can be controlled by an external bias field. Unfolding of Fe-rich layers places the system in a meta-stable local minimum and increases the net moment by 100 times. Upon heating the net moment decreases strongly and goes to negative values before it collapses to zero at the Curie temperature. The observations are described within a mean-field theory framework using a four-sublattice model, which we use to calculate energy landscapes.
3:06 PM
BC-07. Glassy magnetic behavior in Ni,Co:CuMn2O4 spinels
John McCloy1, Clifford Leslie2, Tiffany Kaspar1 and Weilin Jiang1
1Pacific Northwest National Laboratory, Richland, WA; 2University of Washington, Seattle, WA
Copper manganite spinels have been studied for their unique electric [1], thermoelectric [2], and catalyst [3] properties. Mn1.68Cu0.6Co0.24Ni0.48O4, having previously been investigated as a thin film [4], was fabricated as bulk ceramic by slip casting and sintering of powders produced by spray-pyrolysis. Magnetic properties of this composition were measured for the first time, as a function of processing temperature (900 or 1000°C sintering), to study the effects of Cu and Mn valence and site preference. Quantitative x-ray photoelectron spectroscopy data suggested that these samples have a substantial amount of bulk carbon which questioned the assumptions about oxygen stoichiometry. Approximately two-thirds of the Mn was present as Mn4+, with the remainder Mn3+, and the Mn4+/Mn3+ ratio was unchanged with sintering temperature. The amount of Cu2+ remained the same in the two sintering conditions, but the Cu+ content changed from 100% tetrahedral for ~50% tetrahedral/ ~50% octahedral at 900 and 1000°C sintering temperatures, respectively. X-ray diffraction demonstrated that the materials had a cubic spinel structure devoid of tetragonal Jahn-Teller distortion. Helium ion and scanning electron microscopy showed porous structures in the bulk ceramic with grain growth and densification occurring at the higher sintering temperature. AC magnetic susceptibility indicated ferrimagnetic behavior below ~109 K and spin glass behavior belo w ~66-74 K depending on measurement frequency. Magnitudes of the AC magnetic susceptibility, DC magnetization, and Curie-Weiss temperature were lower for samples sintered at 1000°C than for those sintered at 900°C. AC susceptibility freezing temperatures were modeled with the Vogel-Fulcher law and showed characteristics intermediate between canonical spin glasses and cluster glasses. For the sample sintered at 1000°C, the activation energy for magnetic relaxation was decreased and the interaction parameter temperature was increased compared to the 900°C sample. These materials showed promise for advancing fundamental understanding of the consequences of multivalent, mixed site occupancy, cations for the magnetic properties of spinels.
References
[1] B. Gillot, M. Kharroubi, R. Metz, R. Legros and A. Rousset, Phys. Stat. Sol. (a) 124 (1) (1991) 317-325. [2] J. G. Moyer, D. A. Kukuruznyak, N. Nguyen, M. S. Prowse and F. S. Ohuchi, J. Appl. Phys. 100 (8) (2006) 083504-083513. [3] G. Fortunato, H. R. Oswald and A. Reller, 11 (3) (2001) 905-911. [4] D. A. Kukuruznyak, S.-W. Han, M.-H. Lee, K. A. Omland, M. C. Gregg, E. A. Stern and F. S. Ohuchi, J. Vac. Sci. Tech. A 19 (2001) 1923-1928.
3:18 PM
BC-08. Low-Temperature Heat Transport of Spin Gapped Quantum Magnets
Xuefeng Sun1, Xinming Wang1, Wenpao Ke1, Limin Chen1, Cheng Fan1, Zhiying Zhao1 and Xia Zhao2
1Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, China; 2School of Physical Sciences, University of Science and Technology of China, Hefei, China
Low-dimensional or frustrated quantum magnets were revealed to exhibit exotic ground states, magnetic excitations, and quantum phase transitions (QPTs). In the spin-gapped antiferromagnets, the external magnetic field can close the gap in the spectrum, which results in a QPT between a low-field disordered paramagnetic phase and a high-field long-range ordered one. An intriguing finding is that this ordered phase can be approximately described as a Bose-Einstein condensation (BEC) of magnons. In this work, we study the low-temperature and high-field thermal conductivity (κ) of several spin gapped quantum magnets, including the quasi-one-dimensional S=1 chain compound NiCl_2-4SC(NH_2)_2, the layered spin-dimer compound Ba_3Mn_2O_8, and the ferromagnetic-antiferromagnetic alternating chain compound (CH_3)_2CHNH_3CuCl_3. It is found that the magnetic excitations are commonly scattering phonons rather strongly in these materials, particularly at the field-induced QPTs. In some cases they can also act as heat carriers and make a substantial contribution to the heat transport.
References
X. F. Sun, W. Tao, X. M. Wang, and C. Fan, Phys. Rev. Lett. 102, 167202 (2009); W. P. Ke, X. F. Sun et al., unpublished; L. M. Chen, X. F. Sun et al., unpublished.
3:30 PM
BC-09. Field-induced slow spin relaxation in monoclinic Nd2Ti2O7
Hui Xing1, Gen Long2, Hanjie Guo1, Chunmu Feng1, Guanghan Cao1, Zhu-an Xu1 and Hao Zeng2
1Department of Physics, Zhejiang University, Hangzhou, China; 2Department of Physics, University at Buffalo, SUNY, Buffalo, NY
We report a novel field-induced slow spin relaxat ion in the paramagnetic state of the monoclinic Nd2Ti2O7 crystals. AC susceptibility measurements clearly indicate an unusually slow spin relaxation process with relaxation time up to several seconds in all crystallographic directions. The presence of a slow relaxation in the paramagnetic state in Nd2Ti2O7 is highly unexpected. Spin dilution effect by substituting Nd3+ with nonmagnetic La3+ indicates that the relaxation is closely related to spin spin correlations. The significance of the spin correlation is also revealed by the magnetic specific heat measurement which shows that approximately 50% of the total spin entropy freezes before entering the magnetically ordered ground state, indicative of the formation of correlated spin regions at higher temperatures. We propose that the field-induced slow spin relaxation is associated with a cooperative behavior of the correlated regions formed by partially polarized spins in an external field. We further suggest that the field induced slow spin relaxation is a universal behavior for correlated spin systems with frustration, while not specific to detailed crystal field schemes. To confirm our postulation, we investigated the Y-doped Nd2Ti2O7, which stabilizes in the pyrochlore structure due to the smaller ionic radius of Y3+, and accommodates geometrical frustration. AC susceptibility measurements show the existence of field-induced slow spin relaxation as well.
References
1. J. Snyder et al, Nature, 413,48 (2001). 2. Hui Xing et al, Phys. Rev. B., 81, 134426(2010).
3:42 PM
BC-10. Berry Phase of A Randomly Fluctuating Magnetic Field
Ralph Skomski
Physics and Astronomy, Univ Nebraska, Lincoln, NE
Since its discovery, the Berry phase [1] has revolutionized quantum mechanics and perpetrated many areas of physics, including magnetism. It helps us, for example, to understand the orbital magnetic moment of itinerant electrons [2] and the fundamental physics behind the Aharonov-Bohm effect. In fact, the Berry phase of a spin-1/2 particle in an adiabatically changing magnetic field is one of the first examples of the phase [1]. In our presentation, we determine the Berry phase caused by randomly fluctuating magnetic fields, such as environmental fields. In this case, the phase is well-defined when the field forms a closed loop. Furthermore, due to the adiabatic nature of the phase, the field change much must be slow compared to any ongoing quantum-mechanical processes, which is typically the case for macroscopic magnetic fields. The Berry phase is given by the solid angle enclosed by the field, that is, by the corresponding area on the unit sphere. To determine this area, we exploit a very similar problem in polymer physics, namely the two-dimensional random walk [3] with closed loops, which is used to describe topologically constrained polymer chains. As shown by Khandekar and Wiegel [4], the problem can be solved by a path-integral technique and leads to an analytical expression for the area distribution. A key quantity in the polymer random walks is the statistical or Kuhn segment length, whose magnetic analog is the time-correlation function of the fluctuating magnetic field. The second important field-loop parameter is the angular velocity of the field. We calculate the distribution of the Berry phases and the corresponding second moment, that is, the average squared Berry phase, and derive power laws for the Berry phase as a function of the field-loop parameters. In particular, the Berry phase depends only indirectly on the angular velocity of the, namely by affecting the total area, which is in agreement with the adiabatic nature of the phase. — This research is supported by NSF-MRSEC and NCMN.
References
[1] M. V. Berry, Proc. R. Soc. Lond. A 392, 45 (1984). [2] D. Xiao, M.-Ch. Chang, and Q. Niu, Rev. Mod. Phys. 82, 1959 (2010). [3] R. Skomski, Simple Models of Magnetism, University Press, Oxford 2008. [4] D. C. Khandekar and F. W. Wiegel, J. Phys. A 21, L563 (1988).
3:54 PM
BC-11. Manipulation of spin interaction in FM/graphene/FM trilayer structures
Jisang Hong
physics, Pukyong National Univ, Busan, Republic of Korea
Using the full potential linearized augmented plane wave (FLAPW) method, we report the theoretical results that the carrier induced spin switching of magnetic interaction between two magnetic layers in Co(111)/Graphene/Ni(111) (Co/Gr/Ni) is predicted. The Co/Gr/Ni shows an antiferromagnetic (AFM) ground state when there is no external carriers. The antiferromagnetic interaction is still observed for hole carriers. Very interestingly, however, it is observed that the magnetic exchange interaction between Ni and Co layers can be manipulated to display from AFM to ferromagnetic (FM) state by injecting external carriers. Overall, we propose that the potential spin switching by external carriers or electric field can be realized in Co/Gr/Ni. Besides, the calculated DOS feature indicates that the Co/Gr/Ni system may manifest quite different transport property by applying small bias voltage. For instance, the current parallel to the film surface can be completely spin polarized from minority spin electrons. In contrast, the current perpendicular to the film surface will be positively spin polarized from majority spin electrons. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No.R01-2008-000-20014-0) and by the National Research Foundation (NRF) of Korea grant funded by the the Korea government (MEST) (No.2010-0028604).
4:06 PM
BC-12. Metallic state in Eu2Ru2O7 induced by hole creation or orbital overlap in the t2g bands
Susset Muñoz Pérez1, Raiden Andres Cobas Acosta1, Sean Cadogan1, José Albino Oliveira de Aguiar2, Pierre Bonville3, Teresa Puig4 and Xavier Obradors4
1Department of Physics and Astronomy, University of Manitoba, Winnipeg, R3T 2N2, MB, Canada; 2Departamento de Física, Universidade Federal de Pernambuco, Recife, Brazil; 3Service de Physique de l'Etat Condensé, CEA-CNRS, CE-Saclay, 91191 Gif-sur-Yvette, France; 4Institut de Ciència dels Materials de Barcelona, CSIC, Campus de la UAB, Bellaterra 08193, Spain
Two different series of pyrochlore ruthenates with the compositions Eu2-xCaxRu2O7 (0 ≤ x ≤ 0.60) and Eu2Ru2-xRexO7 (0 ≤ x ≤ 0.40) were prepared by solid state reaction and their magnetic and transport properties investigated. The two series correspond to different approaches to inducing a metal - insulator transition in the Eu2Ru2O7 parent compound: i) the substitution of Eu3+ by Ca2+ , resulting in hole creation within the Ru t2g bands, and ii) the partial substitution of Ru4+ by Re4+, resulting in an increase of the orbital overlap. In the first case we have found a structural distortion resulting from the difference between the Eu3+ and Ca< sup>2+ 8-coordinate ionic radii. Such a distortion does not exist in the second case, since the 8-coordinate ionic radii of the Ru4+ and Re4+ are almost identical. This study facilitates the analysis of the role played by the crystal structure in determining the physical properties of these new series. The two systems exhibit a transition from a Mott-insulator to a metallic state with increasing Ca or Re concentration. The dc magnetic susceptibility under zero-field-cooled (ZFC) and field-cooled conditions in Eu2Ru2O7 diverges below 118 K. With decreasing temperature the ZFC susceptibility shows a pronounced peak at around 23 K; the temperature of this maximum decreases with increasing Ca concentration. The suppression of both magnetic anomalies has been observed with increasing Ca or Re concentration. The 151Eu Mössbauer spectra of all samples show that Eu is trivalent and the hyperfine magnetic field at the Eu site decreases by 34 % when the calcium content is equal to x = 0.34. The temperature dependence of the hyperfine magnetic field in Eu1.66Ca0.34Ru2O7 shows a slight change around 23 K. Summarizing, the ruthenium oxides display very appealing magnetic and transport properties which originate solely in the t2g levels of ruthenium, a distinguishing feature compared to the manganites where different bands (t2g and eg) are involved.
4:18 PM
BC-13. Ferromagnetic Resonance in Micro- and Nano-sized Hexagonal Ferrite Powders at Millimeter Waves
Mohammed N. Afsar1, Konstantin A. Korolev1, 3 and John S. McCloy2
1Electrical and Computer Engineering, Tufts Univ, Medford, MA; 2Glass and Materials Science Team, Pacific Northwest National Laboratory, Richland, WA; 3Extremely High Frequency Medical and Technical Association, Moscow, Russian Federation
Ferromagnetic resonance behavior on micro- and nano-sized powdered barium ferrite and strontium ferrite have been studied over a broadband millimeter wave frequency range for the first time. Ferrites and garnets are mostly ferromagnetic oxides with dielectric and magnetic properties that are useful and important for microwave and millimeter wave applications [1]. The garnets and spinels with high quality of crystal structures and material components dispose of different electric and magnetic microwave and millimeter wave characteristics depending on its composition [2]. M-type hexagonal barium ferrite with its stoichiometric chemical formula BaFe12O19 has been well established as permanent magnet material and high density magnetic recording media [3], [4]. A free space magneto-optical approach has been successfully employed to study ferrites in millimeter waves [5]. This technique enables to obtain precise transmission spectra to determine the dielectric and magnetic properties of both isotropic and anisotropic ferrites in the millimeter wave frequency range from a single set of direct measurements. The transmittance spectra of barium and strontium ferrite micro- and nano-powdered materials have been recorded in millimeter waves. A zone of relatively deep and rather wide absorption in transmittance spectra has been observed for both micro- and nanoferrites. This deep absorption is the natural ferromagnetic resonance that shifts to millimeter waves due to the strong magnetic anisotropy of barium and strontium ferrites [5]. The resonance frequency of nanoferrites shifts to the lower frequency compared to micro-sized barium and strontium ferrite materials. The resonance phenomena of the ferrites can be described by both domain wall motion and spin resonance [6]. Polycrystalline ferrites with he xagonal crystal structure have been recently studied in microwave and millimeter waves [7]. The rotation of the magnetization vector in the effective anisotropy field and the demagnetization factors of the grains strongly affect the resonance frequency. The spin rotational relaxation, becoming pronounced in the high frequency range, depends only on the volume loading of the ferrite and the dispersion parameters.
References
[1] J. Smit, H. P. J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1961. [2] Y. W. Buchner, R. Schliebs, G. Winter, K. H. Buchel, Industrial Inorganic Chemistry, V.C.H. Weinheim, 1989. [3] H. Pfeiffer, R. W. Chantrell, P. Görnert, W. Schüppel, E. Sinn and M. Rösler, “Properties of barium hexaferrite powders for magnetic recording”, J. Magn. Magn. Mater. v. 125, pp.373-376, 1993. [4] T. Sakai, Y. Chen, C. N. Chinnasamy, C. Vittoria, and V. G. Harris, “Textured Sc-doped barium-ferrite compacts for microwave applications below 20 GHz”, IEEE Trans. Magn., vol. 42, pp. .3553-3555, Oct. 2006. [5] K. A. Korolev, S. Chen, and M. N. Afsar, “Millimeter-wave transmittance measurements on ferrite near ferromagnetic resonance”, IEEE Trans. Instrum. Meas., vol. 57, pp. 1388-1393, July 2008. [6] G. T. Rado, “Theory of the microwave permeability tensor and Faraday effect in nonsaturated ferromagnetic materials,” Phys. Rev., vol. 89, pp. 529, 1953. [7] V. A. Zhuravlev and V. I. Suslyaev, "Analysis of the microwave magnetic permeability spectra of ferrites with hexagonal structure", Russian Physics Journal, vol. 49, pp. 1032-1037, 2006.
1:30 PM - 4:30 PM, Grand Canyon 9-11
Chair: Sang-Koog Kim, Seoul National University
1:30 PM
BD-01. Tunable negligible-loss energy transfer between dipolar-coupled magnetic disks by stimulated vortex gyration
Hyunsung Jung1, Ki-Suk Lee1, Dae-Eun Jeong1, Youn-Seok Choi1, Young-Sang Yu1, Dong-Soo Han1, Andreas Vogel2, Lars Bocklage2, Guido Meier2, Mi-Young Im3, Peter Fischer3 and Sang-Koog Kim1
1Research Center for Spin Dynamics & Spin-Wave Devices, Nanospinics Laboratory, and Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul Natl Univ, Seoul, Republic of Korea; 2Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg, Hamburg, Germany; 3Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA
A wide variety of coupled harmonic oscillators exist in nature Coupling between different oscillators allows for the possibility of mutual energy transfer between them and the information-signal propagation. Low-energy input signals and their transport with negligible energy loss are the key technological factors in the design of information processing devices. Here, utilizing the concept of coupled oscillators, we experimentally demonstrated a robust new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks - stimulated vortex gyration. Direct experimental evidence was obtained by a state-of-the-art experimental time-resolved soft X-ray microscopy probe (ALS, beamline 6.1.2) [1]. The rate of energy transfer from one disk to the other was deduced from the two normal modes’ frequency splitting caused by dipolar interaction [2]. Control of energy loss during gyration-mediated signal transfer is possible with material engi neering; in fact, almost lossless energy transfer can be achieved by employment of a material having negligible damping. Vortex gyration also can be achieved with low-power consumption through the resonant vortex excitation. This robust mechanism for energy transfer provides the advantages of lossless energy transfer and low-power signal input[1,2,3].
References
[1] H. Jung et al., Appl. Phys. Lett. 97, 222502 (2010). [2] H. Jung et al., arXiv:1011.6399 (2010). [3] A. Vogel et al., Phys. Rev. Lett. 106, 137201 (2011). [4] This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20110000441). *Corresponding author: sangkoog@snu.ac.kr, Phone: +82-2-880-5854, Fax: +82-2-885-1457
1:42 PM
BD-02. Orbital trajectory characteristics of a current-driven magnetic vortex
Kristen S. Buchanan1, S. D. Pollard2, L. Huang2, D. A. Arena3 and Y. Zhu2
1Department of Physics, Colorado State University, Fort Collins, CO; 2Department of Condensed Matter Physics, Brookhaven National Laboratory, Brookhaven, NY; 3National Synchrotron Source, Brookhaven National Laboratory, Brookhaven, NY
Vortices in submicron-sized patterned magnetic elements have been found to exhibit translational dynamic motion at sub-GHz frequencies when excited by magnetic fields or by currents via spin torque effects. There is still debate, however, surrounding the relative contributions of non-adiabatic and adiabatic spin torque effects, a question that is of both technological and fundamental importance. Furthermore, Oersted effects are often non negligible when currents flow through magnetic nanostructures. Here we use micromagnetic simulations to calculate current-driven vortex motion incorporating field (Oersted) as well as both nonadiabatic and adiabatic spin torque effects and compare the obtained vortex trajectories with those calculated using a simple phenomenological model [1]. We examine the effects of the relative contributions of the driving terms on the core trajectory amplitude, tilt (Fig. 1), ellipticity, and phase. The results indicate the orbital characteristics can be used to extract the non-adiabatic spin torque parameter with greatly improved precision using a recently developed dynamic TEM imaging technique. The work was supported by NSF award number 0907706 and U.S. Department of Energy, Office of Basic Energy Science, Material Sciences and Engineering Division, under Contract No. DE-AC02-98CH10886.
References
[1] Kruger et al. PRL 104, 077201 (2010).
1:54 PM
BD-03. Field- and current-induced domain-wall motion in permalloy nanowires with magnetic soft spots
Andreas Vogel1, Sebastian Wintz2, Theo Gerhardt1, Lars Bocklage1, Thomas Strache2, Mi-Young Im3, Peter Fischer3, Jürgen Fassbender2, Jeffrey McCord2 and Guido Meier1
1Institute of Applied Physics, University of Hamburg, Hamburg, Germany; 2Institut für Ionenstrahlphysik und Materialforschung, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 3Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA
Ne w concepts of high-density and ultrafast nonvolatile data storage devices involve the controlled motion of magnetic domain walls (DWs) in nanowires [1]. To realize such a device, reproducible and reliable pinning sites for individual DWs are required. Geometric constrictions are widely used to create local confining potentials acting as pinning sites [2]. As an alternative, pinning sites can be induced via a local modification of magnetic properties by ion irradiation [3]. In this case, a variation in the wire geometry on the nanoscale is not required. Implantation of chromium ions into permalloy is known to cause alloying and structural defects which lead to a reduction in the saturation magnetization MS, and the magnetic anisotropy as well as to a change in the exchange constant and the damping parameter [4]. The strength of the pinning potential can be tuned by the chromium ion fluence applied to induce the so-called magnetic soft spots [3]. Micromagnetic simulations, high resolution magnetic transmission soft X-ray microscopy at beamline 6.1.2 of the Advanced Light Source in Berkeley, CA, USA, and electrical measurements of the anisotropic magnetoresistance are employed to characterize the pinning potential which significantly differs for transverse and vortex walls. We demonstrate field-induced DW pinning and depinning as well as reliable DW depinning by single current pulses in a permalloy nanowire containing a square-shaped magnetic soft spot [5]. Lower requirements on the lithography in comparison to geometric constrictions on the nanoscale, a smaller distribution of properties due to parallel processing during implantation, and fine tunability of the pinning potential via the chromium ion fluence make the magnetic soft spots a promising candidate for applications. Financial support by the Deutsche Forschungsgemeinschaft via Grant Nos. FA314/3-2 and MC9/7-2, the SFB 668 and the GrK 1286 as well as the Forschungs- und Wissenschaftsstiftung Hamburg via the Exzellenzcluster “Nano-Spintronik” is gratefully acknowledged. Operation of the X-ray microscope is supported by the US Department of Energy under Contract No. DE-AC02-05-CH11231.
References
[1] D. A. Allwood, Gang Xiong, M. D. Cooke, C. C. Faulkner, D. Atkinson, N. Vernier, R. P. Cowburn, Science 296, 2003 (2002); S. S. P. Parkin, U. S. Patent No. US 683 400 5 (2004). [2] M.-Y. Im, L. Bocklage, P. Fischer, and G. Meier, Phys. Rev. Lett. 102, 147204 (2009). [3] A. Vogel, S. Wintz, J. Kimling, M. Bolte, T. Strache, M. Fritzsche, M.-Y. Im, P. Fischer, G. Meier, and J. Fassbender, IEEE Trans. Mag. 46, 1708 (2010). [4] J. Fassbender and J. McCord, J. Magn. Magn. Mater. 320, 579 (2008). [5] A. Vogel, S. Wintz, T. Gerhardt, L. Bocklage, T. Strache, M.-Y. Im, P. Fischer, J. Fassbender, J. McCord, and G. Meier, Appl. Phys. Lett. 98, 202501 (2011).
2:06 PM
BD-04. Magnetic vortex core reversal by excitation of spin waves
Hermann Stoll1, Matthias Kammerer1, Markus Weigand1, Michael Curcic1, Matthias Noske1, Markus Sproll1, Arne Vansteenkiste2, Bartel Van Waeyenberge2, Georg Woltersdorf3, Christian H. Back3 and Gisela Schuetz1
1MPI for Intelligent Systems, (formerly MPI for Metals Research), Stuttgart, Germany; 2Department of Solid State Sciences, Ghent University, Ghent, Belgium; 3Department of Physics, Regensburg University, Regensburg, Germany
Thin-film soft magnetic platelets of suited micron or sub-micron size are characterized by an in-plane closed flux magnetization, minimizing the dipolar energy. However, at the center, the exchange energy forces the magnet ization out-of-plane in a small area of only a few exchange lengths in diameter creating the vortex core with a distinct polarization, either up or down. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field vortex core switching by (sub-GHz) excitation of the vortex gyromode was found [1]. This reversal process, based on the creation and subsequent annihilation of a vortex-antivortex (VA) pair as suggested in [1], appeared to be universal and independent of the type of excitation. This discovery did not only open up the possibility for using magnetic vortex core reversal for spintronics applications, but also initiated wide investigations of the physics behind the VA mediated switching mechanism. At frequencies more than an order of magnitude higher than those of the vortex gyromode, vortex state structures possess spin wave modes arising from the magneto-static interaction. The presence of the vortex core modifies the high-frequency spin wave normal mode spectrum by lifting the degeneracy of the azimuthal modes with opposite rotation sense. We have demonstrated experimentally core reversal by exciting spin waves in vortex structures with multi-GHz rotating magnetic fields [2]. The vortex core polarization can be switched unidirectionally, either to up or down, as excitation only takes place when the sense of rotation of the external field and the spin wave mode are the same. These experimental results are in good agreement with micromagnetic simulations [2], which clearly show: (i) the selection rules for this vortex core reversal process, (ii) the creation of a VA pair which is also essential for spin wave mediated vortex core reversal, (iii) asymmetries in vortex - spin wave interaction, caused by the gyrofield of the moving vortex, when spin waves with opposite rotation senses are excited. Reversal times of vortex core polarization down to below 100 ps can be achieved by spin wave excitation. This opens a path to very fast V(ortex)MRAMs.
References
[1] B. Van Waeyenberge, Nature 444, 462 (2006) [2] M. Kammerer et al., Nature Communications 2, 279 (2011)
2:42 PM
BD-05. Thickness dependence of the gyrotropic mode of a single magnetic vortex
Te-Yu Chen1, Michael Erickson1, Andrew Galkiewicz1, Chris Leighton2 and Paul Crowell1
1School of Physics and Astronomy, University of Minnesota, Minneapolis, MN; 2Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN
The ground state of a micron or sub-micron diameter ferromagnetic disk is often a single magnetic vortex, where the gyrotropic mode of the vortex core is the lowest frequency excitation. In thin disks (thickness L << diameter D), the vortex can be treated two-dimensionally (2D) and the gyrotropic frequency (fG) is determined by L/D. Additionally, defects on length scales smaller than the core size cause fluctuations in the confining potential, enhancing fG when a vortex is pinned. However, two questions remain: how the defect-induced enhancement of fG depends on L, and the crossover from 2D to 3D dynamics with increasing L. Using time-resolved Kerr microscopy, we have investigated the gyrotropic mode in 1-μm-diameter Ni80Fe20 disks as a function of L, varied from 20 nm to 200 nm. In thin disks (L < 80 nm) the unpinned fG (squares in Fig. 1) is proportional to L, consistent with 2D micromagnetic simulations (dashed line) and analytical models. In the same regime, the enhancement of fG due to pinning var ies as 1/ L, which reflects the existence of a single length scale characterizing the pinning interaction. For L > 100 nm, we observe a deviation between the experimental values of fG and the 2D simulations, and an additional mode appears at higher frequencies. A quasi-3D micromagnetic simulation (solid lines) reproduces this spectrum, which consists of two gyrotropic modes. In one mode the core oscillates uniformly through the thickness of the disk, while in the other mode the core oscillates with larger amplitude at the surfaces, and with a node in the equatorial plane of the disk. These findings thus provide new insight into the 2D-3D crossover for vortices in ferromagnetic disks.
3:18 PM
BD-06. X-ray Imaging of Nonlinear Resonant Gyrotropic Magnetic Vortex Core Motion in Circular Permalloy Disks
Brooke L. Mesler1, Kristen S. Buchanan2, Mi-Young Im1 and Peter Fischer1
1CXRO, LBNL, Berkeley, CA; 2Dept of Physics, CSU, Fort Collins, CO
We report experimental evidence of nonlinear gyrotropic vortex core motion. Using soft x-ray transmission microscopy we observed the time-averaged dynamic response of a magnetic vortex core in a 2μm diameter, 100 nm thick permalloy (Ni80Fe20) disk as a function of the amplitude and frequency of an applied RF magnetic field. By directly imaging the vortex core, frequencies where the core responded dynamically to the applied field could be identified. At lower amplitude fields a single resonance was observed, but two distinct resonances, above and below the low amplitude resonance frequency, were observed when higher amplitude fields were applied. The results are discussed in the context of a nonlinear vortex energy potential.
References
T. Shinjo, T. Okuno, R. Hassdorf, K. Shigeto, and T. Ono, Science 289, 930 (2000) K. Y. Guslienko, B. A. Ivanov, V. Novosad, Y. Otani, H. Shima, and K. Fukamichi, J. Appl. Phys. 91, 8037 (2002). P. Fischer, M.-Y. Im, S. Kasai, K. Yamada, T. Ono, A. Thiaville, Phys Rev B 83 212402 (2011) K.-S. Lee, S.-K. Kim, Y.-S. Yu, Y.-S. Choi, K. Y. Guslienko, H. Jung, and P. Fischer, Phys Rev Lett 101, 267206 (2008) K. S. Buchanan, P. E. Roy, M. Grimsditch, F. Y. Fradin, K. Yu. Guslienko, S. D. Bader, and V. Novosad Phys. Rev. B 74, 064404 (2006) K. S. Buchanan, M. Grimsditch, F. Y. Fradin, S. D. Bader, and V. Novosad, Phys. Rev. Lett. 99,267201 (2007).
3:30 PM
BD-07. Evidence of vortex-antivortex pair nucleation in the pinned layer of nanocontact vortex oscillators
Ruben M. Otxoa1, 2, Sébastien Petit-Watelot1, 2, Mauricio Manfrini3, 4, Thibaut Devolder1, 2, Joo-Von Kim1, 2, Arne Vansteenkiste5, Ben Van de Wiele6, Wim Van Roy3, 4 and Liesbet Lagae3, 4
1Institut d'Electronique Fondamentale, Paris, France; 2Université Paris-Sud, Paris, France; 3imec, Leuven, Belgium; 4Physics and Astronomy department, K.U. Leuven, Leuven, Belgium; 55. Department of Solid State Sciences, Ghent University, Ghent, Belgium; 6Department of Electrical Energy, Systems and Automation, Ghent University, Ghent, Belgium
The phenomenon of spin transfer torque provides a new method to manipulate magnetization under different geometries without using any external magnetic field[1]. In the case of nanocontact geometry the current passes through a small contact area (~60 nm radius) and is injected into a magnetic multilayer. The Oersted-Ampere field associated with this current can nucleate a vortex under certain conditions[2]. Although the key features of the nucleation process in the free layer have been reported[3], the role of the current on the exchange biased pinned layer remains unknown. We provide conclusive evidence that the pinned layer possesses a vortex-antivortex pair under the nanocontact above certain value of the current. We find that the role of the exchange bias is to confine the pair below the nanocontact. The nucleation of the pair leads to a distinct jump in the frequency of the free layer oscillation mode and a different current tunability (Figure). The coexistence of the two branches indicates the existence of the pair is intermittent, which suggests a thermally-activated phenomenon. Good agreement is found with micromagnetic simulations.
References
[1] M. R. Pufall et al., Phys. Rev. B 75, 140404 (2007). [2] Q. Mistral et al., Phys. Rev. Lett. 100, 257201 (2008). [3] T. Devolder, et al.,Appl. Phys. Lett. 97,072512 (2010).
3:42 PM
BD-08. All Electrical Operation of Vortex Core Memory Cell
Kunihiro Nakano1, Daichi Chiba1, 2, Norikazu Ohshima3, Shinya Kasai4, Tomonori Sato5, Yoshinobu Nakatani5, Koji Sekiguchi1, Kensuke Kobayashi1 and Teruo Ono1
1Institute for Chemical Research, Kyoto University, Uji, Japan; 2PRESTO, Japan Science and Technology Agency, Kawaguchi, Japan; 3NEC Corporation, Sagamihara, Japan; 4Magnetic Material Centre, National Institute for Materials Science, Tsukuba, Japan; 5Department of Computer Science, University of Electro-Communications, Chofu, Japan
The magnetization in a ferromagnetic nanodisk swirls around in the plane of the disk, and at the center of this magnetic vortex it points either up or down. This binary state can be used for information storage, and the magnetic vortex is therefore promising for use in future nonvolatile data storage devices, where the bit data is stored as the direction of the nanometer-scale core magnetization. To realize such a vortex core memory, it is indispensable to develop techniques for electrically switching and detecting the direction of the core magnetization for data writing and reading. Although it was shown that the core magnetization can be switched by using the vortex core dynamics induced by either applying a magnetic field or injecting an electric current into a disk[1,2], the detection of the direction of the core magnetization needs to use microscopic techniques, which are impractical for device applications. Moreover, they are not sensitive enough to detect the single switching event. In the contribution, we demonstrate all-electrical operation of the magnetic vortex core memory cell by using a three-terminal device in which the tunneling magnetoresistance junction is integrated onto a ferromagnetic disk. Binary data corresponding to the core direction can be read out electrically as the amplitude of the device output[3], while the data can be written electrically by applying a pulsed current.
References
[1] K.Yamada et al., Nature Mat. 6, 269 (2007). [2] B. Van. Waeyenberge et al., Nature 444, 461(2006). [3] K. Nakano et al., APEX. 3, 053001 (2010).
3:54 PM
BD-09. Thermo-mechanical sensitivity calibration and magnetometry of permalloy disks via mechanical transduc tion of nano-paddle resonators.
Joseph E. Losby1, 2, Jacob Burgess1, 2, David C. Fortin1, Zhu Diao1, 2, Wayne K. Hiebert2 and Mark R. Freeman1, 2
1Physics, University of Alberta, Edmonton, AB, Canada; 2National Institute for Nanotechnology, Edmonton, AB, Canada
The combination of improving nanofabrication and detection techniques, along with technological demand, are providing an impetus for further understanding of the micromagnetic structure and dynamics of small magnetic devices. The exquisite sensitivity of torsional nanomechanical resonators provides for a means of investigating single magnetic elements. We report a method for displacement sensitivity calibration through the thermo-mechanical noise detected in the devices and perform magnetometry on disks with and without edge defects. The resonators are based on silicon-on-insulator substrates, with a device layer 300nm thick and a 1μm deep sacrificial silicon oxide layer. Following the release of the structures; individual permalloy disks were placed on the paddles through selective shadow masking and collimated e-beam deposition. The detection of the thermo-mechanical noise (the deflection due to Brownian motion at the first torsional resonance frequency) provides for a means of displacement calibration of the resonator, which then allows for the determination of the sensitivity of the device. Through quantitative analysis, the torque sensitivities of the mechanical resonators were determined. Magnetic hysteresis measurements of both a smooth-edged permalloy disk and one with a notched edge defect were acquired through the driven response of the resonators. Micromagnetic simulations support the existence of single magnetic vortices in the ground states of both disks though, due to the pinning and de-pinning of the magnetization in the notched disk, a more complex set of nucleation and annihilation events occur.
4:06 PM
BD-10. Robust switching of swirls for coupled ferromagnetic nanobricks with the Landau structure.
Anthony S. Arrott
Physics, Simon Fraser University, Burnaby, BC, Canada
Swirls on the surfaces of a ferromagnetic nanobrick (120 x 80 x 50 nm3) can be driven (switched) over a distance of 50 nm in 1 ns using the gradient magnetic field from a 1 mA current along the long (x) axis. The switching is a complicated motion of the vortex structure (Bloch wall) that connects the two swirls on opposite largest surfaces of the nanobrick. In switching the swirls pass each other as the magnetostatic energy goes through a maximum with the Bloch structure roughly approximating a simple vortex with its core along the z-axis. The 0.5 T external fields that accompany the swirls have several anticipated uses for nanotechnology, including memory elements that are not sensitive to surface conditions. Two nanobricks with their largest surfaces separated by 4 nm can switch using the smaller fields from 0.1 nm currents if the two nanobricks are in a magnetic state with inversion symmetry. The favorable magnetostatic interaction between the swirls on the facing surfaces of the two nanobricks lowers the magnetostatic barrier for switching. The micromagetics of such devices is investigated using Scheinfein’s LLG simulator for finite grid calculations on 2 nm grids for Fe with a damping parameter α = 0.02. The necessary amplitude and waveform of the current for switching depends on the modes of oscillation of the vortex structure. When the current pumps sufficient energy into the system to overcome the magnetostatic barrier, it is still essential to get the correct timing of the prece ssion to achieve switching with pulses. With small ac currents, though highly non-linear, the system has a resonant response such that, after a few cycles, the vortex structure self-organizes to make it over the barrier. Extensive studies of the dynamic response of these complex systems are presented in this work to find the conditions for highly stable configurations with robust switching characteristics. The period of the ac is doubled from 440 ps for the single nanobrick to 880 ps for the pair of nanobricks when the two Landau structures together preserve inversion symmetry.
4:18 PM
BD-11. Switching rates in exchange-coupled media: moment landscapes
Pieter B. Visscher and Ru Zhu
Physics and MINT Center, University of Alabama, Tusc aloosa, AL
We will describe a new method for the visualization of energy landscapes in magnetic nanostructures, which allows easy identification of switching mechanisms and accurate calculation of their rates. We illustrate the method for the case of switching of a grain of an exchange-coupled recording medium, which switches through domain wall nucleation and motion, but the method is generalizable to other rate processes such as vortex formation and annihilation. The method involves calculating the most probable (lowest energy) switching path [1,2] and projecting the motion onto that path. The motion is conveniently visualized in a two-dimensional projection (Fig. 1) parameterized by the dipole and quadrupole moments of the grain. The motion along that path can then be described by a 1D Langevin equation, and its rate (or equivalently the attempt frequency) can be computed by the classic method of Kramers and Brown[3,4]. This can be done efficiently in high-barrier systems for which direct simulation is impractical. The rate can be obtained numerically, but we will also describe an analytic approximation which gives a formula very similar to Brown's 1963 formula for coherent switching.
References
1. G. Henkelman and H. Jónsson, J. Chem. Phys. 113, 9978 (2000). 2. R. Dittrich, T. Schrefl, D. Suess, W. Scholz, H. Forster, and J. Fidler, J. Magn. Mag. Mat., 250 (2002). 3. H. A. Kramers, Physica VII, 284 (1940). 4. W. F. Brown, Phys. Rev. 130, 1677 (1963).
1:30 PM - 4:30 PM, Grand Canyon 2-3
Co-Chair: Mathew Kramer, Ames Laboratory Materials Science and Engineering Iowa State University; Co-Chair: Parashu Kharel, University of Nebraska, Lincoln
1:30 PM
BE-01. Novel Exchange Anisotropy in Nanostructured MnAl Alloys
T. Sepehrifar1, F. Jimenez-Villacorta1, J. L. Marion1, M. Daniil2, M. Willard3 and L. H. Lewis1
1Department of Chemical Engineering, Northeastern University, Boston, MA; 2Department of Physics, George Washington University, Washington DC, DC; 3Multifunctional Materials Branch, U.S. Naval Research Laboratory, Washington DC, DC
Near-equiatomic AlMn alloys have promising magnetic properties with good potential for advanced permanent magnet applications. While large coercivities have been reported previously in the tetragonal τ-Al50Mn50 compound, the magnetic character of other phases in the Al-Mn system has not been extensively reported . Here we report on unprecedented exchange anisotropy with large values (HE ≈ 13 kOe at 10 K) in nanostructured MnAl alloys fabricated by rapid solidification. It is anticipated that this novel behavior may be understood and developed for advanced permanent magnets. Quenched Mn50Al50 alloys have been fabricated by melt-spinning in He gas at a wheel speed of 64 m/s. X-ray diffraction shows the presence of rhombohedral γ2- and high-temperature metastable hexagonal ε-phases. As-quenched samples were characterized by SQUID magnetometry: temperature-dependent magnetization curves measured at 1 kOe carried out in zero-field-cooled (ZFC) and field-cooled (FC) conditions highlight a blocking temperature around Tpeak ~ 95 K. FC magnetization loops show a prominent exchange bias for T < 95 K that is not present under ZFC conditions. This observed exchange bias is hypothesized to originate from an intimate nanoscaled mixture of antiferromagnetic and ferromagnetic phases with a system blocking temperature of 95 K in the as-quenched state. The current results may be tentatively understood within the framework of a cluster glass that exhibits variations in the local Mn concentrations of the composition, leading to Mn-rich and Mn-poor regions which are likely to exhibit antiferromagnetic and ferromagnetic character, respectively. This research has been funded by the Office of Naval Research (ONR) under Grant # N00014-10-1-0553
1:42 PM
BE-02. Instability of ferromagnetic ground state in Lu2Fe17-XMnX
Zdenek Arnold1, Anatoly G. Kuchin2 and Jiri Kamarad1
1Magnetics and Superconductors, Institute of Physics AS CR, v.v.i., Prague, Czech Republic; 2Institute of Metal Physics, Ekaterinburg, Russian Federation
The substitution of Fe by Mn in Lu2Fe17 leads to an increase of transition temperature from ferromagnetic (FM) to antiferromagnetic (AF) state from 140 K to 272 K for Lu2Fe16.5Mn0.5 and to a complete suppression of AF state for higher Mn concentration [1]. Such strengthening of FM was unexpected in R2Fe17-XMnX compounds and the increase of stability of FM has been ascribed to the volume increase of elementary cell of about 0.3 % going from Lu2Fe17 to Lu2Fe16.3Mn0.7 [2]. To verify the effect of volume changes induced by the Mn substitution, we have studied magnetic properties of polycrystalline Lu2Fe16.5Mn0.5 and Lu2Fe16.3Mn0.7 compounds that are on the verge of AF state, under hydrostatic pressure up to 10 kbar. The measurements were performed in SQUID magnetometer using a CuBe pressure cell in temperature range 5-300 K and at magnetic fields up to 7 T. We have revealed that FM state is very sensitive to the volume changes and even moderate pressure is sufficient to its complete suppression. The critical pressures are 5.8 kbar for Lu2Fe16.5Mn0.5 and 7.1 kbar for Lu2Fe16.3Mn0.7(3.5 kbar for Lu2Fe17 [3] ). The suppression of the FM ground state is observed for practically the same volume of elementary cell (500 Å3), using the compressibility value of 1.0 Mbar-1. The pressure induced Néel temperature TN of both compounds decreases with increasing pressure, dTN/dp = -2.5 K/kbar and dTN/dp = - 3.4 K/kbar for Lu2Fe16.5Mn0.5 and Lu2Fe16.3Mn0.7, respectively (dTN/dp = -1.9 K/kbar for Lu2Fe17 [3]). The remarkable stren gthening of the FM ground state by the Mn substitution and its total suppression by external pressure can be attributed mainly to a subtle competition of positive and negative local direct exchange interactions between Fe atoms located at inequivalent structural sites. These interactions are strongly dependent on the interatomic Fe-Fe distances [4].
References
[1] A.G. Kuchin, W. Iwasieczko, H. Drulis, V.I. Khrabrov, Solid State Comm. 146, 446 (2008) [2] W. Iwasieczko, A. G. Kuchin, H. Drulis, J. Phys.: Condens. Matter 21 306002, (2009) [3] J. Kamarad, Z. Arnold, I. V. Medvedeva, A. G. Kuchin, J. Magn. Magn. Mater. 242, 876, (2002) [4] J. Kamarad, M. Misek, K. Prokes, S. Matas and Z. Arnold, J. Phys.: Conf. Ser., Proceedings ECNS 2011, accepted
1:54 PM
BE-03. Mechanism of MnBi magnetic anisotropy: Role of higher order contributions.
Oleg N. Mryasov1, J. Park2, Y-k Hong2, S. Faleev1 and G. Mankey1
1Physics and MINT, University of Alabama, Tuscaloosa, AL; 2ECE and MINT, Unversity of Alabama, Tuscaloosa, AL
NiAs-type MnBi is known for its unusual magnetic anisotropy characterized by a small negative anisotropy constant at T= 0 K of about -0.2*106 J/m3 and a large positive easy axis anisotropy reaching a maximum value of about +2.4*106 J/m3 at T = 450 K [1,2]. Despite importance of this unusual positive temperature coefficient for applications , the origin of this behavior has not been understood on the quantitative level. We propose a model which explains the experimentally observed temperature dependence of Keff(T) = K1 + K2 + K3. This model is inspired by results of our density functional-local spin density approximation (DFT-LSDA) calculations of the spin and orbital moments and the anisotropy constants. We find that the calculated (T = 0 K) value of Keff(0) = -0.29*106 J/m3 is in a good agreement with available measurements (-0.25*106 J/m3 [1,2]). We also find that |K2| is unusually large (larger than K1) and gives an easy plane. Following these findings we analyse available experimental data on the temperature dependence of effective anisotropy constant Keff (T) and show that the Keff(T) dependence can be well described within the proposed model only with a very large negative K2 constant (Fig.1).
References
[1] Tu Chen and W. E. Stutius , IEEETrans. Magn. 10, 581 (1974). [2] X.Guo et.al., Phys. Rev. B. 46, 1478 (1992).
2:06 PM
BE-04. Structural and electrical properties of half-Heusler LaPtBi thin films grown by 3-source magnetron co-sputtering
Tetsuya Miyawaki1, Nozomi Sugimoto1, Naoto Fukatani1, Tatsuhiko Yoshihara1, Kenji Ueda1, Nobuo Tanaka2 and Hidefumi Asano1
1Dept. of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Nagoya, Japan; 2EcoTopia Institute, Nagoya University, Nagoya, Japan
Topological insulators have attracted much attention due to the dissipationless and spin-polarized electrons on the surface state. Recently, some half-Heusler alloys such as LaPtBi have been predicted as 3D topological insulators (TIs) [1]. Multilayers of TIs and ot her half-Heusler alloys such as semiconductors and ferromagnetic materials will enable all-Heusler-alloy spintronics devices. In this study, we have investigated structural and electrical properties of LaPtBi thin films fabricated on YAlO3 (001) substrate with various growth conditions. By three-target magnetron co-sputtering, we have obtained c-axis oriented, single-phase LaPtBi thin films for the first time. For the precise control of film composition, deposition rates were independently varied for each target. In Fig. 1(a), a LaPtBi thin film deposited at 600°C exhibit only (0 0 2n) peaks without any secondary phases. Metallic behaviors of electrical properties of the sample as shown in Fig, 1(b), despite of semi-metallic bulk samples [2], are attributed to the deviation from stoichiometry or anti-site defects. Also, a few percent of uniaxial lattice distortion is necessary to induce an energy gap for the TI state of LaPtBi. The small lattice distortion of 0.3% in our samples obtained from XRD measurements indicates lattice relaxation due to the large lattice mismatch of 7.2% between LaPtBi and YAlO3.
References
[1] S. Chadov et al., Nat. Mater. 9, 541 (2010). [2] M. G. Haase et al., J. Solid State Chem. 168, 18 (2002).
2:18 PM
BE-05. New magnetic configuration in paramagnetic phase of HoCo2
Claudia M. Bonilla1, 4, C. Castan1, I. Calvo1, 2, A. I. Figueroa1, J. Herrero-Albillos3, J. A. Rodríguez-Velamazán1, 2, D. Schmitz3, E. Weschke3, D. Paudyal4, V. K. Pecharsky4, 5, K. A. Gschneidner Jr4, 5, J. Bartolomé1, F. Bartolomé1 and L. M. García1
1Departamento de fisica de la materia condensada, Universidad de Zaragoza, Zaragoza, Spain; 2Institute Laue Langevin, Grenoble, France; 3Helmholtz-Zentrum Berlin, Berlin, Germany; 4U.S. Department of Energy, Ames Laboratory, Ames, IA; 5Department of Materials Science and Engineering, Iowa State University, Ames, IA
A new magnetic configuration within the paramagnetic phase of ErCo2 was proposed in 2007 as result of small angle neutron scattering (SANS), X-ray circular dichroism (XMCD) and ac susceptibility experiments. Within this magnetic configuration, coined as parimagnetism, the Co moments are disordered at the long-range but a net Co magnetic moment, antiparallel to the applied field (and to Er moment) is found, up to a certain temperature, Tf, well above Tc. The Co moment changes its orientation at Tf recovering the normal paramagnetic configuration. Short-range order between the Co atoms has been identified within this new magnetic configuration. Recently, transverse susceptibility (TS) and SANS measurements, performed above the ordering temperature of HoCo2, allowed us to establish the existence of sizable magnetic short-range correlated regions. The correlation length obtained by SANS has an approximately constant value of 7.5±7 Å in the region of temperatures close to Tf and increases asymptotically to Tc. An XMCD study reveals the inversion of the Co net magnetization in HoCo2, suggesting that parimagnetism is a more general phenomenon among the heavy lanthanide-Co Laves phases. The temperature dependence of Co and Ho moments for different applied fields has been obtained from XMCD measurements, showing that rare earth moment also changes its orientation above Tf, giving rise to an entirely new magnetic configuration at high temperature. Indeed, based on TS measurements and our XMCD study we propose a new magnetic phase diagram for HoCo2. First principles calculations based on LSDA+U approximation have also been performed in HoCo2 to obtain insights on the origin of the Co-short-range correlated volume. Work at Ames Laboratory is supported by US DOE under contract No.DE-AC02-07-CH11358. The authors acknowledge the support of IMANA FEDER funds, neutron source ILL and photon source BESSY. C.M. Bonilla acknowledges Spanish MICINN.
References
[1] J. Herrero-Albillos, L. M. García, F. Bartolomé, A.T Young, T. Funk, J. Campo, G. J. Cuello. Physical Review B 76, 2007. [2] A. I. Figueroa, S. Chandra, M. H. Phan, H. Srikanth, C. M. Bonilla, L. M. García, F. Bartolomé, J. Bartolomé, J. Herrero-Albillos. Journal of Applied Physics 109, 07E118, 2011.
2:30 PM
BE-06. High-field magnetization study of ErCo2
Maurice Guillot1 and Yildirhan Oner2
1CNRS, IPG, Grenoble, France; 2Istanbul Technical Iniversity, Istanbul, Turkey
The cubic Laves-phase compounds like ErCo2 are considered as a model of itinerant metamagnetism (IEM) for decades [1]. Very recently, A.I. Figueroa et al. [2 and ref. therein] have reported the presence of short range correlations at Tc (≈ 32 K) < T < Tf (≈ 55 K). This new magnetic configuration has been identified as “parimagnetism” [2]. Magnetization measurements were performed on powder sample of ErCo2 in the temperature range 4.2-300K under high continuous magnetic field up to 330kOe at 4.2K and in H up to 230 kOe below and above Tc. Two unexpected features are worth noting: whatever the values of T and H are i) the saturation of the magnetization is never attained and noticeable large values of the differential magnetic susceptibility are observed; ii) no field-induced transitions have been found. Besides the remanent magnetization MRem the spontaneous magnetization MSpont deduced from the extrapolation to H=0 of the linear variation of MT(H) in the 40-80 kOe range and the saturation magnetization MSat resulting (for H> 140kOe) from 1/H2 approach law are determined. MRem vanishes at (53± 2 K) while MSpont that is observed up to above 100 K decreases very rapidly when T > 55 K. At 4.2 K MSat is equal to 9.14 μB /f.u. value that confirms that the Co moment is strongly T et H dependent below Tc. It is noticeable that MSat is only weakly temperature up to 100K (5.4 μB /f.u.). All the results presented in this paper show that Tf corresponds to the second order type transitions; Tf (H) increases linearly when H <100 kOe and then saturates towards 62 K. Above this temperature the paramagnetic phase is observed: the reciprocal initial susceptibility obeys a Curie-Weiss behaviour (μeff= 9.14 μB /f.u.) with a positive Curie-Weiss temperature (32.4 K) equal to Tc.
References
[1] E. Gratz and A.S. Markosyan, J. Phys. Cond. Matter 13, R395 (2001) [2] A.I. Figueroa et al. J. Appl. Phys.109, 118 (2011)
2:42 PM
BE-07. Irreversibility in the magnetically ordered state of Laves phase compounds Er1-xDyxCo2 (x = 0.25, 0.33)
R. Nirmala2, 1, Durga Paudyal2, Ya Mudryk2, V. K. Pecharsky2, 3, K. A. Gschneidner Jr.2, 3 and A. K. Nigam4
1Physics, Indian Institute of Technology Madras, Chennai, India; 2The Ames Laboratory, U. S. Department of En ergy, Iowa State University, Ames, IA; 3Department of Materials Science and Engineering, Iowa State University, Ames, IA; 4Tata Institute of Fundamental Research, Mumbai, India
The rare earth Laves phase intermetallic compounds DyCo2 and ErCo2 order ferrimagnetically at ~140 K and ~32 K, respectively. Recent observation of anomalous first order transition in isostructural rare earth dialuminides, namely Er1-xDyxAl2 (x = 0.25, 0.33) and Er0.75Tb0.25Al2 has motivated us to study the cobalt analogues of similar Er:Dy compositions. The present work investigates the Er1-xDyxCo2 (x = 0.25, 0.33) compounds by means of low field magnetization, heat capacity (HC) measurements and theoretical calculations. Low field magnetization data on Er0.75Dy0.25Co2 and Er0.67Dy0.33Co2 reveal an additional transition within ferrimagnetically ordered state. This low temperature state is signified by both irreversibility and metastability as in the dialuminides . Zero field HC of Er0.67Dy0.33Co2 exhibits a sharp peak near the low temperature transition. The signature of first order transition in magnetic and thermal data is unusual in the sense that the ferrimagnetic ordering of the end members ErCo2 and DyCo2 itself is understood to be of first order. This study possibly illustrates the strong role of higher order interactions such as quadrupolar effects when the system hosts rare earth ions of mixed anisotropies. The Er1-xDyxCo2 system indeed presents a platform to test these effects in the midst of interplay of magnetic exchange interactions, crystal field effects (CEF) and, in addition, Co-moment instability and 3d-5d hybridization between rare earth and cobalt moments and thus to arrive at finding a broad theory for systems of mixed rare earths. The electronic structure calculations show antiparallel 5d and 3d spin arrangements confirming the ferrimagnetic state in Er1-xDyxCo2 alloys. The indirect 4f-4f interaction triggers 5d spin polarization and the hybridization between 5d and 3d bands facilitates the 3d spin polarization giving rise to antiparallel coupling of 5d and 3d moments. The split-4f bands of Er and Dy have different energy adding further complexity to the charge density and the CEF of rare earth ions. The correlation between the CEF and exchange interactions has also been explored.
2:54 PM
BE-08. Synthesis and Electronic Structure of Fe1-xInx Thin Films
Adam McClure1, E. Arenholz2 and Y. U. Idzerda1
1Physics, Montana State University, Bozeman, MT; 2Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA
The addition of Ga and Zn to bcc Fe films has been shown to generate materials with higher magnetostrictive properties than pure Fe [1,2]. In each case, the electronic structure of the Fe has shown to follow a band filling picture with dopant concentration [3,4]. We have extended this class of systems by synthesizing Fe1-xInx thin films deposited onto MgO(001) substrates by molecular beam epitaxy (MBE). This is significant because iron-indium is immiscible in the bulk [5]. Reflection high energy electron diffraction (RHEED), performed in-situ during the growth, is consistent with body centered cubic single crystal growths for alloy compositions up to about 10 percent In. Further alloying results in polycrystalline depositions up to 20 percent In, beyond which the In incorporation is inhomogeneous. X-ray absorption spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) conducted at beamline 6.3.1 of the Advanced Light Source confirm the incorporation of In into the Fe, as opposed to forming condensates of each metal. XAS on the Fe L2,3-edges shows a shifting of the L3 peak to lower energy as the In concentration is increased, indicating a charge transfer from the In to the Fe, while XMCD on the same edges reveals that the donated charge preferentially fills the majority band [3,4]. The Fe moment is observed to remain nearly constant to significant In incorporation followed by a dramatic decline (similar to the behavior of the Fe XMCD for Fe1-xGax and Fe1-xZnx).
References
[1] A. E. Clark, J. B. Restorff, M. Wun-Fogle, T. A. Lograsso, and D. L. Schlagel, IEEE Trans. Magn. 36, 3238 (2000). [2] Y. N. Zhang, J. X. Cao, and R. Q. Wu, Appl. Phys. Lett. 96, 062508 (2010). [3] E. Arenholz, G. van der Laan, A. McClure, and Y. U. Idzerda, Phys. Rev. B 82, 180405 (2010). [4] J. M. Hill, R. J. McQueeney, R. Wu, K. Dennis, R. W. McCallum, M. Huang, and T. A. Lograsso, Phys. Rev. B 77, 014430 (2008). [5] Springer Materials link (http://www.springermaterials.com/docs/pdf/10474837_1307.html)
3:06 PM
BE-09. Crystal Structure and Magnetic States in Dy5Ni2In4
Alessia Provino1, 2, Yaroslav Mudryk2, Durga Paudyal2, Pietro Manfrinetti1, 2, Vitalij K. Pecharsky2, 3 and Karl A. Gschneidner Jr.2, 3
1Department of Chemistry, University of Genova, Genova, Italy; 2The Ames Laboratory, U.S. Department of Energy, Iowa State University, Ames, IA; 3Materials Science and Engineering Department, Iowa State university, Ames, IA
The crystal structure of the intermetallic compounds R5Ni2In4 was reported for R = Ho, Er, Tm and Lu [1] (Lu5Ni2In4-type, oP22, Pbam); more recently Dy5Ni2In4 and Sc5Ni2In4 have been identified [2,3]. So far, none of the physical properties have been reported on any of these compounds. We have synthesized and characterized Dy5Ni2In4 and the new isotypic Tb5Ni2In4 and Y5Ni2In4 compounds [4]; in this communication we will present magnetic properties and heat capacity measurements performed on Dy5Ni2In4. This compound shows a ferro-like ordering with a TC ≈ 110 K (Fig. 1), followed by multiple magnetic orderings at lower temperatures. The fit of the inverse susceptibility in the paramagnetic state follows a Curie-Weiss law, giving a μeff. = 10.4 μB (close to the free ion value of 10.64 μB for Dy3+) and a positive θp = 58 K (Ni ions are likely non-magnetic). The heat capacity also shows three reproducible peaks: two weaker ones at 8 and 12 K, and a large one at ≈ 105 K. The first principles electronic structure calculations have also been performed to understand the underlying physics of these materials. Work supported by the US DOE, Division of Materials Science and Engineering (Office of Basic Energy Sciences) under contract N° DE-AC02-07-CH11358.
References
[1] V. I. Zaremba, Ya. M. Kalychak, P. Yu. Zavallii, V. A. Bruskov, Sov. Phys. Crystal logr. 36 (1991) 801-803. [2] Y. B. Tyvanchuk, U. C. Rodewald, Y. M. Kalychak, R. Pöttgen, J. Solid State Chem. 181 (2008) 878-883. [3] M. Lukachuk, B. Heying, U. Ch. Rodewald, R. Pöttgen, Heteroatom Chemistry 16 (2005) 364-368. [4] A. Provino, Y. Mudryk, P. Manfrinetti, D. Paudyal, V.K. Pecharsky, K.A. Gschneidner, Jr., Rare Earth Research Conference 2011 (RERC11), Santa Fe, New Mexico, June 19-23, 2011.
3:18 PM
BE-10. Metamagnetization phase transition in Ni-Cu-Mn-Ga alloys
Panpan Li, Jingmin Wang, Jinghua Liu, Tianli Zhang and Chengbao Jiang
School of Materials Science and Engineering, Beihang University, Beijing, China
NiMnX (X = In, Sn, Sb) ferromagnetic shape memory alloys (FSMAs) have attracted considerable interests due to the metamangetic magnetic-field-induced reverse martensitic transformation (MFIRMT) [1-5], and the produced magnetic shape memory, magnetoresistance, and magnetocaloric effects [7-10]. The MFIRMT is based on a magnetostructural transition between paramagnetic martensite (PM) and ferromagnetic austenite (FA) featured as TCM < TM < TCA, where TM is the martensitic transformation, TCM and TCA are the Curie point of the martensite and austenite, respectively [2-5]. As for NiMnGa alloys, the typical FSMAs developed rather early, there are three kinds of relative relationships in the transition temperatures, i.e. TCM<TM, TC=TM, and TM<TCA. Recently we found that Cu substituting for Ni could significantly decrease TCM, slightly increase TCA, and obviously decrease TM. The MFIRMT was obtained in Ni46Cu4Mn33Ga17 alloy [11]. In this paper, the magnetic and martensitic transitions were investigated in a large composition range of Ni50-xCuxMn30+yGa20-y alloys with 0 ≤ y ≤ 11 for each fixed Cu content x = 6, 8, 12, and 16, respectively. The martensitic transformation temperature TM was monitored over the whole composition range. The Curie temperature, either TCA or TCM, was detected depending on composition. By tailoring the correlation between the martensitic and magnetic transition temperatures, the metamagnetic magnetostructural transition from PM to FA was observed. The PM - FA transition was obtained in a wide range by tuning Cu content over 6 ≤ x ≤ 16 along the relationship 0.4 ≤ (y-31.43)/x ≤ 0.5. Moreover, the PM - FA transition always took place around room temperature, less sensitive to composition changes. Further, metamagnetic magnetic-field-induced reverse martensitic transformation was obtained. The correlation between composition and magnetic-field-induced reverse martensitic transformation was discussed in the viewpoint of Mn-Mn atomic interactions and transition entropy change.
References
[1] Sutou Y, Imano Y, Koeda N, Omori T, Kainuma R, Ishida K. Appl Phys Lett 2004;85(19):4358. [2] Schlagel DL, Yuhasz WM, Dennis KW, McCallum RW, Lograsso TA, Scripta Mater 2008;59:1083. [3] Kainuma R, Imano Y, Ito W, Sutou Y, Morito H, Okamoto S, Kitakami O, Oikawa K, Fujita A, Kanomata T, Ishida K. Nature 2006;439(7079):957. [4] Liu J, Scheerbaum N, Lyubina J, Gutfleisch O, Appl Phys Lett 2008;93:102512. [5] Wang YD, Huang EW, Ren Y, Nie ZH, Wang G, Liu YD, Deng JN, Choo H, Liaw PK, Brown DE, Zuo L, Acta Mater 2008;56:913. [6] Liu J, Aksoy S, Scheerbaum N, Acet M, Gutfleisch O, Appl Phys Lett 2009;95:232515. [7] Pathak AK, Gautam BR, Dubenko I, Khan M, Stadler S, Ali N. J Appl Phys 2008;103(7):07F315. [8] Sharma VK, Chattopadhyay MK, Shaeb KHB, Chouhan A, Roy SB. Appl Phys Lett 2006;89(22):222509. [9] Mañosa L, González-Alonso D, Planes A, Bonnot E, Barrio M, Tamarit JL, Aksoy S, Acet M. Nature Mater 2010;9:478. [10] Li B, Ren WJ, Zhang Q, Lv XK, Liu XG, Meng H, Li J, Li D, Zhang ZD. Appl Phys Lett 2009;95(17 ):172506. [11] Jiang CB, Wang JM, Li PP, Jia A, Xu HB, Appl Phys Lett 2009;95:012501.
3:30 PM
BE-11. A "How To" For Magnetic Carbon
Hendrik Ohldag1, Elke Arenholz2, Tolek Tyliszczak2, Roland Hoehne3, Daniel Spemann3, Pablo Esquinazi3, Magda Ungureneau3 and Tilman Butz3
1Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Center, Menlo Park, CA; 2Advanced Light Source, LBNL, Berkeley, CA; 3Experimentalphysik, University Leipzig, Leipzig, Germany
While conventional wisdom says that magnetic materials have to contain some metallic atoms, the confirmation of intrinsic magnetic order in pure metal free carbon represents an ultimate and general scientific breakthrough because of the fundamental importance of carbon as an elemental building block of organic as well as inorganic matter. The common controversy raised across all disciplines is whether the magnetism of carbon is intrinsic or induced by other elements. We address this controversy by providing clear experimental evidence that metal free carbon can be ferromagnetic at room temperature using dichroism x-ray absorption spectro-microscopy. For this purpose we acquired soft x-ray microscopy images of magnetic structures on a thin carbon film that have been produced by irradiation with a focused 2.25MeV proton beam. Our element specific magnetic probe shows no indication of magnetically ordered Fe, Co or Ni impurities in these samples. In a second step we investigate the particular electronic states that are involved in carbon magnetism and find that the carbon p-states as well as C-H bonds show a magnetic moment, indicating that hydrogenation plays a crucial role in developing the ferromagnetic order. Our surface sensitive approach reveals that the magnetism at the surface of the irradiated graphite samples is much larger than in the bulk of the sample. We observe a surface magnetic moment similar to what is typically present in classical ferromagnetic 3d transition metals.
References
P.Esquinazi et al., Magnetic order in graphite: Experimental evidence, intrinsic and extrinsic difficulties, Journal of Magnetism and Magnetic Materials, Vol 322, 1156 (2010). H. Ohldag et al., π-Electron ferromagnetism in metal free carbon probed by soft x-ray dichroism, Phys. Rev. Lett. 98, 187204 (2007) H. Ohldag et al., The role of hydrogen in room temperature ferromagnetism at graphite surfaces, New J. Phys. 12 123012 (2010)
3:42 PM
BE-12. Inducing Magnetism in Graphene Nanomesh
Hongxin Yang1, Mairbek Chshiev1, Danil W. Boukhvalov2, Xavier Waintal3 and Stephan Roche4, 5
1SPINTEC, UMR CEA/CNRS/UJF-Grenoble 1/Grenoble-INP, INAC, 38054, Grenoble, France; 2School of Computational Sciences, Korea Institute for Advanced Study (KIAS), Hoegiro 87, Dongdaemun-Gu, 130-722, Seoul, Republic of Korea; 3SPSMS-INAC-CEA, INAC, 38054, Grenoble, France; 4CIN2 (ICN-CSIC) and Universitat Autónoma de Barcelona, Catalan Institute of Nanotechnology, Campus UAB, 08193, Barcelona, Spain; 5ICREA, Institució Catalana de Recerca i Estudis Avancats, 08070, Barcelona, Spain
Two-dimensional graphene has emerged as a natural candidate for developing "beyond CMOS" nanoelectronics [1], but to obtain intrinsic magnetism in graphene-based materials is still a hurdle. A po ssible way to induce magnetism in graphene is to create atomic-scale defects (impurities etc.) [2]. It is difficult, however, to control magnetic properties using this approach. On another hand, recent successful fabrication of graphene nanomesh (GNM), using block copolymer lithography [3] may provide a controllable way to obtain graphene-based magnetism. In this work, we present first-principles calculations [4] of electronic and magnetic properties of graphene nanomesh, and found that by varying the shape, size and density of large vacancies, different types of intrinsic ferri(o)magnetic states can be promoted. Systematic studies of non-passivated and hydrogen-passivated GNM are achieved varying the difference ΔAB=|B-A| between missing A and B sites of the underlying bipartite lattice and analyzing different hole geometries. For non-passivated GNM with ΔAB=0, stable non-magnetic (antiferromagnetic) states are found for armchair (zigzag) edge termination. In sharp contrast, when ΔAB≠0, stable ferrimagnetic states are induced with net moment up to 4 μB (per 6 by 6 unit cell) originating from dangling bonds of edge atoms. The hydrogen-passivated GNMs were found to be strongly sensitive to the GNM size and shape, with magnetic moments deviating from the Lieb's theorem [5] trend caused by the delocalization of the unpaired electrons on the zigzag edges. The calculations of the formation energy provide with the evidence for the structural stability of ferromagnetic structures. Furthermore, three magnetic regimes are revealed: (i) quenched magnetic state due to complete chemical bond reconstruction; (ii) highly magnetic isolated GNM obeying Lieb's theorem; and (iii) intermediate regime providing a possible root towards design of magnetic GNM supermeshes. Finally, the calculated large values of the exchange splitting pinpoint promising perspectives for developing room-temperature graphene spintronics. This work has been supported by French ANR Nanosim-Graphene and EU Concept-Graphene projects.
References
[1] K.S. Novoselov et al., Science 306, 666 (2004); K.S. Novoselov et al., Nature 438, 197 (2005); Y. Zhang, et al., Nature (London) 438, 201(2005); C. Berger et al., Science 312, 1191 (2006); M.I. Katsnelson, Mater. Today 10, 20 (2007). [2] P.O. Lehtinen, et al., Phys. Rev. Lett. 93, 187202 (2004) ; O.V. Yazyev, et al., Rep. Prog. Phys. 2010, 73, 056501 and references therein; T. G. Pedersen, et al., Phys. Rev. Lett. 2008, 100, 136804; J. A. Furst, et al., Phys. Rev. B 2009, 80, 115117; X. H. Zheng, et al., Phys. Rev. B 2009, 80, 075413; D. Yu, et al., Nano Res.2008, 1, 56; H.Y. He, et al., J. Appl. Phys. 2010, 107, 114322; L. Chen, et al., Appl. Phys. Lett. 2008, 93, 223106; P. Esquinazi et al., Phys. Rev. Lett. 91, 227201 (2003); J. Cervenka, M. I. Katsnelson, C.F.J. Flipse, Nat. Phys. 5, 840 (2009); D. Martinez-Martin et al., arXiv:1004.4266; M.M. Ugeda et al., Phys. Rev. Lett. 104, 096804 (2010); H. Ohldag et al., New J. Phys. 12, 123012 (2010); M. Sepioni et al., Phys. Rev. Lett. 105, 207205 (2010). [3] J. Bai, et al., Nature Nanotech. 5, 190 (2010). [4] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); G. Kresse and J. Furthmuller, Comput. Mater. Sci. 6, 15 (1996); Phys. Rev. B 54, 11169 (1996); P. E. Blochl, Phys. Rev. B 50, 17953 (1994); J.P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996). [5] E. H. Lieb, Phys. Rev. Lett. 62, 1201 (1989).
3:54 PM
BE-13. Magnetic characteristics of a new cubic defect spinel Li0.5Mg0.5MnO3 for Li-ion batteries
Vivek Singh1, Mohindar S. Seehra1, A. Manivannan2 and P. N. Kumta3
1Physics, West Virginia University, Morgantown, WV; 2National Energy Technology Laboratories, Morgantown, WV; 3University of Pittsburgh, Pittsburgh, PA
Synthesis of a potential new Li-based battery material, Li0.5Mg0.5MnO2.75, by the Pechini method using stoichiometric amounts of Li, Mg and Mn nitrates, citric acid and ethylene glycol is reported. By comparing simulated and experimental x-ray diffraction patterns, its structure is found to be a defect cubic spinel with lattice constant a = 8.2348 Å, crystallite size D = 20 nm and formula: 4(Li0.5Mg0.5MnO2.75) = 3{[Li2/3Mg1/3][Mn4/3Mg1/3V1/3]O11/3]} where V = vacancy. Temperature (2 K to 370 K) and magnetic field (up to ±65 kOe) dependence of the magnetization M under ZFC (zero-field-cooled) and FC (field-cooled) conditions yields a blocking temperature TB = 9 K below which M(FC) and M(ZFC) bifurcate. For T > 30 K, M vs. T data fit the Curie-Weiss law [M = CH/(T - θ)] with θ = 13 K and C yielding μ = 3.96μB as magnetic moment of Mn4+. For T < TB, hysteresis loops are observed with coercivity HC = 160 Oe at 2 K with HC → 0 on approach to TB. The electron magnetic resonance (EMR) spectra at 9.286 GHz yield a single line with g ≈ 1.93 and ΔH ≈ 850 Oe at 300 K, whose intensity increases with lowering T till 30 K. For T< 30 K, a shift towards lower resonance fields and increasing ΔH with decreasing T is observed. With Mn4+ being only on the B-sites, the exchange interaction among Mn4+ ions is found to be ferromagnetic (FM) leading to magnetic clusters responsible for TB, EMR spectra, M vs. T and the short-range order for T < 30 K. However, there is likely no long range FM order due to V sites and non-magnetic of Mg2+ ions. Li exist as Li+ ions. Recently, importance of similar magnetic analysis on the Li-Ni-Co-Mn lamellar oxides for Li-ion batteries has been noted [1].
References
[1]. X. Zhang, C. M. Julien, A. Mauger and F. Gendron, Solid St. Ionics 188 (2011) pp. 148-155
4:06 PM
BE-14. Effect of substrate and chemical doping on the atomic structure and physical properties of thermoelectric Ca3Co4O9 thin films
Dipanjan Mazumdar1, Cihat Boyraz1, Humza Dunya1, Mustafa Ozdemir1, Arunava Gupta1, Qiao Qiao2, A. Gulec2, T. Paulauskas2, R. F. Klie2, S. Kolesnik3 and D. Dabrowski3
1MINT center, MINT Center University of Alabama, Tuscaloosa, AL; 2Physics, University of Illinois, Chicago, IL; 3Physics, Northern Illinois University, Dekalb, IL
The incommensurately layered cobalt oxide Ca3Co4O9 exhibits an unusually high Seebeck coefficient as a polycrystalline bulk material, making it ideally suited for many high temperature thermoelectric applications.It is also speculated that by controlling the spin-state of the Co4+ ion, the seeback coefficient can be improved further through spin-entropy [1]. In this study, we investigate properties of Ca3Co4O9 thin films grown on cubic perovskite SrTiO3, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates and on hexagonal Al2O3 (sapphire) substrates using the pulsed laser deposition technique [2]. Further we study the effect chemically doping Ca3Co4O9 with either Bi or Ti. X-ray diffraction and transmission electron microscopy analysis indicate strain-free growth of films, irrespective of the substrate. However, depending on the lattice and symmetry m ismatch, defect-free growth of the hexagonal CoO2 layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca2CoO3 buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO2 stacking faults is observed, possibly due to weak interlayer interaction in this layered material. We propose that these stacking faults have a significant impact on the Seebeck coefficient and we report higher values in thinner Ca3Co4O9 films due to additional phonon scattering sites, necessary for improved thermoelectric properties.
References
1. Yayu Wang, Nyrissa S. Rogado, R. J. Cava, & N. P. Ong, Nature 423 425(2003) 2. Q. Qiao, A. Gulec, T. Paulauskas, S. Kolesnik, B. Dabrowski, M. Ozdemir, C. Boyraz, Dipanjan Mazumdar, and A. Gupta, and Robert F. Klie, Journal of Physics: Condensed Matter, 23, 305005 (2011).
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BE-15. Structural, Magnetic, and Thermodynamic Properties of Three Metal-organic Frameworks M(N3)2(bpy), M = Ni, Co, Cu
Dusan S. Danilovic3, Youcef Hamida1, Chyan Long Lin1, Tan Yuen1, Kunhao Li2 and Jing Li2
1Physics, Temple University, Philadelphia, PA; 2Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ; 3Physics, Pitt Community College, Greenville, NC
Powder crystalline metal-organic frameworks (MOF), M(N3)2(bpy) (where M = Ni, Co, Cu, and bpy = 4,4’-bipyridine), were successfully synthesized by mixing M(II) salts with solutions of 4,4’-bipyridine and NaN3. Room temperature crystal structures were determined using powder X-ray diffraction. All three MOFs crystallize in an orthorhombic crystal system with the space group Cmmm (No. 65), which is isostructural to that of 2D-Fe(N3)2(bpy), a previously reported magnetic chain system. The M(II) ions are coordinated with four azide ligands (N3) in equatorial plane forming linear magnetic chains. Isothermal magnetization, magnetic susceptibility, and heat capacity measurements were performed on samples of these MOFs. No clear phase transitions were observed. The intra-chain magnetic interaction was found to be ferromagnetic for the Co and Ni compounds, and antiferromagnetic for the Cu compound. The data were fit to theoretical models, and the variation in the obtained exchange interactions was analyzed in terms of the geometric distortions on the octahedral M(II) sites.
1:30 PM - 4:30 PM, Grand Canyon 4-5
Chair: Xiaobin Zhu, Seagate
1:30 PM
BF-01. High-performance Voronoi tessellation-based micromagnetic simulator and the analysis of granular media recording
Marko V. Lubarda1, Marco A. Escobar1, Ruinan Chang1, Shaojing Li1, Jan van Ek2 and Vitaliy Lomakin1
1CMRR, UCSD, La Jolla, CA; 2Western Digital, Longmont, CO
We present a new micromagnetic solver for studying magnetic structures represented via Voronoi tessellation and demonstrate the study of recording on perpendicular granular media using this solver. The Voronoi tessellation is accomplished using an arbitrary seed grid and i t is supplemented by introducing arbitrary distributions of the tessellation parameters. In particular, the cell shape and size can be assigned any desired distribution including distributions in the height. Moreover, a given distribution can be assigned to the separation between the cells and their material parameters. The Voronoi tessellation is performed in two dimensions and in the third dimension the structure can be assumed either having a single layer or any number of layers with prescribed parameters. Voronoi tessellation can also be performed in all three dimensions. The cells can be coupled by any prescribed coupling through closely located cell interfaces. The magnetostatic field between the cells is computed accurately using a new non-uniform fast Fourier transform approach, which allows rapidly computing pairwise summation for non-uniformly distributed sources and observers. The solver is implemented on CPU and massively parallel Graphics Processing Unit (GPU) computer systems. We used these high-performance solvers for the analysis of recording on perpendicular granular media (Fig. 1). In particular, we calculate the M-H loops in the presence of materials and geometry distributions. We also calculate the recorded patterns and compute the signal-to-noise ratios for various distributions and materials properties.
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BF-02. Recording performance and thermal stability in perpendicular media with enhancement of grain isolation as well as magnetic anisotropy field
H. S. Jung, Y. Ikeda, G. Choe and Z. Shi
Media Development, Hitachi GST, San Jose, CA
Simultaneous enhancement of grain isolation and Hk attracts interest in designing perpendicular media with thermally-stable isolated small grains. A 2nm-thick Ru-oxide layer on top of conventional Ru layers provided isolated smaller Ru grains and enhanced Hc by ~2kOe in CoCrPt-oxide layers. [1] Ru-SiO2[2] and Ru-(TiO2 or WO3)[3] were reported. Magnetic clustering, recording performance, and thermal stability are investigated in Hk-gradient CoPtCr-oxide/Cap layers with a thin Ru-oxide layer on top of Ru/NiW. With increasing Ru-oxide thickness from 0 to 1.3nm, Hc and Hs are enhanced by 0.5 and 0.9kOe, respectively. Hn decreases from -2.6 to -2.0kOe. Magnetic correlation length (Dn) significantly decreases (Fig.1) but intrinsic SFD remains unaffected. Ho proportional to Hk increases linearly while KuV/kT remains constant. Compensation of the reduced Dn with the enhanced Ku provides similar KuV/kT. However, thermal decay rate actually degrades from 0.06 to 0.32%/decade, which correlates well with Hn. No change in switching behavior is observed. However, recording performance is significantly improved: narrower MCW at 10T by 10nm and higher SoNR at 2T by 1.0dB are observed even at lower OW by 7dB compared to the media without Ru-oxide. All the recording parameters as a function of Dn correlate well. More detailed analysis of recording performance related to Dn will be discussed in the full paper.
References
1. U. Kwon, et al., IEEE Trans. Magn. 41(10), 3193 (2005). 2. I. Takekuma, et al., J. Appl. Phys. 99, 08E713 (2006). 3. H. Yuan, et al., J. Appl. Phys. 103, 07F513 (2008).
1:54 PM
BF-03. Effectiveness of Medium Noise Suppression in Segmented Media
Jian-Gang Zhu
Data Storage Systems Center, Carnegie Mellon Univ, Pittsburgh, PA
In current perpendicular film media, one of the dominant medium noise sources is the anisotropy field variation in the granular layer with oxide grain boundaries. Both experimental studies and micromagnetic investigation have suggested that segmenting the grains with exchange break layer (EBL) in the perpendicular direction appears to be an effective method for suppressing such noise [1][2][3]. In this talk, we present a micromagnetic modeling analysis in quantifying the effectiveness of the noise suppression with segmentation of the medium grains by preforming recording simulations at area densities near 1Tbits/in2. Figure 1 shows the calculated recording SNR at a linear density of 1.8 MFCI for a segmented medium with each grain having three segments with two EBLs. The red curve represents the case where the inter-segment coupling is relatively weak but adequate and the blue curve the coupling is relatively strong. The SNR is plotted as a function of the standard deviation, σHk , of the assumed Gaussian distribution of the anisotropy field for the bottom segment only while σHk for the top and middle segment are fixed at 3%. If the EBLs provide adequate coupling, increasing σHk would yield little reduction of SNR whereas a strong inter-segment coupling would yield significant SNR degradation as σHk of the bottom segment broadens. It is also found that two EBLs to break a grain into three segments is significantly more effective than just one EBL for two segment grains in terms of achieving high SNR.
References
[1] G. Bertero, et al, TMRC 2010, Paper A2, San Diego (2010). [2] M. Desai, et al, ICMAT 2011, Paper L2-2, Singapore (2011). [3] J.-G. Zhu and Y. Wang, Intermag 2011, Paper GE05, Taipei (2011).
2:06 PM
BF-04. Exchange stiffness in Co thin film alloys
Charles Eyrich1, Wendell Huttema1, Monika Arora1, Eric Montoya1, Capucine Burrowes1, Erol Girt1, Bret Heinrich1, Oleg Myrasov2, Milton From3 and Olof Karis4
1Physics, Simon Fraser University, Burnaby, BC, Canada; 2Physics, University of Alabama, Tuscaloosa, AL; 3Physics, Western Washington University, Bellingham, WA; 4Physics and Astronomy, Uppsala University, Uppsala, Sweden
The exchange stiffness (Aex) is one of the key parameters controlling magnetization reversal in magnetic materials. Recently, we proposed a new method for measuring Aex on the basis of the spin spiral formation in two ferromagnetic films antiferromagnetically coupled across a non-magnetic spacer layer [1]. We used this method [1] and Brillouin scattering to measure Aex for a series of CoX (X = Cr, Ru, Pt) thin film alloys. The results show that Aex of Co alloys does not follow Ms in a simple linear fashion; Aex decreases at the rate of 1.8%, 4.6% and 5.3%, while Ms decreases at the rate of 0.6%, 3.2%, 2.2% per addition of one atomic percent of Pt, Cr, and Ru, respectively. These measured trends have been understood by combining measurements of element specific magnetic moments obtained using X-ray magnetic circular dichroism (XMCD) and material specific modeling based on density functional theory (DFT) within the local density approximation (LDA). XMCD measurements and DFT-LDA calculations show that in CoX alloys, the Cr atom carries a large spin moment of 2.2μB, which is coupled antife rromagnetically to the Co moment. In contrast, Ru and Pt moments couple ferromagnetically to the Co moments, but the moments are much smaller, mRu = 0.42μB and mPt = 0.36μB, respectively. From our DFT calculations we find that it is important to consider the d-d hybridization between Co and atom X to understand the observed trends. This interaction leads to a significant reduction of the magnetic moment and exchange coupling constants in the case of Cr 3d and Ru 4d interacting with the Co 3d electrons.
References
[1] Erol Girt, W. Huttema, O. N. Mryasov, E. Montoya, B. Kardasz, C. Eyrich, B. Heinrich, A. Yu. Dobin, O. Karis, J. Appl. Phys., 109, 07B765 (2011).
2:18 PM
BF-05. Magnetic Reversal Mechanisms in Exchange Coupled Composite Media
Christopher Morrison1, Yoshihiro Ikeda2, Ken Takano2 and Thomas Thomson1
1School of Computer Science, University of Manchester, Manchester, United Kingdom; 2San Jose Research Center, Hitachi Global Storage Technologies, San Jose, CA
Current research on perpendicular magnetic media focuses on fabricating and characterising exchange coupled composite (ECC) materials for use in conventional, heat-assisted or bit-patterned recording. Recently we have explored the reversal mechanisms of well segregated, granular CoCrPt-SiOx single layer continuous media1, which serves as the high anisotropy component of ECC media. Here we extend our work, using the angle dependence of remanence, to a complete bi-layer type ECC system - CoCrPt-SiOx/CoCrPt-based cap. As the cap thickness is increased, the medium reverses by a less coherent mode, as evidenced by the shift in the angle of the minimum switching field2. In single layer granular materials, a shift in the angle of the minimum switching field occurs only through exchange coupling1. In the case of ECC media the figure shows that the minimum angle shifts as a function of temperature and the depth of the minimum increases. This cannot be due to thermal activation, as this leads to the minimum becoming shallower. Our previous work demonstrated that the angle at which the minimum occurs does not change for the high anisotropy CoCrPt-SiOx component1. Hence we hypothesize that the primary effect of temperature is to change the exchange coupling between the two ECC constituents. This observation allows coupling to be explored using temperature and points to the need to consider how reversal in ECC media changes over the operating range of hard disk drives.
References
1 C. Morrison et al., Appl. Phys Lett. (submitted). 2 T. Thomson et al., J. Appl. Phys 103, 07F548 (2008).
2:30 PM
BF-06. Angle dependence of the switching field of recording media at finite temperature
Lalita Saharan1, Christopher Morrison2, Y. Ikeda3, K. Takano3, Jim J. Miles2, Thomas Thomson2, Thomas Schrefl4 and Gino Hrkac1
1Material science and Engineering, University of Sheffield, Sheffield, United Kingdom; 2School of Computer Science, University of Manchester, Manchester, United Kingdom; 3San Jose Reasech Centre, Hitatchi Global Storage Technologies, San Jose, CA; 4St. Pölten University of Applied Sciences, St. Pölten, Austria
A micromagnetic [1] and experimental study has been undertaken to investigate the utility of a one-grain model in describing the switching field Hsw of CoCrPt-oxide recording media as function of applied field angle. The effects of grain diameter, attempt frequency f0 and thermal activation (at T=150-350K) on Hsw were investigated. The cylindrical grains have a diameter of 7.5 to 8.3nm with a height=11nm and material properties taken from experiments [4] and literature. We calculate Hsw using the nudged elastic band method by computing the energy barrier as a function of applied field angle[2,3]. In the case of a well segregated, granular media the single grain model gives good agreement with experiment. The simulated and experimental normalized angle dependent remanence curves, see fig.1, show that thermal activation alone reduces the depth of the Stoner-Wohlfarth minimum. Within the range of values considered here, which are relevant to granular perpendicular media, variations in grain size and f0 only affect the quantitative Hsw values, and not the functional form of the angle dependent switching. This indicates that the model correctly reproduces the magnetization reversal mechanism. The values of the switching fields determined using the model show good agreement with vibrating sample magnetometer measurements, see fig.1. In summary, the one grain model reproduces the angle dependence of Hsw at all temperatures and the value of Hsw for 292K and 350K. We note a discrepancy in the value of Hsw at 150K which may be due to temperature effects on material properties and/or coupling effects.
References
[1] T. Schrefl, G. Hrkac, G. Bance, D. Suess, O. Ertl, and J. Fidler, 2, 765 (2007). [2] R. Dittrich, T. Schrefl, D. Suess, W. Scholz, H. Forster, and J. Fidler, J. Magn. Magn. Mater. 250, L12 (2002). [3] R. Dittrich, T. Schrefl, M. Kirschner, D. Suess, G. Hrkac, F. Dorfbauer, O. Ertl, and J. Fidler, IEEE Trans. Magn. 41, 3592 (2005). [4] C. Morrison, L. Saharan, G. Hrkac, T. Schrefl, Y. Ikeda, K. Takano, J.J. Miles, and T. Thomson, App. Phys. Lett. submitted.
2:42 PM
BF-07. Directly probing magnetization reversal of segmented perpendicular media
Chih-Huang Lai1, Hao-Cheng Hou1, B. J. Kirby2 and Dieter Suess3
1Department of Materials Science and Engineering, National Tsing Hua University, HsinChu, Taiwan; 2NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; 3Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria
Exchange-coupled composite (ECC) media have been widely used to improve media performance. Since the ECC media are composed of exchange-coupled segments, it is quite challenging to experimentally verify the reversal behavior of individual segments. In this work, we report a comprehensive study on depth-dependent magnetization-probing techniques for ECC media. The layer structure of ECC media is CPCS 12nm/[Pt 0.7nm/CPCS 1.1nm]N (CPCS: CoPtCr-SiO2) where CPCS and [Pt/CPCS]N are the hard layer (HL) and soft layer (SL), respectively. To reveal the local magnetization reversal, we co-sputtered Fe with the CPCS layer at varied locations of segmented SL for the measurements of element-specific X-ray magnetic circular dichroism (XMCD) [1]. Fig. 1 shows loops for the sample of N=7. The signals of VSM loops are from both HL and SL, but the XMCD signals are only contributed from the designated segments. The XMCD loops clearly demonstrate that top of the segment in SL is reversed at a lower field to promote the domain wall nucle ation. To further understand the role of exchange coupling on magnetization reversal, we performed polarized neutron reflectometry (PNR), the technique sensitive only to the in-plane magnetization. By performing PNR measurements as a function of increasing in-plane applied field, we could determine how spins of segments at different depths responded to being pulled away from the perpendicular easy-axis direction, and could also characterize the critical length of SL, responding to the exchange coupling from the HL. The observed gradient of in-plane magnetization for N=7 reconfirmed the reversal behavior of exchange springs.
References
[1] H. C. Hou et al., Appl. Phys. Lett., 98, 262507 (2011)
3:18 PM
BF-08. Influence of Magnetic Viscosity on the First Order Reversal Curves of Antiferromagnetically Coupled Perpendicular Recording Media
S. N. Piramanayagam and Mojtaba Ranjbar
A*STAR (Agency for Science, Technology and Research), Data Storage Institute, Singapore, Singapore
First order reversal curves (FORC) technique is emerging as a characterization technique for understanding the exchange and magnetostatic interactions in magnetic films. In FORC contours, a profile along the x-axis (switching field) can give information about the coercivity and switching field distribution and a profile along the y-axis (Hu - magnetic interaction) can give information about the magnetic interactions. In this paper, we study the FORC of the top and bottom layers of an antiferromagnetically coupled perpendicular recording media and report that the contours may be affected by time-dependent magnetization effects. Films of the type Ta(5 nm)/Ru(15 nm)/CoCrPt:SiO2(16 nm)/Ru(0.8 nm)/CoCrPt:SiO2(2 nm) were fabricated on Si substrates. FORC contours were measured at different time-scales of the applied field. Figure 1 shows FORC contours of (a) the stabilizing and (b) recording layers. The following observations can be made: 1. There are four contours observed from the AFC structures, while only two of them represent the bottom and top layers respectively. 2. The Hc or Hu values of the bottom (recording) layer did not change significantly at different time-scales. 3. The Hc and Hu values from the FORC of the top (stabilizing) layer showed a decrease as a function of increasing time-scale, as shown in figure 1c. The decrease of Hc with increasing time-scale is well-understood to be arising from the magnetization relaxation due to thermal activation. The decrease of Hu with time-scale in FORC, arising from viscosity effects, is being reported for the first time. These results indicate that the FORC results, especially in samples exhibiting time-dependent effects, have to be treated with caution. A detailed analysis of the time-dependent Hu will be presented in full paper.
3:30 PM
BF-09. Effects of Grain Size on Short Time Switching Fields in Perpendicular Media
S. H. Florez, C. Boone, F. Zhu, K. Takano and B. D. Terris
Hitachi Global Storage Technologies, San Jose, CA
We study the effects of thermal fluctuations on PMR granular media, as a function of grain size (GS), through sweep time dependent polar kerr measurements on uncapped and thin cap structures. The analysis is based on Sharrock's formalism [1] with varying exponents, including a demagnetization field correction. From this analysis we extract short time switching fields Ho and intrinsic switching field distributions as a function of the magnetization for several exponents. Within cert ain assumptions, the methodology allows comparison of media in absence of the effects of thermal activation. The short time intrinsic switching field, Ho, is found to be nearly independent of GS for the uncapped case in agreement with anisotropy measurements showing no significant reduction of Hk with GS. However, Ho drops with increasing cap thickness and with reducing GS, both trends that increase the magnetic cluster size. While reduction of the Ho/Hk ratio with increasing lateral exchange has been observed in PMR media [2,3,4], our data shows that the observed reductions in Ho partly originate from the shape anisotropy field terms corresponding to the magnetic clusters formed in these media [5].
References
1. M. P. Sharrock, J. Appl. Phys. 76, 6413 (1994). 2. H. Nemoto et al., J. Magn. Magn. Mater., 320, 3144 (2008). 3. H. Zou et al., J. Appl. Phys. 91, 8378 (2002). 4. T. Shimatsu et al., IEEE Trans. Magn. 37, 1567 (2001). 5. T. Shimatsu et al., Nakamura. IEEE Trans. Magn. 39, 2335 (2003).
3:42 PM
BF-10. Rate-dependence of the intrinsic switching field distribution in perpendicular recording materials
Ondrej Hovorka1, Jason L. Pressesky2, Ganping A. Ju2, Andreas Berger3 and Roy W. Chantrell1
1Department of Physics, York University, York, United Kingdom; 2Seagate Technology, Fremont, CA; 3Nanomagnetism Laboratory, CIC nanoGUNE Consolider, Donostia-San Sebastian, Spain
Thermal relaxation leads to a rate dependence of hysteresis loops and complicates the identification of intrinsic materials characteristics in perpendicular recording materials (PMR) [1]. In this work, we discuss the external field-sweep rate (R) dependence of the intrinsic switching field distribution (SFD), which is an important characteristic for the design and optimization of PMR. Using the reference function ΔH(M, ΔM)-method [2] we analyze hysteresis loops for different R obtained from kinetic Monte-Carlo simulations of an interacting Stoner-Wohlfarth particle model of PMR, which includes distributions of anisotropy fields HK and volumes of grains V. We find that this method allows an accurate identification of the SFD even in the interacting case. This allows study of the R-dependence of the SFD by extending the theoretical analysis, leading to the well-known Sharrock equations [3, 4], and deriving a relationship between the standard deviation σS of the SFD and R (given as Fig-inset; [5]). As we will discuss in detail, this scaling relation, which includes explicit dependence on the variances of the anisotropy and volume distributions σK2 and σV2, allows relating the SFD obtained from “slow” laboratory type measurements to fast recording time scales.
References
[1] S. N. Piramanayagam, K. Srinivasan, J. Magn. Magn. Mater. 321, 485 (2009). [2] O.Hovorka, Y. Liu, K. A. Dahmen, and A. Berger, J. Magn. Magn. Mater. 322, 459-468 (2010); O. Hovorka, Y. Liu, K. A. Dahmen, and A. Berger, Appl. Phys. Lett. 95, 192504 (2009). [3] M. P. Sharrock, J. Appl. Phys. 76, 6413 (1994). [4] R. W. Chantrell, G. N. Coverdale, K. O’Grady, J. Phys. D: Appl. Phys. 21, 1469 (1988). [5] O. Hovorka, R. F. L. Evans, R. W. Chantrell, A. Berger, Appl. Phys. Lett. 97, 062504 (2010).
3:54 PM
BF-11. Modeling magnetic column intermediate layer of perpendicular magnetic recording
Zhanjie Li, Daniel Bai, Shaoping Li, Feng Liu and Sin ing Mao
Western Digital, Fremont, CA
The Ru interlayer (IL) of perpendicular magnetic recording media plays a significant role for assuring the recording layer grain growth quality[1-3]. In this regard a thick IL is usually preferred. On the other hand, thick IL will reduce the soft under layer (SUL) imaging effect thus limiting the write field for a given writer design. In this paper, we propose an approach to achieve a lower effective IL thickness to enhance the write field by replacing some Ru grains with the columnar magnetic material without reducing the IL thickness. By using micromagnetic modeling we study characteristics of the spatial distribution of head field in a specific head/media configuration, where a Ru intermediate layer includes periodic magnetic columns, as shown in Figs.1. It was found that the write field increases with the thicker and wider magnetic columns. The write field as a function of magnetic volume ratio of IL layer is plotted in Fig.2. As the magnetic volume in the IL increases, the write field monotonically increases. For a 10% magnetic volume ratio, it is equivalent to reducing IL thickness by 3.5 nm. In addition, the IL magnetic anisotropy, the magnetization and the damping constant effect on the write field are investigated.
References
1) S. Piramanayagan, J. App. Phys, V102, 011301 (2007). 2) H. S. Jung, M.Kuo, S. S. Malhotra, G. Bertero, and A. F. Torabi, IEEE Tran. Magn., V44, 3484(2008) 3) S. Park, J.-G Zhu, and D.E. Langhlim, J. App. Phys, V105, 07B723 (2009).
4:06 PM
BF-12. Possible Impact of Stacking Faults on HCP Co-Based Perpendicular Magnetic Recording Media
Vincent M. Sokalski1, David E. Laughlin1 and Jian-Gang Zhu2
1Material Science & Engineering, Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 2Electrical & Computer Engineering, Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA
The effect of stacking faults on magnetic properties of hexagonal close-packed (HCP) Co & Co84Pt16 thin films is evaluated to understand their possible impact on perpendicular magnetic recording (PMR) media. It’s believed that local breaking of the uniaxial symmetry due to stacking faults will reduce magnetic anisotropy, Ku, of HCP Co-alloys[1]. Here, a series of 15nm Co or Co84Pt16(00.1) thin films are grown by RF sputtering with varying deposition temperature (25-500°C) on MgO(111) single crystal substrates with a 15nm Ru(00.1) buffer layer. Stacking fault content was evaluated by peak broadening (fig. 1) of the (10.3) & (10.4) peaks in a series of x-ray diffraction q-scan measurements. We observe an increase in the magnetic anisotropy with the growth temperature for both Co and Co84Pt16. This dependence is attributed to a decrease in stacking fault density and a concurrent compression of the crystallographic lattice parameter, c. Measured peak broadening is interpreted by comparison with Monte Carlo simulations of diffraction effects for close packed structures. It is determined that for temperatures below ~200°C, Co tends to grow with clustered stacking faults (i.e. stacking faults spanning several layers) while Co84Pt16 tends to have more stacking faults, but are smaller in size (e.g. growth faults spanning only 1 layer). Above 200°C, the stacking fault content is less than 1%. It is believed that the decrease in stacking fault content with increasing temperature is due to thermally activated self-healing during growth[2].
References
[1] Lu, B., et al., Study of stacking faults in Co-alloy perpendicular media. Journal of Applied Physics, 2002. 91(10): p. 3. [2] Busse, C., et al., Stacking-Fault Nucleation on Ir(111). Physical Review Letters, 2003. 91(5): p. 056103.
4:18 PM
BF-13. Si/NiFe seedlayers for Ru intermediate layer in perpendicular magnetic recording tape media
Gaku Saemma1, Shota Takahashi1, Satoshi Matsunuma2, Tetsutaro Inoue2 and Shigeki Nakagawa1
1Dept. of Physical Electronics, Tokyo Institute of Technology, Tokyo, Japan; 2Hitachi Maxell Energy, Osaka, Japan
In order to attain recording tape media with extremely huge capacity, CoPtCr-based granular films with well c-axis orientation should be prepared on soft magnetic underlayer SUL on polymer tape substrate without any heating process. c-axis orientation of the Ru intermediate layer should be required to attain c-axis orientation of CoPtCr-based granular films. (110) oriented FeCoB SUL induces well (001) texture of Ru intermediate layer and a laminated structure of FeCoB SUL assures a high recording capacity[1]. In this study, we suggest a Si/NiFe seedlayers prepared at room temperature to attain better c-axis orientation of Ru intermediate layer. Si/NiFe/Ru/CoPtCr-SiO2 films and Si/NiFe/FeCoB/Si/NiFe/FeCoB/Ru/CoPtCr-SiO2 films were deposited by Facing Targets Sputtering (FTS) system at room temperature. It was observed that diffraction intensity of Ru with Si/NiFe layers became larger than that with laminated FeCoB SULs from XRD diagrams. This means the improvement of (001) crystalline orientation of Ru and CoPtCr crystallites by Si/NiFe layers. Fig.1 shows Δθ50 of CoPtCr(002), Ru(002) and FeCoB(110) peaks of Ru(x nm)/ CoPtCr-SiO2(20 nm) on the laminated crystalline SUL and Si/NiFe layers as a function of Ru thickness. Si/NiFe layers decreased Δθ50 of CoPtCr(002) to 4.6° when the Ru thickness was 20 nm. The Δθ50 slightly increased as the Ru thickness was decreased, but even when the Ru thickness was only 5 nm, the Δθ50 retained small value of 5.8°. In conclusion, we found that a Si/NiFe seedlayers remarkably improved (001) orientation of a Ru intermediate layer.
References
[1] S. Matsunuma, et al., J. Magn. Magn. Mater. (2011).
1:30 PM - 4:30 PM, Grand Canyon 12-13
Chair: Leyi Zhu, Argonne Nat'l Lab
1:30 PM
BG-01. Evolution of spin excitations into the superconducting state in FeTe1-xSex
Mark Lumsden
Oak Ridge National Laboratory, Oak Ridge, TN
Detailed inelastic neutron scattering studies of the spectrum of magnetic excitations are important to understand the link between magnetism and superconductivity in Fe-based superconductors. We have carried out comprehensive measurements of the excitations in both superconducting FeTe0.51Se0.49 (x=0.49) and non-superconducting Fe1.04Te0.73Se0.27 (x=0.27). For superconducting FeTe0.51Se0.49, a spin resonance mode is observed, strongly coupled with the superconducting TC. The resonance in this material occurs at the same wavevector (2d Q=(0.5, 0.5)) as resonance location in doped BaFe2As2 despite the fact that the long-range order in the relevant parent compounds are characterized by different ordering wavevectors. More detailed measurements of the full spectrum of magnetic excitations were performed for both the x=0.27 and x=0.49 samples. The magnetic excitations are two-dimensional in nature and inelastic intensity was observed for energy transfers as high as 300 meV. Extrapolation of the measured dispersion to zero energy transfer reveals excitations which are incommensurate for both concentrations emanating from a wavevector near (0.5, 0.5), the location of the resonance in the superconducting material. The magnitude of the measured incommensuration is larger in the non-superconducting sample perhaps suggesting that excitations more localized near (0.5, 0.5) are important for superconductivity. For lower energy transfers, the spectrum consists of a set of incommensurate spots which exhibit four-fold symmetry about the (1,0) (square lattice (π,π)) wavevector. With increasing energy transfer, these spots evolve into rings centered on Q=(1,0). These excitations are notably different than the spin-wave cones of scattering which would exist in a long-range magnetically ordered material, likely reflecting the itinerant nature of the magnetism. The qualitative evolution of the excitation spectrum is similar to previous studies on several cuprate superconductors and the square lattice incommensurate wavevectors (π±ξ,π) and (π,π±ξ) characterizing the fluctuations share the same parameterization in both FeTe1-xSex and the cuprates (albeit with a different magnitude of ξ).
References
M. D. Lumsden, A. D. Christianson, E. A. Goremychkin, S. E. Nagler, H. A. Mook, M. B. Stone, D. L. Abernathy, T. Guidi, G. J. MacDougall, C. de la Cruz, A. S. Sefat, M. A. McGuire, B. C. Sales, and D. Mandrus, Nature Physics 6, 182 (2010); H. A. Mook, M. D. Lumsden, A. D. Christianson, S. E. Nagler, Brian C. Sales, Rongying Jin, Michael A. McGuire, Athena Sefat, D. Mandrus, T. Egami, Clarina dela Cruz, Phys. Rev. Lett. 104, 187002 (2010).
2:06 PM
BG-02. Neutron scattering studies of the magnetic phase diagram of superconductor parent compound Fe1+xTe
Efrain E. Rodriguez1, Chris Stock1, 3, Pawel Zajdel4, Kathryn L. Krycka1, Charles F. Majkrzak1 and Mark A. Green1, 2
1NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; 2Materials Science and Engineering, University of Maryland, College Park, MD; 3Physics, University of Indiana, Bloomington, IN; 4Physics of Crystals, Institute of Physics, University of Silesia, Katowice, Poland
Several structural families exist in the new iron-based superconductors, the simplest of which is FeCh where Ch represents either tellurium, selenium, or sulfur. As in the high-Tc cuprates, the crystal structures are also highly two-dimensional, and in FeCh it consists of layers of edge-sharing FeCh4 tetrahedra. However, some phases contain interstitial iron located between the layers and this excess iron affects the superconducting and magnetic properties along with the crystal structure. The nonstoichiometry of these compound can be represented as Fe1+xTe where x corresponds to the amount of interstitial iron. We present several neutron scattering studies on the parent compound Fe1+xTe which becomes superconducting upon substitution of Se or S on the Te site. To map out the magnetic phase diagram of this parent phase we have performed neutron powder diffraction and polarized neutron diffraction on single crystals. We find a rich phase diagram including a collinear antiferromagnetic ordering, an incommensurate spin density wave, and a noncollinear helical ordering. In addition, we present inelastic neutron scattering measurements that elucidate the nature of the magnetic exchange interactions corresponding to the different magnetic orderings in Fe1+xTe.
2:18 PM
BG-03. Neutron spin resonance in iron superconductor FeTe0.6Se0.4 under pressure
Karol Marty1, Mark Lumsden1, Andrew Christianson1, Balász Sipos1, Yoshiya Uwatoko2, Antonio Moreira Dos Santos1, Chris Tulk1, Jaime Fernandez-Baca1 and Brian Sales1
1ORNL, Oak Ridge, TN; 2Institute for Solid State Physics, Tokyo, Japan
The interest in unconventional superconductivity has been recently renewed by the discovery of oxypnictide superconductors with Tc up to 55K [1-4]. Since many unresolved questions still exist after decades of studying cuprates, the iron-based compounds have raised new hope in understanding the mechanisms responsible for unconventional superconductivity. Indeed, they exhibit many similarities with the copper oxides, the most prominent probably being a resonant spin excitation observed by neutron scattering in both families [5,6], which indicates a strong correlation between magnetism and superconductivity. In this context, the FeTe(1-x)Sex superconductive series is attracting interest in many respects, as the structure is quite simple and large single crystals can be grown. Extensive studies have already been performed at different doping levels, showing that FeTe(1-x)Sex with x close to 0.5 happens to have the highest TC amongst all concentrations and exhibits a spin resonance [7,8] as well as other striking magnetic excitations [9]. Recent investigations of the pressure effect on the superconducting transition of FeTe0.5Se0.5 [10,11] show that Tc displays a maximum for an applied pressure of 2GPa. This effect is rather large in this material, considering that Tc increases from 13.5K at ambient pressure up to 21K (Tc-offset) or 26K (Tc-onset) at 2GPa [10]. Pressure has always had an important role and drastic effects in superconductor properties [12]. It provides a clean way to study the coupling between Tc and the resonance energy, without inducing doping disorder or splitting the resonance as would occur in the presence of an applied magnetic field [13]. Inelastic neutron scattering experiments were performed on a FeTe0.6Se0.4 sample in a McWhan pressure cell in order to track the resonance energy and link it with Tc as a function of applied pressure. These results will be presented and analyzed during this conference.
References
[1] Y. Kamihara, et al, J. Am. Chem. Soc. 130, 3296 (2008) [2] X. H. Chen, et. al, Nature 453, 761 (2008) [3] G. F. Chen, et. al, Phys. Rev. Lett. 100, 247002 (2008) [4] Z. Ren, et al., Chin. Phys. Lett. 25, 2215 [5] A.D. Christianson, et al., Nature 456, 930-932 (2008) [6] S. Hufner, et al., Rep. Prog. Phys. 71, 062501 (2008) [7] H.A. Mook, et al., arXiv:0904.2178 [8] Y. Qiu, et al., Phys. Rev. Lett. 103, 067008 (2009) [9] M.D. Lumsden, et al., Nature Physics 6, 182 (2010) [10] K. Horigane, et al., J. Phys. Soc. Jpn. 78, 063705 (2009) [11] C.-L. Huang, et al., J. Phys. Soc. Jpn. 78, 084710 (2009) [12] C.W. Chu, et al., Physica C : Superconductivity 469, 385-395 (2009) [13] W. Bao, et al., arXiv:1002.1617
2:30 PM
BG-04. Chemical pressure and electron doping effects in SrPd2Ge2 single crystals
Nak Heon Sung1, B. Y. Kang1 and B. K. Cho1, 2
1School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea; 2Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea
Since discovery of relatively high superconducting temperature (Tc) in pnictide compounds, there have been huge attentions and efforts to increase Tc. In pnictide compounds with ThCr2Si2-type (122) structure, elevated Tc was obtained by electron/hole doping or chemical pressure effect. These approaches are regarded as critical clues for the enhancement of superconductivity, but the origin of superconductivity in the compounds was not understood fully, yet. Therefore, it would be of great interest to investigate the similar experiments in other 122 structure compounds to understand the mechanism of superconductivity. Ternary germanide compound, SrPd2Ge2 (Tc = 2.70 K), has the same 122 structure as pnictide compound without any magnetic elements, such as Fe [1]. Here, we report the chemical pressure effect on superconductivity by doping isovalent elements, such as Pt (at Pd site) and Si (at Ge site), and electron doping effect by doping Ag (at Pd site) in SrPd2Ge2 single crystals. In Sr(Pd1-xPtx)2Ge2 and SrPd2(Ge1-xSix)2, the enhanced Tcs were observed, but decreased Tc was observed in Sr(Pd1-xAgx)2Ge2. We found the optimal dopping ratios were x = 0.035 with Tc = 3.26 K, and x = 0.01 with Tc = 2.76 K for Pt doped and Si doped cases, respectively. Based on these results, we found that chemical pressure by doping isoelectric elements in SrPd2Ge2 single crystal causes the enhanced Tc, similar to pnictide compounds, but that electron doping does not increase Tc, contrary to pnictide compounds. In addition to these results, we will discuss the variation of superconducting critical field and electrical behavior by doping experiments in SrPd2Ge2 single crystals, in detail.
References
[1] N. H. Sung, Jong-Soo Rhyee, and B. K. Cho, Phys. Rev. B 83, 094511 (2011)
2:42 PM
BG-05. Triplet superconductivity in Josephson junctions with barriers of ferromagnetic Heusler alloys
Martin P. Weides1, D. Sprungmann2, H. Kohlstedt3, H. Zabel2 and K. Westerholt2
1National Institute of Standards and Technology, Boulder, CO; 2Institut für Experimentalphysik/Festkörperphysik, Ruhr-Universität Bochum, Bochum, Germany; 3Nanoelektronik, Technische Fakultät Kiel, Christian-Albrechts-Universität Kiel, Kiel, Germany
Superconducting pairing functions with a symmetry different from conventional s-wave singlet pairing are in the focus of interest since the advent of BCS theory. These unconventional pairing states may be induced by the superconducting proximity effect at superconducting/ferromagnetic S/F interfaces. The exchange field in the ferromagnetic layer favors triplet pairing, i.e., a superconducting condensate function with parallel spins for dirty magnets with weak pair breaking at low temperatures. In this talk we present Josephson junctions with barriers prepared from the Heusler compound Cu2MnAl. The Heusler barriers offer the opportunity to change the magnetic state of the barrier by annealing the complete junctions. In the as-prepared state the Cu2MnAl layers are nonferromagnetic and the critical Josephson current density jc decreases exponentially with the thickness of the Heusler layers dF. On annealing the junctions at 240 ○C the Heusler layers develop ferromagnetic order and we observe a dependence jc(dF) with jc strongly enhanced and weakly thickness dependent in the thickness range 7.0 < dF < 10.6nm. We interpret this feature as an indication of a triplet component in the superconducting pairing function generated by the specific magnetization profile inside thin Cu2MnAl layers.
References
D. Sprungmann et al., Phys. Rev. B 82, 060505 (2010)
3:18 PM
BG-06. Exploring triplet superconductivity using an epitaxial exchange-spring ferromagnet/superconductor bilayer*
L. Y. Zhu, Yaohua Liu, J. E. Pearson, Suzanne G. E. te Velthuis, S. D. Bader and J. S. Jiang
Materials Science Division, Argonne National Laboratory, Argonne, IL
Recent theories predict triplet superconductivity in conventional s-wave superconductors (S) via the proximity effect with a ferromagnet (F) of inhomogeneous magnetization at the F/S interface [1]. Magnetic inhomogeneity is crucial to give rise to the effect and determine the strength of the singlet-triplet conversion. But magnetic inhomogeneity is not a parameter that can be unambiguously established and varied in most experiments inferring triplet superconductivity [2]. In this work, we use Nb as the S-layer, and an epitaxial exchange spring Py/Sm-Co bilayer with in-plane uniaxial anisotropy as the F-layer. Due to the exchange coupling between the soft Py and hard Sm-Co layers, a tunable noncollinear spin spiral (i.e. magnetic inhomogeneity) can be achieved by controlling either the direction or magnitude of the external field. The superconducting properties of the exchange spring F/S hybrid are then investigated as a function of the spin spiral winding angle φ. At a fixed temperature within the width of the superconducting transition, starting from a collinear magnetic configuration, as the spin spiral winds up, the resistance (R) initially decreases and the superconducting critical current (Ic) increases. There is an optimized spin spiral winding angle φo, which minimizes R and maximizes Ic, after which R increases and Ic decreases with increasing φ. This observation is consistent with theoretical predictions and suggests spin triplet pairing. More importantly, combining micromagnetic simulations with normal-state anisotropic magnetoresistance measurements, we have experimentally determined the spin spiral structures as a function of the external field's magnitude and direction; and we are able to quantitatively correlate them with the superconducting transport results. Our findings demonstrate that by manipulating the magnetic inhomogeneity at the F/S interface in a single sample, the proximity-effect-induced superconductivity can be tuned at will. * Work supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under contract No. DE-AC02-06CH11357.
References
[1] F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Phys. Rev. Lett. 86, 4096 (2001). [2] See articles in Supercond. Sci. Technol. 24 (2) (2011).
3:30 PM
BG-07. Magnetic irreversibility and quantum tunneling of normal-superconductor interfaces in a type-I superconductor
Saül Vélez1, Antoni García-Santiago1, Ricardo Zarzuela1, Joan Manel Hernandez1, Javier Tejada1 and Eugene Chudnovsky2
1Grup de Magnetisme, Department of Fundamental Physics, University of Barcelona, Barcelona, Spain; 2Physics, Lehman College, The City University of New York, New York, NY
The magnetic dynamics of normal-superconductor interfaces (NSI) in type-I superconductors have recently proven to wear strong ressemblances with that of domain walls in ferromagnets [1,2]. Prompted by this similarity, we have explored the intermediate state of several pure type-I superconducting lead (Pb) samples performing isothermal measurements of first magnetization curves and magnetic hysteresis cycles. The contributions of the geometrical barrier [3,4] and the energy barriers associated to stress defects that act as pinning centers on the NSI [1] have been elucidated [2]. The effect of defects is to enhance the capability to trap magnetic flux along the descending branch of the hysteresis cycle driving the system in a set of metastable states that would originate the occurrence of time-dependent phenomena. The thermal dependence of the magnetic relaxation rate reveals that the dynamics of the intermediate state of samples with defects is ruled by nonthermal processes for low enough temperatures [1]. This is attributed to quantum tunneling of NSI mediated by the formation/flattening of bumps at the defects. The value of the tunneling barriers is estimated in average, and the temperature of crossover from the thermal to the quantum regime is obtained from the Caldeira-Leggett theory. Comparison between theory and experiment points to tunneling of interface segments of a size comparable to the coherence length, by steps of the order of 1 nm. The effect of an applied magnetic field on both the crossover temperature and the quantum relaxation rate is also explored [5]. A decreasing magnetic field dependence of the strength of these energy barriers, which control both the value of the crossover temperature and the onset of metastability of the system, is invoked to explain the results obtained.
References
[1] E. M. Chudnovsky, S. Vélez, A. García-Santiago, J. M. Hernandez, and J. Tejada, Phys. Rev. B 83, 064507 (2011). [2] S. Vélez, A. García-Santiago, J. M. Hernandez, and J. Tejada, e-print arXiv:cond-mat.suprcon/1105.6218. [3] S. Vélez, C. Panadès-Guinart, G. Abril, A. García-Santiago, J. M. Hernandez, and J. Tejada, Phys. Rev. B 78, 134501 (2008). [4] R. Prozorov, Phys. Rev. Lett. 98, 257001 (2007). [5] S. Vélez, R. Zarzuela, A. García-Santiago, and J. Tejada, e-print arXiv:cond-mat.suprcon/1105.6222.
3:42 PM
BG-08. Novel 2D spin system and its interaction with conduction electrons
Tian Gang1, Deniz M. Yilmaz2, Derya Atac1, Elia Strambini1, Saurabh K. Bose1, Aldrik H. Velders3, Michel P. de Jong1, Juriaan Huskens2 and Wilfred G. van der Wiel1
1Group NanoElectronics (NE), MESA+ Institute for Nanotechnology, Enschede, Netherlands; 2Molecular Nanofabrication Group, MESA+ Institute for Nanotechnolo gy, Enschede, Netherlands; 3Biomedical Chemistry Group, MESA+ Institute for Nanotechnology, Enschede, Netherlands
We study the interaction of a dilute 2D spin system with conduction electrons in a metallic host via low-temperature transport measurements. A novel molecular fabrication method is presented, in which the 2D spin system is formed by embedding a self-assembly of spin-1/2 paramagnetic molecules in an Au film. Mixed monolayers of paramagnetic and nonmagnetic organometallic complexes have been used to systematically vary the spin concentration. This method offers great tunability of the nature and density (and hence coupling) of the spins, while avoiding undesired clustering of magnetic impurities often suffered from in alternative methods. The insertion of the paramagnetic molecules leads to a 2D Kondo impurity system with enhanced spin scattering near and above the Kondo temperature. This gives rise to a logarithmic resistivity increase at low temperatures. Our experimental results are very well described by Hamann's expression [1] for the Kondo resistivity correction (Fig. 1). The additional spin scattering also leads to a reduced phase coherence length as demonstrated by weak (anti) localization measurements (Fig. 2). We discuss the relevance of this model system for further study of spin phenomena: the Kondo effect and RKKY interaction.
References
[1] D.R. Hamann Phys. Rev. 1967, 158, 570 [2] S. Hikami, A. I. Larkin,Y. Nagaoka, Prog. Theor. Phys. 1980, 63, 707
3:54 PM
BG-09. Topological Excitations in Nanomagnets
Ralph Skomski1, Zhen Li1, Rui Zhang1, Roger D. Kirby1, Axel Enders1, Eva Schubert2 and David J. Sellmyer1
1Physics and Astronomy, University of Nebraska, Lincoln, NE; 2Electrical Engineering, University of Nebraska, Lincoln, NE
Noncollinear spin structures, such as magnetic skyrmions, have fascinated the magnetism community for many years. However, surprisingly little is known about the physical origin of these structures, and the is no clear-cut definition, except the requirement of a topological charge or winding number. Based on Skyrme's original idea [1], we analyze possible skyrmion mechanisms in terms of underlying physics and provide a simple example for each mechanism. There is no one-to-one analogy between Skyrme's description of baryons and magnetic skyrmions, so that latter cannot be understood properly without specifying the underlying physics. The simplest skyrmions are soliton- and vortex-like domain configurations [2], which are straightforward to understand in terms of magnetocrystalline anisotropy (MCA) and can be also be considered as examples of topological solitons. Dzyaloshinski-Moriya (DM) interactions provide a much more multifaceted picture. DM interactions are operative in all disordered systems, but macroscopic DM effects require crystals with broken inversion symmetry. A second big group of noncollinearities is caused by exchange interactions. Most noncollinear spin structures encountered in practice, such as those in the heavy rare-earth elements, are actually caused by competing exchange and may mask other effects [3]. This is because exchange interactions are non-relativistic and typically much larger than DM interactions and MCA. The Heisenberg interaction responsible for exchange is a simple and basically short-range example of a quantum-correlation effect. A much more complex and intriguing class is quantum phases, which have a far-reaching impact on both quantum mechanics and statistical mechanics [4]. These effects are caused by long-range entanglement of m any-body wave functions and have given rise to structures such as magnetic monopoles, anyons, and topological insulators. As a specific new system we consider magnetic nanospirals produced by glancing-angle deposition (GLAD). In these structures, the topology of a vortex-like spin state is protected by magnetostatic interactions so long as the curvature of the spirals remains sufficiently small. — This research is supported by NSF-MRSEC and NCMN.
References
[1] T. H. R. Skyrme, Nucl. Phys. 31, 556 (1962). [2] E. M. Chudnovsky and J. Tejada,Lectures on Magnetism, Rinton Press, Princeton 2006. [3] R. Skomski, J. Phys.: Condens. Matter 15, R841 (2003). [4] X.-G. Wen, Quantum-Field Theory of Many-Body Systems, University Press, Oxford 2004.
4:06 PM
BG-10. Thermal Defects in Skyrmion Crystals
Matthew Ambrose1 and Robert Stamps2
1School of Physics, University of Western Australia, Crawley, WA, Australia; 2School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
Skyrmion crystals, observed recently in Fe0.5Co0.5Si [1], are exciting examples of small, stable chiral magnetic structures that can be manipulated with applied currents. The Dzyalosinskii Moriya interaction leads to the formation of Skyrmion crystal states with different symmetries and chiralities [2]. At finite temperature a range of features are possible: Domains, lattice dislocations and topological excitations. Results from Monte Carlo simulation of a two dimension classical spin model are presented, with a focus on mechanisms responsible for the breakdown of long range order. Specific heat and the global chirality are calculated.
References
[1] X. Z. Yu et. al., Nature 465, 901 (2010) [2] Su Do Yi et. al., Physical Review B 80 054416 (2009)
4:18 PM
BG-11. Enhancement of the Curie temperature in small particles of itinerant ferromagnets
Lars Peters, Misha Katsnelson and Andrei Kirilyuk
Institute of Molecules and Materials, Radboud University Nijmegen, Nijmegen, Netherlands
The magnetism of metallic nanoparticles is technologically important as well as fundamentally intriguing and still unresolved. Within the picture of Heisenberg localized magnetic moments one could expect that the smaller the cluster size, the weaker the magnetism is, due to a lower coordination of surface atoms. For an itinerant electron system however, the influence of downscaling is not clear at all. For certain cluster sizes the shell effects give rise to an increased density of states near the Fermi level and thus, due to the Stoner criterion, an increased tendency towards ferromagnetism. Here, self consistent renormalization theory of itinerant ferromagnets [1] is used to calculate the Curie temperature of clusters. As an input, the transverse dynamic magnetic susceptibility of a non-interacting electron system is required. For the calculation of this, a description of the cluster’s energy level spectrum is provided by the random matrix theory from which an energy level distribution can be obtained for an ensemble of clusters differing in their atomic surface irregularities only. The remarkable result obtained from these considerations is that the Curie temperature increases for decreasing cluster sizes (see figure). This increase of the Curie temperature originates from the phenomenon of level repulsion i n chaotic quantum systems, which suppresses the spectral density of the spin fluctuations [2]. Since the latter destroy the magnetic order, the resulting Curie temperature increases.
References
[1] T. Moriya, Spin Fluctuations in Itinerant Electron Magnetism (Springer, Berlin, 1985) [2] L. Peters, M.I. Katsnelson, and A. Kirilyuk, Phys. Rev. B, in press (2011).
1:30 PM - 4:30 PM, Grand Canyon 1
Chair: Mingzhong Wu, Colorado State University FC
1:30 PM
BH-01. Tuning the cation distribution and magnetic properties of single phase nanocrystalline Dy3Fe5O12 garnet
Maurice Guillot1, Chins Chinnasamy2, Jean Marc Greneche3 and Vincent Harris2
1Laboratoire National des Champs Magnetiques Intenses, Grenoble, France; 2Electrical Engineering, Northeastern University, Boston, MA; 3Lab Phys Etat Condense, Univ Maine, Le Mans, Cedex, France
Synthesizing single phase nanocrystalline rare earth iron garnet (REIG) is difficult owing due to its complex crystal structure and due to the interaction between [a], (d)and {c} sites. In this report, gram scale, repeatable, single phase Dy3Fe5O12 (DyIG) particles with different grain sizes (bulk, 50,32 and 22 nm) were prepared using the ball milling technique in a controlled atmosphere. The magnetization for the as-prepared DyIG is in quasi perfect agreement with the single crystal values and all the Fe and Dy ions are in trivalent state. When the grain size is reduced below 50 nm, the magnetization is strongly applied field dependent and no saturation is observed even at an applied field of 320 kOe. The compensation temperature (Tcomp) for the nanocrystalline DyIG samples are few degrees higher than that of the bulk(Fig.1a &b). There is no evidence for the presence of Fe2+ charge state in the as prepared garnets. However, Mössbauer spectroscopy studies show that about 14-15 % of Fe2+ content was found in the milled nanocrystalline garnet samples. Oxygen vacancies created during milling process induces the formation of Fe2+ ions in the nanocrystalline Dy3Fe5O12 particles. The results will be explained in terms of the key role played by the cation redistribution using in-field Mössbauer spectroscopy and also by high field magnetic measurements.
1:42 PM
BH-02. Anisotropy Study of Garnet Films Grown Over Substrates Populated with Nanoparticles
Garrett S. Lang1, Charles Krafft2 and Isaak D. Mayergoyz1, 3
1Electrical and Computer Engineering, University of Maryland, College Park, MD; 2Laboratory for Physical Sciences, College Park, MD; 3Center of Applied Electromagnetics, University of Maryland, College Park, MD
Anisotropy of garnet films grown by liquid phase epitaxy (LPE) is an important figure of merit of such films because it by and large determines their magnetic properties. Anisotropy of LPE-grown garnet films is usually controlled by a proper choice of substrates, chemical composition of melt and growth conditions. It has been suggested[1-3] to use plasmon resonances in gold nanoparticles to enhance magneto-optic effects in garnet films. The best enhancement can be obtained if gold nanoparticles are embedded in garnets. T he latter can be achieved by growing garnet films over substrates populated by gold nanoparticles. It turns out that these nanoparticles may affect the growth conditions which may result in different anisotropy properties of garnet films. By selectively populating different areas of substrates with gold nanoparticles, local control of anisotropy of grown garnet films can be potentially achieved. In the paper, the results of the anisotropy study of LPE-grown garnet films over substrates partially populated with gold nanoparticles are reported. In the experiments, thin (about 10 nm) layers of gold have been evaporated on selected areas of (100)-oriented substituted gadolinium gallium garnet substrates and subsequently annealed in air at about 800° C. This annealing resulted in the formulation of gold nanoparticles. Thin garnet films were then grown over such substrates and their magnetic and magneto-optic properties have been studied. In particular, ferromagnetic resonance experiments were performed that reveal appreciably different effective magnetic fields for the parts of garnet films grown over the areas covered and not covered with gold nanoparticles (see Figure below). In the paper, these FMR measurements are discussed and correlated where it is possible with optical loop measurements.
References
1. S. Tomita, T. Kato, S. Tsunashima, S. Iwata, M. Fujii and S. Hayashi, Phys. Rev. Lett., 96, 167402 (2006). 2. R. Fujikawa, A. V. Baryshev, J. Kim, H. Uchida, and M. Inoue, J. Appl. Phys., 103, 07D301 (2008). 3. S. Tkachuk, G. Lang, C. Krafft, O. Rabin, and I. Mayergoyz, J. Appl. Phys., 109, 07B717 (2011).
1:54 PM
BH-03. Growth and Characterization of BaAlxFe12-xO19 Thin Films
Zbigniew Celinski1, Ian Harward1, Yan Nie1, 2, Allen Gardner1 and Lindsay Reisman1
1Physics, UCCS, Colorado Springs, CO; 2Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan, China
Microwave on-wafer devices operating at frequencies above 35 GHz require magnetic materials with high, adjustable perpendicular anisotropies which can be tuned by small magnetic fields. In order to satisfy such requirements we grew a series of aluminum-substituted M-type barium hexaferrite (BaAlxFe12-xO19) thin films on a Pt (111) template and Si wafer using metallo-organic decomposition technique. We varied the composition from x = 0 to x=2 with 0.25 step increments. The physical properties of the films were characterized by XRD, AFM, SQUID and Ferromagnetic Resonance (FMR). XRD patterns confirm highly textured c-axis polycrystalline films while AFM measurements allow us to estimate the lateral grain size to be on the order of 0.5 micron. The microwave properties of these films were studied using a broadband ferromagnetic resonance spectrometer from 40-70 GHz (see example in Figure 1). The measured out of plane effective anisotropy field (Heff = HAni - 4πMs) increases in a nearly linear fashion with increasing Al concentration, between 12.8 kOe for x = 0 and 23 kOe for x = 2. The measured FMR linewidths were relatively low, on the order of 100-300 Oe for compositions below x=1, increasing significantly up to 800 Oe for x= 2. The hysteresis loops exhibit a high degree of squareness, with the remanent magnetization nearly equal to the saturation magnetization.
2:06 PM
BH-04. Skin Effect Suppression for Cu/CoZrNb multilayered inductor
Noriyuki Sato, Yasushi Endo and Masahiro Yamaguchi
Department of El ectrical and Communication Engineering, Tohoku University, Sendai, Japan
Skin effect suppression is important for realizing high-Q RF thin-film inductors. In the present work, a 6-μm-thick Cu/Co85Zr3Nb12 multilayer is used as the conductor of a spiral inductor for 5.2 GHz (IEEE 802.11a) application, taking advantage of Rejaei’s concept [1] of using multilayers of commonly used conductive films and negative-permeability magnetic films. The film thickness ratio of Cu to CoZrNb is fixed at 30:1 since the calculated permeability ratio is 1:(-30) at 5.2 GHz. Fig. 1 shows the change in resistance of reference coplanar waveguides (CPWs) of the same thickness as the inductor. In the case of a Cu thickness of tCu = 1.37 μm, the skin effect is not suppressed and resistance monotonically increases, similarly to that of a Cu single-layered CPW. In the case of tCu = 0.73 μm, the resistance is minimized at approximately 5.2 GHz, indicating that the skin effect is markedly suppressed in the Cu/CoZrNb multilayer when tCu is less than the skin depth of Cu at 5.2 GHz. Fig. 2 shows the resistance of a spiral inductor with tCu = 0.73 µm. The slope of the resistance is lowest in the 4 - 6 GHz range. The inductance is L = 1.3 nH. The quality factor is Q =14.6 at 5.2 GHz, which is comparable to that of a 10-μm-thick Cu single-layered inductor [2]. The proposed inductor can have only 60% of the thickness of a conventional inductor while maintaining a good quality factor. This work was partly supported by Special Coordination Funds for Promoting Science and Technology from the Formation of Innovation Center for Fusion of Advanced Technologies.
References
[1] Behzad Rejaei, Marina Vroubel, J. Appl. Phys., 96, 6863-6686 (2004). [2] Satoshi Fukuda et al., IEEE International Workshop on RF Integration Technology (RFIT), 34-37 (2007).
2:18 PM
BH-05. Soft M-type Hexaferrite for VHF Miniature Antenna Applications
Jaejin Lee1, Yang-Ki Hong1, Woncheol Lee1, Gavin S. Abo1, Jihoon Park1, Won-Mo Seong2, Sang-Hoon Park2 and Wonki Ahn2
1Department of Electrical and Computer Engineering and MINT Center, The University of Alabama, Tuscaloosa, AL; 2Research and Development Center, E.M.W Co., Ltd., Seoul, Republic of Korea
Hexaferrite is effective in the reduction of very high frequency (VHF) antenna size due to its permeability and permittivity (λeff = λ0/(µrεr)0.5; antenna length = λeff/4), but needs to be soft while maintaining a reasonable magnetic crystalline anisotropy. Substitution of Co/Ti for iron cations in M-type hexaferrite (BaFe12O19: BaM) has been studied for permeability at VHF. Magnetic tangent loss (tan δμ) was measured to be about 0.07 at 200 MHz [1], which is still high for antenna applications. In this paper, we report low-loss BaFe9.6Co1.2Ti1.2O19 (Co/Ti-doped BaM: Hc = 29 - 48 Oe, σs = 47 - 52 emu/g) and demonstrate its applicability to terrestrial digital media broadcasting (T-DMB: 174 - 216 MHz) antenna. The Co/Ti-doped BaM powder was prepared by a ceramic process and then sintered by a two-step sintering process at 1,100 °C or 980 °C for the first step and 930 °C for the second step for a desired time. Two different ball-milling times (1.5 h and 8 h) were used. Permeability spect ra of sintered Co/Ti-doped BaM are shown in Fig. 1(a). The permeability increases with increasing the sintering temperature, while tan δμ is much lower for the intensively milled (8 h) Co/Ti-doped BaM. Permeability and tan δμ of the Co/Ti-doped BaM (8 h ball-milled, instant holding at 980 °C, and sintered at 930 °C for 8 h) were measured to be 4.5 and 0.037 at 200 MHz, respectively. This loss is about 47 % of the reported one [1]. Therefore, we have designed and simulated the T-DMB antenna in Fig. 1(b), using the Co/Ti-doped BaM. It was found that the size of soft M-type hexaferrite antenna is 1/3 of dielectric antenna (not shown here).
References
[1] C. Wang, et al., J. Mater. Sci. - Mater. Electron. 13, 713 (2002).
2:30 PM
BH-06. Spinel Ni0.7Mn0.3-xCoxFe2O4 Ferrite for UHF Devices
Jaejin Lee1, Yang-Ki Hong1, Woncheol Lee1, Gavin S. Abo1, Jihoon Park1, Won-Mo Seong2, Sang-Hoon Park2 and Wonki Ahn2
1Department of Electrical and Computer Engineering and MINT Center, The University of Alabama, Tuscaloosa, AL; 2Research and Development Center, EMW Co., Ltd., Seoul, Republic of Korea
Addition of cobalt (Co) to spinel Ni-Mn ferrite increases mangetocrystalline anisotropy, thereby the ferromagnetic resonant frequency (FMR) shifts toward higher frequency [1, 2]. The permeability of iron-rich Ni0.7Mn0.13Co0.04Fe2.1O4 ferrite is 17 at 200 MHz and the ferromagnetic resonance appears at 300 MHz [1]. The FMR of spinel Mn-Ni-Co ferrite used for a miniature T-DMB (terrestrial digital multimedia broadcasting: 174 MHz - 216 MHz) antenna was 400 MHz [3]. In order to achieve higher FMR than 400 MHz, we have developed Ni0.7Mn0.3-xCoxFe2O4 (NiMnCo: x = 0.00 to 0.10) using a combination of sol-gel process, 1150 °C calcination and 1100 °C sintering. Regardless of the Co content, the saturation magnetization remains almost constant at 56 emu/g as shown in Fig. 1(a), while the coercivity increases from 12 Oe (x = 0.00) to 72 Oe (x = 0.10). The law of approach to saturation was used [4] to estimate anisotropy constant (K1). It is found that K1 increased from 4.61×105 erg/cm3 for x = 0.00 to 9.28×105 erg/cm3 for x = 0.10. This is in good agreement with the increase in coercivity with Co content in Fig. 1(a). Complex permeability spectra (µ’− jµ”) of the sintered NiMnCo ferrite are shown in Fig. 1(b). The FMR frequency (at maximum μ”) shifts to GHz range (x = 0.10) from 138 MHz (x = 0.00), but µ’ decreases with Co content. This is because the permeability (μ = Ms/(Hk+Hd)) is inversely proportional to magnetocrystalline anisotropy (Hk = 2K1/Ms) and the FMR frequency (fr = (γ/2π)Hk) increases with the Hk. Our results imply that NiMnCo (x > 0.04) ferrite is applicable to GHz devices.
References
[1] A. Paduraru, et al., J. Optoelectron. Adv. Mater. 5, 945 (2003). [2] J. Lee, et al., J. Appl. Phys. 105, 07A514 (2009). [3] J. K. Ji, et al., IEEE Magn. Lett. 1, 5000104 (2010). [4] A. Franco, Jr., et al., J. Appl. Phys. 109, 07A745 (2011).
2 :42 PM
BH-07. Rotating Field Orientation of Co2Z Hexaferrite Compacts Produced via a Modified Aqueous Approach with a Single Sintering
Andrew P. Daigle1, Eric DuPre2, Jake Modest2, Anton Geiler1, Yajie Chen2, Carmine Vittoria2 and Vincent G. Harris1, 2
1Metamagnetics Inc., Sharon, MA; 2Electrical and Computer Engineering, Northeastern University, Boston, MA
Cobalt substituted barium hexaferrites (Co2Z) possess intrinsically high μ, ferromagnetic resonance values (~1.1 GHz), and have their magnetic orientation in plane perpendicular to the c axis. These characteristics make these materials practical for microwave devices such as antennas and electromagnetic band gap metamaterials (EBG)(1-2). A modified co-precipitation method has been proposed to produce high quality Co2Z hexaferrites, at ~24g/L (3). These particles have been thoroughly characterized by vibrating sample magnetometry (VSM) and X-ray diffraction (XRD) with regard to phase purity and magnetic properties. After formation and subsequent ball milling to sizes on the order of .5 to 2 um, particles were oriented and pressed into compacts inside a rotating field (4). Samples underwent VSM, XRD, and scanning electron microscopy (SEM) to determine the orientation effect. In addition, the complex ε and μ of these samples was measured as a function of applied field. Results show strong orientation, making these ferrites practical for a variety of device applications.
References
(1) Y. Chen, et al., "Electronic tuning of magnetic permeability in oriented Co2Z hexaferrite for application of high frequency electromagnetic devices", Appl. Phys. Lett. 98, 202502 (2011). (2) A. Daigle, et al., "Numeric Simulations of a Novel Wideband Electromagnetic Band Gap Metamaterial Utilizing Oriented Cobalt-Substituted Z-Type Barium Hexaferrites", IEEE Magn. Lett., vol.2, no., pp.0500104, (2011). (3)A. Daigle et al. "Preparation and Characterization of Pure Phase Co2Y Ferrite Powders via a Scalable Aqueous Co-precipitation Method", J. Am. Ceram. Soc. 93 [10] 2994-2997 ,(2010). (4)M. Obol and C. Vittoria, "Magnetic Properties of Co2 Y-Type Hexaferrite Particles Oriented in a Rotating Field," IEEE Trans. Magn., 39, 3103-5 (2003).
2:54 PM
BH-08. New CMOS Integration-Compatible Soft Magnetic Nanocomposite*
Qi Yao1, Lei Lu2, Michael Jantz2, Mingzhong Wu2 and You Qiang1
1Physics, University of Idaho, Moscow, ID; 2Physics, Colorado State University, Fort Collins, CO
The rapid advance in microelectronic technology urgently requires on-chip magnetic films operating at high frequencies up to the gigahertz regime, which is key to enhancing the microchip integration for miniaturization and improvement of next-generation information products.1-2 This presentation reports on novel CMOS integration-compatible nanocomposites made of Fe/Fe3O4 core-shell nanoclusters. The nanocomposites were prepared at room temperature by employing a unique energetic cluster impact (ECI) technology to bombard the spherical core-shell clusters onto a tilted Si substrate.3 XRD and TEM measurements revealed the core/shell structures of the Fe/Fe3O4 nanoclusters with a uniform size distribution of about 25 nm.4-5 The remarkable effects of ECI were clearly confirmed by atomic force microscopy (AFM) characterization, which showed that each Fe/Fe3O4 cluster had a football-like shape with its long axis in the plane of the composite film. The AFM measurements also indicated that the ellipsoidal clusters in the films were aligned in the same direction. A resistivity of 123 micro ohm cm was obtained via a four-probe method. The static magnetic measurements indicated that the Fe/Fe3O4 nanocluster films exhibited a remarkable in-plane uniaxial anisotropy and excellent magnetic softness. The ferromagnetic resonance (FMR) measurements were carried out by a shorted waveguide over 17-40 GHz. The FMR measurements confirmed the in-plane uniaxial anisotropy and yielded an effective Gilbert damping constant on the order of 0.01. The measurements also indicated that both the damping constant and inhomogeneity-caused FMR line broadening vary significantly with the fabrication control parameters, such as the bias voltage. The room-temperature induced in-plane uniaxial anisotropy makes the growth of Fe/Fe3O4 nanocluster films compatible with the underlying Si-based integrated circuits. This CMOS-integration adaptability, combined with the film’s excellent magnetic and electrical properties, is essential to the practical manufacture of novel on-chip magnetic passive components. *: Contact information: youqiang@uidaho.edu. This work was supported by DOE (DE-FG02-04ER46142), NSF and NIST.
References
1: V. Korenivski, J. Magn. Magn. Mater. 215-216, 800 (2000) 2: M. Yamaguchi, K. H. Kim, S. Ikedaa, J. Magn. Magn. Mater. 304, 208 (2006) 3: D. Meyer, M. Faheem, M. Campanell, J. Antony, A. Sharma, Y. Qiang, IEEE Trans. Magn. 43, 3010 (2007) 4: Y. Qiang, J. Antony, A. Sharma, J. Nutting, D. Sikes, D. Meyer, J. Nanoparticle Res. 8, 489 (2006) 5: C. M. Wang, D. R. Baer, J. E. Amonette, M. H. Engelhard, J. Antony, Y. Qiang, J. Am. Chem. Soc. 131, 8824 (2009)
3:06 PM
BH-09. Static and Dynamic Magnetic Properties of the Barium Hexaferrites Co2Z and Co2Y Prepared From Annealed Mixtures of Autocombustion Precursor Powders1
Thomas F. Ekiert1, 2, Derek W. Mirre1, 3, Max D. Alexander, Jr.1, Mark M. Doyle4, 5, Brian G. Kelly4 and Karl M. Unruh4
1Composites and Hybrids Branch, Air Force Research Laboratory, WPAFB, OH; 2Universal Technology Corportation, Dayton, OH; 3Southwester Ohio Council for Higher Education, Dayton, OH; 4Department of Physics and Astronomy, University of Delaware, Newark, DE; 5Department of Physics, Drexel University, Philadelphia, PA
Fine powders of the barium hexaferrites Co2Y (Ba2Co2Fe12O22) and Co2Z (Ba3Co2Fe24O41) have been prepared by annealing precursor powders synthesized by the autocombustion method from aqueous solutions of Ba, Co, and Fe nitrates, citric acid, and ammonium nitrate. After annealing the Co2Y precursor in air at a temperature of 1000 °C for 2 hours, x-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry measurements indicated an essentially single phase Co2Y product consisting of micron sized hexagonal platelets with a static coercivity of about 250 Oe and a saturation magnetization of about 36 emu/g. A similar annealing treatment of the Co2Z precursor also produced well defined micron-sized hexagonal platelets but x-ray diffraction measurements indicated that the product was not single phase Co2Z consistent with the rather large coercivity of about 1000 Oe observed in these samples. At higher annealing temperatures, however, the coercivity rapidly dropped and reached a value of about 25 Oe with a saturation magnetization greater than 50 emu/g following a 1300 °C anneal. The magnetic properties of these samples were consistent with a p redominately Co2Z product despite the presence of minority phases in the diffraction patterns. The high frequency properties of Co2Y and Co2Z composites consisting of 10 volume percent of the hexaferrite powders in a polymeric matrix were measured over the frequency range from 100 MHz to above 1 GHz. The relative permeabilities of the Co2Y and Co2Z composites at 500 MHz were 1.1 and 1.2, respectively, with loss tangents near 0.005 and 0.01. These results indicate that permeabilities greater than 3 should be obtainable in composites with greater hexaferrite volume fractions. Between 100 MHz and 1 GHz the permittivity spectra of both composites were relatively flat with a value between about 3.2 and 3.6 and an electric loss tangent near 0.004.
References
1Approved for unlimited release. Case number 88ABW-2011-3612.
3:18 PM
BH-10. Excessive grain boundary conductivity of spin-spray deposited ferrite/non-magnetic multilayer
Yun Xing1, Ogheneyunume Obi2, Nian X. Sun2 and Yan Zhuang1
1Electrical Engineering, Wright State University, Dayton, OH; 2electrical engineering, Northeastern University, Boston, MA
Low temperature spin sprayed ferrite films (Fe3O4) hold great potential for RF/microwave devices for their high self-biased ferromagnetic resonance (FMR) frequency (>5GHz) and low electrical conductivity[1-2]. In this study, we performed in situ scanning microwave microscopy characterization of a thin Fe3O4 ferrite film at frequencies ranging from 2.0-6.0 GHz. The ferrite film sample was composed of three layers: Fe3O4 (1.2 um)- photoresist (60 nm) - Fe3O4 (1.2 um). The film exhibited an in-plane co-ercivity of 118 Oe, and a saturation magnetization of 398emu/cm3. The resistivity of the film is 7Ωcm and the real permeability is 10 at 300 MHz. The film demonstrated a FMR frequency of 1.2 GHz and an FMR linewidth of 464 at X-band.The Scanning Microwave Microscopy (SMM consists of an atomic force microscope (AFM) and a microwave network analyzer (PNA), recording the one-port scattering parameters (s11) in a broad frequency range from 2.0 - 6.0 GHz. Figure 1 shows the Simultaneous acquisition of the amplitude of the reflected signal s11, and topography. By slightly changing the frequency, the amplitude image contrast of the grain boundary has been switched from bright to dark. Compared to a reference sample, it turns out that the edges appeared to be more conductive.
References
[1] G.M. Yang, X. Xing, A. Daigle, M. Liu, O. Obi, J.W. Wang, K. Naishadham, N.X. Sun, IEEE Trans. Magn., 44, 3091 (2008). [2] G.M. Yang, X. Xing, A. Daigle, M. Liu, O. Obi, S. Stoute, K. Naishadham, N.X. Sun (2010), IEEE Trans. Antennas and Propagation, 58, 648 (2010).
3:30 PM
BH-11. A Magnetically-Tuned Microwave Phase Shifter Using YIG/GGG-GaAs Flip-Chip Structure
Gang Qiu2, Yun Zhu1 and Chen S. Tsai1, 3
1Dept.of EECS, University of California, Irvine, Irvine, CA; 2Broadcom Corp., Irvine, CA; 3The Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan
Microwave phase shifters are important devices widely used in oscillators, phased array antennas, and various measurement systems. Recently, phase shifters based on ferri- or ferro- magnetic thin-film structures have been actively studied[1-5] . Such ferrite phase shifters utilize the tunablity of permeability of the magnetic layer by a bias magnetic field. Variation in the permeability results in changes in the phase velocity and, thus, the signal phase [6]. The phase shifters referred to above exhibited phase shifts ranging from 18 to 52 ο/cm[1-5]. We report hereby an X-band phase shifter using YIG/GGG-GaAs flip-chip structure [7] that has demonstrated significantly larger phase shifts per cm. The schematic of the proposed phase shifter is the same as Fig.1 in Ref.[8]. As the bias field was tuned from 2610 to 3320Oe, a differential phase shift of 53ο (or 117.8ο/cm ) was measured at 9.66GHz from a single segment of stepped-impedance low-pass filter (SILPF) [9-10]. By using 2- and 3-segment SILPFs meander lines, the phase shifts were increased to 104.5ο and 136.7ο(see Fig.1), respectively. The insertion loss (IL) variations of 0.95dB, and the respective return loss (RL) of -12dB and -11dB were also measured. Finally, the phase shifts of 35ο (77.8ο/cm), 46.0ο(102.2ο/cm), and 66.0ο (146.7ο/cm) with IL variations of 1.1dB and the RL lower than -11dB were obtained at 5.2, 7.75 and 11.5GHz, respectively.
References
[1]How, H., P. Shi, et al. J. of Appl. Phys, 87, 4966-4968, 2000. [2]E. Salahum, G.. Tanne, et al. Microwave and Opt. Tech. Lett., 30, 272-276, 2001. [3]J. H. Leach, H. Liu, E. Rowe et. al., J. Appl. Phys.,108, 064106, 2010. [4]A. B. Ustinov, G. Srinivasan, and B. A. Kalinikos, Appl. Phys. Lett. 90, 031913,2007. [5]A. L. Geiler, J. W. Wang, J. Gao, S. D. Yoon, Y. Chen, et.al., IEEE Trans. Magn. 45, 4179-4182, 2009. [6]J.D. Adam, L.E. Davis, G.F. Dionne, E.F. Schloemann, and S.N. Stitlzer, IEEE Trans. Microwave Theory Tech., 50, 721-737, 2002. [7]C.S. Tsai, and J. Su, Appl. Phys. Lett.,74, 2079-2082,1999. [8]G. Qiu, C. S. Tsai, Bert T.Wang, and Y. Zhu, “A YIG/GGG/GaAs-Based Magnetically Tunable Wideband Microwave Band-pass Filter Using Cascaded Band-Stop Filters.” IEEE. Trans. Magn., Vol.44, No.11,pp.3123-3125, Nov.2008. [9]G. Qiu, M. Kobayashi, B. T. Wang, and C. S. Tsai, J. Appl. Phys., 103,.07E915,2008. [10]Chen. S. Tsai, and Gang Qiu,IEEE Trans. Magn., 45, 656-660, 2009.
3:42 PM
BH-12. Tuning Limitations of the Voltage-Controlled Planar Microwave Ferrite Resonator*
Gerald F. Dionne and Daniel E. Oates
MIT Lincoln Laboratory, Lexington, MA
A microwave ferrite microstrip resonator with 15% tunability achieved by mechanical strain alone has been previously demonstrated [1]. The frequency tuning control arises from alignment of the magnetic moments along the propagation axis by application of collinear uniaxial stress. The stress can be produced and controlled by only a voltage applied to a piezoelectric actuator, without a polarizing dc magnetic field H and its current-controlled magnetic circuit [2]. The remarkable feature of the resonator is the reciprocal propagation constant that exists despite the interaction of unpolarized magnetic moments with the transverse magnetic field component of the rf propagating signal. For guidance to performance design, we have examined the conditions required for stress-tuning based initially on an independent single-domain-particle approximation adopted for a ceramic of randomly oriented grains with anisotropy constant K [3]. To account for magnetoelastic effects, the polycrystalline magnetostriction constant λ is assumed to be the weighted average over the principal cubic symmetry axes. The results of the analyses are threefold: (1) a previous relation for the range of tunability is modified to reflect a dependence on stress σ instead of H [4], with the key materials design parameter becoming λ/K; (2) ferrite compositional requirements to create the stress dominating condition is 3σλ >> K for the magnetic vectors along the axis of σ in the absence of H; and (3) for voltage control of the applied stress [1], a model for the fraction of electrostatic potential energy from a piezoelectric actuator that is converted to elastic energy in the ferrite substrate of the rotator’s microstrip circuit. The piezoelectric parameters of the actuator and the elastic constants of both the actuator and ferrite, as well as the geometrical characteristics of the respective components in a given rotator configuration determine the optimum device design. The limits of the device parameters and the tradeoffs available to achieve performance goals will be described in this paper.
References
[1] D.E. Oates, G.F. Dionne, and R.L. Slattery, IEEE IMS Digest, 641 (2009). [2] G.F. Dionne and D.E. Oates, MRS Symposium Proc. 833, 153 (2005). [3] S. Chikazumi, “Physics of Magnetism,” Wiley, New York 1964. Section 12.2. [4] G.F. Dionne and D.E. Oates, IEEE Trans. Magn. 33, 3421 (1997). * This work is sponsored by the Department of the Air Force.
3:54 PM
BH-13. An Active Resonator Based on Magnetic Films for Near Field Microwave Microscopy
Naser Qureshi1, Oleg Kolokoltsev1 and Cesar Ordoñez-Romero2
1Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Mexico, Mexico; 2Instituto de Física, Universidad Nacional Autónoma de México, Mexico, Mexico
A large number of recent implementations of Scanning Near-field Microwave Microscopy in the 1-10GHz frequency range have been based on resonators perturbed by a local probe scanned over a surface of interest [1]. Such resonators, typically designed with a coaxial geometry or with a planar transmission line geometry to take advantage of existing fabrication technologies, can also be considered passive resonators due to the fact that they are fed by an external microwave source. In this work we show results from a new alternative implementation where an active resonator based on magnetic excitations in a yittrium iron garnet (YIG) thin film within a resonant microwave circuit [2] is perturbed by a local probe scanned over a surface of interest. This active resonator has two intrinsic advantages: it has a Q-factor two to three orders of magnitude greater than that of a typical passive resonator, and can be readily tuned by varying an external magnetic field applied to the magnetic YIG film. We report on a YIG-based active resonator tuned to approximately 1.5GHz that has a resonance linewidth in the the KHz range, much smaller than the frequency shifts produced by perturbations from a scanning probe on a typical metal, dielectric or biological surface. Although one would expect this resonator to produce scanning probe images with greatly improved contrast and noise levels, the results are not straightforward. Its sensitivity to variations in external magnetic fields (on the order of nT), and consequently to small temperature changes, limits the sensitivity of this active probe near field scanning microwave microscope. We quantify these factors in detail and show that a magnetic thin-film based active resonator does, in fact, have greater sensitivity than passive designs and provides an attractive approach to Scanning Near-field Microwave Microscopy.
References
[1] B. T. Rosner and D. W. van der Weide, Rev. Sci. Instrum 73, 2505 (2002). [2] e.g. Murakami et. al, U.S. Patent No. 4,887,052, (1989).
4:06 PM
BH-14. On-wafer microwave band stop filter using a high quality barium hexagonal ferrite thin film
Ian Harward1, Justin Shaw2, Travis Hunter1, Allan Gardner1 and Zbigniew Celinski1
1Physics, UCCS, Colorado Springs, CO; 2Electromagnetics Division, NIST, Boulder, CO
Microwave on-wafer devices that employ magnetic resonance and which operate at frequencies above 35 GHz require magnetic materials with high anisotropies and low microwave losses. We have developed a procedure for growing high quality M-type Barium hexagonal ferrite (BaM) thin films using metallo-organic decomposition on a Pt template to fulfill this requirement. Previously we reported our highest quality film as having a full width at half maximum FMR linewidth of 270 Oe at 45 GHz. [1] By modifying the annealing process we have now achieved 140 Oe at 45 GHz, nearly a factor of 1/2 improvement. Furthermore, the new sample retained a hysteresis loop squareness of approximately 0.9, which is not normally seen in this material, and which could eliminate the need for a biasing magnet in a real device. FMR measurements from 28 K to room temperature show a nearly linear increasing temperature dependence of the effective out of plane anisotropy field, Hanis-4πMs, between 10.5 and 13 kOe respectively. Using this material, we built an on-wafer notch filter employing a coplanar waveguide geometry that exhibits attenuation of 12 dB/cm in the 40-50 GHz range. The measured device bandwidth is 0.5 GHz (see example in Figure 1 for a 4 mm long device). This work is supported by ARO (grant # W911NF-10-1-0225).
References
[1] Yan Nie, I. Harward, K. Balin, A. Beaubien, and Z. Celinski, “Preparation and characterization of barium hexagonal ferrite thin films on a Pt template”, J. Appl. Phys., 107, 073903 (2010)
4:18 PM
BH-15. Magneto-Electric Tuning of the Phase of Propagating Spin Waves
Kin L. Wong1, Mingqiang Bao1, Sergiy Cherepov1, Jing Zhao1, Yen-Ting Lin1, Mark Lewis1, Alexandre Bur2, Tao Wu2, Jian Zhu3, Pedram K. Amiri1, Ilya Krivorotov3, Gregory Carman2, Aleksandr G. Khitun4 and Kang L. Wang1
1Electrical Engineering, UCLA, Los Angeles, CA; 2Mechanical and Aerospace Engineering, UCLA, Los Angeles, CA; 3Physics and Astronomy, UCI, Irvine, CA; 4Electrical Engineering, UCR, Riverside, CA
We report a novel approach based on magnetoelectric thin film composite to control, by an electric field, the phase of magnetostatic surface spin waves propagating along thin ferromagnetic films. The magnetoelectric composite consists of laminates of a ferromagnetic thin film (CoFeB or Ni /NiFe) deposited on a ferroelectric substrate (PMN-PT). Propagating spin waves with nonzero k-vectors can travel a finite distance along the film surface and carry phase information. It is shown here the phase of propagating spinwaves can be modulated by an electric field. The phase shift is tuned by the change in magnetic anisotropy of the ferromagnetic film mediated by electric-field-induced strain modulation in the ferroelectric substrate. A quantitative analysis is shown to be consistent with the phase change observed in the spin waves and is in agreement with control measurements performed with Magneto-Optic Kerr Effect (MOKE) and superconducting quantum interference device (SQUID). The anisotropy field changes deduced from the observed spin wave phase shifts is in good agreement with MOKE measurements. The magnetoelectric coefficient of the composite system was also studied in both the linear and non-linear regimes with respect to the strain-voltage response of the PMN-PT substrate, and values up to 60 Oe-cm/kV were measured.
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Young K Kim, Korea University
BP-01. Inhomogeneity and damping in CoFe/Pd multilayers
Justin M. Shaw, Hans T. Nembach and Thomas J. Silva
NIST, Boulder, CO
Perpendicular magnetic materials such as Co/Pd and CoFe/Pd multilayers are of high interest for use in advanced storage technologies as a result of the tunability of the perpendicular anisotropy and saturation magnetization. However, these multilayers have considerable inhomogeneity that can lead to switching field distributions in bit patterned media application as well as distributions in critical currents and threshold currents in spin-transfer torque device applications. Another important property for such technological applications is the damping parameter. The underlying mechanisms that control the damping parameter are not well understood. To address these issues, we performed a systematic investigation using a broadband perpendicular ferromagnetic resonance (FMR) spectrometer to study the inhomogeneity and damping in Co90Fe10/Pd multilayers with different thicknesses and different CoFe:Pd thickness ratios. From the frequency dependence of the FMR linewidth, we are able to separate the inhomogeneous contribution to the linewidth. We find a strong non-linear dependence of the inhomogeneous linewidth broadening on the anisotropy. The intrinsic damping parameter on the other hand is independent of the perpendicular anisotropy. Instead, the dependence of damping on multilayer geometry can be explained in terms of spin pumping from CoFe into Pd and is determined by both the thickness of the CoFe and the Pd layers.
BP-02. The annealing effect of exchange-biased [Co/Pt] multilayer with perpendicular magnetic anisotropy
Jun-Yang Chen1, 2, Jia-Feng Feng1, Xiu-Feng Han2, Wen-Shan Zhan2 and J.m. D. Coey1
1CRANN and School of Physics, Dublin, Ireland; 2Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,Chinese Academy of Sciences, Beijing, China
The exchange bias has been widely used for magnetic devices based on spin valves and magnetic tunnel junctions, such as MRAM, magnetic sensor etc. Usually, the exchange bias was observed in FM-AFM with in-plane anisotropy. Recently, the perpendicular exchange bias has also been observed in [Co/Pt] or [Co/Pd] multilayers (MLs) coupled with an AFM. It is mainly determined by the orientation of the local magnetic moment of Co in the interface between Co and AFM [1-3]. However, the thermal stability of these MLs is very important for technological application. In this report, we demonstrate that it is possible to improve thermal stability by exchange coupling. Two series of [Co/Pt] MLs with compositions: Pt (2)/[Co (0.45)/Pt (2)]3/Pt (2)/Co (tCo)/IrMn (10)/Pt (2) and Pt (2)/IrMn (10)/Co (tCo)/[Pt (2)/Co (0.45)]3/Pt (2) (0 < tCo < 5, in nm,). The samples are annealed at 180 °C for 1 h in a magnetic field (H) of 0.8 T (H is out-of plane). The structure, thickness and magnetic properties are characterized by XRD, XRR, EHE and MFM. The results show thermal stability for bottom-pinned configuration is much better than it for the top-pinned case and such effect is strongly dependent on the Co thickness (tCo) adjacent to IrMn layers (Fig.1). It may be attributed to the additional anisotropy induced by FM-AFM exchange coupling, as well as magnetostatic interactions between Co layers adjacent to IrMn and other [Co/Pt]3 layers.
References
[1] J. Sort, V. Baltz, F. Garcia, B. Rodmacq, B. Dieny. Phys. Rev. B 71, 054411 (2005). [2] S. van. Dijken, J. Moritz, J. M. D. Coey. J. Appl. Phys. 97, 063907 (2005). [3] J.Y. Chen, J.F. Feng, Z. Diao, G. Feng, J.M.D. Coey, X.F. Han, IEEE Trans. Magn. 46, 1401 (2010).
BP-03. Magnetization reversal and change of magnetic anisotropy in Co/Cu Multilayers Nanowires with crossed configuration
Naeem Ahmad, Chen Junyang, Wei Shida and Han Xiufeng
Institute of Physics, Beijing, China
Crossed configurations in Co/Cu multilayers are useful for the application in magnetic sensors to get linear R(H) response. Co/Cu multilayer nanowires fabricated electrodeposition in an array using anodized aluminium oxide (AAO) template has been investigated. Magnetization reversal mode and magnetic anisotropy is found to depend on the Co & Cu layer thicknesses. The reversal modes have been investigated using the magnetic hysteresis loops measured at room temperature for Co/Cu nanowires placed at various angles between the directions of the nanowires axis and external fields using a vibrating sample magnetometer.A transition occurs from nucleation rotation to a combination of nucleation and curling rotation at around Co = 400nm & Cu=10nm.While for Co = 30nm & Cu = 60nm, magnetization reversal occurs by nucleation mode. A change of magnetic anisotropy from out of plane to in plane is observed when thickness of Cu layer tCu= 60 nm & that of Co tCo= 30nm. Magnetic anisotropy is lost when thickness of the Co layer tCo= 400 nm & that of Cu tCu=10nm. Magnetic and transport properties have been explained by the competition between shape anisotropy, magnetostatic interactions, magnetocrystalline anisotropy and interlayer coupling.
References
[1] A. Blondel, J. P. Meier, B. Doudin, and J.-Ph. Ansermet Appl. Phys. Lett. 65, 23 (1994). [2] J. Wong, Peter Greene, Randy K. Dumas, and Kai Liu Appl. Phy. Lett. 94, 032504 (2009). [3] K. Liu, K. Nagodawithana, P. C. Searson, and C. L. Chien, Phys. Rev. B 51, 7381 (1995). [4] B. Doudin, G. Redmond, S. E. Gilbert, and J. P. Ansermet, Phys. Rev. Lett. 79, 933 (1997. [5]L. Piraux, K. Renard, R. Guillemet, S. Mátéfi-Tempfli, M. Mátéfi-Tempfli, V. A. Antohe, S.Fusil, K. Bouzehouane, and V. Cros, Nano Lett. 7, 2563 (2007).
BP-04. Manipulation of permeability spectrum in [ferromagnet/antiferromagnet]N exchange-biased multilayered thin films for wideband microwave noise filter application
Lichuan Jin, Huaiwu Zhang, Xiaoli Tang, Guangduo Lu and Zhiyong Zhong
University of Electronic Science and Technology of China, Chengdu, China
Previous studies generally just focused on using the exchange-biased multilayer structure to push the ferromagnetic resonance frequency to higher frequency. Low frequency magnetic property is suppressed and it is hard to satisfy for wideband noise filter application. Here, an effective and flexible way to manipulate the permeability spectrum and expand the frequency linewidth towards low frequency is proposed. [NiFe/IrMn]6/[NiFe/IrMn]7/NiFe exchange-biased multilayered thin films were prepared by sputtering. Both the static and the microwave magnetic properties in frequency range from 10 MHz to 6 GHz have been systematic investigated. A wideband multi-peak permeability spectra with a 3.1 GHz linewidth was experimentally obtained by overlapping the spectrum of different [ferromagnet/antiferromagnet]N stacks. The study shows that the linewidth of the samples can be feasibly tuned through controlling the proper exchange bias fields of each stack. Moreover, theoretical fitting based on the Landau-Lifshitz-Gilbert (LLG) equation were also carried out to quantitatively identify the effective anisotropy fields and the Gilbert damping parameters of each peak. The results are extraordinary interesting and these exchange-biased multilayered thin films have potential application for a tunable wideband high frequency noise filter.
References
[1] Nguyen N. Phuoc, Feng Xu, and C. K. Ong, Appl. Phys. Lett. 94, 092505 (2009) [2] X. Chen, Y. G. Ma, and C. K. Ong, J. Appl. Phys. 104, 013921 (2008) [3] Changjun Jiang, Desheng Xue, Dangwei Guo, and Xiaolong Fan, J. Appl. Phys. 106, 103910 (2009) [4] J. Fassbender and J. McCord, Appl. Phys. Lett. 88, 252501 (2006) [5] Guozhi Chai, Yuancai Yang, Jingyi Zhu, Min Lin, Wenbo Sui, Dangwei Guo, Xiling Li, and Desheng Xue, Appl. Phys. Lett. 96, 012505 (2010) [6] J. F. Sierra, F. G. Aliev, R. Heindl, S. E. Russek, and W. H. Rippard, Appl. Phys. Lett. 94, 012506 (2009) [7] Nguyen N. Phuoc, Feng Xu, and C. K. Ong, Appl. Phys. Lett. 94, 092505 (2009) [8] Y. Lamy and B. Viala, IEEE Trans. Magn. 42, 3332 (2006) [9] N. N. Phuoc, S. L. Lim, F. Xu, Y. G. Ma, and C. K. Ong, J. Appl. Phys. 104, 093708 (2008) [10] D. Y. Kim, C. O. Kim, M. Tsunoda, M. Yamaguchi, S. Yabugami, and M. Takahashi, J. Appl. Phys. 101, 09E511 (2007)
BP-05. Relaxation effects evidenced on first-order reversal curves in hard/soft magnetic multilayers
Alexandru Stancu1, Petronel Postolache1, Laurentiu Stoleriu1, Thomas Bakas2, Nikoleta Siadou3, M. Androutsopoulos3 and Ioannis Panagiotopoulos3
1Faculty of Physics, "Alexandru Ioan Cuza" University of Iasi, Iasi, Romania; 2Department of Physics, University of Ioannina, Ioannina, Greece; 3Department of Materials Science and Engineering, University of Ioannina, Ioannina, Greece
Magnetic relaxation measurements performed on superstructures of the form [Ni/Pt]6/Pt(x)/[Co/Pt]6 in which the coupling between the [Co/Pt]6 “hard layers” and the [Ni/Pt]6 (“soft layers”) is adjusted by the thickness x of a Pt interlayer, prepared as presented in [1], have shown that in particular cases (when the average characteristic relaxation time of the magnetic entities is in the 1-100 seconds range) the hysteresis loop shape is strongly dependent on the waiting time in each point. When this analysis is extended to the use of first-order reversal curves (FORC) we have observed that some features appearing on the FORC diagram produced as described in many publications (e.g. [2]) are rapidly changing shape and some are even disappearing if one waits sufficiently. In this paper we present a systematic experimental analysis of relaxation processes measured on the first-order reversal curves. A relaxation process is measured in each point of a FORC (Fig.) in order to evidence how strong the dependence on the waiting time is and how this influences the FORC diagram of the system. One observed that even when the applied field is negati ve the relaxation at a certain critical field reverse tendency from a clear decrease towards an increase during relaxation, essentially due to the coupling. In the full paper we shall analyze and explain the position of these critical points on the FORCs. Financial support from CNMP - 12093 HIFI, IDEI FASTSWITCH 1994 and POSDRU/89/1.5/S/49944 Projects is acknowledged.
References
[1] N. Siadou et al, J. Magn. Magn. Mater., 323(12) 1671 (2011) [2] A. Stancu, C. Pike, L. Stoleriu, P. Postolache, D. Cimpoesu, J. Appl. Phys. 93 6620 (2003)
BP-06. Intrawire Magnetic Interactions in Electrodeposited Co/Cu and Co/Au Multilayer Nanowires
Tawab Dastagir and Hongbin Yu
Arizona State University, Tempe, AZ
Arrays of ferromagnetic nanowires have been studied as candidates for patterned magnetic recording media[1-2]. Specially, multilayer nanowire arrays in which each wire has a combination of alternating magnetic and non-magnetic layers are very useful for the application of giant magnetoresistance (GMR) with the current perpendicular to the plane. For the arrays of the nanowires to be useful in patterned magnetic area, it is important to understand the characteristics of these closely packed nanowires with the magnetization perpendicular to the plane. [3] In this study intrawire magnetic interactions inside aluminum oxides (AAO) membrane have been studied. Co/Cu and Co/Au multilayers were electrochemically synthesized inside the membrane. Co layers were separated by nonmagnetic Cu or Au layers. Thickness of Cu and Au layers were tuned to study the coercivity of the multilayer nanowires. Coercivity is found to increase as the nonmagnetic layer thickness is reduced between to Co layers. It suggests that intrawire magnetic interaction plays a vital role to determine the characteristics of multilayer magnetic nanowires. Vibrating Sample Magnetometry (VSM) was used to study these evolutions. Using micromagnetic simulation tool OOMMF we also validated our experimental result which shows coercivity increases as non magnetic layer thickness decreases.
References
1. C. A. Ross, M. Hwang, M. Shima et al., "Magnetic properties of arrays of electrodeposited nanowires," Journal Of Magnetism And Magnetic Materials 249 (1-2), 200-207 (2002). 2. K. Nielsch, R. B. Wehrspohn, J. Barthel et al., "High density hexagonal nickel nanowire array," Journal Of Magnetism And Magnetic Materials 249 (1-2), 234-240 (2002). 3. L. Piraux, J. M. George, J. F. Despres et al., "Giant Magnetoresistance In Magnetic Multilayered Nanowires," Applied Physics Letters 65 (19), 2484-2486 (1994).
BP-07. Effect of interfaces on the magnetic properties of SnO2/Cu-Zn ferrite multilayers
S. Saipriya, Joji Kurian and Rajender Singh
School of Physics, University of Hyderabad, Hyderabad, India
Magnetic multilayers (ML) composed of ferromagnetic (FM) and non magnetic (NM) components have been of great interest in the recent years 1-3 . The objective of the present work is to study the interfacial effects on the magnetic properties of [SnO2/Cu-Zn Ferrite (CZF)]n (n = 5, 10, 15 and 20) ML deposited by rf-magnetron sputtering technique. The total thickness of the CZF layer is kept constant. The FMR signal is asymmetric for the ML with n = 5 and it becomes symmetric as n increases. The FMR signal peak to peak intensity, effective and saturation magnetization and coercivity exhibit oscillations as a function of n. The peak to peak linewidth remains same irrespective of n indicati ng same level of inhomogeneties in the ML. In the parallel configuration the resonance field increases from 2526 to 2735 Oe with increase in n, presumably due to the decrease in the active layer to dead layer thickness ratio. The interlayer coupling is strong in the ML with n = 5 leading to the formation of spin waves. The interlayer coupling and anisotropy of the ML decreases with increasing n. The magnetic moment shows increasing and then decreasing trend as the temperature decreases indicative of evolving AFM interactions with decreasing temperature. The oscillatory behavior of the magnetic properties can be ascribed to the non monotonic variations in the structure and the geometry of the interfaces
References
[1]S. Saipriya, Joji Kurian and R. Singh, IEEE Trans. Magn. (2011)(in press). [2] P. Bruno, Eurohys. Lett., 23 615(1993). [3] K. Benkirane et al. , J Alloy Compd. 388 186 (2005).
BP-08. Effect of layer thickness (MgO and Ta) on perpendicular magnetic anisotropy of [Ta/Co60Fe20B20/MgO]5 multilayer films
Fu-Te Yuan1, Jen-Hwa Hsu1, Yi-Hung Lin2, Po-Cheng Kuo2 and J. K. Mei3
1Physics, National Taiwan University, Taipei, Taiwan; 2Materials Science & Engineering, National Taiwan University, Taipei, Taiwan; 3Department and Institute of Electrical Engineering, Minghsin University of Science and Technology, Hsin Chu, Taiwan
Ta/Co60Fe20B20/MgO films with strong perpendicular magnetic anisotropy and high anisotropic field Hk of around 3 kOe at room temperature were recently reported [1]. The TMR junction based on Co60Fe20B20 exhibits a remarkable TMR ratio of 120%, making it effective for spintronic applications. Since most spintronic devices have a multilayered structure, this study examines the magnetic properties of [Ta (tTa)/Co60Fe20B20(1.3 nm)/MgO (tMgO)]5 multilayer films. Samples were deposited on glass substrates at room temperature (RT) by sputtering followed by post-annealing at 250°C for 30 min. tMgO significantly affected perpendicular magnetic anisotropy. The optimal value of tMgO is 1 nm. tTa also crucially influences magnetic properties. Figure 1 plots the dependence of the RT anisotropic field (HkRT) on tTa for as-deposited and annealed samples with tMgO = 1 nm. Although perpendicular anisotropy was obtained in the samples with tTa ≥ 1 nm, HkRT ≥ 2 kOe when tTa ≥ 4 nm, as displayed in Fig. 1(a). In the RT-prepared samples, HkRT increases with tTa from 2.4 kOe to a maximum of 4.7 kOe at tTa = 10nm, which exceeds the previously reported values for annealed Co60Fe20B20 samples [2,3]. In contrast to results presented elsewhere [4], post-annealing reduced HkRT in the multilayers herein. Furthermore, with tTa = 10 nm, the perpendicular magnetic anisotropic energy at RT (KuRT), obtained from Figs. 1(b) and 1(c), decreases from 1.2×106 to 7×105 erg/cm3. X-ray reflectivity results suggest that the roughened interface increased by increasing tTa and post-annealing may account for the degradation of Ku and Hk.
References
[1] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno, Nat. Mater. 9, 721 (2010). [2] P. K. Amiri, et al., Appl. Phys. Lett. 98, 112507 (2011). [3] D. C. Worledge, et al., Appl. Phys. Lett. 98, 022501 (2011). [4] Yamanouchi, et al., J. Appl. Phys. 109, 07C712 (2011).
BP-09. Perpendicular magnetic anisotropy in CoFe/Tb multilayers
Jiiafeng Feng, Huseyin Kurt, Munuswamy Venkatesan and Michael Coey
CRANN and School of Physics, Trinity College, Dublin 2, Ireland
Perpendicular magnetic anisotropy (PMA) is investgated in fully amorphous [Co50Fe50(t)/Tb(t)]n multilayers with Ta and MgO cap layers, where the layer thickness t is 1.0 - 3.0 nm The CoFe layer does not crystallize when sandwiched between Tb layers and the mechanism for PMA is similar to that in amorphous TbCoFe ternary alloys. These multilayers can be very versatile compared to the ternary compounds as their magnetic properties can be fine tuned by changing the thicknesses of CoFe and Tb. Figure 1 (a) and (b) show the perpendicular magnetization measured by extraordinary Hall effect (EHE) and the coercivity (Hc) as a function of t for (Co50Fe50/Tb)2 /Co50Fe50 multilayers with a Ta cap in the as-grown state. Post deposition annealing weakens PMA in all samples, probably due to partial crystallization, but the Ta-capped samples remain perpendicular on annealing at temperatures up to 350 °C. The anisotropy energy is of the order of 106 J/m3 for these samples, which is similar to that of (Co/Pt)n multilayers [2]. These multilayers can be useful for bottom pinned perpendicular spin vales and magnetic tunnel junctions.
References
[1] Toshio Suzuki, Takanori Kiya, Naoki Honda, 2000 IEEE Trans. Mag. 36 2418 [2] Yakushiji K, Saruya T, Kubota H, Fukushima A, Nagahama T, Yuasa S, and Ando K, 2010 Appl. Phys. Lett. 97 232508
BP-10. The strain, thickness and electric field effects on the magnetic anisotropy of the thin FeCo/MgO(001) films: A first principles study
Kaihua He1, Jingsheng Chen1 and Yuanping Feng2
1Department of Materials Science & Engineering, National University of Singapore, Singapore, Singapore; 2Department of Physics, National University of Singapore, Singapore, Singapore
The magnetocrystalline anisotropy energy is a key parameter for the technological application of magnetic materials. Experimental studies showed that the perpendicular magnetic anisotropy of the thin FeCo/MgO films could be tuned by electric field and thickness [1,2]. To date, there is no theoretical work to elucidate the magnetic anisotropy and many related physical properties. In this work, several effects on magnetic anisotropy of FeCo/MgO films were studied by first principles calculations. The results indicate that the shorter in-plane lattice constant prefers out-of-plane anisotropy, and with increasing the lattice constant, the out-of-plane anisotropy is transited to in-plane anisotropy. The FeCo/MgO film with two FeCo layers is of out-of-plane anisotropy (in-plane lattice constant: 0.0405 nm), while the configurations with more FeCo layers show in-plane anisotropy. Moreover, the in-plane anisotropy can be tuned to out-of-plane by electric field (Fig. 1(a)). With elongating the interface distance, the hybridization between Fe/Co 3d and O 2p orbitals is weakened, which prevents the electron charges transferring from O 2p to Fe/Co 3d orbitals and increases the symmetries of the Fe/Co 3d bands. Therefore the MAE is reduced significantly by tensile strain (Fig. 1(b)). The calculated magnetoelectric coefficients indicate that FeCo/MgO interfaces have stronger magnetoelectric effect than those of Fe/MgO and clean Fe films, which attribute to that Co atom mainly determines the difference of orbital moments between different magnetization directions.
References
[1] M. Endo, S. Kanai, S. Ikeda, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 96, 212503 (2010). [2] T. Nozaki, Y. Shiota, M. Shiraishi, T. Shinjo, and Y. Suzuki, Appl. Phys. Lett. 96, 022506 (2010).
BP-11. Texture induced CoFe(110) phases study in multilayer [CoFe(x)/Os]n films
Dp Chiang1, 2, Yd Yao3, Dh Wei4, Sy Chen3 and Hm Lin2
1Center of General Education, Ming Hsin Univ. of Sci. and Tech., Hsinchu, Taiwan; 2Department of Materials Engineering, Tatung University, Taipei, Taiwan; 3Institute of Applied Science and Engineering, Fu Jen University, Taipei, Taiwan; 4Dept. of Mechanical Engn., National Taipei University of Technology, Taipei, Taiwan
Microstructure and magnetic properties of multilayer [CoFe(x)/Os(1)]n films on glass substrate (with x varied from 1, 2, 5, 10, 25, 50, to 100 nm and its associated n value of 100, 50, 20, 10, 4, 2, and 1, respectively) have been investigated. Os has good effect on preventing inter diffusion between layers [1-3]. In this study, Os layers were used to control the texture of magnetic layers and the grain size of CoFe layers by its thickness. As shown in Fig. 1a, the XRD patterns of all the samples have demonstrated that the CoFe(110) phases can be induced in samples with x small than 2 nm by the texture effect between Os and CoFe layers. The texture growth of disordered (110)bcc CoFe phase in sample with x = 2 nm and the ordered (110)bct phase in sample with x = 1 nm were observed as shown in Fig. 1b. After annealing at 500 C, the XRD peak for sample with x = 1 nm is unchanged, however, for sample with x = 2nm, the bcc(1 1 0) peak is gradually split into two: one corresponding to the ordered bct phase, and the other, to the disordered bcc phase. From the magnetic studies, the variation among the multi-domain, single domain, and super-paramagnetism is demonstrated for samples with x varied from 100 to 1 nm. Finally, the Os layers can be a good candidate to control the texture, grain growth and the inter diffusion among CoFe layers. This work is supported by the Sapintia Education Foundation.
References
1. S. Y. Chen, Y. D. Yao, and J. M.Wu, J. Magn. Magn. Mater., 310, 1914 (2007). 2. T. Y. Peng, C. K. Lo, Y. D. Yao, and S. Y. Chen, Appl. Phys. Lett., 90, 121904 (2007). 3. D. P. Chiang, S. Y. Chen, Y. D. Yao, H. Ouyang, C. C. Yu, Y. Y. Chen, and H. M. Lin, J. Appl. Phys., 109, 07A732 (2011).
BP-13. Thermal stability of CoFeB/Pt multilayers with perpendicular magnetic anisotropy
Yanyan Zhu, Zongzhi Zhang, Bin Ma and Qingyuan Jin
Department of Optical Science and Engineering, Fudan University, Shanghai, China
Giant magnetoresistive devices with perpendicular magnetic anisotropy (PMA) are known to have great advantages over those with in-plane anisotropy for applications in high density spin-torque MRAM.[1-3] CoFeB/Pt multilayers (MLs) are one of the promising candidate materials used in spin valves or magnetic tunnel junctions. In this work, CoFeB/Pt MLs of glass/Ta(30)/Cu(17)/[CoFeB(t1)/Pt(t2)]N /Ta(30) (thickness unit: Å) were studied. The polar MOKE loops exhibit a square shape when t1 is between 2.2 and 5.0Å and t2 is over 10Å, suggesting MLs with appropriate CoFeB and Pt thicknesses display out-of-plane anisotropy. The thickness influence of CoFeB and Pt layer on magnetic thermal stability has been investigated for MLs showing PMA. Typical polar loops of [CoFeB (2.2 and 4.5Å)/Pt(14.8Å)]5 after annealing at different temperatures (Ta) are shown in Fig.1. No obvious change occurs in the loop shape of the thin CoFeB sample (Fig.1a-d), while the perpendicular coercivity of the ML with thick CoFeB gradually increases with increasing Ta (Fig.1e-h). Note that the original square loops for both samples become slant after annealing at the same temperature of 300 degree, suggesting the perpendicular anisotropy degrades upon thermal treatment. XRD results will be given to clarify the different varying behaviors.
References
[1] S. Mangin, D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris, and Eric E. Fullerton, Nat. Mater. 5, 210 (2006). [2] M. Nakayama, T. Kai, N. Shimomura, M. Amano, E. Kitagawa, T. Nagase, M. Yoshikawa, T. Kishi, S. Ikegawa, and H. Yoda, J. Appl. Phys. 103, 07A710 (2008). [3] L. Liu, T. Moriyama, D. C. Ralph, and R. A. Buhrman, Appl. Phys. Lett. 94, 122508 (2009).
BP-14. Curie temperatures of CoPt ultrathin continuous films
Wupeng Cai1, 2, Ji Shi2, Yoshio Nakamura2, Wei Liu1 and Ronghai Yu3
1Department of Materials Science and Engineering, Tsinghua University, Beijing, China; 2Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo, Japan; 3School of Materials Science and Engineering, Beihang University, Bejing, China
CoPt thin films have attracted much attention due to their high magnetocrystalline anisotropy [1, 2]. However, curie temperatures (Tc) of CoPt thin films are rarely reported. This may be due to the formation of island structure instead of continuous layer, when the thickness of CoPt film (tCoPt) is only several nanometers. We have found that CoPt layers in CoPt/AlN multilayer structure remains continuous even when tCoPt = 1 nm. Subsequently Tc of CoPt thin films are studied in this work.
Fig.1 shows the experiment results of Tc. For as-deposited films, with decreasing tCoPt, Tc remains nearly constant until 3 nm and abruptly decreases thereafter. This behavior may be caused by the surface effect and loss of spin-spin interactions [3]. There exists lattice deformation during the heating process of Tc measurement, which will change the exchange interaction parameter and influence Tc. In-situ high temperature XRD results reveal that the (111) spacing of as-deposited and 300 °C annealed films will go to almost the same value when the temperature is higher than 300 °C. Hence 300 °C annealed films have nearly the same Tc with as-deposited films. We have found ordering transformation from FCC to FCT in 600 °C annealed films [4]. Tc of bulk FCT CoPt is about 100 K lower than FCC for present composition Co44Pt56 [5]. Therefore the ordering transformation can significantly decrease Tc, as shown in fig. 1 when tCoPt ≥ 3 nm. However, 1.5 nm and 2 nm thick CoPt films have higher Tc than as-deposited. This may be induced by the low degree of ordering [4] and different high-temperature lattice deformation of such thin CoPt films.
References
[ 1] K. Barmaka, J. Kim, L. H. Lewis, K. R. Coffey, M. F. Toney, A. J. Kellock, and J.-U. Thiele, J. Appl. Phys. 98, 033904 (2005).
[2] D. Alloyeau, C. Ricolleau, C. Mottet, T. Oikawa, C. Langlois, Y. L. Bouar, N. Braidy, and A. Loiseau, Nat. Mater. 8, 940 (2009).
[3] R. J. Zhang, and R. F. Willis, Phys. Rev. Lett. 86, 2665 (2001).
[4] W. P. Cai, J. Shi, Y. Nakamura, W. Liu, and R. H. Yu (unpublished).
[5] J. M. Sanchez, J. L. Moran-Lopez, C. Leroux, and M. C. Cadeville, J. Phys.: Condens. Matter 1, 491 (1989).
BP-15. Magnetism of compositionally modulated Ti1-xVxO2/TiO2 multilayers
Damien Le Roy1, 2, Shah Valloppilly2, Ralph Skomski1, 2, Sy-Hwang Liou1, 2 and David J. Sellmyer1, 2
1Physics and Astronomy, University of Nebraska, Lincoln, NE; 2Nebraska Center for Materials and Nanoscience, Lincoln, NE
Anatase TiO2 is a promising host material for wide-gap ferromagnetic semiconductors. When doped with cobalt, the ferromagnetic order persists up to 400 K [1]. Recently, Yamada et al. have demonstrated the ability to manipulate the magnetization of Co-doped anatase TiO2 layers by applying an external electric field [2], which furthers the idea of building fully electrically driven spintronic devices. V-doped anatase TiO2 is predicted to fulfill the requirements of a magnetic semiconductor as (i) the V atoms carry a magnetic moment of 1 μB when substituting for Ti and (ii) the large solubility of the V atoms in the anatase TiO2 of up to 21% exceeds the percolation threshold of the ferromagnetic interaction of 5.6% [3]. Our focus is on the magnetism of epitaxially grown anatase Ti1-xVxO2/TiO2[001] multilayers by means of pulsed-laser deposition. The composition is modulated along the growth direction by sequential deposition of V and TiO2, and post-annealing. The V total content x varies from 0 to 25%, covering the whole domain of solubility in anatase TiO2. The structure is analyzed by cross-section transmission electron microscopy and X-Ray diffraction, and the magnetization is measured using a commercial superconducting quantum interference device magnetometer. Throughout the range of investigated compositions, the anatase TiO2 structure is conserved, and the crystal structure is only moderately affected by the V dopants. The full width at half maximum of the rocking curves around the anatase Ti1-xVxO2/TiO2[001] Bragg-reflection increases from 0.4° to 1° as x increases from 0 to 25%. It is demonstrated that the V dopants induce ferromagnetism in anatase TiO2, as expected for partial substitution of Ti by V with an average magnetic moment of 0.5 μB per V atom. The effects of V local environments and ferromagnetism will be discussed. This research is supported by ARO, NSF-MRSEC and NCMN.
References
[1] Y. Matsumoto et al., Science 291, 854 (2001). [2] Y. Yamada et al., Science 332, 1065 (2011). [3] J. Osorio-Guillén et al., Phys. Rev. Lett. 100 (2008).
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Brian Kirby, NIST
BQ-01. Stability and Magnetoelectronic Properties of Indium Nitride Quantum Dots Doped With Co and Ni Atoms
Liudmila A. Pozhar
Department of Physics, University of Idaho, Moscow, ID
Rich electronic properties,1,2 and in particular sensitivity of the band gap of indium nitrides to their nitrogen content, accommodates a wide range of device applications. Doping such systems with Co and/or Ni atoms can extend their applications to magneto-optics, quantum electronics and spintronics. In this work conditional and unconditional total energy minimization procedures based on ab inition many-body-theoretical methods2,3 were used to model nucleation of small non-stoichiometric In-N and In-As-N quantum dots (QDs) doped with Co or Ni atoms in quantum confinement and “vacuum” (i.e., in the absence of foreign atoms). Nitrogen atoms play the leading role in stabilization of the studied QDs enabling electron charge delocalization and serving as electron charge sinks. It was found that dopant nickel atoms significantly enhance performance of nitrogen atoms. In particular, dopant Ni atoms mediate indium electron charge redistribution providing for the further accumulation of the electron charge in the "volume" of the QDs. "Surfaces" of such Ni-doped QDs become either electroneutral or electropositive, as opposed to those of the corresponding undoped QDs that are electronegative. This results in a decrease of the ground state energy of the Ni-doped QDs by up to 320 H as compared to that of the corresponding undoped systems. In contrast, similar substitution doping of the original InN- and InAsN-based QDs with Co atoms leads to destabilization of the original QDs in the majority of the studied cases. In Co-doped QDs uncompensated electron spins remain localized on Co atoms, while in the case of substitution doping with Ni atoms the electron spin density distribution is such that uncompensated electron spins are localized "between" nickel and nitrogen atoms. The results of the reported study are in agreement with experimentally observed sensitivity of the band gap of indium nitride systems to their nitrogen and substitution defect content.
References
1. L.A. Pozhar, Physica Status Solidi (accepted; 2011). 2. L.A. Pozhar, EuroPhys. J. D 57, 343(2010). 3. L.A. Pozhar, A.T. Yeates, F. Szmulowicz and W.C. Mitchel, Phys. Rev. B 74, 085306 (11) (2006).
BQ-02. Magnetic Bipolarons in Quantum Dots
Rafal Oszwaldowski1, Igor Zutić1 and Andre G. Petukhov2
1Physics, University at Buffalo, Buffalo, NY; 2Physics, South Dakota School of Mines and Technology, Rapid City, SD
Magnetic Quantum Dots (QDs) are quasi-zero-dimensional structures of a semiconductor material doped with transition metal ions, most often Mn. They attract a great deal of attention owing to their potential in the field of spintronics [1]. Magnetism in QDs could be controlled by methods not available in bulk semiconductors [2]. Magnetic polarons consist of a single confined carrier or exciton, which aligns Mn spins in its vicinity through exchange interaction. This effect has been observed in both self-assembled and colloidal QDs [3,4] and persists to high temperatures [5]. In this work, we discuss the surprising finding that two carriers can also induce magnetic ordering of Mn spins, despite their ground state possessing zero carrier-spin [6]. We use a transparent analytical approach, taking into account the strong spin-orbit interaction in the valence band. We show that both time-reversal and spatial symmetry of the system can be broken. We propose experimental tests of our predictions and the magnetic-dot structures to perform them. This work was supported by DOE-BES, U.S. ONR, AFOSR-DCT, NSF-ECCS, and CAREER.
References
[1] I. Zutić, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] R. M. Abolfath, A. G. Petukhov, and I. Zutić, Phys. Rev. Lett. 101, 207202 (2008). [3] J. Seufert et al., Phys. Rev. Lett. 88, 027402 (2002). [4] R. Beaulac et al., Science 325, 973 (2009). [5] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B 82, 195320. [6] R. Oszwaldowski, I. Zutić, A. G. Petukhov, Phys. Rev. Lett. 106, 177201 (2011).
BQ-03. Exchange bias effect of Ge1-xMnxTe with antiferromagnetic MnTe and MnO materials
Sze Ter Lim1, 2, Hui Lu1, 2, Jing Feng Bi1, Thomas Liew1, 2 and Kie Leong Teo1
1Electrical & Computer Engineering Department, National University of Singapore, Singapore, Singapore; 2Data Storage Institute, Singapore, Singapore
Recent progress in the molecular epitaxial growth (MBE) of Ge1-xMnxTe has shown that Tc as high as 200 K can be achieved. [1] On the other hand, only a narrow window of growth conditions exists to achieve single phase of the material. Due to phase separation, secondary phase of antiferromagnetic (AFM) hexagonal MnTe can be formed in Ge1-xMnxTe. The coexistence of MnTe and ferromagnetic (FM) Ge1-xMnxTe can result in exchange-bias (EB) effects. [2] In this work, the EB effect of Ge1-xMnxTe with AFM MnTe and MnO materials has been investigated. Both MnTe and MnO is either deposited directly adjacent to Ge1-xMnxTe, or were separated from it by nonmagnetic spacer layers ZnTe. We observed that the Ge1-xMnxTe-MnTe system only leads to a modification of coercivity field, which could be attributed to compensated spins at the FM/AFM interface. On the other hand, a negative EB shift in the hysteresis loop is observed for Ge1-xMnxTe-MnO bilayer when it is cooled in applied field.
References
[1] M. Hassan, G. Springholz, R. T. Lechner, H. Groiss, R. Kirchschlager, G. Bauer, J. Crystal Growth 323, 363 (2010). [2] R. T. Lechner, G. Springholz, M. Hassan, H. Groiss, R. Kirchschlager, J. Stangl, N. Hrauda, and G. Bauer, Appl. Phys. Lett. 97, 023101 (2010).
BQ-04. Ferromagnetic Behavior in Ytterbium-doped and Ion Implanted GaN Semiconductor
Ratnakar Palai1, J. Wu1, H. Tanaka2, J. Wang2, W. M. Jadwisienczak2 and H. Huhtinen3
1Dept. of Physics, University of Puerto Rico, San Juan; 2School of EECS, Ohio University, Athens, OH; 3Department of Physics, University of Turku, Turku, Finland
Thin films of ytterbium (Yb)-doped and ion implanted GaN have been grown using plasma assisted molecular beam epitaxy (MBE) and metalorganic chemical phase deposition (MOCVD) on (0001) sapphire substrate at different growth conditions. We have performed systematic and comparative studies of the magnetic and optical (photoluminescence and cathodeluminescence) properties of in situ doped epitaxial thin films (50-100 nm) by MBE and Yb-ion implanted epilayers (300 nm-1 μm) by MOCVD. The magnetic measurements were carried using superconducting quantum interference device (SQUID), alternating gradient force microscopy (AGFM), and Kerr Effect. X-ray diffraction (XRD) and magnetic force microscopy (MFM) measurements show the absence of phase segregation and clustering of rare earth ions. The surface morphology of the films was studied by atomic force microscopy (AFM) and the surface roughness was found to be about 2-3 nm over a large scanning area. Magnetic hysteresis loop measurements of Ga1-xYbxN and Yb-implanted GaN thin films reveal the existence of ferromagnetism at room temperature, which is a desired property for device applications. The temperature variation of magnetization reveals the non-zero magnetization at room temperature. The observation of higher magnetization in MBE grown thin films compared to the ion implanted MOCVD films could be related to the epitaxial strain and lattice strain due to in situ doping. This in turn confirms that the, RE3+ ion in general, and Yb3+ ion in particular unlikely by themselves induce the strong ferromagnetism in GaN at 300 K. Furthermore, all studied ytterbium doped/implanted GaN samples exhibit excellent semiconducting properties similar to pure the GaN as determined by Hall effect measurement and good optical quality confirmed by photoluminescence at different temperature. These properties have been analyzed in the framework of currently accepted theory describing magnetic behavior in rare earth-doped III-N dilute magnetic semiconductors and will be discussed in details.
BQ-05. Magnetic properties of Mn-implanted 6H-SiC single crystal
Maya Al-Azri1, Mohammed El-zain1, Khalid Bouziane1, Mourad Chérif2, Yves Roussigné2, Alain Declemy3, Michel Drouet3 and Lionel Thomé4
1Physics, Sultan Qaboos University, Muscat, Oman; 2LSPM (CNRS-UPR 34071), Université Paris 13, 93430 Villetaneuse, France; 3PhyMat (CNRS UMR 6630), Université de Poitiers, Marie et Pierre Curie SP2MI, France; 4CSNSM-Orsay, Université d’Orsay, F-91405 Orsay, France
Energy stability, electronic structure and magnetism of Mn-doped 6H-SiC have been investigated using ab-initio calculation. Various configurations of Mn site and vacancy type have been considered. For a Mn atom at substitutional sites without vacancies, it was found that Mn atom at Si sites possesses a much larger magnetic moment as compared to vanishing moments of Mn atoms at C site. When a Mn atom resides at a substitutional site in presence of a neighboring vacancy, it was found that Mn at Si site with a neighboring C vacancy is more stable than a Mn atom at C site with a neighboring Si vacancy. The calculation shows that the Mn atom possesses a magnetic moment for both types of substitution (either Si or C). However, the magnetic moment of Mn atoms at Si site is still larger than the corresponding moments for Mn at C site. Two doping Mn atoms within a unit cell are also considered. Ferromagnetic and antiferromagnetic coupling between two neighboring Mn atoms at substitutional and/or interstitial in the presence of vacancy have also been explored. Our calculation shows that two Mn atoms in neighboring Si sites (MnSi-MnSi) in the presence of a C vacancy prefer the antiferromagnetic configuration. Relaxation effects were also studied. It was found that more stable antiferromagnetic configurations are obtained for Mn atoms slightly displaced from the regular substitutional sites These results are described in the light of our experimental data obtained for Mn-implanted 6H-SiC for various Mn fluencies (different Mn content).
BQ-06. Clustering and Magnetism of Nickel in situ Doped Amorphous AlN Thin Films
Hiroki Tanaka1, Gang Chen2, Congshang Wan2, Marty E. Kordesch2, Savas Kaya1 and Wojciech M. Jadwisienczak1
1EECS, Ohio University, Athens, OH; 2Department of Physics and Astronomy, Ohio University, Athens, OH
In this paper we investigate magneto-optics of nickel in situ doped amorphous AlN layers (a-AlN) deposited by rf sputtering on Silicon (0001) substrates. As-grown material exhibits weak ferromagnetic behavior as evidenced by the magneto-optic Kerr effect measurement at room temperature. The three orders of magnitude increase of the Kerr rotation and ellipticity values were observed after post-growth thermal annealing at 450C in nitrogen. The abrupt decrease in material magnetization was observed after annealing at higher ambient temperature. The morphology of as-grown and annealed a-AlN:Ni films were characterized by small angle x-ray scattering and high resolution TEM and MFM. It is found that the as-deposited film contains nano-particles of different sizes with average diameters between 10 and 30 nm. The size distribution of nano-particles in the thermally annealed a-AlN:Ni films was studied as a function of annealing time and temperature, and the results correlate well with those obtained from the magnetism measurements. Furthermore, the morphological changes of a-AlN:Ni due to annealing with and without magnetic field stimulus, Al and Ni atoms clustering, hybridization of Ni atoms and/or Ni clusters with immediate amorphous surrounding are studied and correlated with the magnetic domains boundary formation and their role in observed ferromagnetism.
BQ-07. Structural and compositional phase separation in ferromagnetic semiconductor (Zn,Cr)Te
Hiroaki Kobayashi1, Yotaro Nishio1, Koichiro Ishikawa1, Ken Kanazawa1, Shinji Kuroda1, Masanori Mitome2 and Yoshio Bando2
1Institute of Materials Science, University of Tsukuba, Tsukuba, Japan; 2International Center For Materials Nanoarchtectonics, National Institute for Materials Science, Tsukuba, Japan
In the search of high-temperature ferromagnetic semiconductors, it has sometimes been observed that magnetic impurities incorporated aggregate in the host semiconductor, instead of being uniformly distributed. In some cases, the magnetic impurities aggregate into precipitates of an extrinsic phase of different crystal structure from the host matrix, while in other cases the aggregation takes place as the “coherent” phase separation into regions with high and low contents of the magnetic impurity keeping the same crystal structure. In our recent study on (Zn,Cr)Te, one of the candidates of room-temperature ferromagnetic semiconductors, it was revealed that the Cr aggregation could be controlled by co-doping donor impurity iodine and ferromagnetic properties were significantly enhanced[1]. In the present study, we have investigated how the details of Cr aggregation in I-doped (Zn,Cr)Te changes depending on the growth conditions. As a result of analyses on a series of films grown by MBE under various growth conditions, we have found that the growth temperature has a dominant effect on the details of Cr aggregation. In particular, for a relatively high average Cr content around x = 0.2, the shape of Cr-aggregated regions changes with the growth temperature TS[2]; Cr atoms aggregate into isolated clusters at an intermediate growth temperature of TS = 300oC, whereas Cr-aggregated regions become elongated or columnar shape at a higher temperature of TS = 360oC. The structural and compositional analyses using element-specific probes of EDX and EELS, it is revealed that the Cr-aggregated regions consist of an extrinsic phases which is presumably assigned as NiAs-structure CrTe or Cr1-δTe of the hexagonal structure derived by incorp orating Cr deficiency. In addition, it is found that these hexagonal precipitates have a particular orientational relationship with the zinc-blende structure of the host matrix. The result of our investigation on the films with different Cr compositions and growth temperatures will be presented at the conference.
References
[1] S. Kuroda et al., Nature Mater. 7, 440 (2007). [2] Y. Nishio et al., Mater. Res. Soc. Symp. Proc. vol. 1183, 9 (2010).
BQ-08. Mn5Ge3 clustered in n-type ferromagnetic semiconductor (Mn,H):Ge
Dang Duc Dung1, 2, Wuwei Feng1, Younghun Hwang1, Duong Anh Tuan1 and Sunglae Cho1
1Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea; 2Department of General Physics, Ha Noi University of Science and Technology,, 1 Dai Co Viet road, Ha Noi, Viet Nam
The effect of cluster Mn5Ge3 on n-type ferromagnetic semiconductor (Mn,H):Ge has been observed. The room temperature ferromagnetism with TC ~ 300 K were obtained for Mn:Ge annealed under hydrogen environment at 300oC. The negative anomalous Hall effect was shown up to room temperature which suggest for spin polarization. The only negative magnetoresistance (MR) were obtained for as-grown samples while the negative MR at low temperature change to positive at high temperature which was relative with present of Mn5Ge3 clusters. The result may help us to deep understand the role of carrier and clusters in Mn:Ge which could be open the way to control the magnetism of diluted magnetic semiconductor.
BQ-09. Room-temperature Ferromagnetism in Homogeneous Cr-doped GaN
Praveen Suggisetti1, 2, Tarkeshwar Patil1, 2, Ramabhadra R. Adari1, 2, Debashree Banerjee1, 2, Tanmoy Pramanik1, 2, Dipankar Saha1, 2 and Swaroop Ganguly1, 2
1Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, India; 2Centre of Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai, India
Magnetically-doped semiconductors are of considerable interest for a variety of spintronics applications1. GaN, which is one of the most important semiconductor materials for high-speed and other applications, has also been of considerable interest as a potential high-temperature ferromagnetic semiconductor2,3. Here we report room-temperature ferromagnetism in GaN doped by thermal diffusion of Cr. The samples were fabricated by electron-beam evaporation of Cr on GaN-on-sapphire wafers followed by controlled rapid thermal processing (RTP) in a number of one-minute time cycles. Detailed magnetic, structural and optical investigations were carried out to elucidate the properties of the Cr-doped GaN. SQUID magnetometry shows clear hysteresis behavior along with remanant magnetization at room-temperature. The remanance is seen to initially increase and then decrease with the number of RTP cycles. This indicates that, as in (Ga,Mn)As, optimization of the annealing conditions will be essential to increasing the Curie temperature in this system4. HRTEM was carried out at several locations on the sample to investigate the precipitation of ferromagnetic clusters, which is common in this class of materials5. We see that the Cr-doped GaN exhibits a single diffraction pattern - that of GaN - as well as clear GaN lattice planes. The HRTEM images showed no (macro/micro-scopic) secondary-phase clusters anywhere. T his suggests that the diffused Cr is indeed substituting at the Ga sites. The absence of other ferromagnetic impurities in the GaN film was verified by EDX profiling. This shows the Cr diffused uniformly throughout the GaN film. Control experiments with shallow Cr-doping confirm that the observed ferromagnetism is not due to Cr2O3. Finally, the band-edge emission of GaN with and without Cr-doping was observed to coincide from photoluminescence measurements. This indicates the absence of a Cr-induced impurity band. Magneto-electric and/or magneto-optic measurements can clarify the role of carriers in the exchange interactions responsible for ferromagnetism in this material.
References
[1] D. Awschalom et al., Nature Physics 3, 153 (2007) ; [2] G.T. Thaler et al., Appl. Phys. Lett. 80, 3964 (2002); [3] H. X. Liu et al., Appl. Phys. Lett. 85, 4076 (2004); [4] T. Dietl, Nature Mater. 9, 965 (2010) and references therein; [5] L. Gu et al. J. Magn. Magn. Mater. 290-291, 1395 (2005).
BQ-10. Layer by Layer investigation on the magneto-transport properties of ferromagnetic Ge:Mn prepared by pulsed laser annealing
Danilo Bürger, Shengqiang Zhou, Marcel Höwler, György Kovacs, Helfried Reuther, Wolfgang Skorupa, Manfred Helm and Heidemarie Schmidt
Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
The fabrication of ferromagnetic semiconductors is one big step to realize spintronic devices. Beside GaAs:Mn also Ge:Mn is a very interesting material system [1-2]. Moreover, the lattice inversion symmetry of Ge leads to weak spin-orbit interaction and enhanced spin-lifetime and spin-diffusion lengths of more than 100 µm. This is shown in Ge nanowires at 4.2 K [3]. Also a spin diffusion up to 225 K could be shown in weakly n-doped Ge [4]. In a previous work we investigated ferromagnetism in Ge:Mn thin films. Hysteretic Hall resistance after Mn implantation and short-time pulsed laser annealing (PLA) has been observed [5]. Similar to low temperature molecular beam epitaxy [1], the clustering of Mn inside the material has to be prevented. Especially after PLA, the Mn segregation leads to the formation of a Mn enriched surface layer. In our work, we performed layer by layer magneto-transport measurements after different etching steps. The hysteretic properties are vanishing after removing the segregated Mn-rich phases. In conclusion, we think that the observed hysteretic Hall resistance may have its origin in the scattering of spin-polarized holes on these Mn-rich phases. Such hysteretic transport properties have not yet been reported for Ge with embedded Mn-rich clusters. The vanishing hysteresis at 30 K and above can be interpreted by the increasing decoherence between the individual Mn rich clusters or/and the reduced magnetization of the clusters itself.
References
[1] M. Jamet et al., Nature Materials 5, 653 (2006) [2] Y.D. Park et al., Science 295, 651 (2002) [3] E. S. Liu et al., Nano Letters 10, 3297 (2010) [4] Y. Zhou et al. arXiv:1103.5095v1 (2011) [5] S. Zhou et al., Phys. Rev. B 81, 165204 (2010)
BQ-11. Ferromagnetic InMnSb Multi-Phase Films Study by Aberration-Corrected Scanning Transmission Electron Microscopy
Leonardo Lari1, 2, Caitlin Feeser3, Bruce W. Wessels4 and Vlado K. Lazarov1, 2
1Department of Physics, University of York, York, United Kingdom; 2The York JEOL Nanocentre, University of York, York, United Kingdom; 3Department of Ch emical Engineering, Northwestern University, Evanston, IL; 4Department of Materials Science & Engineering and Materials Research Center, Northwestern University, Evanston, IL
Dilute (III, Mn)V magnetic semiconductors are candidate materials for applications in spintronic devices as magnetic field sensors, spin transistors and reconfigurable logic devices. In such materials, substitutional Mn atoms, are responsible for a carrier-mediated ferromagnetism by acting as sources of both magnetic moment and holes for the hosting III-V semiconductor matrix. However, for the realization of magnetic layers in spintronic devices, ferromagnetism at room temperature is a strict requirement for the constituent materials. In this work we report a study by aberration corrected (scanning) transmission electron microscopy (S)TEM of ferromagnetic InMnSb epilayers showing a Curie temperature Tc in excess of 550 K. In particular, the structural and chemical composition of In1-xMnxSb films grown by metalorganic vapor phase epitaxy on GaAs(001) substrates were studied as a function of Mn concentration. High resolution TEM and Z-contrast STEM imaging show that films are single phase up to 2 % of Mn and multi-phase for higher Mn concentrations. The microstructure, chemical composition, size and the population density within the matrix of the different types of precipitates was studied by cross-sectional (S)TEM and correlated to field-cooled and zero-field-cooled magnetization curves as determined by SQUID magnetometry.
BQ-12. Magnetic and Optical Properties of Rare Earth-Doped GaN Semiconducting Thin Films
Ratnakar Palai1, K. Dasari1, J. Wu1, W. M. Jadwisienczak2 and H. Huhtinen3
1Dept. of Physics, University of Puerto Rico, San Juan; 2School of EECS, Ohio University, Athens, OH; 3Department of Physics, University of Turku, Turku, Finland
The rare-earth (RE) doped GaN semiconducting materials are promising candidates for spintronic and optoelectronic applications. The higher magnetic moment of RE ions compared to the transition metal ions makes RE-doped GaN as materials of choice for dilute magnetic semiconductor. RE-doped GaN materials have shown the ability to tune the direct bandgap from the ultraviolet through visible to the near infrared region. GaN-doped with Er3+ and Yb3+ are of great interest because of their emission in visible and infrared regime, respectively. The magnetic and electrical properties of thin films of GaN:Yb and GaN:Er have been studied by us for the better understanding of magnetoptic properties. Yb-doped GaN shows room temperature ferromagnetism but Yb ions not optically active. Whereas, Er-doped GaN shows very good visible emission (536nm, 558nm and 668nm) but the ferromagnetism was observed below 150 K. The main objective of our work is to make RE ions in GaN both magnetically and optically active at room temperature. Here we report, the magnetic and optical properties Er+3 and Yb+3 codoped GaN epitaxial thin films grown by molecular beam epitaxy (MBE). Thin films of GaN:ErYb have been grown on Si (1111) and Sapphire (0001) substrates using MBE. The doping concentration and elemental analysis have been calculated from the x-ray photoelectron spectroscopy (XPS). X-ray diffraction and atomic force microscopy (AFM) were used to examine the phase purity and smoothness of the films. The concentration and mobility of carriers were calculated from the Hall effect measurements. The magnetic properties of GaN:ErYb thin films were measured using SQUID and magnetotransport measurements. The optical properties of the films were analyzed by photoluminescence and cathode luminescent spectroscopy. The magnetic and optical properties o f the GaN:ErYb films will be compared with Er and Yb-doped GaN films and will be discussed in details.
BQ-13. Giant magnetocaloric of half-metallic Ba0.08Mn0.92As epitaxial film grown on Al2O3(0001) substrate
Duong Anh Tuan1, Dang Duc Dung1, 2, Wuwei Feng1, Duong Van Thiet1, Yooleemi Shin1 and Sunglae Cho1
1Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea; 2Department of General Physics, Ha Noi University of Science and Technology, 1 Dai Co Viet road, Ha Noi,, Viet Nam
The MnAs compound is still promising to application even it has long history cause it exhibit the half-metallic and display the giant magnetocaloric [1]. In addition, the giant room temperature magnetocaloric were enhanced by substitution Mn by Fe, Co, Cu or Cr and substitute As by Sb [2]. Other substitutions included S, Se, Te, Bi and P, but they showed no giant magnetocaloric effect [3]. However, there is no/less formation about dopants of group II such as Ca, Ba, Sr in MnAs. In this work, we report the magnetism and transport properties of epitaxial Ba0.08Mn0.92As thin film on Al2O3(001) substrate. The RHEED and XRD results indicated that film were epitaxially grown on Al2O3(0001) without the impurity phases. The resistivity linearly proportional to the square of temperature at below transition temperature with suggests for hall-metallic nature of film likely the zinc-blende MnAs [4]. The anomalous Hall effect showed which further suggest for spin polarization in Ba0.08Mn0.92As film. The magnetic measurement indicates that the samples are ferromagnetic with TC above room temperature and display the giant magnetocaloric. Moreover, the magnetic domain of film was observed at room temperature by using the magnetic force microscope.
References
[1] L. Daweritz, Pep. Prog. Phys. 69, 2581 (2006). [2] P. F. Xu et al. Appl. Phys. Lett. 97, 042502 (2010). [3] Daniel L. Rocco et al. Appl. Phys. Lett 90, 242507 (2008). [4] H. C. Jeon et al., Appl. Phys. Lett. 89, 112517 (2006).
BQ-14. Kondo effect and carrier transportation in α-In2O3:Cr diluted magnetic oxide thin films
Cheng-Pang Lin1, Yu Ju Lee1, Chun-Yu Hsu1, Shih Jye Sun2 and Hsiung Chou1
1Physics, Natl Sun Yat-Sen University, Kaohsiung, Taiwan; 2Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan
For the technological applications of spintronics, diluted magnetic semiconductor or oxide is one of the promoting candidates. The unclear magnetic origins and poor reproducibility in oxide materials, such as ZnO doped with 3d transition metals, significantly limits the practical applications. Indium oxides (In2O3) recently have been widely utilized to be a transparent conducting oxide and also as an alternative candidate for realization of spintronic devices. Room temperature (RT) ferromagnetism for transition metal doped In2O3 has been demonstrated by many research groups[1,2], but the relationship between the spin transport properties of the charge carriers and the ferromagnetic coupling remains unclear. In2O3 doped with 0%, 2.8% and 4% Cr (CIO) thin films of 50nm thick were prepared by RF sputtering at RT onto (001)-oriented Si substrates. The crystalline structure of the CIO films, as examined by x-ray diffraction and transmission electron microscopy, are amorphous. The valence states of doped Cr elements are determined to be 3+ by X-ray photoemission spectroscopy. Electrical tran sport measurement demonstrates two electric transitions near 250K and 50K, which respectively corresponds to a semiconducting and metallic behavior above and below 250K. The transitions near 50K for Cr doped In2O3 indicate a strong Kondo interaction between conducting carriers and localized spins of doped magnetic ions. The combination of the Kondo model and the s-d electron scattering model can qualitatively match the ρ-T curves. Both magnetic and Hall measurements as functions of temperatures exhibit qualitatively identical behavior discloses the carrier mediated conduction and ferromagnetic coupling mechanisms. However, the origin of the semiconducting-metallic transtion are yet unclear and worthy for further study.
References
1. J. Philip, et al. Nat. Mater. 5, 298 (2006). 2. R. P. Panguluri, et al. Phys. Rev. B 79, 165208 (2009).
BQ-15. The origin of the ferromagnetic ordering of zinc vacancies in Sc-doped ZnO: bulk versus thin-films
Mohammed B. Kanoun, Souraya Goumri-Said, Udo Schwingenschlogl and Aurelien Manchon
Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
Recent experiments have shown possible room temperature ferromagnetism in Sc-doped ZnO thin films [1]. During the growth process, doping by Sc atoms may create vacancies in this material. We considered both bulk and thin-films ZnO doped with Sc in the presence of Zn vacancies. For bulk ZnScO, we have performed first-principles calculations to investigate the effects of Zn vacancies on the electronic and magnetic properties. ZnScO is half metallic with a total magnetic moment of 2.012 μB per supercell (Fig. 1). The stability of Sc-doped ZnO, even in bulk, compared to vacancies in pure ZnO is comforting the idea to use ZnScO instead of the binary alloy. For Sc-doped ZnO in thin-films structure, we have carried out first principles calculation to determine the favorable location of Scandium inducing magnetism. We considered four configurations (Fig.2), where the ZnO thin film was modeled by a (1x2) seven-layer slab having [11-20] surface orientation which contains a total of 56 f.u of ZnO. The magnetic moment is 0.21 μB by Sc, and agrees well with the experimental value found 0.31 μB. To understand the origin of the magnetism, calculations of total and local density of states as well as the spin-density contours will be presented.
References
[1] M. Venkatesan, C. B. Fitzgerald, J. G. Lunney, and J. M. D. Coey, Phys. Rev. Lett. 93, 177206 (2004).
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Michel de Jong, Twente
BR-01. Spin-dependent transport properties of single-molecule magnet Mn3
Hua Hao1, Xiao-Hong Zheng1, Zhi Zeng1 and Hai-Qing Lin2
1Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui, China; 2Department of Physics and Institute of Theoretical Physics, The Chinese University of Hong Kong, Hong Kong SAR, China
Single-molecule magnets (SMMs) are particularly attractive to molecular spintronics, due to their exceptional chemical and physical characteristics[1]. Recently, transport properties of Mn12 molecule were studied by Sanvito et al.,[2] showing that inner electronic structures of Mn12 molecule have different response to bias voltages for its ground state (GS) and spin-flip (SF) state. Consequently, it results in dramatic difference in I-V characteristics for GS and SF. In the present work, electron transport properties of SMM Mn3 are investigated within an ab initio framework combining the nonequilibrium Green’s function approach with density-functional theory. A new physical mechanism is explored to modulate I-V characteristics by changing spin states of Mn3 molecules. Here, two states of Mn3 molecule are considered in Fig.1(a): one is the ground state (GS); the other is the spin-flip state (SF). As demonstrated in Fig.1(b), different eigenstates of Ag (100) nanoelectrodes are coupled with GS and SF of Mn3 molecule near the Fermi level. As a result, the equilibrium transmission peak of GS is split into two lower peaks of SF. More importantly, the negative differential resistance (NDR) of GS vanishes in the SF of Mn3 molecule due to the same reason.
References
[1] L. Bogani and W. Wernsdorfer, Nature Materials 7, 179 (2008). [2] C. D. Pemmaraju, I. Rungger, and S. Sanvito, Phys. Rev. B 80, 104422 (2009).
BR-02. Magnetic fringe field control of electronic transport in an organic film
Fujian Wang2, Ferran Macià1, Andrew D. Kent1, Markus Wohlgenannt2 and Michael E. Flatté2
1Physics and Astronomy, University of Iowa, IOWA, IA; 2Physics, New York University, New York, NY
Random, spatially-uncorrelated nuclear hyperfine fields in organic materials dramatically affect electronic transport properties such as the electrical conductivity, photoconductivity, and electroluminescence. The influence of these nuclear hyperfine fields can be overwhelmed by a uniform external applied magnetic field. As a result, in applied magnetic fields of about 10mT the kinetics of exciton formation, bipolaron formation, and single-carrier hopping are all modified, leading to changes in room-temperature electrical transport properties in excess of 10 % in many materials. We demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially-varying magnetic fringe fields of an unsaturated magnetic electrode located underneath the organic film. The effect of these magnetic fringe fields is hysteretic, anisotropic, can be much stronger than the nuclear hyperfine fields, and depends sensitively on the distance of the organic material from the ferromagnetic electrode. The inhomogeneous stray fields from the magnetic electrodes directly affect and dominate the electronic transport; the stray fields set the number of hops along the organic layer because they control the spin quantization axis along the electronic path. The size of the effect at non-zero applied field resembles the effect at zero field (with several % of change for Alq3). Uncorrelated magnetic fields act as random hyperfine fields. We have shown that turning on and off the stray fields we can fix the resistance state of the organic layer at will independently of the value of the overall external field. Our devices, which do not rely on spin injection, tunneling anisotropic magnetoresistance or spin-valve behavior, may provide a simple approach to integrating magnetic metals and organics for hybrid spintronic devices. Organic layer may serve as detectors of uncorrelated magnetic fields (from magnetic domain structures, spin-dynamics…).
BR-03. Spin-flip induced magnetoresistance in positionally disordered organic solids
Nicholas Harmon and Michael Fla tté
Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, IA
Spintronics in organic materials has generated considerable interest due to their long spin lifetimes of organic semiconductors as well as the flexibility, low cost and chemical tunability. Spin transport properties are connected to the electrical transport properties, so although spin transport through inorganic semiconductors have been extensively explored, novel features should be expected in organics due to their different electronic transport properties. The understanding of spin transport in organics has been challenged by the discovery of large magnetic field effects on properties such as conductivity and electroluminescence [1,2,3,4]; and characterized by magnetoresistances of 10-20 % in magnetic fields as small as 10~mT. We present and solve a model for magnetoresistance in positionally disordered organic materials using percolation theory [5]. The model describes the effects of spin transitions on hopping transport by considering the effect of spin dynamics on an effective density of hopping sites. Faster spin flips open up `spin-blocked' pathways (see figure) to become viable conduction channels and hence produces magnetoresistance. The magnetoresistance can be found analytically in several regimes, including when the spin flip time is faster than the hopping time. The ratio of hopping time to spin flip time is a crucial quantity in determining the shape of magnetoresistance curves.
References
[1] K. Kalinowsk et. al Chem. Phys. Lett. 380, 710 (2003) [2] V.N. Prigodin et al. Synthetic Metals 156, 757 (2006) [3] P. Desai et al. Phys. Rev. B 76, 235202 (2007) [4] P.A. Bobbert et al. Phys. Rev. Lett. 99, 216801 (2007) [5] arXiv:1106.3040v1
BR-04. Depth of metal penetration at the organics/ferromagnet interface
Hui-Ching Chang1, Chia-Hao Wang2, Meng-Ruei Chiang2, Yuet-Loy Chan1, Chih-Hao Lee1, 3, Pen-Cheng Wang3, Yao-Jane Hsu1, 4 and Der-Hsin Wei1
1National Synchrotron Radiation Research Center, Hsinchu, Taiwan; 2Graduate Program for Science and Technology of Synchrotron Light Source, National Tsing Hua University, Hsinchu, Taiwan; 3Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan; 4Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan
Spins transport through a ferromagnet/organic semiconductor/ferromagnet (FM/OSC/FM) hybrid structure is currently the subject of intense research. In particular, the direct deposition of a hard metallic electrode over a soft organic layer has raised a concern if the interface thus formed can efficiently transport the spins. Recently, an X-ray spectroscopy and microscopy study indicated that the interface separating the top FM electrode and OSC layer does not display the same magnetic signatures as the one that sits between the OSC layer and bottom FM electrode [1]. Furthermore, an independent magnetoresistance measurement conducted with a ~2 nm top contact demonstrated that the hybrid structure avoiding most of the ill-defined interface can exhibit a magnetoresistive response as large as 300% at 2 Kelvin [2]. To help ascertain what might have occurred at the FM-OSC interface that lead to the unexpected complication, we examined the significance, as well as the depth, of metal penetration. The systems under study were a series of pentacene/cobalt (Pn/Co) bilayers, and the characterization method used was a non-invasive near-edge X-ray absorption fine structure (NEXAFS) measurement. According to the intensity of photoelectron yield recorded on Pn(x nm)/Co bilayers, we were able to quantitatively estimate the depth of Co penetration. The numerical value reported here gives the least Pn thickness needed to avoid electrical shortage between Co contacts. Our observation further implies the spins transporting across Pn/Co interface would behave different from those systems having an ideal interface.
References
References 1. Chan, Y.-L., et al., Magnetic Response of an Ultrathin Cobalt Film in Contact with an Organic Pentacene Layer. Physical Review Letters, 2010. 104(17): p. 177204. 2. Barraud, C., et al., Unravelling the role of the interface for spin injection into organic semiconductors. Nat Phys, 2010. 6: p. 815.
BR-05. Hybridization and exchange coupling at organic semiconductor/ferromagnetic heterojunctions
Andrew Pratt1, 2, Xia Sun3, Luke Dunne4, Mitsunori Kurahashi1 and Yasushi Yamauchi1
1National Institute for Materials Science, Tsukuba, Japan; 2York Institute for Materials Research, University of York, York, United Kingdom; 3University of Science and Technology of China, Anhui, China; 4Department of Physics, University of York, York, United Kingdom
The magnetic interaction of an organic semiconductor (OSC) adsorbed on a ferromagnetic (FM) substrate plays a vital role in determining the injection and extraction of spin carriers in an organic spintronics device [1]. On various metal FM surfaces, a strong interaction occurs that induces spin-polarization in the orbitals of adsorbed π-conjugated molecules through hybridization of electronic states of the molecule and substrate [2]. Due to its localization at the interface, detecting this induced magnetism presents experimental difficulties and is beyond the surface sensitivity of spin-resolved photoelectron spectroscopy. In this work, we use a spin-polarized metastable helium (He*) beam in order to probe the adsorption of tris(8-hydroxyquinolino)-aluminum (Alq3) on Fe and Fe3O4 substrates [3]. As the cross-section for de-excitation of He* is so large, only the electronic states at the outermost surface are detected rendering this technique ideal for investigating the surface magnetism at interfaces relevant to organic spintronics. We find that the strong interaction between Fe and Alq3 leads to exchange coupling between substrate FM states and the molecular orbitals of the adsorbate providing a pathway for efficient spin injection [4]. However, we also detect the presence of π* states at the Fermi level induced by the hybridization of the Alq3 molecular orbitals with the Fe substrate. This opens up a conducting channel and leads to Ohmic-like metallicity that, although promoting efficient charge transport, is detrimental to spin injection (the conductivity mismatch problem). As a means to prevent this effect we investigated the adsorption of Alq3 on the more passive surface of Fe3O4 and found that exchange coupling also occurs for this system but that the induced π* states at the interface are much reduced. This suggests that the use of an inert ferromagnetic oxide surface promotes more efficient spin injection and presents a better candidate for organic spintronic devices.
References
[1] V. A. Dediu, L. E. Hueso, I. Bergenti, and C. Taliani, Nature Mater. 8, 707 (2009). [2] X. Sun, Y. Yamauchi, M. Kurahashi, T. Suzuki, Z. P. Wang, and S. Entani, Chem. Phys. Lett. 452, 156 (2008). [3] A. Pratt, M. Kurahashi, X. Sun, and Y. Yamauchi, J . Phys. D: Appl. Phys. 44, 064010 (2010). [4] Y. Zhan et al., Adv. Mater. 22, 1626 (2010).
BR-06. Ab-initio understanding of the spin injection in organic molecular semiconductors systems
Souraya Goumri-Said, Mohammed Benali Kanoun, Udo Schwingenschlogl and Aurelien Manchon
Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
The field of organic spintronics has also grown in order to open new avenues to cheap, low-weight, mechanically flexible, chemically inert and bottom-up fabricated spin-devices. Furthermore, the control and understanding of new hybrid , will enable considerable progress in organic spintronics for technological purpose, including processing elements, sensors, spin-valves[1, 2]. Using density-functional theory calculations we have investigated how the metallic surface can spin-polarizes molecules such as Benzene, Pentacene and Alq3 (see Figure 1). Understanding the complexity of the charge and spin transport in organic/metallic interfaces in these systems make their study extremely challenging. Two DFT codes are conjointly used: DMol3 [3] and Wien2k code [4] for Benzene/Fe (100) system. For a Benzene/Fe (100) system, the surface was modeled by a six-layers Fe slab through a 6x6 unit cell with a vacuum region of 20 Å (see Figure 2). We demonstrate that in the spin-up channel, the pz atomic type orbitals which originally form the p-molecular orbitals hybridize with the majority d states of the Fe atoms forming molecule-metal hybrid states with bonding and anti-bonding character.
References
[1] V. A. Dediu et al., Nature Mater. 8, 707 (2009). [2] A. J. Drew et al., Nature Mater. 8, 109 (2009).[3] B. Dellay, J. Chem. Phys. 113, 7756 (2000). [4] P. Blaha et al, An Augmented plane wave + local orbital method for calculating crystal properties (Technical University of Vienna, (2001). [5] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996).
BR-07. Magnetic Specific Heat Studies of Two Ising Spin ½ Chain Systems M(N3)2(bpy)
Tan Yuen1, Youcef Hamida1, Dusan S. Danilovic1, Kunhao Li2 and Jing Li2
1Physics, Temple University, Philadelphia, PA; 2Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ
M(N3)2(bpy) [where M = Cu(II), Co(II), N3 = azide, and bpy = 4,4’-bipyridine] are two newly synthesized metal-organic framework (MOF) systems, in which the divalent M ions are connected though the azide ligands forming almost ideal magnetic 1D chains. Specific heat measurements were performed on these compounds and the magnetic specific heat were deduced using appropriate methods for estimating the lattice specific heat. The magnetic specific heat data were analyzed and fit to Ising spin ½ model. The exchange interaction J/kB values of 8.8 K for Cu(N3)2(bpy) and 18.56 K for Co(N3)2(bpy) were obtained and compared to the J values from fitting the measured magnetic susceptibility data.
BR-08. Large change of perpendicular magnetic anisotropy in Cobalt ultrathin film induced by varying capping layers
Xianmin Zhang1, Shigemi Mizukami1, Takahide Kubota1, Hiroshi Naganuma2, Mikihiko Oogane2, Yasuo Ando2 and Terunobu Miyaz aki1
1WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan; 2DepartDepartment of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai, Japan
Spin transport mechanism in organic semiconductors (OSEs) spacer has been extensively investigated since the organic spin valves (SVs) were reported [1, 2]. The spin injection and diffusion in OSEs were confirmed by measuring a spin diffusion length in organic SVs [3, 4]. This was also observed in our recent work [5]. Meanwhile, spin-conserved tunneling through ultrathin Alq3 was reported [6, 7]. Comparatively, little work has been investigated about the influence of OSEs on the ferromagnetic electrode itself. Recently, Chan et al. found that the Co/Pc interface had slight effect on the in-plane hysteresis behavior, but Pc/Co displayed a reacted interface with a magnetically dead layer, showing significant difference of hysteresis behavior [8]. Here, we fabricated the Co ultrathin films (0.5-1.8 nm) on Pt buffered Si/SiO2 substrate, which was caped ontop by different layers (Pt, pentacene and Alq3). Magnetic properties are measured in both out-of-plane and in-plane directions. It was found that the critical thickness of Cobalt for maximum perpendicular magnetic anisotropy (PMA) was around 0.7 nm in these three group films. The out-of-plane coercive strengths of samples with pentacene layer were similar with that using Pt layer. Notablely, the out-of-plane coercive strength of films with Alq3 layer waas much lower than that of pentacene capped films. The coercive strengths were about 250 Oe and 360 Oe for Alq3 and pentacene capping films with 0.7 nm of cobalt, respectively. Their corresponding effective magnetic anisotropy energies were estimated around 3 x 106 erg/cc and 5 x 106 erg/cc, agreeing well with the change of coercive strengths. The XRD results showed pentacene film was polycrystalline but Alq3 film was amorphous. The different interactions of ontop organic layers with ultrathin Cobalt were investigated and their influences on PMA of Cobalt were systemically discussed. The investigation is valuable to develop advanced substances for organic spintronics.
References
[1] V. A. Dediu, et al. Room temperature spin polarised injection in organic semiconductor. Solid State Commun. 122(2002) 181. [2] Z. H. Xiong, et al. Giant magnetoresistance in organic spin-valves, Nature. 427(2004) 821. [3] J. H. Shim, et al. Large Spin Diffusion Length in an Amorphous Organic Semiconductor, Phys. Rev. Lett. 100 (2008) 226603. [4] A. J. Drew, et al. Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation, Nature Mater. 8 (2009)109. [5] X. M. Zhang, et al. Spin transport in Co/Al2O3/Alq3/Co organic spin valve. IEEE Trans. Magn. (to be published). [6] T. S. Santos, et al. Room-Temperature Tunnel Magnetoresistance and Spin-Polarized Tunneling through an Organic Semiconductor Barrier, Phys. Rev. Lett. 98 (2007) 016601. [7] J. J. H. M. Schoonus, et al. Magnetoresistance in Hybrid Organic Spin Valves at the Onset of Multiple-Step Tunneling, Phys. Rev. Lett. 103 (2009) 146601. [8] Y.-L. Chan, et al. Magnetic Response of an Ultrathin Cobalt Film in Contact with an Organic Pentacene Layer, Phys. Rev. Lett. 104 (2010) 177204.
BR-09. The half-metallic properties of the soft ferrimagnet [MnII(enH)(H2O)][CrIII(CN)6]H2O : A first principle study
Neng Li1, 2, Guohua Zhong1 and Haiqing Lin1, 3
1Center for Photovoltaics and Solar Energy, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China; 2School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, China; 3Physics, The Chinese University of Hong Kong, Hong Kong, Hong Kong
We have investigated the electronic and the magnetic properties of the two-dimensional achiral magnet [MnII(enH)(H2O)][CrIII(CN)6]H2O (1), en = 1,2-diaminoethane using first-principles, namely density-functional theory (DFT) with with local spin density approximation plus coulomb potential (LSDA+U) method. The relative stability of the ground state, the electronic band structure, the magnetic and the conducting properties were investigated. Our results reveal that the compound has a stable ferrimagnetic ground state and has a metallic ferromagnetic metastable state, which are in good agreement with the experiment. The spin magnetic moment of per molecule is about 1.0μB, which is mainly assembled at the Cr(III) and Mn(II) ions at the same time, with a little contribution from the O1, N5. The ferrimagnetic properties come from the spin delocalization effect. By analysis of the band structure, we find that the compound has half-metallic properties.
References
1 Meng-Qiu Cai, Ji-Cheng Liu, Guo-Wei Yang, Yun-Lun Cao, Xin-Yi Chen, Yan-GuoWang, Ling-ling Wang,and Wang-Yu Hu,J.Chem.Phys. 126 (2007)154708 2 M. Fiebig, T. Lottermoser, D. Frohlich, A. V. Goltsev, Nature. 418(2002) 818 3 Y. Tokunaga, T Lottermoser, Nature. 5(2006) 937
BR-10. Permalloy and Co50Pd50 as ferromagnetic contacts for TMR measurements in carbon nanotube-based devices
Caitlin Morgan1, 2, Klaus Schmalbuch2, 3, Carola Meyer1, 2 and Claus M. Schneider1, 2
1Peter Grünberg Institute 6, Forschungszentrum Jülich, Jülich, Germany; 2JARA Jülich Aachen Research Alliance, Jülich, Germany; 3II. Physikalisches Institut, RWTH Aachen University, Aachen, Germany
In addition to exhibiting ballistic transport, carbon nanotubes (CNTs) are thought to have small spin-orbit interaction and relatively few spin nuclei (13C), suggesting a long spin relaxation length. This makes CNT-based devices interesting for studying TMR effects and potential applications in the field of spintronics, especially as electric-field control of the magnetic behavior of samples has been observed. [1,2] In order to create a reliable device, the spin injector and detector - in this case ferromagnetic contacts - must be optimized to have both high in-plane magnetic polarization, and form a stable electronic interface with CNTs. In our work, we compare two materials, the well-studied weak ferromagnet permalloy, and an alloy of cobalt and palladium, which has a more complex magnetic behavior, but would be expected to have a higher polarization[3], and form stable contacts to CNTs. CNTs are grown by chemical vapor deposition on lithographically prepatterned substrates. Contacts are fabricated via electron beam lithography and either e-beam evaporation of permalloy (Py) or CoPd. 3-terminal devices of Py-CNT-Py show reproducible magnetic and electronic behavior. The shape of the contacts and the gate-dependent current are shown to have a strong influence on the TMR signal. The results will then be compared to first magnetoresistance measurements on CoPd-CNT-CoPd devices, which are supplemented with magnetic force microscopy measurements. Domain structure and temperature are shown to strongly affect the magnetic hysteresis of the contacts and thus change the TMR signal.
References
[1] S. Sahoo et al. Nat. Phys. 1, (2005) [2] A. Jensen et al. Phys. Rev. B 72, 035419 (2005) [3] H. Vinzelberg et al. JMMM 290-291, 2, (2005)
BR-11. Spin-orbit force in graphene with Rashba spin-orbit coupling
Chien-Liang Chen1, 2, Yu-Hsin Su1, 2 and Ching-Ray Chang1, 2
1Department of Physics, National Taiwan University, Tapei, Taiwan; 2Center for Quantum Science and Engineering (CQSE), National Taiwan University, Taipei, Taiwan
Recent experiments observed Rashba spin splitting in graphene on Ni (111) substrate with [1] and without an intercalated Au monolayer [2]. To understand the Rashba effect in graphene, theorists have presented a tight-binding model to calculate the electronic structure [3,4]. In this work, we further show that the tight-binding Hamiltonian with Rashba spin-orbit (SO) coupling can generate unusual SO force. The SO force in graphene has several interesting features, some of which are different from that in a conventional two-dimensional electron gas [5]. Firstly, the SO force in graphene depends on both the out-of-plane and in-plane components of electron spin. Electrons with out-of-plane spin orientation experience the transverse SO force. However, for in-plane spin orientation electrons, the SO force even doesn’t depend on momentum. Secondly, electrons with the energy closer to the energy neutral point experience stronger SO force. According to the SO force picture, strong spin Hall effect near the energy neutral point is predicted.
References
[1] A. Varykhalov et al., Phys. Rev. Lett. 101, 157601 (2008). [2] Y. S. Dedkov, M. Fonin, U. Rudiger, and C. Laubschat, Phys. Rev. Lett. 100, 107602 (2008). [3] E. I. Rashba, Phys. Rev. B 79, 161409(R) (2009). [4] M-H Liu, and C-R Chang, Phys. Rev. B, 80, 241304 (2009). [5] J. Li, L. Hu, and S.-Q. Shen, Phys. Rev. B, 71, 241305 (2005).
BR-12. Slow Magnetic Relaxation and Superexchange Interactions in Actinide-based Molecules
Nicola Magnani
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2Institute for Transuranium Elements, European Commission, Joint Research Centre, Karlsruhe, Germany
Polymetallic complexes (often called clusters) displaying intramolecular exchange coupling are the subject of intensive research in such diverse fields as high-density memory recording, quantum information processes, magnetocaloric refrigeration and spintronic applications. However, the low anisotropy energy barrier typical of transition-metal cluster limits the interesting physical behaviour of these systems to very low temperature, so that their use in real devices is presently unfeasible. It has been suggested that f-electron elements can considerably improve these properties; in particular the use of actinides, whose unfilled 5f shell could carry a large ligand-field anisotropy barrier together with significant magnetic superexchange, has been envisaged. With this goal in mind, we studied the magnetic properties of several actinide-based molecules. In this presentation I will summarize our recent progress in this field; in particular, we investigated the cause of the magnetic moment reduction in U(III)-based single-ion magnets, demonstrated that sizeable superexchange coupling can be achieved in polymetallic actinide clusters which display slow magnetic relaxation, and discovered sizeable memory effects in the magnetization curve of a Np-based organometallic complex.
BR-13. Spin transition pressure pulses investigation in the perspective of a modified dynamical model
Radu Tanasa1, Alexandru Stancu1, Francois Varret2, Jorge Linares2, Epiphane Codjovi2 and Jean-Francois Letard3
1Department of Physics, "Alexandru Ioan Cuza" University, Iasi, Romania; 2Groupe d’Etude de la Matiere Condensee, Universite de Versailles CNRS-UMR8635, Versailles, France; 3Institut de Chimie de la Matiere Condensee de Bordeaux, Universite Bordeaux I, UPR 9048 CNRS, Bordeaux, France
The inorganic spin transition compounds(STC) preserved their novelty during the last decades even if are known for a while already. It is mostly due to their potential in recording industry as a viable replacement of the conventional materials used to store data[1]. In the simplest STC, one observes two molecular states named high spin(HS) and low spin(LS) with different magnetic properties, volume or color that might be switched under a external perturbation like temperature, pressure, magnetic field or electromagnetic radiation[2]. The effect of a pressure pulse compared to the one produced by a high magnetic field was studied experimentally[3] and theoretically[4] in the past. However, the main focus was on STC response to the magnetic field because it was easily to control than the pressure. In this paper we are presenting systematic experimental data of [Fe(PM-BiA)2(NCS)2] reaction to positive and negative pressure pulses. Basically, we applied a pressure pulse and followed the system variation, i.e. nHS - fraction of the molecules in the HS state. We observed that for consecutive states on the heating thermal hysteresis loop branch, when the pressure increases the system arrives to LS state and when it reduces back to its starting value, the initial nHS is recovered. Once the pressure variation was inverted (first decrease and after that increase which was possible due to 500 bars baseline) the nHS fraction raises and surprisingly, remained close to unity even if the pressure was brought back to 500 bars. That is a clear evidence of the hysteretic behavior of the STC. The model we are proposing as a supplementary tool in understanding complex features experimentally observed is based on the master equation[5]. Using the gap energy between LS and HS and interaction parameter, we were able to reproduce qualitatively the transient states the system undergoes under pressure variation. The model was extended to include the interaction and gap energy distributions extracted from FORC data[6]. In the full paper, we will provide details on the implementation of this new method and discuss our results in the conjunction with the pressure induced structural phase which this compound is known to exhibit[7]. [8]
References
[1] A. Bousseksou, et al. Chem. Soc. Rev., 40 (2011) 3313; [2] P. Gutlich, et al. (ed.), Top. Curr. Chem., vv 233, 234, 235 (2004); [3] A. Bousseksou, et al. C. R. Chim. 6 (2003) 329; [4] S. Bonhommeau, et al., Phys. Rev. B 74 (2006) 064424; [5] K. Boukheddaden, et al., Phys. Rev. B 62 (2000) 14796; [6] R. Tanasa, et al. J. Appl. Phys. 103 (2008) 07B905; [7] A. Rotaru et al, J. Appl. Phys. 106 (2009) 053515. [8] Work supported by PD-CORELSYS 294 and TE-PROCORE 185 grants.
BR-14. Tailoring and Understanding Metal/Organic Semiconductor Interfaces Using a Thin Oxide Layer
Nyun Jong Lee1, Yu Jeong Bae1, Tae Hee Kim1, Hyunduck Cho2, Changhee Lee2, Luke Fleet3, Atsufumi Hirohata3 and Eisuke Ito4
1Department of Physics, Ewha Womans University, Seoul, Republic of Korea; 2School of Electrical Engineering and Computer Science, Seoul National University, Seoul, Republic of Korea; 3Department of Electronics, The University of York, York, United Kingdom; 4Flucto-Order Functions Research Team, RIKEN Advanced Science Institute, Wako, Japan
Spin transport in molecular organic semiconductors (OSC) has attracted tremendous attention for new science and future molecular based technology.[1-3] For realistic applications, a fundamental prerequisite is basic knowledge about the mechanism of spin injection and spin transport. Engineering and controlling the electronic properties of hybrid metal/organic (M/O) structures are important subjects to optimize the key process of OCS devices in which poor reproducibility due to little control of the interface properties still remains as a big obstacle. In this work, we focus on understanding interface electronic states and energy level alignment of a hybrid structure consisting of a highly-qualified interface between transition metals (Co and Fe)/OSC Cu-phthalocyanine (CuPc) by using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. In order to tune the alignment between the ferromagnetic (FM) electrode Fermi level (EF) and the OCS molecular levels, a few-Angstrom thick oxide layer, such as Co-O, Fe-O and MgO(100) was inserted between M/O films. The experimental determination of energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states of CuPc films with respect to the EF of the FM films was carried out. The work function of the FM electrodes with and without the oxide film was also measured by Kelvin-probe force microscopy. Based on these values, the energy barriers for hole and electron injection from the metal to the organic layer were estimated. Our results show that the shifts of CuPc HOMO level strongly depending on the FM metal surface: while a slight shift observed for the metal electrodes of naturally oxidized Fe(100) and poly crystal Fe (or Co) films, the bigger shift of the HOMO level observed for the epitaxial MgO(100) film gown on the Fe(100) film. This shift increases monotonically as the CuPc thickness decreases ranging from 10 to 1 nm. Moreover, the interface microstructure of these hybrid M/O structures was investigated by using high-resolution TEM. This work provides important information of the interface to understand the spin transport behavior in Si(100)/MgO/Fe/MgO/CuPc/Co magnetic tunnel junctions.
References
[1] Z. H. Xiong et al., Nature 427, 821 (2004). [2] V. Dediu et al.. Nature Mater. 8, 707 (2009). [3] L. Schulz et al.. Nature Mater. 10, 39 (2010). * e-mail: taehee@ewha.ac.kr
BR-15. Local Electronic Structure of Complex Magnetic Nanostructures
Ralph Skomski1, Pankaj Sahota2, 1, Axel Enders1, Jeff Rojas1, Arti Kashyap2 and David J. Sellmyer1
1Physics and Astronomy, Univ Nebraska, Lincoln, NE; 2IIT, Jaipur, India
Modern physics is based on quantum mechanics, but many-body wave functions are rarely known, and often they are not even necessary [1]. In fact, a local partition-function theorem by Kohn and Yaniv [2] shows that properties described by DFT are largely determined on a local scale, even if the wave functions extend to infinity. This locality can be exploited very efficiently in macroscopic systems and in quantum-mechanical problems where precise energy differences (less than 100 meV) are of secondary importance. Our resear ch is concerned with self-assembled organic molecules on metal substrates [3], specifically metallated tetraphenyl porphyrin molecules. In these structures, a pattern of porphyrin molecules is deposited onto a Cu surface, in order to accommodate magnetic atoms. The theoretical modeling of the corresponding STM images requires the knowledge of the local density of states D(r, E) of the atoms probed by the STM tip and the consideration of a huge number of atoms, 318 or more for a single porphyrin molecule on Cu. Conventional first-principle calculations determine the wave functions of the whole system, which is a cumbersome and time-consuming process. However, according to Kohn and Yaniv, one can define a local partition function (LPF) ρ(r, β) = ∫ D(r, E) exp(-βE) dE that is determined by the external potential V(r) in the vicinity of r = 0. Using a complete set of wave functions φa centered around r = 0, for example harmonic-oscillator wave functions, yields D(r, E) = ∑μ φμ* φμ δ(E - Eμ), where the φμ are eigenfunctions of the matrix <φa|H|φb> and H is the full Hamiltonian. The quality of D(r, E) depends on the number of wave functions and works best for r = 0. In the tight-binding (TB) approximation, this procedure is equivalent to the use of the moments theorem in the continued-fraction technique, but it does not suffer from the challenge of constructing the DOS from the calculated moments. In our presentation, we compare the Kohn-Yaniv predictions results with available experimental STM images and with VASP supercell calculations. The agreement is reasonable, although the charge transfer is rather poorly described in the TB approximation. — This research is supported by NSF-MRSEC and NCMN.
References
[1] W. Kohn, Rev. Mod. Phys. 71, 1253 (1999). [2] W. Kohn and A. Yaniv, Proc. Natl. Acad. Sci. USA 75, 5270 (1978). [3] A. Enders , R. Skomski, and J. Honolka, J. Phys.: Condens. Matter 22, 433001 (2010).
1:00 PM - 4:00 PM, Saguaro Ballroom
Chair: Hideki Saito, AIST
BS-01. Spin accumulation created electrically in an n-Ge channel using Schottky tunnel contacts
Yuzo Baba1, Kenji Kasahara1, Kohei Masaki1, Yuichiro Ando1, Yusuke Hoshi2, Kentarou Sawano2, Masanobu Miyao1 and Kohei Hamaya1, 3
1Department of Electronics, Kyushu University, Fukuoka, Japan; 2Research Center for Silicon Nano-Science, Tokyo City University, Tokyo, Japan; 3PRESTO, Japan Science and Technology Agency, Tokyo, Japan
For high-performance spintronic applications compatible with Si-LSI technologies, we have so far studied the electrical spin injection and detection in Ge. Here we report on the first experimental demonstration of the detection of spin accumulation created electrically in an n-Ge channel using Schottky tunnel contacts. For three-terminal Hanle-effect measurements [1,2], we fabricated the lateral devices consisting of a phosphorus-doped n-Ge(111) channel with a thickness of ~100 nm (n = ~1×1018 cm−3) and three Schottky tunnel contacts [Co (20 nm)/Fe3Si (10 nm)/n+-Ge (5 nm, n = ~ 1×1019 cm−3)]. Each contact has a lateral dimension of 100 × 200 µm2, 40 × 200 µm2 or 100 × 200 µm2. With increasing perpendicular magnetic fields from zero to ±2000 Oe, we clearly observed a drop of three-terminal voltage (~25 µV) at a bias current of +10 µA at 50 K, in which the positive bias-current condition showed the spin extraction from Ge. This voltage drop is caused by the depolarization of spin-polarized electrons in a Ge channel. Such voltage change was not seen at a bias current of -10 µA, where the negative bias-current condition indicated the spin injection into Ge. Because of the asymmetric detectability depending on the bias-current polarity, the Hanle-effect signals with almost the same magnitude were observed even in the two-terminal voltage measurements. These features are consistent with the previous work of Fe/GaAs devices reported by Lou et al [2]. In our device, the electrical creation of the spin accumulation was observed up to ~150 K.
This work was partly supported by Industrial Technology Research Grant Program from NEDO and Grant-in-Aid for Young Scientists (A) from JSPS.
References
[1] S. P. Dash et al., Nature (London) 462, 491 (2009). [2] X. Lou et al., Phys. Rev. Lett. 96, 176603 (2006).
BS-02. Electrical creation of spin accumulation in a Si channel using a Schottky tunnel contact
Yuichiro Ando1, 2, Yuya Maeda1, Kenji Kasahara1, Yuzo Baba1, Yusuke Hoshi3, Kentarou Sawano3, Masanobu Miyao1 and Kohei Hamaya1, 4
1Electronics, Kyushu University, Fukuoka, Japan; 2INAMORI Frontier Reseach Center, Kyushu University, Fukuoka, Japan; 3Advanced Research Laboratories, Tokyo City University, Tokyo, Japan; 4PRESTO, Japan Science and Technology Agency, Tokyo, Japan
For realizing silicon-based spintronics with low parasitic resistance, we have demonstrated spin injection and detection using ferromagnet/silicon structures without an insulating tunnel barrier.[1] In this study, we show the first experimental detection of the spin precession in Si though a Schottky tunnel contact. We fabricated the Si-based three-terminal devices with one single-crystalline Co60Fe40 spin injector (40 × 200 µm2) and two AuSb ohmic contacts (100 × 200 µm2). The 200-nm-thick Si channel (n = ~ 2×1018 cm-3) was fabricated on an undoped FZ-Si(111) substrate by ion implantation. Prior to the growth of the epitaxial CoFe layer, an Sb δ-doped Si layer (n = ~5×1019 cm-3) was grown on top of the Si channel formed. By applying magnetic fields perpendicular to the film plane, we performed three-terminal Hanle effect measurements. When the magnetic field increased from zero to ±100 Oe, a drop of three-terminal voltage (24 µV) was clearly observed at a small bias current of +0.1 µA at 25 K,[2] where the positive bias-current condition represented the spin extraction from Si. This voltage drop is caused by the depolarization of the accumulated spins in Si. On the other hand, such voltage change was not detected at a small bias current of -0.1 µA, where the negative bias-current condition indicated the spin injection into Si. This asymmetric detectability is consistent with Fe/GaAs-based devices reported by Lou et. al.[3] When the negative current was increased, the Hanle signals can be detected because of the enhancement in the spin accumulation. As a result, the signals were observed even at room temperature, showing the first demonstration of room-temperature creation of the spin accumulation in a Si channel without using an insulating tunnel barrier. This work was partly supported by PRESTO-JST and STARC
References
[1] Y. Ando et al., Appl. Phys. Lett. 96, 182105 (2009).
[2] Y. Ando et al., Appl. Phys. Lett. 98, (2011) accepted.
[3] X. Lou et al., Phys. Rev. Lett. 96, 176603 (2006).
BS-03. Tunneling anisotropy in Si/Al2O3/ferromagnet devices
Sandeep Sharma1, 2, Saroj P. Dash3, Hidekazu Saito1, Shinji Yuasa1 and Ron Jansen1
1Spintronics Research Centre, AIST, Tsukuba, Japan; 2Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands; 3Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg, Sweden
A recent development [1] in silicon spintronics is the electrical creation and detection of a spin accumulation in silicon at room temperature, and the observation of the Hanle effect from which the spin lifetime and thereby the spin-diffusion length was determined. Several control experiments [1] have proven unambiguously that the large room-temperature spin signal is genuine and originates from spin-polarized tunneling and spin accumulation in the Si bands. The latter was also confirmed by optical detection [2]. This now enables the systematic study of various parameters that influence spin injection into silicon [3], of which our understanding is still rather limited. A feature that provides more insight is the anisotropy of the tunnel conductance, denoting the change in the tunnel conductance when the magnetization direction of the ferromagnetic layer is rotated, for instance from in-plane to out-of-plane. Here we present a systematic study of the anisotropic tunnel conductance of Si/Al2O3/ferromagnet spin tunnel devices at 300K. Measurements are done by keeping the tunnel current constant and detecting the voltage as a function of magnetization angle. We describe and compare characteristic differences in the tunneling anisotropy between devices with different ferromagnets on p-type and n-type Si. Depending on the materials, we observe a non-trivial variation of the signal as a function of magnetization angle that does not follow the expected sinusoidal variation. Another salient feature is the systematic variation of anisotropic tunnel conductance with bias voltage, and its correlation with the bias variation of the electrically-induced spin accumulation. We will discuss various factors that may affect spins in Si when the magnetic moments in the magnetic layer are rotated.
References
[1] S.P. Dash, S. Sharma, R.S. Patel, M.P. de Jong and R. Jansen, Nature 462, 491 (2009). [2] R. Jansen, B.C. Min, S. P. Dash, S. Sharma, G. Kioseoglou, A. T. Hanbicki, O. M. J. Van’t Erve, P. E. Thompson and B. T. Jonker, Phys. Rev B 82, 241305 (2010). [3] S.P. Dash, S. Sharma, J.C. Le Breton, H. Jaffrès, J. Peiro, J.M George, A. Lemaître and R. Jansen, http://arxiv.org/abs/1101.1691.
BS-04. The effect of ferromagnetic Gd marker on the effective work function of Fe in contact with Al2O3/Si
Andrei V. Zenkevich1, Yuri A. Matveyev1, Yuri Y. Lebedinskii1, Roberto Mantovan2, Marco Fanciulli2, 3, Sebastian Thiess4 and Wolfgang W. Drube4
1NRNU, Moscow Engineering Physics Institute, Moscow, Russian Federation; 2CNR-IMM MDM Laboratory, Agrate Brianza ( MB), Italy; 3Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, Milano, Italy; 4Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
The injection of spin polarized electrons from ferromagnetic (FM) materials into Si is generally difficult due to the high conductivity mismatch at the FM/Si interface. An accepted route in order to overcome such an electrical mismatch is to employ the tunneling of spins through an oxide barrier such as Al2O3. Further improvement in the spin injection efficiency could be achieved by engineering the Fe/Al2O3 interface with the insertion of a low-work-function Gd layer at FM/tunnel barrier interface [1]. In particular, it has been shown that by varying the thickness of Gd interlayer in Ni80Fe20/Gd/Al2O3/Si structure, the electrical mismatch at the FM/Si interface can be effectively controlled, while simultaneously maintaining a reasonable tunnel spin polarization [1]. However, the direct measurement of Fe effective work function (WFeff) in contact with Al2O3 as a function of Gd thickness has not been performed so far. In this contribution, we systematically investigate the effect of Gd marker interlayer thickness on the Fe WFeff in a Fe(6 nm)/Gd(0.2-2 nm)/Al2O3(10 nm)/Si stack produced by combining atomic layer deposition and pulsed laser deposition. Hard X-ray photoemission spectroscopy (HAXPES) analysis is employed to determine the WFeff of Fe in the Fe/Gd/Al2O3/Si using the well-known Kraut methodology [2]. We observe a monotonous change in the band alignment at the Fe/Al2O3 interface, with WFeff ranging from 4.6 to 3.7 eV for Gd thickness in the range 0 ÷ 2.7 nm. For spin injection purposes, it is equally important to investigate the structure and the magnetism at the Fe/Gd interface. Interface-sensitive 57Fe conversion-electron Mossbauer spectroscopy performed in a 54Fe(6.5 nm)/57Fe(0.5 nm)/Gd(0.2-2 nm)/Al2O3(10 nm)/Si stack evidences that the majority (>80%) of 57Fe atoms are coordinated in the pure α-Fe phase (hyperfine magnetic fields ∼33 T) in contact with Gd, with the rest being characterized by lower hyperfine magnetic fields at the interface.
References
[1] B.-C. Min, K. Motohashi, C. Lodder and R. Jansen, Nature Mat. 5 817 (2006). [2] E. A. Kraut, R. W. Grant, J. R. Waldrop, and S. P. Kowalczyk, Phys. Rev. Lett. 44, 1620 (1980).
BS-05. Spin transport calculation for the three-terminal device of zigzag graphene nano-ribbon
Hsin-Han Lee1 and Ching-Ray Chang1, 2
1Physics, National Taiwan University, Taipei, Taiwan; 2Center for Quantum Science and Engineering (CQSE), National Taiwan University, Taipei, Taiwan
Recently, graphene is a popular material for study. People usually have new ideals for this honeycomb structure. One of the interesting shaped is graphene nano ribbon. Zigzag graphene nano-ribbon(ZGNR) is a kind of two dimensional topological insulator[1]. In this case, we used the non-equilibrium Green’s function of Landauer formula method[2][3] to calculate the transport properties for quantum coherence system and we don’t consider coulomb repulsion. We focus on two kind of fork(branch) shaped device of ZGNR there are three terminals(There are also some issues about three-terminal graphene device[4][5][6][7]). The one is the ZGNR separate two narrow ZGNR channel from the boundary. The other one is a Semi-finite ZGNR connect with two 2D sq uare lattice channel. And we can find the current will be polarized by QSHE. Those two device all exist a interesting phenomenon: If we adjust the onsite energy of Tight-binding Hamiltonian on both branch channels, it will change the current polarizations on each channels. For experiment, maybe we can control the current polarization by adjust the gate voltage, and help for nano device study in the future.
References
[1] C. L. Kane and E. J. Mele. Quantum spin hall effect in graphene. Phys.Rev. Lett. 95(22), 226801 (November 2005). [2] Supriyo Datta. “Quantum Transport: Atom to Transistor". Cambridge University Press, Cambridge (2005). [3] Supriyo Datta. “Electronic transport in mesoscopic systems". Cambridge University Press, Cambridge (1995). [4] Håvard Haugen, Daniel Huertas-Hernando, and Arne Brataas. PHYSICAL REVIEW B 81, 174523 (2010) [5]Antonis N. Andriotis, Madhu Menon. APPLIED PHYSICS LETTERS 92, 042115 (2008) [6] Tao Ouyang, Yuanping Chen, Yuee Xie,X. L. Wei,Kaike Yang, Ping Yang, and Jianxin Zhong. PHYSICAL REVIEW B 82, 245403 ([|#1#|]2010)[|#2#|] [7] A. Jacobsen,[|#2#|] I. Shorubalko, L. Maag, U. Sennhauser, and K. Ensslin. A. Jacobsen, I. Shorubalko, L. Maag, U. Sennhauser, and K. Ensslin. APPLIED PHYSICS LETTERS 97, 032110 [|#2#|](2010)[|#3#|]
BS-06. Magnetic Properties of Nanostructured-Co/Graphite interface studied by X-ray Magnetic Circular Dichroism
Ping Kwan J. Wong, Michel P. de Jong, Martin H. Siekman and Wilfred G. van der Wiel
NanoElectronics Group, MESA+ Institute for Nanotechnology, University of Twente, Enschede, Netherlands
System comprising interfaces between ferromagnetic metals (FMs) and carbon-based materials holds good promise for spin-based applications, because the latter inherently provide potentially weak hyperfine and spin-orbit interactions, thus resulting in long spin lifetimes [1]. So far, spin valve effects are mainly demonstrated in lateral spin valve devices that employ mechanically exfoliated graphene sheets as spin transport media [2]. Karpan et al. have put forward a stimulating idea to combine epitaxial sandwich structure containing Co and/or Ni electrodes separated by multilayer-graphene/graphite (Gr), where, at the epitaxial interfaces of FM and Gr, a perfect spin filtering due to k-vector conservation would be possible [3]. In this study, we investigate the adsorption of individual Co adatoms, ultrasmall clusters (e.g. dimers, trimers), and nanostructures on freshly cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) measurements show that Co atoms and nanoislands nucleate actively on graphite, which might be attributed to the formation of Co carbide as clearly evidenced in the X-ray absorption spectra (XAS). The magnetic properties of the interface between Co and a graphite surface have been characterized by X-ray magnetic circular dichroism (XMCD), which has been made possible by the inclusion of a ferromagnetic Fe cap atop the superparamagnetic Co nanostructures. Remarkably, a 40% reduction and a 140% enhancement in the spin and orbital moment, respectively, have been revealed when compared to the bulk. These observations provide an insight into the magnetic robustness of direct spin contacts of 3d transition ferromagnetic metals on (single- as well) few-layer graphene for carbon-based spintronic devices.
References
[1] W.J.M. Naber et al., J. Phys. D: Appl. Phys. 40, R205 (2007); V.A. Dediu et al., Nature Mater. 8, 707 (2009) [2] N. Tombros et al., Nature 448, 571 (2007); W. Han et al., Phys. Rev. Lett. 102, 137205 (2009); C. Jozsa et al., Phys. Rev. Lett. 100, 236603 (2008). [3] V.M. Karpan et al., Phys. Rev. Lett. 99, 176602 (2007)
< div height="0">BS-07. Non-uniform current density and spin accumulation in a 1 µm thick n-GaAs channel
Bernhard Endres1, Mariusz Ciorga1, Robert Wagner1, Sebastian Ringer1, Martin Utz1, Dominique Bougeard1, Christian Back1 and Günther Bayreuther1, 2
1Universität Regensburg, Regensburg, Germany; 2Max-Planck-Institut für Mikrostrukturphysik, Halle, Germany
The spin accumulation in an n-GaAs channel produced by a lateral electron flow from the semiconductor into a ferromagnetic GaMnAs contact across an Esaki diode structure was measured with a method which provides 2-dimensional cross-sectional images of the spin polarization in GaAs even beneath a ferromagnetic contact [1,2]. An unexpected spin density distribution was observed in a 1 µm thick n-GaAs channel with the maximum polarization near the contact edge opposite to the maximum charge current (see Fig 1). Furthermore the decay of the spin polarization on this side of the contact could not be described by a single exponential decay but showed a faster decay close to the contact edge depending on the bias voltage. This behavior cannot be explained within a frequently used one-dimensional model of spin diffusion and electron drift. However, numerical simulations (COMSOL) of the two-dimensional electron drift and spin diffusion in the GaAs channel reproduced the observed spin density distribution quite well (see Fig 2): the pronounced non-uniform electron drift beneath the contact area shifts the polarization peak towards the diffusion side. Due to the inhomogeneous current density it is not sufficient to describe the electron spins solely by diffusion on this side of the channel. As a consequence, if Hanle measurements are fitted with a one-dimensional drift-diffusion function as usually done they yield spin lifetimes which may strongly depend on the distance to the contact and the applied bias voltage. It is shown that a two-dimensional drift-diffusion equation must be used to evaluate correct spin relaxation times and spin diffusion constants from Hanle effect data in such a lateral geometries.
References
[1] P. Kotissek et al., Nature Phys. 3, 872 (2007) [2] B. Endres et al., J. Appl. Phys. 109, 07C505 (2011)
BS-08. Schottky Barrier Distributions in Fe/GaAs Devices
Luke R. Fleet1, K. Yoshida2, 3, H. Kobayashi4, Y. Ohno4 and A. Hirohata1, 5
1Physics, The University of York, York, United Kingdom; 2Nagoya University, Nagoya, Japan; 3Nanostructures Research Laboratory, JFCC, Nagoya, Japan; 4RIEC, Tohoku University, Sendai, Japan; 5PRESTO, JST, Kawaguchi, Japan
Fe on GaAs remains one of the leading candidate systems to achieve efficient spin-polarized injection. Theoretical calculations suggest that the atomic interfacial structure plays a key role in the formation of the Schottky barrier [1]. As the Schottky barrier is crucial for the injection of spin-polarized carriers [2], a detailed understanding of the interfacial structure is essential for the development of spintronic devices [3]. We have used scanning transmission electron microscopy (STEM) to analyse the atomic structure of the Fe/GaAs interface. Two different interfacial structures were observed; (a) abrupt and (b) Fe mixing between the As atoms. For systems with different interfacial structures, a distribution of barrier heights may exist, creating preferential regions for tunnelling which would dominate the device characteristics. The transport properties of three terminal devices, fabricated from the same films, were explored through an all electrical method. I-V-T characteristics are used to estimate the barrier distributions through analysis of the ratio of thermionic emission and tunnelling over a range of temperatures. JST-EPSRC grant EP/G051631/1.
References
[1] D. O. Demchenko and A. Y. Lui, Phys. Rev. B 73, 115332 (2006). [2] E. I. Rashba, Phys. Rev. B 62, R16267 (2000). [3] L.R. Fleet et al., J. Appl. Phys. 109, 07C504 (2011).
BS-09. Effect of Drift on Spin Polarization in a Spin-LED
D. Banerjee1, 2, T. Pramanik2, R. Adari2, T. Patil2, P. Suggisetti2, Swaroop Ganguly2 and Dipankar Saha2
1IITB-Monash Research Academy, CSE Building, Indian Institute of Technology Bombay, Mumbai, India; 2Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, India
Efficient injection and transport of spin-polarized electrons in semiconductors are among the most important aspects in any spintronic device1,2. We have studied drift-assisted spin transport3 in a spin-LED. We have shown that drift helps in transporting spin-polarization to a longer distance, which in turn enhances the output circular polarization (Πc) in a InGaAs/GaAs quantum-dot spin-LED. The n-type region of the spin-LED is made relatively long and doped moderately (5×1016 cm-3). A ferromagnetic Fe contact injects spin-polarized electrons through a heavily n-doped Schottky tunnel barrier4. Πc is measured as a function of out-of-plane magnetic field. A peak polarization of ∼4% is measure. The normalized out-of-plane magnetization of Fe contact follow the measured Πc. A control LED with non-magnetic n-contact shows negligible polarization. The spin-LED is studied at high bias where the potential drop across the resistive n-doped region becomes substantial and the incremental voltage appears in this region increasing the electric field, which facilitates spin drift. Πc is measured as a function of bias for a magnetic field H=3T. Πc initially increases before it begins to saturate. Πc decreases at very high bias and becomes negligible. The initial increase in Πc can be attributed to the improvement of injection efficiency through the tunnel barrier, which is observed earlier5. The effect of spin drift is negligible in this region as most of the voltage appears across the depletion region. This is also confirmed through simulation of Poisson and spin drift-diffusion equations. At high bias, the incremental potential drop across the n-doped region contributes to most of the voltage changes which increases the electric field. The spin-polarized electrons drift through the n-doped region. The spin polarization in the active region therefore increases which lead to enhanced output circular polarization. At even higher bias voltages, the spin relaxation at the Schottky barrier and electron-hole spin relaxation (BAP mechanism) dominate and Πc is found to decrease to a smaller value6,7. Theoretically calculated enhancement of electron spin-polarization in the active region is found to match well with experiments.
References
[1] Fabian et al., Acta Phys. Solv. 57, 565 (2007), [2] Awschalom et al., Nature Phys. 3, 153 (2007), [3] Yu et al., Phys. Rev. B 66, 235302 (2002), [4] Fert et al., Phys. Rev. B 64, 184420 (2001), [5] Saha et al., Appl. Phys. Lett. 92, 022507 (2008), [6] Bosco et al., Mat. Sci. & Engg. B 126, 107 (2006), [7] Wang et al., Phys. Rev . B 72, 153301 (2005).
BS-10. Optically oriented electron spin transport across a Heusler alloy/GaAs quantum well interface
Yasuhiro Shirahata1, Hironobu Muraoka1, Mitsuru Itoh1, Yukiko K. Takahashi2, Kazuhiro Hono2, Masahito Yamaguchi3 and Tomoyasu Taniyama1
1Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Japan; 2Magnetic Materials Unit, National Institute for Materials Science, Tsukuba, Japan; 3Department of Electronics, Nagoya University, Nagoya, Japan
An understanding of the spin transport across a ferromagnetic metal/semiconductor interface is of crucial importance to develop spintronic devices. Optical spin orientation method is one of the most useful tools to obtain information on the spin transport across a ferromagnetic metal/GaAs interface by detecting spin dependent photocurrent [1-3]. In this report, we investigate the spin transport across a Heusler alloy/GaAs quantum well (QW) interface using the combined approach of electroluminescence and optical spin orientation analysis. The sample used has the layer sequence of Co2Fe(Ga0.5Ge0.5)/n-AlGaAs/i-GaAs QW/p-AlGaAs capped with a Au layer. Co2Fe(Ga0.5Ge0.5) is one of the highly spin polarized Heusler alloys and CPP-GMR using Co2Fe(Ga0.5Ge0.5) showed a large magnetoresistance [4]. Prior to the examination of the spin filtering effect, electroluminescence measurement was done to determine the optical excitation energy for the spin dependent photocurrent measurement. Also, spin injection from the Heusler alloy into the QW was confirmed in the circular polarization analysis of the electroluminescence. Spin dependent photocurrent (ΔI) was then measured in applied magnetic fields up to ± 5 T perpendicular to the film plane at an excitation of 1.523 eV. The ΔI exhibits clear magnetic field dependence, ensuring that spin filtering effect occurs at the interface. We also note that two noticeable features appear in the bias dependence of ΔI at -0.26 and 0.40 V. These features are likely to be associated with spin transport via interface states, while the result is in contrast to a spike-like dip in the bias dependent ΔI for an Fe/GaAs QW interface[1] The combined results indicate that the spin dependent photocurrent is very sensitive to the interface localized states and the spin dependence can be controlled by tuning the bias voltage at the interface. Work supported in part by Industrial Technology Research Grant Program in 2009 from NEDO of Japan.
References
[1] E. Wada et al., Phys. Rev. Lett. 105, 156601 (2010). [2] E. Wada et al., Appl. Phys. Lett. 97, 172509 (2010). [3] Y. Shirahata et al., J. Appl. Phys. 109, 07E105 (2011). [4] Y. K. Takahashi et al., Appl. Phys. Lett.98,152501 (2011).
BS-11. Magnetism and transport properties of epitaxial Fe-Ga thin film on GaAs(001)
Dang Duc Dung1, 2, Duong Anh Tuan1, Vo Thanh Son3, Yooleemi Shin1 and Sunglae Cho1
1Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea; 2Department of General Physics, Ha Noi University of Science and Technology,, 1 Dai Co Viet road, Ha Noi, Viet Nam; 3Centers for Nanobioenineering and Spintronics, Chungnam National University, Daejon 350-746, Republic of Korea
Injecting spin into nonmagnetic semi conductors has recently attracted great interest due to potential spin-dependent electronic devices [1]. The Fe-Ga is promising to real application because it has high Curie temperature (TC), high saturation magnetization (MS) beside the unique magnetostrictive properties [2,3]. Recently, electrical spin injection from Fe0.5Ga0.5 produces an electron spin polarization above 70% in GaAs(001) and its remain for Ga content below x< 50% in Fe1-xGax alloy [4]. However, the out-of-plane saturation field and magnetization decrease rapidly with Ga content. In addition, there is less information about transport and magnetism properties of the epitaxial Fe-Ga thin film which grown on semiconductor substrates. Here, single crystal Fe-Ga thin films have been grown on GaAs(001) by molecular beam epitaxy in disordered BCC α-Fe crystal structure (A2). Unlike grown on MgO(001) and ZnSe(001), VSM results of Fe-Ga alloy indicated that the easy direction is in-plane while the hard exit is out-of-plane. The MS decreased from 1371 to 1105 emu/cm3 with increasing the Ga concentration from 10.5 to 24.3% at room temperature. The anomalous Hall effect was obtained at room temperature is evident for spin polarization of epitaxial film. The resistively dependent temperature shown the typical metallic behavior, however the resistively of low Ga concentration is larger than high Ga concentration which may rise scattering between conduction electrons and local moments.
References
[1] S. A. Wolf et al., Science 294, 1488-1495 (2001). [2] H. Okamoto, Bull. Alloy Phase Diagrams. 11, 576 (1990). [3] A. E. Clark et al., J. Appl. Phys. 93, 8621 (2003) [4] O. M. J. van ‘t Erve et al., Appl. Phys. Lett. 91, 122515 (2007).
BS-12. Complex Spin Detection Behaviour at the Epitaxial Fe/GaAs Interface Following Post-growth Annealing
Chen Shen, Theodossis Trypiniotis and Crispin Barnes
Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
The post-growth annealing (PGA) of the Fe/GaAs interface at a mild temperature plays a role in determining the interfacial structure and spin transport properties across such interfaces. High resolution transmission electron microscopy (TEM) studies have suggested the interface quality be improved after the PGA. However, in the microscopic scale, three different atomic arrangements at the interface have been proposed by TEM studies so far [1-3]. The effect of PGA on spin injection has been investigated by several groups [4, 5]. They found the spin injection efficiency was improved after PGA by detecting the spin polarisation of the emitted light using spin-LED devices [1, 4, and 5]. Some of the experiments demonstrate that even the sign can be changed after the annealing process [4]. These results suggest the complexity of change of the interface states induced by the PGA. Most studies have concentrated on the electrons spin transport across the interface around the Fermi level of the metal. In this study, we report on an investigation of the effect of PGA of an epitaxial Fe/GaAs interface on the spin detection for energies above the Fermi level of the metal. Electrical characterisation and spin detection measurement were carried out before and after PGA at 200°C for 15 minutes in an N2 atmosphere. The annealing process is expected to modify the Fe/GaAs contact only rather than the Fe and GaAs bulk structure [5] and the Schottky barrier height was found to increase. The spin detection efficiency before and after annealing shows a complex bias-dependent behaviour. In the low forward bias regime, the spin detection efficiency is higher before the PGA, whereas, at a high forward bias level, the PGA enhances the efficiency. These results show that, unlike spin injection of electrons near the F ermi level of the FM, in the specific case of spin detection the increase in the efficiency is only observed at higher energies after PGA. At lower energies, the increased Schottky barrier height and width results in a longer dwell time in the GaAs channel and consequently the higher spin-flip probability and lower efficiency.
References
[1] T. J. Zega, A. T. Hanbicki, S. C. Erwin, I. Zutic, G. Kioseoglou, C. H. Li, B. T. Jonker and R. M. Stroud, Phys. Rev. Lett. 96, 196101 (2006). [2] J. M. LeBau, Q. O. Hu, C. J. Palmstrøm, and S. Stemmer, Appl. Phys. Lett. 93, 121909 (2008). [3] L. R. Fleet, H. Kobayashi, Y. Ohno, J.-Y. Kim, C. H. W. Barnes, and A. Hirohata, J. Appl. Phys. 109, 07C504 (2011). [4] B. D. Schultz, N. Marom, D. Naveh, X. Lou, C. Adelmann, J. Strand, P. A. Crowell, L. Kronik, and C. J. Palmstrøm, Phys. Rev. B 80, 201309(R) (2009). [5] C. Adelmann, J. Q. Xie, C. J. Palmstrøm, J. Strand, X. Lou, J. Wang, and P. A. Crowell, J. Vac. Sci. Technol. B 23, 1747 (2005).
BS-13. Magnetic field controlled threshold resistive switching in magnetic granular systems
Ajeesh M. Sahadevan, Alan Kalitsov, Surya N. Jammalamadaka, Kalon Gopinadhan, Charanjit S. Bhatia, Guangcheng Xiong and Hyunsoo Yang
Department of Electrical and Computer Engineering, NUSNNI-Nanocore, National Univ Singapore, Singapore, Singapore
We propose a theoretical model for magnetic field dependence of threshold resistive switching (RS) in magnetic granular system. We consider a simple tight-binding like model of a two-dimensional (20 by 15) cluster with classical localized moments which mimic magnetic granules and a spin-independent hopping integral between nearest neighbors. The model is based on the self-trapped electrons mechanism [1,2]. We support our theoretical calculations by measuring the magnetic field dependence of threshold RS behavior in Co/Al2O3 granular multilayers and the results are in good agreement with the proposed theory. Figure 1 shows both the calculated and measured I-V characteristics of RS system for different external magnetic field (H) which clearly shows that the switching voltage may be significantly decreased with increasing H. The underlying mechanism is the influence of magnetic field on electron occupation of the conduction band, which depends on the materials used in magnetic granular system, concentration of magnetic granules in the insulating matrix, applied voltage, and the charge accumulation on the granules. Density of states (DOS) calculations shows that after charging, the center of the conduction band moves closer to the Fermi level and its occupation depends on the magnetic field.
References
[1]. D. M. Ramo et al., Phys. Rev. Lett. 99, 155504 (2007). [2]. Y. S. Chen et al., J. Appl. Phys. 106, 023708 (2009).
BS-14. Tunable positive magnetoresistance effect of Co-doped amorphous carbon films
Yucheng Jiang, Ju Gao and Zhenping Wu
Physics, Hongkong University, Hong Kong, China
Co-doped amorphous carbon (a-C) films were deposited on n-type Si substrates by pulsed laser deposition method. The a-C film is a unique material which consists of two species of hybridized carbon atoms: sp2 and sp3. Many electrical and magnetic properties can be explained by the sp2/sp3 ratio on basis of the typical structural model that sp2 clusters are embedded in the sp3 matrix.1 Current-voltage characteristics of the Co:a-C/Si junctions, investiga ted at various magnetic fields, show an apparent positive magnetoresistance (PMR) effect which turns out being tuned by the bias voltage and reaches a peak at a particular voltage.2,3 MR-bias relations were further studied at the temperatures of 50 K, 100 K, 150 K and 200 K, respectively. Raman spectra results demonstrate that Co doping favors the formation of graphitic sp2 sites. The mechanism of the PMR effect is attributed to the interactions between the applied magnetic field and Co ions, which lead to the transition from sp2 to sp3 sites and increase the resistance.
References
1. Y. Miyajima, A. A. D. T. Adikaari, S. J. Henley, J. M. Shannon, and S. R. P. Silva, Appl. Phys. Lett. 92, 152104 (2008) 2. H. S. Hsu, P. Y. Chung, J. H. Zhang, S. J. Sun, H. Chou, H. C. Su, C. H. Lee, J. Chen, and J. C. A. Huang, Appl. Phys. Lett. 97, 032503 (2010) 3. Lanzhong Hao, Qingzhong Xue, Xili Gao, Qun Li, Qingbin Zheng, and Keyou Yan, J. Appl. Phys. 101, 053718 (2007)
BS-15. Frequency-dependence of magneto-conductance of Co doped amorphous carbon films
Hua Shu Hsu1, Cheng Hung Ko1, Wan Ting Liao1, Pei Yu Chuang2, Chih Hao Lee2 and Hui-Chia Su3
1Department of Applied Physics, National Pingtung University of Education, Pingtung, Taiwan; 2Department of Engineering and System Science, National Tsing-Hua University, HsinChu, Taiwan; 3Industrial Application Office, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
Amorphous carbon (a-C) is an amorphous semiconductor with a mixture of graphite-like sp2 and diamond-like sp3 bond characters, which feature the extra degrees of freedom by tuning the sp2/sp3 ratio. Recently, bias dependent magneto-resistance effects were discovered at room temperature of transition metal doped a-C films.1,2. These findings suggest that the carbon-based thin films have potential for spintronics applications. In this work, we present the considerable intrinsic alternating current (a.c.) magneto conductance behavior of these Co:a-C films. We found this effect is frequency dependent. For the RT grown sample, the ratio of the ac magneto conductance initially remains almost at a small value, follows by an up step in the frequency range of 104 to 105 Hz, and reaches a saturated value about 20% at 0.4T. However, no obvious frequency-dependent magneto conductance effect has been observed for Co:a-C film grown at 573K, implying that the a.c. magneto-transport property strongly depends on the growth condition. The Raman spectra results support the appearance of the frequency-dependent magneto conductance effect strongly depends on the sp2 states and Co dopants. A phenomenological model related to orbital Zeeman splitting is introduced to describe how the a.c. conductance is controlled by frequency and magnetic field.
References
[1] H. S. Hsu, P. Y. Chuang, J. H. Zhang, S. J. Sun, H. Chou, H. C. Su, C. H. Lee, J. Chen, and J. C. A. Huang. Appl. Phys. Lett. 97, 032503 (2010). [2] X. Zhang, X. Zhang, C. Wan, and L. Wu, Appl. Phys. Lett. 95, 022503 (2009).
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Claudia Felser, University of Mainz
BT-01. Investigation of m agnetic ordering in Bi4Ti3O12-n BiFeO3 solid solutions
Preethi Meher K R. Srinivasan1, Satish Vitta2 and Varma Kalidhindi B. R.1
1Materials Research Centre, Indian Institute of Science Bangalore, Bangalore, India; 2Department of Metallurgical engineering and Materials Science, Indian Institute of Bombay, Mumbai, India
As a part of search for single phase magnetoelectric materials, we specifically focus on the magnetic properties of Bi4Ti3O12 - n BiFeO3 (where n=3 and 5), belonging to the Aurivillius family of oxides. Recently, a number of studies on these systems have claimed their promising ferroic properties due to the simultaneous presence of ferroelectric and magnetic ordering 1. Infact the structural and ferroelectric properties of Bi4Ti3O12-5BiFeO3 (BFT5) system was systematically studied by us 2. Similar studies have been conducted on Bi4Ti3O12-3BiFeO3 (BFT3) along with La substituted systems of composition Bi4Ti3O12-3LaxBi1-xFeO3 (x=0.1 and 0.2) (LBFT3). Therefore, it can be very interesting to correlate the nature of magnetic ordering in a wide range of compositions belonging to the same family. The magnetic properties of these compounds were measured in a wide temperature range (300 K to 10 K) in an applied field of 100 Oe under both field-cooled (FC) and zero-field cooled (ZFC) conditions. The susceptibility (χ) Vs temperature (T) behavior of these compounds was observed to be rather complex with continuously changing slope down to 10 K that indicates multiple strengths of interaction. A weak magnetic transition temperature (Tc) was found to be lying above room temperature for BFT5 whereas the BFT3 exhibited a Tc around 100 K. A marginal increase in the magnetization values observed in the case of LBFT3 samples was expected as La substitution was found to improve the ferromagnetic interaction between the Fe3+ magnetic moments. We also propose possible models of magnetic ordering in such complex oxides based on our experimental results.
References
1 M.A.Zurbuchen et al, Appl. Phys. Lett. 91,033113 (2007); Xiangyu Mao et al, Appl. Phys. Lett. 95,082901 (2009) 2 K.R.S.Preethi Meher and K.B.R. Varma, J. Appl. Phys.106,124103 (2010)
BT-02. Structure and magnetism of BaTi1-xFexO3 multiferroics
Van-Dang Nguyen1, 3, Ha M. Nguyen1, 2, Pei-Yu Chuang2, Jie-Hao Zhang5, Dang-Thanh Tran1, Chih-Wei Hu2, 4, Tsan-Yao Chen2, 4, Hung-Duen Yang5, Dinh-Lam Vu1, Chih-Hao Lee2, 4 and Van-Hong Le1
1Vietnam Academy of Science and Technology, Hanoi, Viet Nam; 2Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan; 3Faculty of Physics, College of Science, Thai Nguyen University, Thai Nguyen, Viet Nam; 4National Synchrotron Radiation Research Center, Hsinchu, Taiwan; 5Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan
BaTi1-xFexO3 (0.0 ≤ x ≤ 0.12) materials were synthesized using the solid state reaction. Their structure and magnetism were characterized using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), Raman spectrosco py, and magnetization measurements. We aim investigating how Fe dopant content affects on the phase transformation, local environment of Fe ions, and magnetism. The XRD data show that samples with x ≤ 0.01 exhibit the single tetragonal (Tetra) phase (P4mm) while this phase is transformed completely into the hexagonal (Hexa) one (P63/mmc) in sample x = 0.12. There is a coexistence of the Tetra and Hexa phases in sample x = 0.10 (~ 14 % Tetra and 86% Hexa in volume content). Raman spectroscopy has been used to verify the quantitative analysis of XRD to show that a tinny Hexa phase likely in the amorphous form is still present in sample x = 0.01. The quantatitive analysis of local environment surrounding Fe dopant ions using Fe K-edge XAS data confirms that Fe ions substitute in Ti positions in the Hexa phase rather than in the Tetra one. The increase in intensity of the pre-edge peak of 1s→3d indicates the progressive distortion of Fe centrosymmetric environment from octahedral to tetrahedral sites, which is consistent with the increasing Fe 3d-O 2p mixing covalency indicated by the peak intensities of multiple-electron 1s→4p plus ligand-to-metal charge transfer shake-down transition. The increase of edge energy with Fe content evidences the increasing Fe valence to charge neutralize with oxygen deficiency. The the first-shell double-peak feature of the Fourier transformation magnitude of the partial radical distribution function apparently evidence the strong Jahn-Teller distortion around Fe ions. Our structure analyses may help to rule out the possibility that the multiple magnetic phases observed from the hysteresis curve for sample x = 0.1, which is not present for sample x = 0.12, is due the contributions of Tetra phase and Hexa phase. This may suggest that multiple magnetic phases are likely due competition between super exchange and double exchange interactions in the Hexa phase with Fe3+/Fe4+ mixed valence states as in colossal magnetoresistance perovskites.
BT-03. Conductivity across barriers as origin of high-temperature dielectric response in BaTiO3 and (Ba,Ti) doped BiFeO3 multiferroic ceramics
Tzu-Hsiang Wang1, Chi Shun Tu1, 2 and Kun Tung Wu2
1Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, Taipei County, Taiwan; 2Department of Physics, Fu Jen Catholic University, New Taipei City, Taiwan
Temperature- and frequency-dependent dielectric permittivity (ε′ and ε′′) and conductivity (σ′ and σ′′) have been carried on multiferroic ceramics (1-x)BiFeO3-xBaTiO3 and (Bi1-xBax)(Fe1-xTix)O3 for x=0.0, 0.05, and 0.1, which were prepared by the solid state reaction method. The results indicate two separate phenomena for both ceramics. At higher temperatures a phase-shifted conductivity is evident, while at lower temperatures a permittivity feature not associated with conductivity dominates. The nearly Arrhenius behavior at higher temperature is the major effect of this conductivity mechanism. The phase shift of this conductivity is also evident in the sharp upturns of ε′. An apparent relaxation for that step appears as the curved deviations from the barrier-model fits in both ceramics in the low-temperature region. If this were Debye behavior given by ε=ε∞ +(εs-ε∞)/[1+(iωτo )1-α ] if α=1 or a frequency-broadened Debye-like feature for 0< <1, the height of the step in ε′ would be independent of frequency. Such behavior is typical for dipolar relaxation independent of a phase transition. Because the step height decreases with frequency, we can surmise that these frequency-dependent plateaus in ε′ are instead likely act ivated by the antiferromagnetic (AFM)-paramagnetic (PM) transition in the 620-670 K range. The synchrotron XRD of BFO revealed a minimum in rhombohedral angle αR near 670 K, due to the changes in positions of Bi3+ and Fe3+ ions, and this temperature is near the range of the permittivity step. To understand the large conductivity and dielectric response in the high-temperature region, a one-dimensional barrier model was derived, in which B represents the intrinsic barriers every lattice spacing a. B+Δ are extrinsic barriers spaced a distance d apart. Good qualitative fits of conductivities in the high-temperature region are obtained with d~40 nm roughly for BFO, (1-x)BiFeO3-xBaTiO3, and (Bi1-xBax)(Fe1-xTix)O3 ceramics. (Bi0.9Ba0.1)(Fe0.9Ti0.1)O3 has higher intrinsic barriers about B=11000 K than B=8200 K of BiFeO3. A step-like dielectric relaxation appears in the 500-650 K range, which is possibly caused by the magnetoelectric coupling.
BT-04. Photovoltaic Phenomena in BiFeO3 Multiferroic Ceramics
Li Shian Jou1, Tzu Hsiang Wang2, Chun Wei Chen1, Wei De Yen1, Chi-Shun Tu2 and Y. Yao2
1Department of Physics, Fu Jen Catholic University, Taipei, Taiwan; 2Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, Taipei, Taiwan
Photovoltaic phenomena and magnetization were investigated in BiFeO3 (BFO) and 0.01 mol% WO3-doped BiFeO3 ceramics, which were prepared by the solid state reaction method with starting powders of Bi2O3, Fe2O3, and WO3. BFO and WO3-doped BiFeO3 ceramics exhibit a significant photo-current under near-ultraviolet illumination and an antiferromagnetic behavior, whose magnetization curves are linear with the field at room temperature. The magnetic susceptibilities of BFO and WO3-doped BiFeO3 ceramics are about 1.9×10-5, and 9.0×10-6 (emu/g.Oe), respectively. The photovoltaic response strongly depends on wavelength and intensity of laser illumination. As shown in Figure, the photo-current increases with illumination power (or intensity). "ON" and "OFF" indicate with and without illumination. The photo-current reaches a saturated value as laser power increases. The photo-current exhibits an exponential decay before reaching an equilibrium value. By using I(t)=Ie+I0exp(-t/α) to fit the data, the decay constant α appears in the range of 5-20 seconds. Ie and I0 are the equilibrium and peak photo-currents, respectively. The α decreases with increasing intensity of illumination. The photo-current exhibits an equal and opposite sign while the connections were switched. A similar photo-current phenomenon was observed under laser illumination of λ=632 nm.
BT-05. Fabrication of highly ordered ferromagnetic BiFeO3 nanotubes by AAO template method
Luiz Oliveira and Kleber R. Pirota
Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, Campinas, Brazil
One-dimensional nanostructures, such as nanowires and nanotubes, have become the focus of studies because of their unique size-dependent properties and their relevant applications in mesoscopic physics [1]. One perovskite oxide of particular interest is BiFeO3 (BFO), which exhibits the coexistence of ferroelectric and anti ferromagnetic orders up to quite high temperature (> 300 K). The relationship between the size of BFO nanotubes and their physical properties has also aroused much interest due to their special applications such as ultra-high density vertical magnetic recording. In this work, we report the fabrication of highly ordered BFO nanotubes by a sol-gel technique using two-step anodic aluminum oxide (AAO) as template [2]. We succeeded to lower the dimensions of the BFO nanotubes to 65 nm in diameter and 3 μm in length [3,4], as confirmed by scanning electron microscopy (SEM) measurements, Figure 1. The obtained nanotubes present the expected pure phase (BiFeO3) as confirmed by energy-dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscopy (HRTEM). In addition to the antiferromagnetic behavior, the magnetization curves of the BFO nanotubes also present a ferromagnetic response, which holds from 2 to 300 K, Figure 1. This behavior can be associated to the uncompensated spins located at the surface. Comparison between parallel and perpendicular magnetic measurements shows a small shape anisotropy.
References
[1] X. Zhu, Z. Liu and N. Ming, Journal of Materials Chemistry 20, 20 (2010). [2] Hernandez-Velez M., Pirota K.R., Paszti F, et al., Applied Physics A, 80, 8 (2005). [3] X. Xu et all., Chemistry Letters 36, No.1 (2007) [4] X. Y. Zhang, C. W. Lai, X. Zhao, D. Y. Wang, and J. Y. Dai, Applied Physics Letters 87, 143102 (2005)
BT-06. Sputter-prepared BiFeO3(001) films on L10 FePt(001)/glass substrates
H. W. Chang1, F. T. Yuan2, C. W. Shih3, C. R. Wang1, W. C. Chang3 and S. U. Jen4
1Department of Physics, Tunghai University, Taichung, Taiwan; 2Department of Physics, National Taiwan University, Taipei, Taiwan; 3Department of Physics, National Chung Cheng University, Chia-Yi, Taiwan; 4Institute of Physics, Academia Sinica, Taipei, Taiwan
BiFeO3 (BFO) thin films have received much attention in the recent years [1-3]. Comprehensive studies have been made on BFO films grown on single crystal substrates by pulse laser deposition. However, research on sputter-prepared BFO is less due to the difficulties in obtaining high quality samples. In this paper, we present the preparation of BFO films by sputtering at reduced temperature of 450oC on glass and commercial Pt/Ti/SiO2/Si(001) substrates. Besides, underlayers with different orientations were prepared on glass substrates including highly textured Pt(111), isotropic Pt/FePt underlayers, and L10 FePt(001) by rapid thermal annealing. Figure 1 shows XRD patterns for the optimized 200-nm-thick BFO films grown on different underlayers. Isotropic perovskite BFO grains with size of about 200 nm formed on the commercial substrates, showing large surface roughness. Pt(111) suppresses BiFeO3 phase, and Pt/FePt underlayer results in isotropic growth of it. Single phase perovskite BFO with strong (001) texture and fine grain size was formed on L10 FePt(001) buffer. The reduced surface roughness results in significantly improved ferroelectric properties, and consequently an enhancement over 100% in remanent electrical polarization was obtained in the FePt(001) underlayered sample as compared to the film on commercial substrate. Thickness and growth temperature dependence of BFO layer as well as growth mechanism will be discussed in details.
References
[1] J.R. Teague et al., Solid State Commun. 8, 1073 (1970). [2] M. Bibes et al., Nature Materials 7, 425 (2008). [3] J. Wang et al., Science 299, 1719 (2003).
BT-07. Annealing temperature dependence of exchange bias in BiFeO3/CoFe bilayers
Tian Yu1, Hiroshi Naganuma2, Wen-Xiu Wang1, Xiu-Feng Han2 and Yasuo Ando1
1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China; 2Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai, Japan
Recently, much effort has been devoted to multiferroics which simultaneously combine both magnetic and ferroelectric orders1. Among various single phase multiferroic materials, BiFeO3 (BFO) is one of the most promising candidates because of its large ferroelectric polarization and high ordering temperatures. However, since BFO is a G-type antiferromagnet2 and only small magnetization can be observed3, it is difficult to exploit its multiferroic properties alone. Instead, exchange bias in BFO hybrid structures 4-6 are employed, where the BFO is severed as an antiferromagnet and manipulated electrically4. In previous studies, the exchange bias were established by applying a magnetic field during film deposition6, and rare works have been made using post-annealing though it is one of the standard methods to induce exchange bias and widely used in spintronic devices fabrication. Besides, post-annealing provides a simple and unique way to investigate the annealing temperature dependence of exchange bias properties. In this work, we present our experimental study on post-annealing temperature dependence of exchange bias in BFO/CoFe bilayers with different textures. Annealing below 100 °C, exchange bias field is enhanced as increasing annealing temperature which can be well understood based on thermal fluctuation aftereffect model7; whereas, it is degraded at higher annealing temperature which may be due to interface diffusion and reaction. The effect of BFO texture on exchange bias is also investigated and no remarkable difference is observed. Our results suggest that the interface direct coupling plays an important role for the annealing temperature dependence of exchange bias in BFO/ferromagnet system.
References
[1] W. Erenstein, N. D. Mathur, J. F. Scott, Nature 442, 759 (2006) [2] P. Fischer, M. Polomska, I. Sosnowska, M. Szymanski, J. Phys. C: Solid State Phys. 13, 1931 (1980) [3] J. Wang, J. B. Neaton, H. Zheng, et.al., Science 299, 1719 (2003) [4] S. M. Wu, S. A. Cybart, P. Yu, et.al., Nature Mater. 9, 756 (2010) [5] H. Bea, M. Bibes, F. Ott, et.al., Phys. Rev. Letts.100, 017204 (2008) [6] H. Naganuma, M. Oogane, and Y. Ando, J. Appl. Phys. 109, 07D736 (2011) [7] E. Fulcomer and S. H. Charap, J. Appl. Phys. 43, 4190 (1972)
BT-08. Correlation of spin and structure in doped Bismuth ferrite nanoparticles
Je Wei Lin1, 2, Teddy Tite1, Yung-Hsiu Tang1, Chin Shan Lue2, Yu-Ming Chang1 and Jauyn Grace Lin1
1CCMS, NTU, Taipei, Taiwan; 2Phyics, NCKU, Tainan, Taiwan
The mutiferroic Bi1-xEuxFeO3 samples with x = 0 to 0.4 are studied by x-ray diffraction (XRD), Raman using a 632.8 nm He-Ne laser and electron spin resonance (ESR) with X-band (9.53GHz). Among several rare earth dopants, europium (Eu) has been found to greatly enhance magnetization via the magnetic coupling between Eu3+ and Fe3+ ions . In this work, we report a systematic study on the structural and optical properties of BFO nanoparticles in a wide range of Eu-concentration (0≤x≤0.40). The values of bond lengths are obtained from x-ray diffraction (XRD) and the fitting of crystalline structure with GSAS (General structure analysis system), in orderto investigate the relation of bond length with x. The lattice structure is further examined by performing the spontaneous Raman spectroscopy on these Eu-doped BFO samples, where the Raman spectra reveal a blue shift and eventually a structural disorder at high Eu doping level. A correlation between the change of Bi-O bond length and the Raman blue shift is found. These structural data are further related to the change of ESR spectra which shows a doping induced suppression of cycloid spin structure.
References
[FZQian] F. Z. Qian, J. S. Jiang, S. Z. Guo, D. M. Jiang, and W. G. Zhang, J. Applied. Phys. 106, 084312 (2009) [Eerenstein] W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 442, 759-765 (17 August 2006) [AJHauser] A. J. Hauser, J. Zhang, L. Mier, R. A. Ricciardo, P. M. Woodward, T. L. Gustafson, L. J. Brillson, and F. Y. Yang, Appl. Phys. Lett., 92, 222901 (2008) [JDing] J. Ding, X. Lü, H. Shu, J. Xie, H. Zhang, Materials Science and Engineering B, 171, 31-34 (2010)
BT-09. Magnetoelectric effects in BiFeO3/CoFe2O4 nano-composites
Nicolas Aimon, Dong Hun Kim, Hong Kyoon Choi and Caroline Ross
MIT, Cambridge, MA
Magnetoelectric composites composed of nanostructured magnetic and ferroelectric phases show high coupling between the magnetic and electric order parameters. The electrostriction of the ferroelectric phase and the magnetoelasticity of the ferromagnetic phase enable this coupling via the transfer of strain from one phase to the other at the interface. Films were grown by pulsed laser deposition using a KrF excimer laser pulsed at 10Hz to ablate ceramic targets of CoFe2O4 and Bi1.2FeO3. Substrates were (001) single crystal SrTiO3 and (001) SrTiO3-buffered Si. The codeposition of immiscible CoFe2O4 (CFO) and BiFeO3 (BFO) formed vertical nanocomposite thin films in which rectangular ferrimagnetic CFO pillars of spacing <100 nm were embedded in a ferroelectric BFO matrix. VSM and SQUID magnetometry showed that the nanocomposites exhibit a strong out-of-plane anisotropy, consistent with the sum of the shape anisotropy of the high aspect ratio structures and the magnetoelastic anisotropy induced by the out-of-plane compressive strain state of the CFO phase which has a negative magnetostriction coefficient. We quantified the full stress state using XRD reciprocal space mapping and HRTEM imaging. Finite element analysis simulations allow us to quantify transfer of strain from the piezoelectric BFO matrix to the adjacent CFO pillars when the former is electrically poled. The as-deposited and electrically induced strain distribution within the CFO induce spatial variations in magnetic anisotropy. We will relate the magnetic properties of the nanocomposite to the calculated and measured strain state in the CFO pillars and BFO matrix.
BT-10. Strain relaxation in Bi0.9Pb0.1FeO3/SrRuO3/SrTiO3 heterostructure
Murtaza Bohra, Hsiung Chou, H. J. Yeh and C. P. Wu
Physics, Natl Sun Yat-Sen University, Kaohsiung, Taiwan
We report a detailed study of strain relaxation in Bi0.9Pb0.1FeO3/SrRuO3(45nm)/SrTiO3 heterostructure fabricated by off-axis RF-sputtering with varying Bi0.9Pb0.1F eO3 layer thickness (100-403nm). Fig.1(a)&(d) show reciprocal space maps (RSM) depicting that the top Bi0.9Pb0.1FeO3 (a=3.994-4.034Å) layer exhibit the largest in-plane lattice constants than the bottom SrRuO3(a=3.938-3.964Å) layer which is also larger than the SrTiO3 (a=3.905Å)substrate. This phenomenon indicates a strain relaxation of all layers. Interestingly, both Bi0.9Pb0.1FeO3 (V=63.5-65.9Å3) and SrRuO3 (V=62.4-60.8Å3) layers undergo large unit cell expansion compared to their bulks counterpart (62.4Å3) and (60.7Å3) respectively. Strikingly, with increasing the top Bi0.9Pb0.1FeO3 layer thickness, the bottom SrRuO3 unit cell volume decreases and approaches to its bulk value for 400nm thick top Bi0.9Pb0.1FeO3 layer (Fig.1(d)). This result is very contrary to the general concept that the bottom layer should control the strain state of the top layer. This controversial results strongly indicates that the top Bi0.9Pb0.1FeO3 layers is responsible for the shrinkage of unit cell volume of bottom SrRuO3 layers in the heterostructure.
BT-11. Indication of Magnetoelectric Properties in BiFeO3/(00l)SrFe12O19 Bilayers
Yukiko Yasukawa, Xiaoxi Liu and Akimitsu Morisako
Information Engineering, Shinshu University, Nagano, Japan
Highly-(00l) oriented strontium ferrite (SrFe12O19) thin film has been sputtered on thermally oxidized silicon substrate. Afterward, bismuth ferrite (BiFeO3) film was deposited on the SrFe12O19 film through rf sputtering, thereby BiFeO3/(00l)SrFe12O19 bilayer was obtained (Figs. 1(a) and 1(b)). Based on XRD measurements, BiFeO3 phase was found to be random orientation but our sample exhibited essentially pure phase. The most possible secondary phases such as Bi2Fe4O9, α-Fe2O3 (non-magnetic) and γ-Fe2O3 (magnetic) were not detected. Magnetic properties of bilayers were evaluated by vibrating sample magnetometer (VSM) and magnetic domain structures were confirmed by magnetic force microscopy (MFM). Estimated size of the magnetic domain was approximately ∼300 nm. Moreover, naturally-generated electric domain structures were observed by electrostatic force microscopy (EFM). In the case of (00l)SrFe12O19 thin film, no electric domain structure was observed. On the other hand, electric domains were clearly revealed in bilayers (Figs. 1(c) and 1(d)). When a conductive EFM probe was biased at 0.8V and -0.8 V, two color tones seen in figures were flipped over. This indicates the electrostatic force could be reversed due to in-situ electric field generated by the probe.
BT-12. Ferromagnetism in Multiferroic BiFeO3: Facts and Artifacts
Ratnakar Palai1 and H. Huhtinen2
1Dept. of Physics, University of Puerto Rico, San Juan; 2Department of Physics, University of Turku, Turku, Finland
Magnetoelectric (ME) multiferroics are technologically and scientifically promising because of their potential applications in data storage, spin valves, spintronics, quantum electromagnets, microelectronic devices, etc. Ferroelectricity originates from off-center structural distortions (d0 electrons) and magnetism is involved with local spins (dn electrons), which limit the presence of off-center structural distortion. These two are quite complementary phenomena, but coexist in certain unusual multiferroic materials. BiFeO3 (BFO) is one o f the most widely studied multiferroic material because of its interesting ME properties, i.e., ferroelectricity with high Curie temperature (Tc ~810-830 0C) and antiferromagnetic properties below TN ~370 0C, along with cycloidal spin structure with G-type ordering. Ferromagnetism with high magnetic moment (150 emu/cc) has been reported by several researchers in BFO thin films and polycrystalline samples at room temperature, which is quite intriguing. We studied magnetic properties of a stoichiometric single domain single crystal, polycrystalline samples, and 300 nm thin films using SQUID. We did not observe any such high magnetic moment with ferromagnetism at room temperature contrary to earlier reports. We strongly believe that the reported ferromagnetism with high magnetic moment at room temperature could be due to the phase segregation or valence fluctuation of Fe ions. It has been reported that presence of γ-Fe2O3 could have magnetic moment of about 300 emu/cc. It our conjecture that very thin epitaxial films (< 100 nm) might display weak ferromagnetism due to the spin canting because of the epitaxial strain. We will review and compare the present observation with all the earlier reports.
BT-13. Structural transformation in Pb-doped BiFeO3 (00l) epitaxial thin films
Murtaza Bohra, C. P. Wu, Y. H. Yeah and H. Chou
Department of Physics, National Sun Yat-Sen University, Kaohsiung, Kaohsiung, Taiwan
We demonstrate that Pb doped BiFeO3(BPFO) thin films grown on strain relaxed SrRuO3(SRO) bottom layer on SrTiO3 (STO) substrate undergo a structural transformation which has a tight association with the growth temperature, Ts. With increasing Ts, the crystal structure of BPFO films develops from near cubic symmetry to a mixture of two pseudo-tetragonal symmetries with c/a<1 and c/a>1, as shown by high resolution x-ray reciprocal space mappings in Fig.1(a). Both the near cubic BPFO (a=4.05Å) films and pseudo-tetragonal films exhibit larger lattice constant than bulk rhombohedral BiFeO3 (a=3.95Å), but similar to the predicted high temperature tetragonal(>1300) and cubic(>1400K) phases of BiFeO3.[1] The saturation magnetization (Ms), as shown in Fig. 1(b), at 300K declines sharply for higher Ts films ranges from 0.22-0.05 μB/Fe+3. The variation of Ms can be linked with observed huge lattice expansions, which in turn change Fe-O bond length/angles that could affect canted spin structure of Fe+3 sub-lattices. The structural transformation also results in the reduction of coercivity value of 2.5kOe at 5K (Inset of Fig.1(b)) in which each loop is composed of two sub-loops which are the contribution of the top BPFO film and bottom SRO film (Tc-SRO=160K). *H. Chou: hchou@mail.nsysu.edu.tw
References
[1] Arnold et al.Adv. Funct. Mater. 2010, 20, 1-8
BT-14. Multiferroic behaviour of disordered bismuth-substituted zinc ferrite
Shinichiro Mito, Hiroyuki Takagi, Alexander V. Baryshev and Mitsuteru Inoue
Toyohashi University of Technology, Toyohashi, Japan
It is known that zinc ferrite (ZnFe2O4) exhibits a stable antiferromagnetic phase below 10 K due to Fe+ ions in the octahedral sites (B sites). Also, ferromagnetic state can be observed at room temperature when ZnFe2O4 is quickly cooled from sintering temperatures. This ferromagnetic behavior is attributable to the exchange interaction between Zn2+ in the A site (tetrahedral site) and Fe3+ in the B site takes place; this state of ZnFe2O4 is so-called disordered zinc ferrite [1]. On the other hand, ferroelectricity from iron valance ordering in the charge-frustrated system rare earth ferrite (RFe2O4) was reported in Ref. [2], where disordered RFe2O4 is considered as a promising candidate of multiferroic materials. In this work, we have investigated ferromagnetic and ferroelectric properties of disordered bismuth-substituted zinc ferrites (disordered Bi:ZnFe2O4). Bi:ZnFe2O4 films were deposited by using the RF-magnetron sputtering method. Composition of the fabricated films was Bi0.3Zn0.7Fe2O4. As-deposited films exhibited a ferromagnetic hysteresis and a large Faraday rotation angle in the short wavelength range (~400 nm): a saturation magnetization of 120 gauss and the Faraday rotation angle of -1.2 degs./μm were observed. Ferroelectric hysteresis was measured using the Sawyer-Tower method. Dielectric constant of Bi:ZnFe2O4 was measured in a frequency range from 120 Hz to 100 kHz, showing the ferroelectric hysteresis. For 120 Hz, the dielectric constant of 800 was observed at room temperature. By contrast, the dielectric constant of disordered zinc ferrite was 5.5 at 120 Hz. Our studies showed that bismuth substitution in ZnFe2O4 enhanced the dielectric constant drastically. -Furthermore, in the films, voltage-induced alteration of the magneto-optic Kerr rotation angle was identified.
References
[1] K. Tanaka, et al., J. Appl. Phys., 99, 106103 (2006) [2] N. Ikeda, et al., nature, vol 436, 25, 1136-1138 (2005)
BT-15. Crystal structure and magnetic properties of La and Ba codoped Bi0.8La0.2-xBaxFeO3 (0≤x≤0.2) multiferroics
Jianjian Ge, Guofeng Cheng, Jun Du and Xiaoshan Wu
Lab of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing, China
Multiferroic materials exhibiting both spontaneous magnetization and spontaneous polarization have recently attracted a surge of attention due to their potential applications for new devices [1-3]. As the only known room temperature single-phase multiferroic material, BiFeO3 (BFO) has been intensively investigated [3]. However, the bulk BFO is characterized by serious current leakage problems making it difficult to attain high resistivity and hamper the development of BFO applications. Fortunately, chemical substitution is proved to an efficient way to reduce these issues [2]. In this paper, systematic studies of crystalline structures and magnetic properties were performed on a series of ceramic Bi0.8La0.2-xBaxFeO3 (BLBFO, x=0-0.20) samples. The crystalline structure changes from pseudo-tetragonal to pseudo-cubic eventually when x varies from 0 to 0.20. The remnant magnetization (Mr) decreases quickly from 0.29 emu/g to 0.002 emu/g with increasing x from 0 to 0.05; however, it increases quickly from 0.002 emu/g to 0.8 emu/g with x increasing further from 0.05 to 0.20. This non-monotonic change behavior of magnetic property in BLBFO samples is suggested to be resulted from the relative structural variation. When the crystalline structure is pseudo-tetragonal, i.e. x = 0, it is much deviated from the rhombohedral BFO structure, which suppresses the cycloid spin structure causes a significant spontaneous magnetization appeared. When La and Ba co-dopped in BFO, the two kinds of suppression may compensate each other, e.g. x = 0.05, resulting in the magnetization decreased abruptly. When the crystalline structure is pseudo-cubic, i.e. x = 0.20, the cycloid spin st ructure may be suppressed and the latent ferromagnetic moment is released again. In addition, in this case, the magnetization is stronger than that of La-doped BFO, which is possibly due to that the spiral spin structure is destructive.
References
[1] G. Catalan and J. F. Scott, Adv. Mater. 21, 2463 (2009). [2] K. F. Wang, J. M. Liu, Z. F. Ren, Adv. Phys.¬ 58, 321 (2009). [3] L.W. Martin, Y. H. Chu, R. Ramesh, Mater. Sci. Eng. R 68, 89 (2010).
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Victorino Franco, Sevilla University
BU-01. Magnetocaloric Effect in Gd/Fe Heterostructures
Christopher Bauer, Dustin D. Belyea, Priyanga B. Jayathilaka and Casey W. Miller
Applied Physics, University of South Florida, Tampa, FL
Polarized neutron reflectivity experiments demonstrated a reduction of the magnetic moments of the Gd atoms at the interfaces in MgO/[W(50Å)/Gd(300Å)]8/W(50Å), which in turn reduced the magnetocaloric effect (MCE) of the structure [1]. Based on those results and reports of increased moment of Gd atoms when interfaced with Fe [2], we have investigated the MCE of Si/SiOx/Ta(50Å)/[Gd(200Å)/Fe(x_Å)]8/Ta(50Å) heterostructures, where x = 10, 20, 30, 40 and 50Å. X-ray reflectivity shows that these structures have sharp interfaces.Figure 1 shows that the magnetic entropy change (ΔS) has a novel table-like temperature dependence for the low temperature region (i.e., T < ~280K). While we show ΔS for a field change of 3 Tesla, this behavior persists from 500 Oe to 5T field changes. Noting that the relative cooling power is estimated by the full width at half max, these structures reveal that interface engineering may be a potential route for perturbing the MCE. Supported by AFOSR-YIP.
References
[1] B.J. Kirby, J.W. Lau, D.V. Williams, C.A. Bauer, C.W. Miller, J. Appl. Phys. 109, 063905 (2011). [2]Y.Choi, D. Haskel, R.E. Camley,D.R. Lee, J.C. Lang, G. Srajer, J.S. Jiang and S.D. Bader, Phys. Rev. B. 70, 134420 (2004)
BU-02. Improved magnetocaloric properties in partially Co-substituted Gd65Fe20-yCoyAl10X5 (X = Si, B) melt-spun ribbons
Ivan Skorvanek1, Jozef Marcin1, Jozef Kovac1, Zbyszek Sniadecki2 and Bogdan Idzikowski2
1Magnetism, Institute of Experimental Physics Slov. Acad. Sci., Kosice, Slovakia; 2Institute of Molecular Physics, Poznan, Poland
Among the recently developed magnetic refrigerant materials, the Gd(Fe,Mn)Al-based glassy alloys prepared by melt-spinning combine favourable magnetic entropy characteristics with sufficiently high effective magnetic moment per volume, which makes them good candidates for magnetic refrigeration in a wide operating temperature range below 200 K [1,2]. In this work, we report on a beneficial effect of partial Co substitution for Fe on the magnetocaloric behaviour of melt-spun GdFeAl-based alloys. Specimens with the composition Gd65Fe15Co5Al10Si5, Gd65Fe5Co15Al10Si5 and Gd65Fe10Co10Al10B5 were prepared in the ribbon form by the melt-spinning method. Amorphous structure of samples was confirmed by X-ray diffraction. The values of magnetic entropy change, ΔSM, were calculated using the Maxwell relation from the isothermal magnetization curves measured in applied magnetic field up to 50 kOe by SQUID magnetometer at different temperatures ranging from 5 to 275 K. The value of the maximal magnetic entropy change under 50 kOe for the Gd65Fe10Co10Al10B5 ribbon reached 7.02 J/kg K at 150 K. This value is markedly higher than that reported for the parent Co-free Gd65Fe20Al10B5 alloy [2], where the maximal ΔSM reached 5.17 J/kg K at 197 K. The values of refrigeration capacity, RC, were determined as the area below the ΔSM peak with the integration limits corresponding to the temperatures at its half maximum. The RC value under 50 kOe for Gd65Fe10Co10Al10B5 ribbon was calculated to be 766 J/kg, which is slightly higher that that reported for the parent Co-free alloy [2]. The maximal ΔSM value for the Gd65Fe5Co15Al10Si5 specimen under 50 kOe is 6.81 J/kg K at 155 K. The Gd65Fe15Co5Al10Si5 sample shows slightly lower values of the magnetic entropy change and the maximum of the ΔSM is shifted to 195 K. The enhanced values ΔSM extended over a wide temperature range together with the good magnetic softness leading to the low hysteresis losses make the partially Co-substituted GdFeCoAl(Si,B) amorphous ribbons promising magnetic refrigerants in the temperature range from 80 to 220 K.
References
[1] S. Gorsse, B. Chevalier, G. Orveillon, Appl. Phys. Lett. 92 (2008), 122501. [2] Y.K. Fang, C.H. Lai, C.C. Hsieh, X.G. Zhao, H. W. Chang, W.C. Chang W.Li, J. Appl. Phys. 107 (2010), 09A901.
BU-03. Magnetocaloric powder composite from alloys of the series Gd1-xPrxNi2 to be used in an Ericsson-cycle magnetic refrigerator
Alexandre M. Carvalho1, Ana Teresa G. Mendes2 and Adelino A. Coelho2
1Materials Metrology, INMETRO, Duque de Caxias, Brazil; 2Applied Physics, UNICAMP, Campinas, Brazil
In this work, we have studied the magnetic and magnetocaloric properties of five alloys of the series Gd1-xPrxNi2 (x = 0, 0.25, 0.5, 0.75 and 1) and calculated a composite prepared from the powder of these alloys. The alloy PrNi2 (x = 1) is paramagnetic [1] and substituting 25% of the Pr atoms by Gd atoms, we obtained the alloy Gd0.25Pr0.75Ni2 (x = 0.75), which is ferrimagnetic below 20 K. The alloys with x = 0.5 and 0.25 are also ferrimagnetic at low temperatures while the alloy GdNi2 is ferromagnetic below 75 K [1,2]. The magnetocaloric effect (MCE) data for the five alloys of the series are presented in the Figure 1. These magnetocaloric data are important to evaluate the quantities of the alloys to be used in the preparation of the powder composite. The solid curve in the Figure 1 is the calculated MCE for the composite to be prepared from the alloys with x = 0, 0.25, 0.5, 0.75 and 1. This theoretical powder composite could be used as the refrigerant in an Ericsson-cycle magnetic refrigerator able to work with great efficiency at temperatures lower than 85 K.
References
[1] J. J. Melero et al., J. Magn. Magn. Mat. 140, 841 (1995). [2] M. Mizumaki et al., Phys. Rev. B 67, 132404 (2003).
< /p>BU-04. Large magnetocaloric effect and refrigerant capacity in Gd-Co-Ni metallic glasses
Xichun Zhong1, Pengfei Tang1, Zhongwu Liu1, Dengchang Zeng1, Zhigang Zheng1, Hongya Yu1, Wanqi Qiu1 and Hu Zhang2
1School of Materials Science and Engineering, South China University of Technology, Guangzhou, China; 2State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing, China
Metallic glasses have many unique properties, such as high electrical resistance, high thermal stability, enhanced corrosion resistance and excellent mechanical properties. The magnetic metallic glass materials with a high glass-forming ability have also shown good soft magnetic properties with tunable Curie temperature. Most interestingly, these materials have exhibited the same or even larger magnetic entropy change with respect to the crystalline refrigeration working substance in certain temperature range [1] However, the magnetocaloric effect of metallic glasses has not been fully studied. Here we report on a large magnetocaloric effect and refrigerant capacity in Gd-Co-Ni metallic glasses. The Gd-Co-Ni metallic glass was prepared by melt-spinning method. Their structure was studied by using X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The XRD analysis revealed that the Gd-Co-Ni ribbons showed fully amorphous structure. The DSC revealed that the Gd-Co-Ni metallic glass has high thermal stability. With the increase of Co/Ni ratio, the Curie temperature TC of Gd-Co-Ni metallic glass increases from 140K to 175K and 192K. The peak values of magnetic entropy change (-ΔSM)max of Gd55Co25Ni20, Gd55Co30Ni15 and Gd55Co35Ni10 metallic glasses for a magnetic field change of 0-5 T are 6.04, 6.30 and 6.47 J kg-1 K-1, respectively. The refrigerant capacities for Gd55Co25Ni20, Gd55Co30Ni15, and Gd55Co35Ni10 alloys reach 450, 487, and 502 J kg-1, respectively. These values are comparable with that of La(Fe0.88Si0.12)13 [2] and are slightly larger than that of the known crystalline magnetic refrigerant compound Gd5Si2Ge1.9Fe0.1 [3]. A large magnetic entropy change and refrigerant capacity values together with high thermal stability make them an attractive candidate among the magnetic refrigeration materials working in the temperature of 100-230 K.
References
[1] Q. Luo, D.Q. Zhao, M.X. Pan and W.H. Wang. Appl. Phys. Lett. 89: 081914(2006). [2] A. Fujita, K. Fukamichi, J. Alloys Compd. 404-406: 554 (2005). [3] V. Provenzano, A.J. Shapiro, R.D. Shull, Nature (London) 429: 853 (2004).
BU-05. The effect of Fe/Al-ratio on the thermal properties and magnetocaloric effect of Gd55FexAl45-x (x=15-35) glassy alloy ribbons
Fang Yuan, Qian Li and Baolong Shen
Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China
Re (Gd, Dy, Ho and Er )-Tm (Mn, Co, Ni)-Al metallic glasses have been investigated thoroughly due to their large refrigeration capacity (RC), high thermal stability, sufficiently soft magnetic properties and outstanding mechanical properties. However, the curie temperature (Tc) are mostly below 100 K. Fe-based glassy alloys were reported to have a relatively high Tc but a low maximum entropy change (-ΔSm) and RC values. Accordingly, t he potential applications of these mentioned metallic glasses are limited greatly. In our study, the effect of Fe/Al-ratio on the thermal properties, magnetocaloric effect and RC of Gd55FexAl45-x (x=15-35) glassy alloy ribbons have been investigated. With Fe/Al-ratio increasing, while -ΔSm keeps a relatively high value, the temperature at the -ΔSm (TΔsm), the Tc, as well as the full width at half maximum of the -ΔSm (δTFWHMT) increase gradually. The maximum RC of 868 J kg-1 was obtained for x=20, which makes the Gd55FexAl45-x (x=15-35) glassy alloy ribbons attractive candidates for magnetic refrigeration materials. Furthermore, the glass transition phenomenon tends to disappear with the increasing Fe/Al-ratio. The high working temperature, δTFWHMT and RC values could be attributed to the forming of some structure phases during rapid cooling and the existence of multiple second-order transitions (SOT).
References
[1] L. Liang et al., J. Alloys Compd. 463, 30 (2008). [2] Q. Luo et al., Appl. Phys. Lett. 90, 211903 (2007). [3] V. Franco et al., Appl. Phys. Lett. 88, 132509 (2006). [4] S. Gorsse et al., Appl. Phys. Lett. 92, 122501 (2008). [5] X. G. Zhao et al., J. Appl. Phys. 109, 07A911 (2011).
BU-06. The magnetocaloric effect and critical behavior in amorphous Gd60Co40-xMnx
Zheng Zhigang, Zhong Xichun, Yu Hongya, Liu Zhongwu and Zeng Dechang
School of Materials Science & Engineering, South China University of Technology, Guangzhou, China
The magnetic materials with high magnetocaloric effect have attracted considerable attention for magnetic refrigeration applications. Recently, Gd based soft magnetic amorphous alloys have been proposed as promising candidates for magnetic refrigeration because of the special advantages such as the negligible magnetic hysteresis losses, higher electrical resistivity, tunable Curie temperature Tc, corrosion resistance and mechanical flexibility. Zhong et al. [1] found that amorphous Gd68-xNi32+x have large refrigerant capacity (RC) and maximum RC value is 724 Jkg-1. For Gd-Co amorphous alloys [2], the magnetic entropy changes ΔSM under a magnetic field of 10 kOe are −3.1 Jkg-1K-1 for Gd71Co29 sample. In this work, the effects of Mn on magnetic properties and magnetocaloric of Gd60Co40-xMnx alloys are investigated. Furthermore, to investigate the nature of the ferromagnetic to paramagnetic phase transition, it performed a critical exponent study. The amorphous ribbons Gd60Co40-xMnx (x=0, 5, 10, 20) were obtained by arc-melting and melt-spinning method. Changing Mn content, Curie temperatures can be tuned from 191 to 201 K. The ΔSMax of Gd60Co40-xMnx under a magnetic field of 50 kOe are −7.7, −7.1, −6.2 and −5.4 Jkg-1K-1 for x=0, 5, 10 and 20 samples respectively. As for magnetic phase transition, due to substituting Mn, the value of critical exponent β firstly decreases and then increases, while value of critical exponent γ increases. Physically, β describes how the ordered moment grows below TC while γ describes the divergence of the magnetic susceptibility at TC. The smaller the value of β, the faster is the growth of the ordered moment. The β value decreases for the x=5 sample, compared with x=0 sample, reflecting a faster growth of the ordered moment with decreasing temperature. For all samples, the critical exponents are close to the 3D Ising model. This Ising-like behavior suggests that there are greater magnetocrystalline anisotropy in samples, which can be explained by the microstrain produced by different atomic radius between Mn (1.366Å)and Co (1.253Å).
References
[1] X. C Zhong., P. F. Tang, Z. W. Liu, D. C. Zeng, Z. G. Zheng, H. Y Yu. J. Alloy. Compd. 2011, 509: 6889-6892. [2] C. L. Zhang, D. H. Wang, Z. D. Han, H. C. Xuan, B. X. Gu, Y. W. Du. J. Appl. Phys. 2009, 105: 13912-13915.
BU-07. Withdrawn
BU-08. Large Cryogenic Magnetocaloric Effect in Superparamagnetic DyCuAl Nanoparticles at 1 T Magnetic Field
Xianguo Liu1, 2, Qiang Zhang2, Jingjing Jiang2, Dianyu Geng2, Zhidong Zhang2 and Siu Wing Or1
1Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China; 2Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
The conventional core/shell-structured superparamagnetic nanoparticles have larger magnetic entropy change because of their larger effective magnetic moments in single magnetic particles than the magnetic moment of the constituent atoms. However, the shell materials with low thermal conductivity decrease their application potential in a refrigeration process. In order to overcome this problem, DyCuAl nanoparticles, which are stable in air without any shell protection and show superparamagnetism between 3 K and the Curie temperature 24 K, are prepared by the modified arc-discharge technique. The phase transition from antiferromagnetic order to ferromagnetic order in the temperature range of 3-24 K as found in bulk DyCuAl disappears in the DyCuAl nanoparticles. The observation is ascribed to uncompensated surface spins induced by the size effect. The DyCuAl nanoparticles have a large magnetic entropy change (-ΔSm) of 3.8 Jkg-1K-1 at 3 K in a magnetic field change from 0 to 1 T.
BU-09. Isothermal Entropy Changes in Nanocomposite Co:Ni67.7Cu32.3
Steven A. Michalski, Ralph Skomski, Xingzhong Li, Damien Le Roy, Tathagata Mukherjee, Christian Binek and David J. Sellmyer
Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska-Licoln, Lincoln, NE
The magnetic field dependence of the isothermal entropy change of granular Co:Ni67.7Cu32.3 nanocomposites is investigated. The motivation of this research is to discover new materials for magnetic cooling which are free of rare-earth elements. This work focuses on two-phase nanocomposite materials where nanoclusters (Co) are embedded in a matrix (Ni67.7Cu32.3). The nanostructuring of the Co clusters lowers the quasi-equilibrium Curie temperature due to finite-size effects, which we model in terms of a continuum Ornstein-Zernike approach. Ni-Cu alloy was chosen for the matrix since it is readily available and cost effective. Also the Curie temperature can be tuned by varying Ni concentration. Bulk Ni70Cu30 has a Curie temperature of Tc = 300 K, which decreases with decreasing Ni concentration at rate of about 11 K per atomic% Ni [1]. Several Co:Ni67.7Cu32.3 composite films were produced by cluster deposition where the volume fraction of the cobalt clusters was varied from 11-25 percent. The avera ge Co cluster size and size distribution depends on the deposition conditions and can be tuned by varying the deposition conditions. Our experiments have focused on small Co clusters having average particle sizes of 2.0 nm in a matrix with composition Ni67.7Cu32.3. Isothermal magnetization curves were measured at various temperatures 140K < T < 340 in steps of 10 K, as well as hysteresis loops and FC and ZFC magnetization curves. The samples were relatively magnetically soft with coercivities of order 0.1 T at 20 K. The entropy changes ΔS were calculated using the Maxwell relation. The entropy changes were, -ΔS = 0.1 J/kg K in a field change of 3 T and 0.30 J/kg K in field change of 7 T at 220 K. — This research is supported by NRI, NSF MRSEC, and NCMN.
References
[1] S. A. Ahern, M. J. C. Martin and W. Sucksmith, Proc. Roy. Soc. London, A 248, 145 (1958).
BU-10. The magnetic and magnetocaloric properties of NdFe12-xMox compounds
Yuanhua Xia, Honglin Du, Jianhui Xu and Jinbo Yang
School of Physics, Peking University, Beijing, China
Compared with the conventional vapour-cycle refrigeration, magnetic refrigeration especially near room temperature which is based on the magnetocaloric effect (MCE) has been extensively studied in the due to its energy-efficient and environment-friendly features [1]. Because of the good permanent magnetic properties, the NdFe12-xMox compounds and their nitrides were studied in detail [2,3]. However, some intrinsic properties of the NdFe12-xMox are not yet clear. So, in this paper the crystal structure and magnetic properties (particularly magnetocaloric property) of NdFe12-xMox compounds have been investigated. The structure of all the compounds is ThMn12 type. The magnetization and Curie temperature obviously decrease with the increase of Mo content, due to the magnetic dilution and exchange interaction reduction effects. For NdFe9.5Mo2.5, the maximum values of -ΔSM reaches 2.38 J /kg K at 290 K for field change of 5 T, which is twice as the value of YFe9.5Mo2.5 around room temperature [3]. Although the maximum values of -ΔSM is not as large as Gd5Si2Ge2 and MnFeP0.45As0.55 [4,5], the very large temperature range of magnetic refrigeration makes the compound owns considerable magnetic refrigeration capacity (225 J/kg). In addition, the low material costs and easy fabrication make it a promising candidate material for a commercial magnetic refrigerator.
References
1. K.A. Gschneidner, Jr., V. K. Pecharsky, and A. O. Tsokol, Rep. Prog. Phys. 68, 1479 (2005). 2. Y.Z. Wang, B.P Hu, X.L. Rao, G.C. Liu, L. Yin, and W.Y. Lai, J. Appl. Phys. 73, 6251 (1993). 3. Z.H. Wang, D.Y. Geng, J. Li, W. Liu, and Z.D. hang, J. Magn. Magn. Mater. 322, 3000 (2010). 4. V. K. Pecharsky and K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1997). 5. O. Tegus, E. Brück, K. H. J. Buschow, and F. R. de Boer, Nature (London) 415, 150 (2002)
BU-11. Magnetocaloric effect in SmCo2-xFex alloys
Luis A. Burrola, Cristina Grijalva, Carlos Santillán and José A. Matutes
Centro de Investigación en Materiales Avanzados, Chihuahua, Mexico
SmCo2 and SmFe2 are magnetic cubic Laves phases with Curie temperatures of 220 and 669 K respectively. In this work Co was partially replaced by Fe aiming to increase the Curie temperature for potential application to room temperature magnetic refrigeration. The samples with composition x=0 and 0.2 were prepared by arc melting method in a high purity argon atmosphere. The effect of iron content on structural and magnetocaloric properties was studied by X ray diffraction, scanning electron microscopy and vibrating sample magnetometry. The volume cell of the 1:2 compound, determined by the Rietveld method, increased as Fe replaced Co. For the SmCo2 compound (x=0) a maximum magnetic entropy change of 3.61 J/kgK at 175 K was determined for an applied magnetic field of 15 kOe. This magnetic entropy change occurred due a spin reorientation transition from [011] to [111] direction and is greater than the magnetic entropy change observed at its Curie temperature (220 K). The Curie temperature for composition SmCo1.8Fe0.2 increased to 300K and a lower magnetic entropy change of 0.60 J/kgK for an applied magnetic field of 50 kOe was determined, while the spin reorientation transition at 175 K was barely detected. For the same composition a homogenization heat treatment was carried out at 773 K during 50 hours obtaining a maximum magnetic entropy change of 0.77 J/kg K at the Curie temperature of 300 K.
References
Kapusta Cz., Oliveira I.S., Riedi P.C., 1998. A nuclear magnetic resonance study of SmCo2. Journal of magnetism and magnetic materials. 177, 1121-1122. Dunhui W., Shaolong T., Songling H., 2003. The origin of the large magnetocaloric effect in RCo2 (R=Er, Ho and Dy). Journal of alloys and compounds. 360, 11-13. Niraj Singh K., Suresh K.G., Nigam A.K., 2007. Itinerant electron metamagnetism ans magnetocaloric effect in RCo2- based laves phase compounds. Journal of magnetism and magnetic materials. 317, 68-79. von Ranke P.J., de Oliveira N.A., de Sousa V.S.R., 2007. The influence of the spin reorientation process on the magnetocaloric effect: application to PrAl2. Journal of magnetism and magnetic materials. 313, 176-181. van Diepen A.M., de Wijn H. W., Buschow K.H.J., 1973. Temperature dependence of the Cystal-Field-Induced Anisotropy in SmFe2. Physical Review B. 8, 1125-1129.
BU-12. Large refrigerant capacity of RGa (R=Tb and Dy) compounds
Xin-Qi Zheng1, Jing Chen1, Jun Shen2, Zhi-Yi Xu1, Jian-Feng Wu2, Feng-Xia Hu1, Ji-Rong Sun1 and Bao-Gen Shen1
1State key laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing, China; 2Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China
Magnetic refrigeration based on the magnetocaloric effect (MCE) is expected to be a promising alternative technology to the conventional gas compression refrigeration because of its higher energy efficiency and friendly environment. To improve the application of this cooling technology, many efforts have been made to explore advanced magnetic refrigerant materials that possess not only large value of magnetic entropy change (ΔSM) but also considerable refrigerant capacity (RC). Recently, the magnetic and magnetocaloric properties of RGa (R=Gd, Ho and Er) have been studied in detail. In this paper, we report on the magnetic properties, MCEs and RC in RGa (R=Tb and Dy) and a large reversible RC is observed in TbGa compound. The XRD patterns confirm that the prepared RGa (R=Tb and Dy) compounds are of single phase, crystallizing in the orthorhombic CrB-type structure (space group Cmcm). Magnetization measurements were carried out as functions of temperature and magnetic field by using a superconducting quantum interference device magnetometer. The TbGa compound exhibits two successive magn etic transitions: spin-reorientation transition at TSR=27 K and reversible second-order ferromagnetic (FM)-paramagnetic (PM) transition at Curie temperature TC=154 K, while the DyGa compound only undergoes a FM-PM transition with a TC=113 K. The ΔSM of RGa (R=Tb and Dy) was calculated from magnetization isotherms by using the Maxwell relation . The maximal values of ΔSM for TbGa and DyGa compounds are 7.1 and 7.2 J/kg K for a field change of 0−5 T, respectively. It is noteworthy that a broad distribution of the ΔSM peak is observed. The values of RC for TbGa and DyGa compounds are found to be 703 and 356 J/kg for a field change of 5 T, respectively. The RC value of TbGa is much larger than those of many magnetic refrigerant materials with similar transition temperature. The large RC of TbGa originates from the combined contribution from SR and FM-PM transitions, which enlarge the temperature span of large MCE. The reversible ΔSM as well as large RC with negligible hysteresis loss in RGa (R=Tb and Dy) are very useful for applying it to the magnetic refrigeration.
BU-13. Magnetic phase transition, magnetocaloric effect and magnetotransport in Tb3Co
Bing Li, W. J. Ren, Yuqin Zhang, Zhenhua Wang, Ji Li, Jianlin Yang and Zhidong Zhang
Shenyang national laboratory for materials science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
The magnetization both in dc and in ac modes, as well as resistivity have been investigated as functions of magnetic fields and temperature in Tb3Co compound with successive magnetic phase transitions from ferromagnetic (FM) to antiferromagnetic (AF), and to paramagnetic state, at transition temperature Tt and TN, respectively. The temperature dependences of dc magnetization under zero-field-cooled and field-cooled conditions show the frozen state at lower temperature due to large magnetic anisotropy, and two well-defined phase transitions in agree with the ac counterpart. There is no distinguishable dynamics around the FM to AF phase transition since the frequency dependence of ac magnetic susceptibility doesn’t display any systematic change. Increasing magnetic fields drives Tt to higher temperature while TN to lower temperature, and thus rendering an overlapping tendency of two transitions. It exhibits two noticeable peaks at the temperature dependence of magnetic entropy change, associated with the two phase transitions above, at relative low magnetic-fields. As a rare case, a normal magnetocaloric effect (MCE) with a value of -4.5 J kg-1K-1 under magnetic-field changes of 20 kOe has been obtained with magnetic-field-induced AF to FM transition, although such transition usually leads to an inverse MCE, as reported in variety of materials. A negative magnetoresistance larger than -10% has been observed around TN at magnetic-field change of 20 kOe. No remarkable magnetoresistance was observed at Tt, differing from that giant magnetoresistance is often found in most materials with this kind of transition. The relationship among magnetic phase transitions, MCE and magnetoresistance has been discussed.
BU-14. Anomalous magnetic ground state in PrSi evidenced by the magnetocaloric effect
Jasper L. Snyman and Andre M. Strydom
Physics Department, University of Johannesburg, Johannesburg, South Africa
Previous investigations into the physical properties of PrSi have shown this compound to order ferromagnetically at 54 K [1,2]. However, the magnetic ground state of this compound has not yet been determined unambiguously. P rSi crystallises in the orthorhombic FeB-type structure. Typically the crystalline electric field (CEF) would uplift the degeneracy of the J=4 Pr3+ free-ion ground state multiplet, yielding nine (normally non-magnetic) singlets. It is known that magnetic order in such systems may be established via the admixture of two singlets into a doublet state, analogous to TmNi which crystallizes in the same structure [3]. From symmetry considerations collinear ferromagnetically ordered moments should lie parallel to the crystallographic b-axis. However neutron diffraction experiments have shown that the easy magnetic axis lies in the ac-plane. This is the first suggestion that the ground state in PrSi is not determined by the CEF alone. Here we investigate the ground state properties of PrSi by analyzing the specific heat. A Schottky contribution associated with the thermal population of CEF-split energy levels is absent from the 4f-electron contribution to the specific heat and the magnetic configurational entropy points to a full nine fold degenerate J=4 multiplet ground state. The strongest evidence for such a ground state is found when the magnetocaloric effect (MCE) in this system is considered. Furthermore, the MCE indicates the presence of a significant higher order exchange term in the magnetic Hamiltonian.
References
[1] Nguyen et al., Solid State Communications 23 (1977) 821 [2] Pinguet et al., Journal of Alloys and Compounds 348 (2003) 1 [3] Gignoux et. al. Physica Status Solidi (a) 14 (1972) 483
BU-15. Neutron diffraction and magnetization studies on the effect of Cr disorder in Cr1-xTe
Efrain E. Rodriguez1, Virgil Provenzano2, Olivier Gourdon3 and Robert Shull2
1NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; 2Metallurgy, National Institute of Standards and Technology, Gaithersburg, MD; 3Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN
Transition metal pnictides, in particular manganese-based ones, have been widely studied for their magneto-caloric properties. Here, we present studies on an alternative transition metal intermetallic system Cr1-xTe where x ≈ 0.13. These materials vary their Tc based on the the ratio of Cr to Te, and here we show that the nature of order of the Cr site vacancies also has an effect on the magnetic properties. Depending on the preparation conditions, we can prepare phases where the Cr site vacancies are completely disordered leading to a NiAs-type crystal structure or ordering of the sites leading to a Cr7Te8 phase adopting a monoclinic structure. Tc's are between 320 K to 350 K and each phase has a drop in magnetization below 100 K, which is explained as canting of the moments from powder neutron diffraction. The magnetic entropy change is also calculated for the two phases and we present the magnetic ordering found from the neutron diffraction measurements at different temperatures. Based on these results we also present possible alloying of the Cr site to boost the magnetic entropy change.
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Ming Yue, Beijing University of Technology
BV-01. Precise Measurement of Magnetization Characteristics in High Pulsed Field
Yasushi Nakahata1, 2, Bartosz E . Borkowski1, 2, Hiroyasu Shimoji1, 2, Koji Yamada3, 1, Takashi Todaka2 and Masato Enokizono2
1Regional Technological Collaboration Promotion Bureau, Oita Prefectural Organization for Industry Creation, Oita, Japan; 2Oita University, Oita, Japan; 3Saitama University, Saitama, Japan
Permanent magnets, especially Nd-Fe-B magnets are very important engineering elements that are widely used in many applications such as information, communication, acoustic and medical equipment. Detailed design of electrical and electronics equipment using permanent magnets, requires precise measurement of magnetization characteristics. High pulsed magnetic fields can be used to measure the magnetization characteristics of Nd-Fe-B magnets in easy and hard magnetization axes up to the high magnetic field Errors influencing the measurements stems from relations between tested material, pick-up sensor configuration, and excitation coil. In this paper we present the analysis of these relations and their influence on accuracy of measurements of material magnetic properties.
BV-02. The study of permanent magnet vibration-to-electric generation for human vibration
Zhihua Wang1, Bowen Wang1, 2, Na Zhang1, Li Wang1, Qing Li1 and Weili Yan1
1Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Hebei University of Technology, Tianjin, China; 2International Center for Materials Physics, Academia Sinic, Shenyang, China
The application of human portable wireless communication devices and health monitoring sensors, etc. is growing rapidly. However, the power system which based on batteries seriously hampers the service duration and function expansion of these electric devices. Permanent magnet vibration-to-electric technology, which can be used to convert vibration energy into electric energy, is meaningful to overcome the power supply restriction of potable electric devices. Furthermore, the previous mathematical models of permanent magnet vibration-to-electric generation are all on the basis of ideal sinusoidal vibration [1-3]. A new mathematical model of the permanent magnet vibration-to-electric generation is built under the condition of actual human vibration. On the basis of the model, the relative displacement and acceleration between the stator and oscillator are calculated. The electromechanical coupling model, which can guide the design of the permanent magnet vibration-to-electric generator, is further derived from output electromotive force model. A permanent magnet vibration-to-electric generator is designed and fabricated with the NdFeB oscillator on the basis of the human vibration. The prototype was placed in different parts of human body and tested on the condition of walking and running. When the prototype is placed on the wrist, the testing results show that the maximum output power, peak-to-peak value and virtual value of output voltage are up to 9.25 mW, 7.20 V and 1.49 V, respectively. The experimental result and the mathematical model can describe the output characters of permanent magnet vibration-to-electric generator. And this study will improve the application of the permanent magnet vibration-to-electric generator for human portable electric devices.
References
[1] C.R. Saha, T. O’Donnell, N. Wang, P. McCloskey, “Electromagnetic generator for harvesting energy from human motion,” Sensors and Actuators A, vol.147, pp. 248-253, Sep. 2008. [2] Dibin Zhu, Stephen Roberts, Michael J.,Tudor, Stephen P, “Beeby.Design and experimental characterization of a tunable vibration-based electromagnetic micro-generator,” Sensors and Actuators A, vol.158, pp.284-293, Mar. 2010. [3] Zhihua Wang, Bowen Wang, Minwei Wang, Huijuan Zhang, Weiping Huang, “Model and Experimental Study of Permanent Magnet Vibration-to-Electrical Power Generator,” IEEE Trans. on Appl. Supercond., Vol. 20, 1110-1113, Jun. 2010.
BV-03. Characteristic analysis and comparison of axial flux machines according to magnetization pattern for 500W-class wind power generator application
Seok-Myeong Jang, Yu-Seop Park, Kyoung-Jin Ko and Jang-Young Choi
Electrical Engineering, Chungnam National University, Daejoen, Republic of Korea
For the characteristic analysis of electrical machins, the finite element method (FEM) is widely applied. However, differntly from other types, axial flux machines must employ 3D FEM with very long analysis time owing to their structural features. Therefore, this paper applied analytical method realizing high reliablity as 3D FEM. Furthermore, this paper compares Halbach and vertical magnetization types [1]-[2], and the analysis models according to the magnetization patterns are shown in Fig. 1(a) and (b). Here, Fig.1(c) shows the flux density by one pole of vertical type PM in air-gap, and for accurate characteristic analysis, it should be dealt with. Fig.1(d) shows the contour lines to comfirm its consideration. In particular, this paper will introduce the compensation factor for the consideration. The analysis results by both analytical method and 3D FEM are presented in Fig.2(a). Based on the obtained flux density, this paper calculate back-EMF, and for the comparison for the magnetization patterns, Fig.2(b) presents its Fast Fourier Tranform (FFT) results. In addition, this paper determined the vertical type to manufacture in that it is well known that the 3rd harmonic component does not highly affect on the electromagnetic torque ripple, and the model is now being constructed as shown in Fig.2(c) to be presented in later full paper with experimental results.
References
[1] Changliang Xia, Hongfeng Li and Tingna Shi, “3-D Magnetic Field and Torque Analysis of a Novel Halbach Array Permanent-Magnet Spherical Motor”, IEEE Trans. on Magn., vol.44, no.8, pp.2016-2020, 2008. [2] Miroslav Markovic and Yves Perriard, “Optimization Design of a Segmented Halbach Permanent Magnet Motor Using an Analytical Model”, IEEE Trans. on Magn., vol.45, no.7, pp.2955-2960, 2009.
BV-04. Wide Aperture Permanent Magnet Solenoid
Brad W. Hoff1, Christina H. Chen2, John C. Horwath3, Michael D. Haworth1, Peter J. Mardahl1 and Susan L. Heidger1
1Directed Energy Directorate, Air Force Research Laboratory, Kirtland AFB, NM; 2GE Global Research, Niskayuna, NY; 3Propulsion Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH
Various configurations of rare earth permanent magnet solenoids with a large bore were simulated using finite element computer modeling. Each configuration’s effect on the axial field Bz and its uniformity ΔBz was analyzed. The magnetic orientation of individual segments and the aspect-ratio of the solenoid have significant effect on BZ and ΔBz. Two-layered configurations assembled with trapezoidal or arc segments result in a more uniform Bz compared to the configuration assembled with five-faced polyhedrons. The two-layered configuration also has the advantage of easier fabrication. A permanent magnet solenoid prototype, based on the two-layered arc segment configuration and built using Nd-Fe-B magnets, was experimentally tested. The solenoid was designed to have a length to inner diameter ratio of approximately 5:3 and peak axial field magnitude of 1.8 kGauss. Experimental measurements agree with simulation predictions to within < 5%.
BV-05. Improvement of the corrosion resistance of the Nd-Fe-B sintered magnets by Cu nanoparticles doping
Chao Sun, Weiqiang Liu, Hao Sun, Ming Yue, Dongtao Zhang and Jiuxing Zhang
Beijing University of Technology, Beijing, China
In this paper, the corrosion and oxidation resistance of Nd-Fe-B permanent magnets with a small amount of Cu nano-particles doping were studied. Cu nanoparticles were prepared by inert gas condensation method. The doping amount of the Cu particles was set as 0, 0.2, 0.5, and 0.8 wt %. The Nd-Fe-B powders and the Cu nanoparticles were mixed homogeneously and pressed together with rubber isotropic pressing (RIP). Nd-Fe-B permanent magnets with a small amount of Cu nanoparticles doping were prepared by conventional sintered method. Effect of Cu content on corrosion resistance and oxidation properties of the magnets were studied. The autoclave and salt atmosphere test results show that the weight losses of the magnets decrease with the Cu content increases, which indicates better corrosion resistance. The magnet with 0.8wt % Cu doping possesses optimal corrosion resistance that corrosion rate decreases by 47.1% in salt atmosphere condition and 97.2% in autoclave condition compared to the magnet without Cu doping, respectively. The oxidation tests show that the weight gains of the magnets decrease with the Cu content increases, which indicates better oxidation resistance. The magnet with 0.8wt % Cu doping possesses optimal oxidation resistance that the oxidation rate decreases by 50.5% in high temperature conditions compared to the magnet without Cu doping. The magnetic properties of magnets with and without Cu doping were measured. After oxidation, the maximum energy product losses of the magnets without and with 0.2 wt% Cu are 6.13% and 0.99%, respectively. It is, therefore, Cu nanoparticles doping is a promising way to enhance the corrosion and oxidation resistance of sintered Nd-Fe-B magnet.
BV-06. Effect of hydriding degree on the microstructure and magnetic properties of NdFeB magnets
Shuai Guo1, 2, Youhao Liu1, 2, Bicheng Chen1, 2, Changjiang Yan1, 2, Don Lee1, 2 and Aru Yan1, 2
1Key Laboratory of Magnetic materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chiense Academy of Sciences, Ningbo, China; 2Zhejiang province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chiense Academy of Sciences, Ningbo, China
HD technology is widely used to produce rare-earth permanent magnet.1,2 The degree of crushing depends on the absorption content of hydrogen, and affects the magnetic properties of final magnet. In this paper, the coarse powders with different hydriding degree were obtained by changing the parameters in the hydrogen absorption process. Then, the final magnets were prepared by powder metallurgy route. The magnetic properties of the sintered magnets are significantly related to the hydriding degree of HD powders, as shown in fig. 1. The remanence increases monotonously and the coercivity decreases sharply with the increasing of hydriding degree. DSC analysis (fig.2) indicates that the Nd-rich phase hydro genates firstly, and the main phase hydrogenates with carrying out of HD process. Insertion of hydrogen atom leads to a significant increase of the unitcell volume. And the crush mechanism depends on the internal stress resulting from differences in the expansion rates of the two phases. Besides, the microstructure and the flowability of magnetic powder can greatly affect the microstructure and magnetic properties of final magnets.
References
1 P. J. McGuiness and I. R. Harris, Journal of Applied Physics 64, 5308 (1988). 2 I. R. Harris and P. J. McGuiness, Journal of the Less Common Metals 172-174, 1273 (1991).
BV-07. Corrosion protection of Nd-Fe-B sintered magnets by surface phosphate treatment
Liang Qiao, Jingwu Zheng, Wei Cai, Shenglei Che and Liqiang Jiang
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, China
In this work, a direct medium-temperature phosphate treatment of the magnet surface was made to improve the corrosion resistance of sintered NdFeB magnets. The morphology of the phosphate film was checked by using scanning electron microscopy (SEM), while the composition of the crystalline intergranular and the host lattice phases were investigated by EDS. The corrosion resistance of the phosphated magnet was studied in a 3% NaCl solution by measuring polarization curves and electrochemical impedance spectroscopy (EIS). Meanwhile, the forming mechanism of the phosphate film was investigated. The corrosion resistance of the magnet shows a significant enhancement, due to that the phosphate film covers on the magnet surface and fills pores. The phosphate film is different in the crystalline intergranular phase and the host lattice phase. The crystalline intergranular phase is thicker and includes more NdPO4 than the host lattice phase.
BV-08. Rotor Losses Minimization Techniques using Combination of Double Different Layer Sleeve of High Speed PM Machines based on Electromagnetic Field Theory
Seok-Myeong Jang1, Jihun Ahn1, Kyoung-Jin Ko1, Sung-Ho Lee2 and Yong-Bok Lee3
1Dept. of Electrical Engineering, Chungnam National university, Daejun, Republic of Korea; 2Korea Institute of Industrial Technology Gwangju Research Center, Gwangju, Republic of Korea; 3Korea Institute of Science and Technology, Seoul, Republic of Korea
Recently more attention is paid to minimizing the eddy current losses for higher efficiency of permanent magnet synchronous machine (PMSM) and many researchers have been continuously developed. But, eddy current loss may cause significant heating of the PM with the copper loss and core loss, due to relatively poor heat dissipation from the rotor, result in partial irreversible demagnetization, and decrease the motor efficiency [1]. For these reasons, this paper deals with rotor loss minimization techniques using the combination of different materials for the sleeve based on resistance and electromagnetic field theory. Resistance of sleeve and PM is in inverse proportion to conductivity, and resistance is changed by area of conductor, so it can find rapidly minimum eddy current loss point of rotor loss according as materials and thickness. In this paper, it shows the change pattern of eddy current losses are predicted by difference materials of sleeve, and it compared trend that calculated by resistance and electromagnetic field theory, and eddy current losses are compared with 2-d transient finite element (FE) method . Case 1 shows the eddy current loss when using one sleeve, and case2 and case3 shows the eddy current loss when using double sleeve that low conductivity and high conductivity, respectively. Fig. 1 (a) shows the distribution of eddy current loss that minimum point and maximum point, and (b) shows the eddy current loss change according to radius, and (c) shows the FEM results. The more detailed analysis results, discussion and mathematical expressions will be given in final paper.
References
F. Zhou, J. Shen, W. Fei and R. Lin, “Study of Retaining Sleeve and Conductive Shield and Their Influence on Rotor Loss in High-Speed PM BLDC Motors”, IEEE Transaction on Mgnetics, vol.42, No.10, October 2006
BV-09. Design of Permanent Magnet Eddy Current Brakes for an Electromagnetic Launch System
Shigui Zhou1, 2, Haitao Yu1, Minqiang Hu1 and Lei Huang1
1Research Center for Motion Control of MOE, School of electrical Engineering, Southeast University, Nanjing, China; 2School of Electrical Information and Automation, Qufu Normal University, Rizhao, China
Recently, the development of eddy current brakes in high speed applications is increasing popular due to the adhesion limit. The permanent magnet (PM) eddy current brake can be obtained a high efficiency and reliability. In this paper, a variable pole pitch PM eddy current brake is proposed for an electromagnetic launch system. As shown in Fig. 1 is the topology of the PM eddy current brake, of which the pole pitches are different (160mm, 140mm and 120mm respectively) for better effect of braking. When the mover is traveling through the air gap, by Faraday’s law, there is an induced eddy current in the conduct sheet. As a result, these eddy currents generate a velocity-dependent braking force. The simulated curves by FEA are plotted in Fig. 2a. As can be seen, the peak of the braking force is generated at different speed when the pole pitch is varied. The measured waveform of the mover speed in different position of rails is shown in Fig. 2b, which concluded that the PM eddy current brake can carry out the braking effectively, at an initial speed of 20m/s, within 2m. A variable pole pitch PM eddy current brake is proposed. Moreover, the calculations and the test are both verified that the design is correct and effective.
References
[1] S. Jang and S. Lee, “Comparison of three types of permanent magnet linear eddy-current brakes according to magnetization pattern,” IEEE Trans. on Magn., vol.39, no.5, pp. 3004-3006, 2003. [2] M. T. Thompson, “Practical issues in the use of NdFeB permanent magnets in maglev, motors, bearings, and eddy current brakes,” Proceedings of the IEEE, vol.97, no.11, pp. 1758-1767, 2009.
BV-10. Electromagnetic-thermal-mechanical coupled analysis of dual mechanical port machine for wind power application
Xikai Sun, Ming Cheng, Sa Zhu and Jianzhong Zhang
School of Electrical Engineering, Southeast University, Nanjing, China
Recent years, permanent magnet (PM) dual mechanical port (DMP) machine [1] is widely researched for its prospect in wind power application and hybrid electrical vehicles as a continuous variable gear-box. Because the wounded inner rotor of the DMP machine is surrounded by the stator and outer-rotor, thermal dissipation in the inner rotor is difficult. Hence, thermal analysis for the DMP machine is essential. Also because of the existence of vulnerable cuppe d PM outer-rotor, mechanical deflection, electromagnetic as well as thermal performance influence mutually and mechanical analysis should be done to guarantee the mechanical strength and deflection under different working conditions. In this paper, electromagnetic-thermal-mechanical coupled finite element analysis is modled for DMP machine as shown in Fig.1. The detailed procedure is as follows [2]. Step 1, initial electromagnetic model deduced by analytical method is testified by mechanical analysis to guarantee the electromagnetic and thermal design within mechanical safety zone. Step 2, calculate the electromagnetic performance with the calculated mechanical deflection and initial temperature. Steps 3, electromagnetic parameters are output to the thermal model, and step 4, the calculated temperature is input to the electromagnetic model. Step 5, calculate the mechanical deflection with detailed electromagnetic and thermal parameters. Finally, with the coupled analysis, a 10kW prototype DMP machine is designed,fabricated and tested. The results are given to validate the theoretical analysis and the effectiveness of the coupled design.
References
[1] Longya Xu, “Dual-mechanical-port electric machines-concept and application of a new electric machine to hybrid electrical vehicles,” IEEE Industry Application Magazine, Vol. 15, No. 4, pp. 44-51, July/August 2009. [2] Arnaud Monnier, B. Froidurot, C. Jarrige, and et.al., “A mechanical, electrical, thermal coupled-field simulation of a sphere-plane electrical contact,” IEEE Transaction on Components and Packaging, Vol. 30, No. 4, pp. 787-795, Dec. 2007.
BV-11. Research on a Permanent Magnet Tubular Linear Generator
Haitao Yu, Bang Yuan, Minqiang Hu, Lei Huang and Shigui Zhou
Research Center for Motion Control of MOE, Southeast University, Nanjing, China
Permanent magnet linear generators (PLMG) have directly been implemented to the drive of wave motion [1]. In the paper, a novel permanent magnet tubular linear generator with Halbach array is proposed. Assistant teeth are introduced to reduce the end effects and the oscillatory detent force. The structure of a permanent magnet tubular linear generator (PMTLG) with Halbach array is shown in Figure 1. For 2-D axisymmetric electromagnetic problems for a PMTLG, the governing equation can be derived from Maxwell Equations as follows, ▽(1/μ)(▽A)=Js (1) where A is magnetic vector potential, and µ is the magnetic permeability. Js=(Hc×n) , is equivalent current density for a PMTLG. Hcis the coercivity of permanent magnet. An integral equation at a local element can be obtained by using Galekin method into (1), ∫S(1/μ)▽Wi●▽AdS-∮l(1/μ)Wi●▽Adl= ∫SJsdS (2) where Wi is weighing function equal to a corresponding interpolation function Ni. Virtual work method is presented for computing the detent force on x direction. The detent forces for 7-slot and 13-slot linear generators with and without assistant teeth are computed and shown in Figure 2a. The corresponding experimental result is lower than the simulated result by 4.6%, which is shown in Fig. 2b. A PMTLG has been proposed, and the calculations by FEM and the test verify that the design and the analysis are correct and effective.
References
[1] T.W. Thorpe, M.J. Picken, “Wave energy devices and the marine environment”, IEE Proceedings-A., vol. 140, no. 1, pp. 63-70, 1993. [2] Faiz J, Ebrahimi-Salari M, Shahgholian G., “Reduction of cogging force in linear permanent-magnet generators”, IEEE Transactions on Magnetics, vol.46, no.1, pp135-140, 2010.
BV-12. Magnet Eddy Current Loss Anal ysis of Interior Permanent Magnet Synchronous Motor for railway vehicles
Chan-Bae Park1, 2, Hyung-Woo Lee2, Byung-Song Lee2 and Ju Lee3
1Electrical Engineering Department, Hanyang University, Seoul, Republic of Korea; 2Korea Raillroad Research Institute, Uiwang-si, Gyeonggi-do, Republic of Korea; 3Division of Electrical & Biomedical Engineering, Hanyang University, Seoul, Republic of Korea
Interior Permanent Magnet Synchronous Motor (IPMSM), permanent magnets are embedded inside the rotor, has a high-efficiency and wide-variable speed range properties. IPMSM has expanded its scope of application of propulsion motors for railway vehicles as well as compressors, spindle and electric vehicle motors in recent years. In order to apply IPMSM to the propulsion system of the Light Railway Vehicle (LRV), 110 (kW) class IPMSMs with high-power density are designed as a concentrated winding model and a distributed winding model in this study. The concentrated winding model designed in this study is 6 poles/9 slots and the distributed winding model is 6 poles/36 slots. The base speed of each motor is 2400 (rpm) and the maximum speed is 6000 (rpm). In general, the eddy current losses in the permanent magnets of IPMSM are caused by the slot harmonics. The thermal demagnetization of the magnet by the eddy current losses at high rotational speed often becomes one of the major problems in the IPMSM with a concentrated windings especially. A design to reduce eddy current losses in permanent magnets design is important in IPMSM for the railway vehicle propulsion system which requires high-speed operation. Therefore, a method to put some slot on the surface of the permanent magnet is proposed to reduce the eddy current losses in permanent magnet in this study. In general, paths of the eddy currents become short by the slots of the permanent magnet surface and the eddy current losses are reduced because the eddy current occurs along the surface of the permanent magnet. Therefore, authors analyze the variation characteristics of the eddy current losses generated in each permanent magnet of the concentrated winding and the distributed winding model by changing the depth and the number of the slots carved on the surface of the permanent magnets.
References
[1] K. Yamazaki, Y. Fukushima and Y. Seto, “Loss Analysis of Permanent-Magnet Motors With Concentrated Windings-Variation of Magnet Eddy-Current Loss Due to Stator and Rotor Shapes”, IEEE Trans. Ind. Appl. ,vol.45, no.4, pp.1334-1342, Jul./Aug. 2009. [2] K. Yamazaki, and H. Ishigami, “Rotor-Shape Optimization of Interior-Permanent-Magnet Motors to Reduce Harmonic Iron Losses”, IEEE Trans. Ind. Elec. ,vol.57, no.1, pp.61-69, Jan. 2010. [3] S. H. Han, T. M. Jahns, and Z. Q. Zhu, “Analysis of rotor core eddy-current losses in interior permanent magnet synchronous machines,” IEEE Trans. Ind. Appl., vol.46, no.1, pp.196-205, 2010. [4] K. Yamazaki and Y. Seto, “Iron loss analysis of interior permanent magnet synchronous motors-Variation of main loss factors due to driving condition,” IEEE Trans. Ind. Appl., vol.42, no.4, pp.1045-1052, Jul./Aug. 2006.
BV-13. Core-loss reduction on the permanent magnet for a traction motor with concentrated winding
Chanbae Park and Hyung-Woo Lee
High-Speed Railroad System Research Team, Korea Railroad Research Institute, Uiwang, Republic of Korea
This paper presents the optimal permanent magnet shape on the rotor of an interior permanent magnet motor to reduce the core losses. As permanent magnet has conductivity inherentl y, it causes huge amount of eddy current losses by the slot harmonics with concentrated winding. This loss is roughly 100 times larger than that of distributed winding in high speed operation and it cannot be ignored, especially on traction motors. Even though it has serious core losses, as the power density of the motor with concentrated winding is better than the distributed winding and power density is one of the most important concerns in railway traction motors, accordingly core loss reduction on the motor with concentrated winding is crucial. The aim of this paper is to analyze the magnetic property, calculate the performance and find the optimal shape to reduce the core losses by varying the angle of the V-shape permanent magnet and length of the permanent magnet with same area. Each core loss on permanent magnet has been investigated in detail by using FEM(Finite Element Method) instead of EMCNM(Equivalent Magnetic Circuit Network Method) in order to consider saturation and non-linear magnetic property. Simulation-based DOE(Design Of Experiment) is also applied to avoid large number of analyses according to each design parameter and consider expected interactions among parameters. Consequently, the optimal design to reduce the core loss on the permanent magent significantly while maintaining motor performance is proposed by an optimization algorithm using regression equation derived and lastly the core loss reduction on the proposed shape of the permanent magnet is verified by FEM.
References
[1]R. Schiferl and T. A. Lipo, “Core loss in buried magnet permanent magnet synchronous motors,” IEEE Trans. on Energy Conversion, vol. 4, No. 2, pp. 279-284, June 1989. [2]Nobuyuki Matsui and et al., “Design and Control of IPMSM,” Ohmsha Ltd., Japan, 2001. [3]Katsumi Yamazaki, Yu Fukushima and Makoto Sato, “Loss Analysis of Permanent-Magnet Motors With Concentrated Windings—Variation of Magnet Eddy-Current Loss Due to Stator and Rotor Shapes,” IEEE Trans. on industry Applications, vol. 45, No. 4, pp. 1334-1342, July/August 2009. [4]Katsumi Yamazaki, Yuji Kanou and et al., “Reduction of Magnet Eddy-Current Loss in Interior Permanent-Magnet Motors with Concentrated Windings,” IEEE Trans. on industry Applications, vol. 46, No. 6, pp. 2434-2441, Nov./Dec. 2010. [5]KEPCO report, “High efficiency magnetic materials and their applications,” 2011.
BV-14. Field Weakening Capability Investigation of an Axial Flux PMSM with Radially Sliding Permanent Magnets Used for Electric Vehicles
Jing Zhao1, Ping Zheng2, Xiangdong Liu1 and Chengde Tong2
1School of Automation, Beijing Institute of Technology, Beijing, China; 2Department of Electrical Engineering, Harbin Institute of Technology, Harbin, China
Due to the advantage of high power density compared to the conventional radial flux machines, the axial flux permanent-magnet synchronous machines (PMSMs) are very suitable candidates for the power train of electric vehicles (EV). However, most of the axial flux PMSMs are with surface permanent magnets and big air gap, which makes the field weakening difficult to execute for wide range operation by using traditional electrical methods. To satisfy the speed requirement for the drive motor of the EV, this paper investigates a new mechanical method of adopting radially sliding permanent magnets (PMs) to fulfill field-weakening control and to improve the operating speed range. The field-weakening structure and principle of the axial machine with radially sliding PMs are proposed and analyzed. The PMs slide radially beyond the base speed under the action of centrifugal force, electromagnetic force, mechanical force, friction force, and so on. When the speed increases or decreases, the forces on PMs are discussed by analytical method and calculated by 3D finite-element method (FEM), and the motion law of PMs versus the speed are investigated. The regulation laws of electromagnetic performances by radially slided PMs is researched. The influence of the PMs’ position on the flux density distribution is evaluated with 3D FEM, which verifies the magnetic field adjustment of this method. The inductance and flux linkage versus the radially slide permanent magnets, which directly affects the field-weakening capability, is analyzed based on magnetic circuit and validated by FEM. To meet the typical operating demand of EV, the control strategy is investigated for this kind of field weakening operation. The traditional electrical method and this mechanical one are compared and combined. Due to the optimized combination of the two methods, the field-weakening capability of the machine is much improved and the maximum speed can achieve five times of the base speed with constant power, which is very satisfying for EV drive application. The design method of this kind of axial flux PMSM is put forward at last.
References
[1] Ping Zheng, Jing Zhao, Qian Wu, Weiguang Fan, Lin Shen, Lina Li, Ranran Liu, “Evaluation of the Magnetic Coupling Degree and Performance of an Axial-Axial Flux Compound-Structure PMSM Used for HEVs,” Journal of Applied Physics, 2008, vol. 103, no.7, pp. (07F113-1)-(07F113-3). [2] Luca Del Ferraro, Federico Caricchi and Fabio Giulii Capponi, “Analysis and Comparison of a Speed-Dependant and a Torque-Dependant Mechanical Device for Wide Constant Power Speed Range in AFPM Starter/Alternators”, IEEE Transactions on Power Electronics, 2006, vol. 21, no.3, pp. 720-729.
BV-15. Improved Analytical models for Predicting the Electromagnetic Field Distribution in Brushless Permanent-magnet Machines
Wu Yuanyuan, Deng Zhiquan, Wang Xiaolin, Ling Xing and Ma Xiaohan
Jiangsu Key Laboratory of New Energy Generation and Power Conversion, Nanjing, China
Past two decades, the development of electromagnetic-field analysis technology of permanent magnet BLDC motors can be summarized into three stages. In the first stage [1-2, 6, 8], the eddy and velocity effect were ignored. Laplace or Poisson equations were solved to get magnetic-field solutions. However, it was governed by static magnetic-field theory so that the parasitic electric-field distribution couldn’t be derived. In the second stage [3-5, 7], the eddy effect was considered but velocity effect. Based on time-varying electromagnetic-field theory, the eddy equation was solved to obtain the electromagnetic-field distribution. However, the ignored current density induced by velocity effect could also cause loss [10]. In the last stage [9-10], the eddy and velocity effect were both considered. However, Markovic [9] didn’t derive the electric-field solution induced by velocity effect in the permanent magnet. Markovic [10] derived the eddy current density referred to the stationary co-ordinate that should be referred to the rotating co-ordinate, and the mechanical angular velocity didn’t match with the electric angular velocity, it didn’t conform to reality. This paper extends analytical models described in [6-7, 9-10] and proposes two improved models for predicting electromagnetic-field distributions due to the open-circuit and armature-reaction field respectively. It’s a harmonic analysis method, which considers the eddy and velocity effect and applies double Fourier series to solve problems in complex domain. The current density induced by velocity effect in permanent magnet is derived, and the eddy current density is derived referred to the rotating co-ordinate that conforms to reality. The proposed models a re more general that means they are applicable to different motor configurations. Analytically calculated field distributions from both models are compared with finite element predictions for a slotted motor having a radial magnetized rotor and overlapping stator windings.
References
[1] Z. Q. Zhu, D. Howe, E. Bolte, and B. Ackermann, "Instantaneous magnetic field distribution in brushless permanent magnet DC motors. I. Open-circuit field," Magnetics, IEEE Transactions on, vol. 29, pp. 124-135, 1993. [2] Z. Q. Zhu and D. Howe, "Instantaneous magnetic field distribution in brushless permanent magnet DC motors. II. Armature-reaction field," Magnetics, IEEE Transactions on, vol. 29, pp. 136-142, 1993. [3] D. Fang, "Commutation-caused eddy-current losses in permanent-magnet brushless DC motors," Magnetics, IEEE Transactions on, vol. 33, pp. 4310-4318, 1997. [4] D. Fang and T. W. Nehl, "Analytical modeling of eddy-current losses caused by pulse-width-modulation switching in permanent-magnet brushless direct-current motors," Magnetics, IEEE Transactions on, vol. 34, pp. 3728-3736, 1998. [5] D. Fang, "Improved analytical modeling of commutation losses including space harmonic effects in permanent magnet brushless DC motors," presented at Industry Applications Conference, 1998. Thirty-Third IAS Annual Meeting. The 1998 IEEE, 1998. [6] Z. Q. Zhu, D. Howe, and C. C. Chan, "Improved analytical model for predicting the magnetic field distribution in brushless permanent-magnet machines," Magnetics, IEEE Transactions on, vol. 38, pp. 229-238, 2002. [7] Z. Q. Zhu, K. Ng, N. Schofield, and D. Howe, "Improved analytical modelling of rotor eddy current loss in brushless machines equipped with surface-mounted permanent magnets," Electric Power Applications, IEE Proceedings -, vol. 151, pp. 641-650, 2004. [8] D. Ishak, Z. Q. Zhu, and D. Howe, "Eddy-current loss in the rotor magnets of permanent-magnet brushless machines having a fractional number of slots per pole," Magnetics, IEEE Transactions on, vol. 41, pp. 2462-2469, 2005. [9] M. Markovic and Y. Perriard, "An Analytical Determination of Eddy-Current Losses in a Configuration With a Rotating Permanent Magnet," Magnetics, IEEE Transactions on, vol. 43, pp. 3380-3386, 2007. [10] M. Markovic and Y. Perriard, "Analytical Solution for Rotor Eddy-Current Losses in a Slotless Permanent-Magnet Motor: The Case of Current Sheet Excitation," Magnetics, IEEE Transactions on, vol. 44, pp. 386-393, 2008.
1:00 PM - 5:00 PM, Saguaro Ballroom
Co-Chair: Atsufumi Hirohata, Uiversity of York; Co-Chair: Chris Palmstrom, U.California Santa Barbara
BW-01. Spin reorientation in Ni/NiO Core-shell Nanowires
Yen-Chun Huang and Chih-Huang Lai
Materials Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan
Ni/NiO core/shell nanowirse (NWs) with the diameter from 40 to 100nm, composed of a ferromagnetic Ni core and an antiferromagnetic NiO shell, are successfully fabricated via post-heat treatment method within anodic aluminum oxide templates. The as-synthesized Ni NWs show anisotropy with an easy axis along the axial direction due to the demagnetization effect. By annealing Ni NWs in the air, the obtained Ni/NiO NWs showed the enhanced coercivity after annealing due to the exchange coupling. By changing the annealing time of Ni NWs, we can adjust the thickness ratio R (tNi/tNiO) between Ni and NiO. When R=7, the dominated anisotropy is still along the axial direction, but when R is decreased to 1, the dominated anisotropy is changed to the radical direction , as shown in Fig. 1. These results implied that the extra anisotropy was introduced after oxidation, which competed with the shape anisotropy and was related to the coupling strength between the FM core (Ni) and AFM shell (NiO). A perpendicular coupling, resulting in the orthogonal alignment of FM and AFM magnetization, has been also reported in Fe/FeF21 and Fe/MnPd2 bilayer system. The spin-flop state3, which is ascribed to the strong interfacial exchange coupling, was proposed to explain the reorientation of the magnetization in the FM layer. From the TEM analyses, the interface between Ni and NiO was dominated by (200) NiO planes, spin compensated interfaces which might promote the spin-flop state. We also performed the micromagnetic simulations by introducing extra radical anisotropy. With increasing the radical anisotropy, the frustration of the easy axis along the axial direction occurs, suggesting that the change of easy axis may originate from the presence of the spin-flop state.
References
1. T. J. Moran, J. Nogués, D. Lederman, and I. K. Schuller, Appl. Phys. Lett. 72 (1998) 617 2. Q. F. Zhan and K. M. Krishnan, Appl. Phys. Lett. 96 (2010) 112506 3. N. C. Koon, Phys. Rev. Lett. 78 (1996) 4865
BW-02. Effect of the exchange bias angle on the magnetoimpedance response in multilayered (FeNi/IrMn)5 films
Carlos Garcia1, 2, Juan M. Florez2, 3, Patricio Vargas3 and Caroline A. Ross2
1Physics, Bogazici University, Istanbul, Turkey; 2Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA; 3Dpto de Fisica, Universidad Técnica Federico Santa María, Valparaiso, Chile
The magnetoimpedance effect (MI) has been widely used in sensitive magnetic field sensors, but its intrinsic nonlinear characteristics are disadvantageous for sensor applications near zero field. In multilayer systems different methods were performed to establish an asymmetrical MI (AMI) [1], including the use of exchange -bias (EB) [2-4]. The magnitude of the shift produced by the EB in both the hysteresis loop and the GMI response strongly depends on the thickness of the ferromagnetic material [4]. The asymmetrical MI peak positions are shifted to higher magnetic field (MF) as the probe frequency increases, so linear MI behavior can be obtained around zero external field by tuning the frequency [3]. Here, AMI was studied in multilayer strips of exchange-coupled [FeNi(60 nm)/IrMn(35 nm)]x5. To study the angle dependence three stripe samples were prepared with EB induced; parallel, perpendicular and forming an angle of 45o with the direction of the wire. Fig. 1 shows the magnitude of the impedance as a function of magnetic field when the EB is applied at 45o, and the MF makes an angle of 45 and 315o with the direction of the wire. At this 45o the EB magnitude is maximum. The MI peak shifts with the magnitude of the EB. In addition, the EB angle also affects the number of peaks in the MI response. Similar behavior was seen for the other two samples: When the MF is parallel to the direction of the EB an AMI response is observed and the asymmetry increase with angle reaching a maximum when the MF is perpendicular to the EB direction. This shows a way to control the asymmetric behaviour of the MI curve which has practical applications in sensors.
References
[1] J. Torrejon, M. Vazquez, L.V. Panina, Journal of Applied Physics, 105, 033911 (2009) [2] R. B. da Silva, A. D. C. Viegas, V. P. Nascimento, M. A. Corrêa, L. F. Schelp, E. Baggio-Saitovitch, an d R. L. Sommer, Appl. Phys. Lett. 94, 042501 (2009). [3] C. García, J. M. Florez, P. Vargas, and C. A. Ross. Appl. Phys. Lett. 96, 232501 (2010) [4] C. García, J. M. Florez, P. Vargas, and C. A. Ross. Journal of Applied Physics, 109, 07D735 (2011)
BW-03. Underlayer controlled exchange bias in room-temperature deposited Ta/FeMn/NiFe thin films
Fu-Te Yuan1, Jen-Hwa Hsu1, Punnet Sharma3, Chau-Yi Tsai2 and Yi-Hung Lin2
1Physics, National Taiwan University, Taipei, Taiwan; 2Institute of Materials Sciences and Engineering, National Taiwan University, Taipei, Taiwan; 3School of Physics adn materials Science, Thapar University, Patiala, India
Exchange bias (EB), the coupling between a ferromagnetic (FM) and antiferromagnetic (AFM) layers, is a key phenomenon that has been used to design advanced spintronic devices such as spin valve read head and magnetic memory. Many structural factors were reported to affect EB significantly including the thickness of AFM, FM, and underlayer, interface roughness, grain size of AFM layer, etc [1,2]. Contradicting results were reported, leaving it unresolved. In this study, we demonstrate that the EB can be controlled by manipulating the condition of Ta underlayer. Samples of Ta(t nm)/FeMn(20 nm)/Ni80Fe20(5 nm) were deposited by sputtering at room temperature (RT) with an external magnetic field of 1.5 kOe at the substrate holder. Ta underlayer was prepared at different argon pressures from 1 to 10 mtorr and the thickness of Ta underplayer t were varied from 0 to 50 nm. The exchange bias field Heb are found to increase with t and level off at about 200 Oe with t = 30 nm as shown in Fig. 1(a) and (b). The coercivity Hc rises from several Oe to 20-25 Oe with increasing of Heb, suggesting the spin-flop in the FeMn layer during magnetic reversal. Structural analysis indicates that the crystallinity of FeMn layer is sensitive to the Ta underlayer. High-quality Ta layer obtained with t ≥ 30 nm or deposited at low working pressure induces well-grown FeMn layer, stabilizing the AF spin structure and resulting in enhanced Heb. Additionally, reduced surface roughness as shown in Fig. 1(c) can also increase Heb by promoting the exchange coupling across interface [3]. This study highlights the influence of AF microstructure as well as crystallinity on EB and provides an effective way to enhance Heb of RT-prepared EB films.
References
[1] J. Nogues and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999). [2] K. O’Grady, L. E. Fernandez-Outon, and G. Vallejo-Fernandez, J. Magn. Magn. Mater. 322, 883 (2010). [3] M. Pakala, Y. Huai, G. Anderson, and L. Miloslavsky, J. Appl. Phys. 87, 6653 (2000)
BW-04. Exchange bias in sputtered FM/BiFeO3 (FM: Fe, Co, and NiFe) thin films
H. W. Chang1, F. T. Yuan2, C. W. Shih3, W. L. Li1, C. R. Wang1, W. C. Chang3 and S. U. Jen4
1Department of Physics, Tunghai University, Taichung, Taiwan; 2Department of Physics, National Taiwan University, Taipei, Taiwan; 3Department of Physics, National Chung Cheng University, Chia-Yi, Taiwan; 4Institute of Physics, Academia Sinica, Taipei, Taiwan
BiFeO3 (BFO) with outstanding multiferroic properties of ferroelectricity (TC=810 oC) and antife rromagnetism (AFM) (TN=380 oC) has received much attention.[1-3] The switching of electric polarization in BFO alters AFM order, providing a mechanism to control magnetic polarization of a ferromagnete (FM) through exchange bias (EB) [2]. In epitaxial FM/BFO films, EB field (Heb) was reported to be dominated by the configuration and size of ferroelectric domain [3]. However, EB in polycrystalline is rarely reported. In this study, we report EB of sputtered FM/BFO films on Pt/Ti/SiO2/Si(100) substrate. Isotropic BFO single phase was obtained at deposition temperature (Td) of 400-500 oC. A 5-nm-thick Co film is sequentially sputtered on BFO at room temperature (RT). A field cooling process was applied to align the spin of BFO before magnetic measurements. Large Heb at RT of 102 Oe and coercivity (Hc) of 1700 Oe appear for Td = 400 oC as shown in Fig. 1. The increase of Td to 500 oC reduces Heb to 50 Oe but enhances Hc to 4130 Oe. Microstructure change may account for the variation of magnetic properties. In the film prepared at 500oC, increased grain size weakens the pinning of AFM domain wall, decreasing Heb; and roughened surface enhances localized shape anisotropy of FM layer, resulting in large Hc. Effect of BFO thickness and different FM layer were also investigated and discussed in detail.
References
[1]H. Bea et al., J. Phys. : Condens. Matter 20, 434221 (2008). [2]Y. H. Chu et al., Nat. Mater. 7, 478 (2008). [3]Y. H. Chu et al., Nanoletters 8, 2050 (2008).
BW-05. Current-induced switching of exchange bias in nano-scaled magnetic tunnel junctions with SAF pinned layer
Chien-Tu Chao1, Cheng-Yi Kuo1, Lance Horng1, M. Tsunoda2, M. Takahashi2 and Jong-Ching Wu1
1Physics, National Changhua University of Education, Changhua, Taiwan; 2Department of Electronic Engineering, Tohoku University, Sendai, Japan
Current switching in MgO-based magnetic tunnel junction (MTJ) has been widely studied due to high scalability for the future applications. The key for such switching scheme is to reverse the magnetization of the free layer back and forth using bipolar currents, in which a spin-torque transfer takes place in the free layer. More recently, a similar current switching scheme has been proposed but it occurs in the pinned layer due to exchange bias switching [1-2]. Herein, a current-induced switching of exchange bias is presented using bipolar currents, without external magnetic field, in nano-scaled MTJs. The MTJ stack film, consisting of a synthetic antiferromagnetic (SAF) pinned layer of CoFeB/CoFe/Ru/CoFe/IrMn, was patterned into an elliptical shape of 120 nm × 270 nm. The photoresist was employed as etching mask followed by depositing insulator directly. The photoresist can be removed and then the top of MTJ cell was exposed to atmosphere. This self-aligned technique is beneficial to make the top contact. Figure 1 shows a MR curve of MTJ cell that was measured to confirm the high/low resistance. Afterward, to eliminate the thermal effect, a pulsed current with duration of 100 ns was used to switch the orientation of magnetic layer. Before applying pulse current, the MTJ cell was initialized as high state by external field. In figure 2, the resistance is switched from high to low state in the absence of external field either by positive or negative current at 2.95 mA and -2.80 mA, respectively. After each current-induced switching, the MR curve was remeasured and one may clearly see that the MR curve is reversed, as shown in figure 3. In particular , this behavior was independent on the polarity of pulsed current. As a result, it is believed that the exchange bias in SAF pinned layer changed its direction by sufficient pulsed current.
References
[1] Sergei Urazhdin and Nicholas Anthony, “Effect of Polarized Current on the Magnetic State of an Antiferromagnet,” Phys. Rev. Lett. 90, 046602 (2007) [2] Xiao-Li Tang, Huai-Wu Zhang, Hua Su, Yu-Lan Jing, and Zhi-Yong Zhong, “Spin torque-driven switching of exchange bias in a spin valve,” J. Appl. Phys. 106, 073906 (2009)
BW-06. Antiferromagnetic exchange coupling in Fe3O4 / Fe (001) epitaxial films grown by a conventional sputtering technique
Kohei Miura, Hideto Yanagihara, Makoto Myoka and Eiji Kita
Inst. Appl. Phys., U. Tsukuba, Tsukuba, Japan
An interfacial antiferromagnetic coupling (AFC) between Fe3O4 and Fe(001) layers was recently discovered in the bilayers grown by MBE technique and the AFC constant is comparable to that of the Co/Ru/Co trilayer [1]. We show that the AFC in Fe3O4/Fe(001) can be easily realized by a conventional planer magnetron sputtering method. We also found that the detailed growth conditions critically affect the AFC. Samples were prepared with a conventional magnetron sputtering technique [2]. The bilayers of Fe3O4(20 nm)/Fe(4.5 nm) were grown on MgO substrates at different conditions. The epitaxial growth was confirmed with TEM and XRD. When the Fe3O4(001) layers were grown at 300 °C and the Fe layers at room temperature, the MH curves show high saturation fields at around 20 kOe (Fig. 1(a)), while each single layer of the Fe and Fe3O4 film easily saturates at less than 2 kOe. Moreover, the saturation moment is close to the sum of those two ferromagnetic layers, and the remanence is almost zero, indicating that the strong AFC at the interface exists between Fe3O4 and Fe layers. The AFC constant estimated from the shaded area in Fig. 1(a) is approximately -0.7 erg/cm2. When the growth temperature for the top Fe layer was changed, the AFC disappeared with no reduction of the total magnetic moment(Fig. 1(b)), suggesting that the subtle interfacial differences cause significant change in the AFC. The results indicate that the Fe3O4 /Fe films with the strong AFC can be obtained by a conventional sputtering technique of a high productivity by choosing the appropriate growth conditions.
References
[1] H. Yanagihara et al., Appl. Phys. Express 1 (2008).[2] M. Myoka et al. in preparation.
BW-07. Spin transport in lateral spin-valve devices with single-crystalline Heusler compounds
Naoki Hashimoto1, Soichiro Oki1, Shinya Yamada1, Yuya Maeda1, Takashi Kimura2, 3, Masanobu Miyao1, 3 and Kohei Hamaya1, 4
1Department of Electronics, Kyushu University, Fukuoka, Japan; 2INAMORI Frontier Research Center, Kyushu University, Fukuoka, Japan; 3CREST, Japan Science and Technology Agency, Tokyo, Japan; 4PRESTO, Japan Science and Technology Agency, Tokyo, Japan
The generation of a pure spin current using Heusler compounds with high spin polarization has not been demonstrated yet. Here we report on the room-temperature generation and detection of pure spin currents by using Heusler-compound elec trodes, Co2FeSi(CFS) and Fe3Si(FS). Figure 1 shows a scanning electron micrograph of a lateral spin-valve (LSV) device fabricated. Here the CFS or FS film was grown on undoped Si(111) by low-temperature molecular beam epitaxy [1,2]. We confirmed that the interface resistances of CFS/Cu and FS/Cu junctions are less than 0.1 fΩm2. In nonlocal measurements, we clearly observed spin-valve signals depending on the magnetization configuration between injector and detector for both CFS/Cu and FS/Cu LSVs, which are the first experimental detections of pure spin currents generated by Heusler-compound electrodes. The magnitude of the spin signals for the CFS/Cu and FS/Cu LSVs were 2.7 mΩ and 1.2 mΩ, respectively, at room temperature. Note that the spin signal for the CFS/Cu LSV is more than twice as large as that for the FS/Cu LSV though the resistivity of the CFS electrode is less than half of that of the FS one. From the analysis based on the one-dimensional spin diffusion model, the spin polarizations for CFS and FS were found to be 0.7 ± 0.1 and 0.3 ± 0.05, respectively. We attribute the large spin signal for the CFS/Cu LSV to high spin polarization for bulk CFS, as predicted theoretically [3]. This work was partly supported by CREST-JST.
References
[1] K. Hamaya et al., Appl. Phys. Lett. 93, 132117 (2008). [2] S. Yamada et al., Appl. Phys. Lett. 96, 082511 (2010). [3] B. Balke et al., Sci. Technol. Adv. Mater. 9, 014102 (2008).
BW-08. Synthesis of half metallic nanowires in anodized alumina membrane by annealing
Seonggi Min1, Jin-Hee Lim2, Leszek Malkinski1 and John B. Wiley2
1Advanced Materials Research Insitute(AMRI), The University of New Orleans, New Orleans, LA; 2Department of Chemistry, University of New Orleans, New Orleans, LA
This paper reports on the fabrication and magnetic properties of magnetite nanowires in anodized alumina membrane (AAM). Magnetite is considered to be a half-metal with 100% spin polarizations. Therefore, it has a great potential for applications in spintronics. The technology of magnetic Fe3O4 nanowires is far less developed than that of the spherical magnetite nanoparticles which are commonly used in biomedical applications. Initially, Fe nanowires, 60 nm in diameter, were synthesized by electrodeposition within AAM template. They were transformed into Fe3O4 nanowires using a two-step annealing process. First, Fe nanowires were annealed at 500 oC for 2 hours in pure oxygen. This oxidation led to the formation of nanowires with mixed composition of Fe3O4 and Fe2O3. A subsequent 2 hour annealing in pure flowing hydrogen at the temperature of 350 oC reduced the oxygen content and transformed the nanowires to Fe3O4. The crystalline structures of iron and iron oxide nanowires after heat treatments were studied with XRD. TEM images showed that the polycrystalline nanowires were compact and uniform. SQUID magnetometry was used to determine magnetization characteristics of the magnetite nanowires embedded in the alumina membrane. Field cooled (FC) and zero-field cooled temperature dependent magnetization curves for Fe3O4 nanowires indicate the Verwey transition at 130 K. Hysteresis loop measurements revealed strongly anisotropic magnetization characteristics. Room temperature coercivity of Fe3O4 nanowires was 732 Oe for the magnetic field parallel to nanowire axis and 310 Oe for the perpendicular field; the saturation fields were 2910 Oe and 8290 Oe, respectively. The authors acknowledge support through LA Board of Regents grants NSF(2010-15)-RII-UNO and LEQSF(2007-12)-ENH-PKFSI-PRS-04
BW-09. Nucleation of magnetic nano domains in CMR-manganites
Tsukasa Koyama1, Yoshihiko Togawa2, 3, Kazuya Takayanagi2, Masakazu Kobayashi1, Ken Harada1 and Shigeo Mori1, 3
1Department of Materials Science, Osaka Prefecture University, Sakai, Japan; 2Nanoscience and Nanotechnology Research Center, Osaka Prefecture University, Sakai, Japan; 3CREST, Japan Science and Technology Corporation (JST), Tokyo, Japan
Transition-metal oxides present a rich variety of structural, electronic and magnetic properties due to strongly-correlated interaction between various degrees of freedom. La1-xSrxMnO3 (LSMO) is one of interesting materials with strongly-correlated electrons, wherein a complex variety of ground states are generated depending on the Sr doping concentration x. Slight changes of external and/or internal parameters such as temperature, electromagnetic fields, and chemical compositions might significantly alter its ground state. Indeed, with decreasing temperature, the ferromagnetic (FM) metallic phase appears as a ground state at x > 0.15 in LSMO. In the FM metallic phase, the structural phase transition strongly affects the magnetic ground state; narrow magnetic stripe domains are spontaneously formed in the orthrombic structure at low temperature (e.g., below 185 K for x = 0.175), while rather large magnetic domains appear at high temperature [1]. In this respect, the nucleation and growth mechanism of such characteristic magnetic domains, i.e., drastic responses to subtle changes of external parameters are still not clarified in detail, which may give rise to novel types of magnetic ground states and new insights of generating multi-functionality in manganites. In this work, we have microscopically investigated changes of the magnetic states by applying magnetic fields in single crystals of LSMO by using Lorentz microscopy. In the specimen with x = 0.175, the magnetic stripe domains appear at regular intervals of about 200 nm as a magnetic ground state in zero magnetic field at 110 K. Importantly, we have clarified that magnetic domains as large as 100 nm are generated in the magnetic stripe domains in vertical magnetic fields and take a form of the magnetic vortex with tilted magnetic components. To the best of our knowledge, these magnetic domains are new kinds of magnetic ground states (spin textures) in manganites. We will explain detailed responses of magnetic vortices to various experimental parameters of external magnetic fields and chemical compositions and discuss the nucleation and growth mechanism of magnetic vortices and the expected functionality in manganites.
References
1. S. Mori et al., J. Appl. Phys., 107, 09D306, (2010)
BW-10. Spin polarization ratio and exchange bias properties of (111) Fe4N thin films
Xuan Li1, Micheal S. Osofsky2, Kevin L. Jensen2 and Jian-Ping Wang1
1The Center for Micromagnetics and Information Technologies, Department of Electrical and computer engineering, University of Minnesota, Minneapolis, MN; 2Naval Research Laboratory, Washington, DC
Fe4N was simulated with half metal behavior as a good candidate for future CPP GMR device1. We deposited (111) textured Fe4N thin films on silicon wafer with different under layers. Formation temperature of (111) Fe4N was reduced to 150°C. The spin polarization of (111) Fe4N thin films was measured using the point-contact Andreev reflection (PCAR) technique. Spin polarization ratios of (111) Fe4N films were found to be 50% by fitting the measured data based on a modified BTK model, as shown in Fig.1. Measurement results were compared with that of Fe films prepared by the same method. It was found that the spin polarization (P) for (111) textured Fe4N is evidently larger than that of Fe (P=0.44). We also investigated the exchange bias properties of (111) Fe4N films with IrMn antiferromagnetic layer. The insertion of a thin CoFe layer was used to enhance the exchange bias field, as shown in Fig.2.
References
1 Satoshi Kokado, Nobuhisa Fujima, Kikuo Harigaya, Hisashi Shimizu, and Akimasa Sakuma, Phys. Rev. B 73, 172410 (2006).
BW-11. High Temperature Magnetic Behavior of Heusler Alloy Thin Films and Nanowires (Co2XAl, X = Fe or Mn)
Keshab R. Sapkota2, 1, Parshu Gyawali1, Ian L. Pegg2, 1 and John Philip2, 1
1Vitreous State Laboratory, Catholic University of America, Washington, DC; 2Physics Department, Catholic University Of America, Washington, DC
Cobalt based Heusler alloys (Co2XAl, X = Fe or Mn) have been extensively explored for spin-based device application due to their half-metallic nature. We have grown Co2MnAl (CMA) and Co2FeAl (CFA) nanowires and thin films and studied their magnetic behavior. High temperature magnetic studies of these compounds are not well explored in the literature. Both Mn and Fe-based Co2XAl exhibit high temperature ferromagnetic transition. The figure shows the magnetization versus temperature plots of CMA and CFA thin films. CMA shows a Tc around 700 K, on the other hand CFA displays interesting magnetic behavior at higher temperature above 600 K and the Tc appears to be above 1000 K. The magnetic properties of nanowires and thin films vary according to processing conditions. We will present detailed high temperature magnetization studies of both CMA and CFA thin films and nanostructures.
BW-12. Spin dependent transport at Fe3O4/graphene interface
Zhi-Min Liao1, 2, Han-Chun Wu1 and Igor Shvets1
1Physics, Trinity College Dublin, Dublin, Ireland; 2State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing, China
Graphene is a two-dimensional material with promising applications for spintronics [1]. The weak spin-orbit coupling of the carbon atoms makes it a promising candidate for spin information transformation and as an ideal material spin-filter [2]. Recently, many experiments have been carried out focusing on spin-valve devices utilizing ferromagnetic metal as electrodes [3]. However, ferromagnetic metals deposited on top of the graphene/metal interface tend forming clusters [4]. Moreover, the strong chemical interaction between graphene and the top ferromagnetic electrode reduces the stability of the graphene layer. Those and other shortcomings motivate us to search for new ferromagnetic materials as electrodes. As an alternative ferromagnetic material, ferromagnetic oxides are expected to be superior to metals as a spin injection source because of the existence of interfacial barriers between the oxide electrode and carbon material. Fe3O4 is an archetype magnetic material with a high Curie temperature of about 850 K and i s expected to have a fully spin polarized electron band at the Fermi level, making it attractive for spintronic devices [5]. In this work, we fabricated the Fe3O4-graphene-Fe3O4 junctions and systematically studied the spin dependent transport at the Fe3O4/graphene interface. It is found that a barrier with a barrier height of 0.1 eV exists at the Fe3O4/graphene interface and the Fe3O4/graphene interfaces work as a tunnel junction as the electrical behavior of the interface meets the proper tunneling criteria. Firstly, there is a nonmetallic temperature dependence of interface resistance as derived above. Secondly, the device displays nonlinear I-V curves at low bias voltage. Thirdly, the dI/dV vs. V shows a quasi-parabolic relationship. Significantly, spin dependent transport at the interfaces contributes -1.6% MR to the whole device at room temperature. Temperature dependent and bias dependent spin transport at the Fe3O4/graphene interface were also investigated. Our work shows that ferromagnetic oxide electrodes may be valuable for the realization of graphene-based spin devices operated at ambient temperature.
References
[1] K. S. Novoselov, A. K. Geim, S.V. Morozov, D. Jiang, M. I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, and A. A. Firsov, Nature (London) 438, 197 (2005). [2] V. M. Karpan, G. Giovannetti, P. A. Khomyakov, M. Talanana, A. A. Starikov, M. Zwierzycki, J. van den Brink, G. Brocks, and P. J. Kelly, Phys. Rev. Lett. 99, 176602 (2007). [3] S. Cho, Y.-F. Chen, and M. S. Fuhrer, Appl. Phys. Lett. 91, 123105 (2007). [4] J. Lahiri and M. Batzill, Appl. Phys. Lett. 97, 023102 (2010). [5] H. C. Wu, S. K. Arora, O. N. Mryasov, and I. V. Shvets, Appl. Phys. Lett. 92,182502 (2008).
BW-13. Study of Powder Magnetoresistance in Magnetite
Renee J. Sáenz, Carlos R. Santillán and José A. Matutes
Integración y diseño de Materiales Compuestos, Centro de Investigación en Materiales Avanzados,S.C., Chihuahua, Mexico
Powder magnetoresistance is a type of tunnel magnetoresistance with large contact resistance in which point contacts are established between the grains of a half-metallic powder. There are good prospects for manipulating this effect by surface treatment of the powder and especially by alignment of the easy axes of the particles in a magnetic field. Under a magnetic field the powder grains tipically form a mixture of tunnel barriers and ballistic point contacts, and the resistance is determined by the most conductive paths.Spin-dependent Coulomb blockade is observed in the smallest particles [1-3]. Powder magnetoresistance is an extrinsic effect strongly depending on microstructural features like particle shape, average size, particle size distribution and compactation degree, etc. Half-metallic oxides with powder magnetoresistance are candidates for magnetic field sensor and other applications. Magnetite is a half-metallic oxide with potential for powder magnetoresistance applications. In this work magnetite nanoparticles were synthetized by the hydrothermal method starting from ferrous sulfate and hydrogen peroxide used as oxidizing agent. The synthesis was carried out in a reactor at temperatures ranging from 160°C to 180°C. Average crystal sizes ranging from 15 to 120 nm were determined by X-ray diffraction using the Scherrer’s formula and confirmed by transmission electron microscopy. The powders were pressed into rectangular tablets under a pressure of 1.3 GPa and sintered at 300°C during two hours under a flow of Ar/H2. Magnetoresistance values(ΔR/R in percent) of 4.8% and 3.0% for a 2T magnetic field were determined for non-sintered and sintered at 160°C com pacts with crystal sizes of 24 and 28 nm respectively.
References
[1] Powder magnetoresistance, J.M.D. Coey, J. Appl. Phys. 85, 8 (1999)5576. [2] Nanomagnetism and spintronics, Teruya Shinjo, Elsevier 2009. [3] Spintronics: A Challenge for Materials Science and Solid-State Chemistry, Claudia Felser et al, Angew. Chem. Int. Ed. 2007, 46, 668- 699.
BW-14. Band structure calculations of Co2FeGa1-xGex Heusler alloys
Vijay Karthik Sankar1, Oleg Mryasov2, Ananthakrishnan Srinivasan3, S. Faleev2, Yukiko Takahashi4 and Kazuhiro Hono4
1University of Notre Dame, Midwest Institute for Nanoelectronics (MIND), South Bend, IN; 2Materials for Information Center, University of Alabama, Tuscaloosa, AL; 3Department of Physics, Indian Institute of Technology, Guwahati, India; 4Magnetic Materials Center (MMC), Nationsl Institute for Materials Science (NIMS), Tsukuba, Japan
Recently, Takahashi et. al. [1] reported the world’s highest current-perpendicular-to-plane giant magneto-resistance (CPP-GMR) using Co2FeGa0.5Ge0.5 Heusler alloys. Also, from the point contact Andreev reflection (PCAR) technique, the spin polarization of Co2FeGa1-xGex has been found to increase with increasing x from P=0.62±0.02 at x=0.25 to a maximum of P=0.69±0.02 at x=0.5 and gradually decreases to P=0.66±0.02 at x=0.75 [1]. In this work, we have performed band structure calculations within high fidelity GW theory for a detailed understanding on this new high spin polarization system. Our GW calculations predicts that only Co2FeGe end point L21 alloy possess a truly half metallic electronic structure and thus reveals that Co2FeGa1-xGex alloying has benefit of stabilizing right phase and shifting Fermi level closer to the center of minority band gap for 0.5 ≤ x ≤ 0.75 compositions. The ab-initio calculation results are discussed in view of the experimental spin polarization measurements of the alloy.
References
1. Y. K. Takahashi, A. Srinivasan, B. Varaprasad, A. Rajanikanth, N. Hase, T. M. Nakatani, S. Kasai, T. Furubayashi, and K. Hono, Appl. Phys. Lett., 98 152501 (2011).
BW-15. Co2MnZ (Z = Al, Ga) compounds: Structural, electronic, and magnetic properties
Rezki R. Mebsout
Physics Departement, Modelling and Simulation in Materials Science Laboratory (LMSSM), Sidi Bel Abbes, Algeria
The purpose of this Work is to further the understanding of the structural, electronic and magnetic properties of ternary inter-metallic compounds using density functional theory (DFT) based electronic structure calculations will be performed by using the full-potential linear augmented plane wave (FP-LAPW)[1]. Four main problems are addressed. First, a detailed analysis on the ternary Heusler compounds is made. It has long been known that many Heusler compounds (X2Y Z; X and Y transition elements, Z main group element) exhibit interesting half-metallic and ferromagnetic properties. In order to understand these, the dependence of magnetic and electronic properties on the structural parameters, the type of exchange-correlation functional and electron-electron correlation was examined. It was found that almost all Co2Y Z Heusler compounds exhibit half-metallic ferromagnetism. It is also observed that X and Y atoms mainly contribute to the total magnetic moment. The magnitude of the total magnetic moment is determined only indirectly by the nature of Z atoms, and s hows a trend consistent with Slater-Pauling behaviour in several classes of these compounds. We study the electronic structures and magnetic properties of a eries Co2MnZ (Z=Al, Ga) compounds with L21 structure which onsists of four inter-penetrating fcc sub-lattices. Experimentally, we successfully synthesized the Co2MnZ (Z=Al, Ga) compounds and we will use the general gradient approximation method (GGA)[2,3]. The magnetic interaction is quite complex in these alloys then it is shown that calculated lattice constants and spin magnetic moments are in good agreement with experimental values. It is found that the compounds with Z=Al, Ga are half-metallic ferromagnet.
References
[1] Blaha P, Schwarz K and Luitz J 1997 WIEN97 Vienna University of Technology This is an improved and updated Unix version of the copyrighted WIEN code: Blaha P, Schwarz K, Sorantin P and Trickey S B (1990) Comput. Phys. Commun. 59 399 [2] Perdew J P and Wang Y Phys. Rev. B 45 13 244 (1992) [3] Perdew J P, Burke S and Ernwerhof M Phys. Rev. Lett. 77 3865
8:30 AM - 11:30 AM, Grand Canyon 6
Chair: Berry Jonker, Naval Research Lab
8:30 AM
CA-01. Graphene spintronics
Nikolaos Tombros
Physics of Nanodevices, Zernike Institute for Advanced Materials, Groningen, Netherlands
I will give an overview of electron spin injection, spin transport, spin precession and spin manipulation in graphene field effect devices with ferromagnetic contacts. I will show that spins can be transported through a graphene layer with a spin relaxation length of about 1.5 micrometer. By applying a perpendicular magnetic field Hanle spin precession can be studied and information about spin relaxation and the carrier diffusion can be obtained [1]. Typical values for the spin relaxation time in graphene deposited on a silicon oxide substrate are around 200 ps. We have observed a scaling between the spin relaxation times and lengths and the carrier mobility in graphene [2]. This scaling indicates that the spin relaxation mechanism is of extrinsic origin. Improving the electronic quality of graphene should therefore result to a considerable increase of the spin relaxation length. Currently we obtain very high mobilities (>5.0 104cm-2/Vs) for graphene deposited on a boron nitride crystal [3] and for suspended graphene [4]. Spin transport experiments on such high mobility graphene will clarify the origin of spin relaxation in this system.
References
[1] N. Tombros et al., Nature 448, 571 (2007) [2] M. Popinciuc et al., Phys. Rev. B 80, 214427 (2009) [3] P. J. Zomer et al., in preparation [4] N. Tombros et al., Nature Physics, doi:10.1038/nphys2009 (2011)
9:06 AM
CA-02. Single spin readout and control of nanopositioned defects in diamond
Joerg Wrachtrup
1University of Stuttgart, Stuttgart, Germany; 23rd Institute of Physics, Stuttgart University, Stuttgart, Germany
Optical readout allows single spin access in wide band gap materials such as diamond. For defects in diamond it is particularly easy to coherently control single electron or nuclear spins even under ambient conditions. By integrating electronic control structures, for e.g. changing surface potentials in FET, structures an interplay between optical readout and electronic control and potentially readout emerges. In such settings, electric excitation of single defects is feasible. The talk will describe those scenarios and demonstrate results for diamond spins but other spins systems, e.g. rare earth doped materials as well.
9:42 AM
CA-03. Electrical injection and detection of spin accumulation in Si at 500 K with magnetic metal/SiO2 contacts
Connie H. Li
Code 6361, Naval Research Laboratory, Washington, DC
Electrical spin injection into Si (001) from a FM metal through an Al2O3 tunnel barrier has been demonstrated1. However, the utilization of SiO2 as the tunnel barrier can have significant impact on the development of Si based spintronics. Here we demonstrate the electrical injection, detection and precession of spin accumulation in Si, via injection from FM contacts such as Ni0.8Fe0.2 & Co0.9Fe0.1 through a SiO2 tunnel barrier2. The injection of spin-polarized carriers produce a net spin polarization and an imbalance in the spin-dependent electrochemical potential under the contact, which is detected as a voltage at the same contact. The decrease of this voltage with increasing out-of-plane magnetic field due to spin dephasing, i.e., Hanle precession of the electron spin, is observed up to 500 K (Fig.). We observe Hanle precession of electron spin accumulation in Si for a wide range of bias, and demonstrate that the spin lifetime (extracted from the Lorentzian fit to the Hanle data) varies with Si carrier density. These results confirm spin accumulation in the Si transport channel to 500 K rather than trapping in localized interface states, and demonstrate the practical aspect of spin-based semiconductor device technology. Supported by ONR.
References
[1] B. T. Jonker et al., Nature Phys. 3, 542 (2007); S. P. Dash et al., Nature 462, 491 (2009). [2] C. H. Li et al., Appl. Phys. Lett. 95, 172102 (2009).
10:18 AM
CA-04. Spin injection, detection and modulation in (Mn)Ge
Kang Wang, Faxian Xiu and Yi Zhou
Electrical Engineering, UCLA, Los Angeles, CA
The control of collection of spins is critical in accomplishing room-temperature spin field effect transistors (Spin-FETs). In this talk, we will present the growth of high Curie temperature MnxGe1-x quantum dots. We will discuss the use of metal-oxide-semiconductor capacitor with the MnxGe1-x quantum dots as the channel material and by controlling the electrical field at the gate, the ferromagnetism of the quantum dots are shown to be modulated by controlling the hole concentration inside the channel (up to 300 K). These results provide a base for the understanding and realization of high efficiency Ge-based Spin-FETs. To realize functioning spin FETs, we also demonstrate electrical spin injection and detection in bulk Ge up to room temperature by using an epitaxially grown Fe/MgO/n-Ge tunnel junction. High quality MgO not only depins the Ge Fermi level to favor electronic transport, but also leads to an enhanced spin injection efficiency due to the symmetry induced spin filtering property of the single crystalline MgO. 4-terminal non-local spin valve measurement is performed to demonstrate spin accumulation in Ge.
References
Xiu, F. et al. Nat. Mater. 9 (4), 337-344 (2010).The work was in part supported by Western Institute of Nanoelectronics.
10:54 AM
CA-05. Electric field control of Spin waves in BiFeO3: towards magnonic analog and digital logic devices.
Pauline Rovillain1, Rogerio de Sousa2, Yann Gallais1, Alain Sacuto1, Dorothée Colson3, Anne Forget3, Manuel Bibes4, Agnès Barthélémy4 and Maximilien Cazayous1
1Laboratoire Matériaux et Phénomènes Quantiques Université Paris 7, PAris, France; 2Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada; 3Service de Physique de l'Etat Condensé, CEA Saclay, IRAMIS, SPEC (CNRS URA 2464), Gif sur Yvette, France; 4Unité Mixte de Physique CNRS/Thales, Palaiseau, France
Magnonics, a particularly exciting new approach for information processing, relies on the manipulation of spin waves, just as photonics are based on the control of light waves. With wavelengths much shorter than electromagnetic waves, spin waves are well suited for the miniaturization of fast devices operating from gigahertz to terahertz frequencies. In addition, magnonic devices can be combined with spintronics architectures, e.g. through spin transfer. A key goal of magnonics is, just as in spintronics, to write non-volatile spin-based information with minimal or no energy consumption. So far, spin waves have been controlled by external magnetic fields, or through the injection of strong currents. While the latter approach is more local and more energy-efficient, only small variations of spin wave frequency have been achieved. Multiferroic materials having at least two coupled magnetic and ferroelectric orders combine these individual properties, leading to electrical control of magnetic effects and vice-versa. Multiferroic materials are thus potentially interesting as a medium for spin-wave-based information processing. Recently we have shown that in multiferroic BiFeO3 spin wave excitations can be controlled by an electric field at room-temperature, low power and in a non-volatile way [1]. Our experimental results clearly demonstrate that the frequency of spin waves can be varied by more than 30%, which represents a tunability several orders of magnitude larger than previously reported. We find that the ferroelectric polarisation acts as a handle to manage this tuning and demonstrate through Landau theory calculations that it originates from an electric-field-driven linear magnetoelectric coupling mechanism. Our results not only qualify BiFeO3 as a promising medium for magnonic devices but also, and more generally, extend the potential of this multifunctional material to transform information from one state variable into another (e.g., charge, spin, light, etc) in future hybrid computational architectures. We anticipate that they will inspire novel concepts in magnonics and encourage further exploration of coupled phenomena in multiferroics and other oxide-based systems.
References
[1] Electric-field control of spin waves at room temperature in multiferroic BiFeO3, P. Rovillain, R. de Sousa, Y. Gallais, A. Sacuto, M. A. Méasson, D. Colson, A. Forget, M. Bibes, A. Barthélémy, M. Cazayous, Nature Materials 9, 975 (2010).
8:30 AM - 11:30 AM, Grand Canyon 7
Chair: Sybille Flohrer, Siemens
CB-01. Changes in Structural and Magnetic Properties on Crystallization of Fe-rich FeSiBPCu Nano hetero-amorphous Alloys
Akihiro Makino, Masanori Yokoyama, Sung-Chul Kim and Parmanand Sharma
Institute for Materials Research, IMR, Sendai, Japan
We have recently developed new Fe83.3-86SiBPCu0.7-1.0 nanocrystalline alloys exhibiting high magnetic flux density(Bs) ~ 1.8-1.9 T and excellent magnetic softness [1,2]. The most outstanding property of the alloys is the extremely low core loss (W) at high magnetic flux density of ~ 1.5-1.7T, which is an order of magnitude lower or half of the silicon steels that are currently used in transformers and motors. On replacement of core material from silicon steels to our nano-crystalline alloys in transformers and motors, the electrical power loss is estimated to be reduced to 1% from 3-4% of total electric power consumption in Japan, and it is ~ 70 % lower than the core loss occurring in silicon steels. The conventional nano-crystalline alloys contain non-magnetic early transition metals (Zr, Nb et.), resulting in lower Bs. The high Bs of our nanocrystlline alloys basically result from high Fe content exceeding the limit for the formation of a fully amorphous phase, and not-containing any non-magnetic early transition metal elements. The excellent magnetic softness is due to very low magnetostriction and very weak magnetic anisotropy originating from a uniform nanocrystalline structure composed of α-Fe grains (size ~ 10-20nm) which are separated by very thin residual amorphous phase. The uniform nanocrystalline structure forms under optimum annealing conditions because of unusual hetero-amorphous structure present in as-spun ribbons. Hetro-amorphous structure in compositionally optimized P and Cu-added Fe -rich FeSiB alloys is made from a large number of α-Fe clusters of size < ~ 2-3nm embedded in an amorphous matrix. Origin of unusual structure lies into nanoscale phase separation into Fe-metalloids amorphous and P+Cu-enriched phases during melt-spinning. Heterogeneous nucleation of α-Fe is triggered by the P+Cu-enriched phase. This interpretation is supported by entropy of mixing between the constituent atoms, TEM, 3D-atomprobe observations, and the simulations of interfacial energy between α-Fe and Cu3P. The coercivity decreases to 3- 6A/m from 12A/m and the B at 800A/m increases to 1.8-1.9T from 1.5T on nanocrystallization of as-quenched hetero-amorphous structure for Fe85-86Si1-2B8P4Cu1 alloys.
References
[1]A. Makino, T. Kubota, K. Yubuta, A. Inoue, A. Urata, H. Matsumoto, and S. Yoshida, J. Appl. Phys. 109, 07A302 (2011). [2]A. Makino, H. Men, T. Kubota, K. Yubuta, and A. Inoue, Mater. Trans. 50, p. 204- 209 (2009).
8:42 AM
CB-02. Domain structure and magnetization loss in a toroidal core based on an Fe-based amorphous alloy
Daichi Azuma1, 2, Ryusuke Hasegawa2, Shin Saito3 and Migaku Takahashi4
1Hitachi Metals, Ltd., Yasugi-shi, Japan; 2R&D, Metglas, Inc, Conway, SC; 3Electeric engineering, Graduated school of engineering, Tohoku University, Sendai, Japan; 4New industry creation hatchery center, Graduated school of engineering, Tohoku University, Sendai, Japan
There are few reports on domain structure in Fe-based amorphous alloy-based core showing square BH loops. We succeeded in observing domain structure in a toroidal core based on amorphou s Metglas® 2605HB1 alloy utilizing a Kerr-effect microscope designed to image domain patterns on curved surfaces with an area of about 10x14 mm2[1]. We found that the number of domain walls in a torodial core at different demagnetizing frequencies (f=60 - 1 kHz) and excess eddy current loss per cycle are proportional to the square root of the exciting frequency as shown in Fig. 1. Also variation of domain width becomes smaller with increase of demagnetizing frequencies. The frequency dependence obtained is then analyzed in terms of Bertotti’s eddy current loss model [2]. The analysis indicates that the effective number of active magnetic objects, i.e. active domain walls, increase with frequency as predicted, but domain width was not even especially at low frequencies. The results then should help clarify the physical meaning of the parameters such as magnetic objects used in the model.
References
[1]S. Meguro, S. Saito, M. Kobayashi, K. Akahane and M. Takahashi, Digest of 33rd Annual Conference of Magnetic Society of Japan, (2009). [2]G. Bertotti, J. Appl. Phys. 57, 2118 (1985).
8:54 AM
CB-03. Effect of P to B concentration ratio on soft magnetic properties in FeSiBPCu alloys
Fanli Kong, He Men and Baolong Shen
Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, China
Si steels are widely used in generators, motors and transformers for their high saturation magnetic flux density (Bs). However, more and more serious environment and energy issues require development materials with better magnetic softness to decrease wasteful dissipation of energy. Over the past two decades, nanocrystalline alloys of FINEMET and NANOPERM have been developed. Unfortunately, these nanocrystalline alloys contain a large amount of non-magnetic metal elements, which cause Bs of the alloys lower than 1.7 T and costs increased. High Bs is demanded to minimize the physical dimensions of electrical equipment. Recently, nanocrystalline FeSiBCu and FeSiBPCu soft magnetic alloys with high Bs more than 1.8 T have been developed. Although the Bs of these alloys somewhat drops by about 10% compared to that of Si steels, the core loss is much lower than that of Si steels. Moreover, the raw materials of these alloys are low cost and high productivity, so FeSiBCu and FeSiBPCu alloys are promising soft magnetic materials for the future. In order to reduce the costs and further improve the soft magnetic properties of FeSiBCu, P was added to substitute for B. In this work, the influence of P to B concentration ratio on magnetic properties and microstructure of Fe82.65Si2B14-xPxCu1.35(x=1-6) nanocrystalline alloys were investigated. The results showed that the magnetic softness was greatly improved with appropriate P to B concentration ratio. The annealed Fe82.65Si2B9P5Cu1.35 alloy showed the minimum average grain size of 15 nm and most superior soft magnetic properties. The alloy exhibits a high Bs of 1.81 T, a low Hc of 1.1 A/m and low core loss of 0.22 W/kg (1.0 T, 50 Hz), respectively.
References
1 Y. Yoshizawa, S. Oguma, and K. Yamauchi, J. Appl. Phys. 64, 6044 (1988). 2 K. Suzuki, A. Makino, N. Kataoka, A. Inoue, and T. Masumoto, Mater. Trans., JIM 32, 93 (1991). 3 B. Shen, H. Kimura, and A. Inoue, Mater. Trans. 43, 589 (2002). 4 M. Ohta and Y. Yoshizawa, Appl. Phys. Lett. 91 062517 (2007). 5 A. Makino, H. Men, K. Yubuta, and T. Kubota, J. Appl. Phys. 105, 4 (2009). 6 F. Kong, A. Wang, X. Fan, H. Men, B. Shen, G. Xie, A. Makino, and A. Inoue, J. Appl. Phys. 109, 07A303 (2011).
9:06 AM
CB-04. Microstructure and Magnetic Anisotropy in Amorphous and Nanocrystalline Materials
Masato Ohnuma
National Institute for Materials Science, Tsukuba, Japan
Though there are several structural models for the origin of induced magnetic anisotropy, little evidence of structural anisotropy have been reported in amorphous (AM) and nanocrystalline (NC) alloys. In this talk, the observation results of structural anisotropy of AM and NC ribbons annealed under stress are studied using transmission X-ray diffraction (t-XRD). For the NC ribbons, the clear anisotropy in the peak position has been observed in t-XRD. The strain calculated from the anisotropy shows linear dependence to the applied stress, indicating that the elastic strain induced by stress annealing process is the origin of magnetic anisotropy. [1] In contrast, the anisotropy of AM ribbons induced by stress annealing are not straightforward because of AM single phase and a large “plastic” (non-recovered) elongation. Figure 1 shows the 1st halo measured along the direction parallel (solid line) and perpendicular (dotted line) to the applied stress in AM ribbons annealed under small (~30 MPa), σs and large stress(~300 MPa), σL at 350C for 30 sec. Although the ribbon annealed under σs shows little anisotropy of the halo top position, the ribbon annealed under σL shows clear anisotropy. The anisotropy defined as the difference in halo top position becomes small when it is post-annealed up to 360C, indicating that the observed anisotropy is the elastic origins. The relation between magnetic and structure anisotropy is discussed based on the magneto-elastic effects. The mechanism of constrained stress in the amorphous single phase is also discussed in this talk.
References
M.Ohnuma, T.Yanai, K.Hono M.Nakano, H.Fukunaga, Y.Yoshizawa. G.Herzer, J.Appl.Phys. 108, art#093927(2010)
9:42 AM
CB-05. Secondary Crystallization in (Fe65Co35)79.5+xB13Nb4-xSi2Cu1.5 and (Fe65Co35)83B10Nb4Si2Cu1 Nanocomposite Alloys
Samuel J. Kernion1, Vladimir Keylin2, Joe Huth2 and Michael E. McHenry1
1Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA; 2Magnetics Technology Center, Division of Spang & Company, Pittsburgh, PA
High induction, low loss HTX002-type nanocomposite alloys[1,2] with the compositions (Fe65Co35)79.5+xB13Nb4-xSi2Cu1.5 (x=0-4) and (Fe65Co35)83B10Nb4Si2Cu1 were studied by x-ray diffraction and high temperature vibrating sample magnetometry (VSM) to determine nanocomposite stability. Secondary crystallization deteriorates soft magnetic properties [3,4]. Planar flow casting was used to produce amorphous ribbon of each composition. In the (Fe65Co35)79.5+xB13Nb4-xSi2Cu1.5 alloys, the Curie transition, Tc, of a (Fe,Co,Nb)2B (2-1) phase is seen on cooling from 700°C at 670°C. The x=0 alloy also shows an increase in magnetization at 540°C due to th e Tc of the (Fe,Co,Nb)23B6 (23-6) phase. The formation of the 23-6 phase is explained by the enrichment of the residual amorphous matrix in Nb and Co, which act to lower the free energy of the 23-6 phase [5,6]. The magnetization of the as-cast (Fe65Co35)83B10Nb4Si2Cu1 alloy was measured to 700°C, 800°C, and 900°C, with cooling to 300°C between each step (Fig. 1). The Curie transitions of the 2-1 phase and 23-6 phase are seen on cooling from 700°C. After heating to 800°C, the Tc of the 2-1 phase is absent. This indicates that the 23-6 phase is more stable than the 2-1 phase in the (Fe65Co35)83B10Nb4Si2Cu1 alloy due to the high Nb-content. The kinetics of secondary crystallization are important to determine long-term ageing effects on the metastable microstructure at elevated temperatures.
References
[1] K.J. Miller, A. Wise, A. Leary, D.E. Laughlin, M.E. McHenry, V. Keylin and J. Huth, J. Appl. Phys. 107, 09A316 (2010). [2] S. J. Kernion, K. J. Miller, S. Shen, V. Keylin, J. Huth and M. E. McHenry, accepted to appear in IEEE Trans. Mag. (2011). [3] A. Hsiao, Z. Turgut, M. A. Willard, E. Selinger, M. Lee, D. E. Laughlin, M. E. McHenry and R. Hasegawa, MRS Res. Symp. Proc. 577, 551 (1999). [4] M. E. McHenry, F. Johnson, H. Okumura, T. Ohkubo, A. Hsiao, V. R. V. Ramanan and D. E. Laughlin, Scripta Mat. 48, 881 (2003). [5] J. Long, P. R. Ohodnicki, D. E. Laughlin, M. E. McHenry, T. Ohkubo and K. Hono, J. Appl. Phys., 101, 09N114 (2007). [6] P. R. Ohodnicki, N. C. Cates, D. E. Laughlin, M. E. McHenry and M. Widom; Phys. Rev. B 78, 144414 (2008).
9:54 AM
CB-06. In-situ Investigation of Phase Formation in Nanocrystalline (Co97.5Fe2.5)89Zr7B4 Alloy by High Temperature XRD
Samuel J. Kernion1, Paul R. Ohodnicki Jr.2 and Michael E. McHenry1
1Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA; 2Chemistry and Surface Science Division, National Energy Technology Laboratory, Pittsburgh, PA
Crystallization and phase evolution in a (Co97.5Fe2.5)89Zr7B4 amorphous alloy was studied by high temperature x-ray diffraction (HTXRD) and transmission electron microscopy (TEM). Co-based nanocomposite alloys exhibit zero magnetostriction and a strong response to magnetic field annealing, making them interesting for sensor and high frequency inductor applications[1]. This work employs in-situ crystallization studies to confirm previously reported calculations and experimental results on the phase identity and composition of phases observed at various stages of crystallization in Co-rich nanocomposite alloys[1-3]. Amorphous alloys, synthesized by melt-spinning, develop a nanocomposite structure in primary crystallization. After annealing at 540°C for 1 hr, TEM images and diffraction patterns confirm a grain size of 19 nm and the presence of multiple crystalline phases, respectively. HTXRD results show formation of fcc, bcc, and hcp phases at various stages of the crystallization process (Fig. 1) and only the fcc phase remained at temperatures above 600°C. Upon heating, the lattice parameter of the fcc phase increases at a rate higher than expected from thermal expansion effects alone suggesting potential Fe enrichment as the Fe-rich bcc phase is consumed. A model of the Fe:Co ratio of the fcc phase as a function of temperature is proposed, based on the observed lattice parameters.
References
[1] P.R. Ohodnicki, J. Long, D.E. Laughlin, M.E. McHenry, V. Keylin, J. Appl. Phys. 104, 113909 (2008). [2] P.R. Ohodnicki, Y.L. Qin, D.E. Laughlin, M.E. McHenry, M. Kodzuka, T. Ohkubo, Acta Mater. 57, 87 (2009). [3] P.R. Ohodnicki, D.E. Laughlin, M.E. McHenry, M. Widom, Acta Mater. 58, 4804 (2010).
10:06 AM
CB-07. High temperature properties of (Fe81Co19)84Ta9B7 alloy for high frequency applications
Zafer Turgut1, 2, Emily Michel1, 3, John C. Horwath1, Lee Semiatin1 and Matthew S. Lucas1, 4
1Air Force Research Laboratory, Wright Patterson Air Force Base, OH; 2UES Inc., Dayton, OH; 3Wright State University, Dayton, OH; 4UTC Inc., Dayton, OH
FeCo based nanocrystalline materials have excellent soft magnetic properties, but are limited to low frequency applications due to their relatively low electrical resistivity, which result in high eddy current losses [1]. Recently, the alloy (Fe81Co19)84Ta9B7 was identified as having a high resistivity of 140 μΩ-cm after annealing at the crystallization temperature of 552 °C for 45 minutes [2]. In this study, we present results from in-situ x-ray diffraction and resistivity measurements as a function of temperature. The high temperature xrd measurements reveal that crystallization begins at 500 °C, with the crystallite sizes determined from the Scherrer analysis growing from 13±4 nm at 525 C to 31±3 nm at 600 °C. The grainsize of 17±2 nm measured at 550 C is consistent with previously reported values for the samples annealed at this temperature. The resistivity of the amorphous alloy was 161±10 μΩ-cm, which is significantly reduced after heating to 650 °C. The results clearly show that the large resistivity of the (Fe81Co19)84Ta9B7 alloy annealed at 552 °C is due to the smaller grainsize and larger phase fraction of the amorphous phase. These results indicate that the annealing temperature may be used to tune the electrical resistivity and grainsize, which may be used to optimize the high frequency magnetic properties for specific applications.
References
[1] M.E. McHenry, M.A. Willard, D.E. Laughlin, Progress in Materials Science 44 (1999) 291. [2] M. S. Lucas, W. C. Bourne, A. O. Sheets, L. Brunke, M. D. Alexander, J. M. Shank, E. Michel, S. L. Semiatin, J. Horwath, and Z. Turgut, “Nanocrystalline Hf and Ta containing FeCo based alloys for high frequency applications”, Materials Science and Engineering B, doi:10.1016/j.mseb.2011.05.044
10:18 AM
CB-08. Soft magnetic properties of bulk FeCoMoPCBSi glassy core prepared by copper mold casting
Mingxiao Zhang, Fanli Kong, Chuntao Chang and Baolong Shen
Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China
Compared to conventional Si-steels, Fe-based amorphous alloys exhibit superior magnetic properties such as low coercive force (Hc) and core loss, and have been extensively used in various magnetic-electric devices. However, these soft-magnetic alloys have usually been prepared in thin sheet, wire, and film due to the low glass-forming ability (GFA), which strongly limits their application fields. On the other hand, some bulk glassy cores have been synthesize d with Fe-based glassy alloy powders by the consolidation method in the supercooled liquid state, but the sintered cores show relatively low maximum permeability (μmax). Therefore, it is necessary to develop large size bulk glassy cores with good magnetic properties. In this work, Fe-Mo-P-C-B-Si alloy system was selected as candidate because of its high GFA. By optimizing its compositions and substituting Fe with Co, the GFA of this alloy system was further increased. In addition, we have successfully prepared bulk Fe66Co10Mo3.5P10C4B4Si2.5 glassy cores of 10 mm in outer diameter, 6 mm in inner diameter, and 1 mm in thickness by copper mold casting. Magnetic properties, thermal stability and structure of these cores were also investigated. The isothermal treatments were carried out in the range of 648-748 K for 600 s under a vacuum of 10-3 Pa for improving soft-magnetic properties through structural relaxation. The resulting bulk glassy core exhibits good magnetic properties, i.e., high saturation magnetic flux density of 1.23 T, low Hc of 1.0 A/m, high μmax of 450000 after an optimum heat treatment, respectively. The core loss values of this glassy core are 0.4 W/kg, 22 W/kg and 121 W/kg at f = 50 Hz, 400 Hz, 1000 Hz and Bm = 1.0 T, respectively. The bulk Fe66Co10Mo3.5P10C4B4Si2.5 glassy Core should be very useful for inductive applications because of simple production process and excellent magnetic properties.
References
1 S. Yoshida, T. Mizushima, A. Makino, J. Jpn. Inst. Met. 63, 1097 (1999). 2 B.L. Shen, M. Akiba, A. Inoue, Appl. Phys. Lett. 88, 131907 (2006).
10:30 AM
CB-09. Influence of Nb Content on Nanocrystallization and High Temperature Magnetic Properties of FeCo Based High Induction Alloys
Rajat K. Roy1, Sam J. Kernion2, Shen Shen2 and Michael E. McHenry2
1Material Science and Technology Division, National Metallurgical Laboratory, Jamshedpur, India; 2Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
The nanocrystallization behavior of (Fe0.65Co0.35)85-x Si2.8B11.2NbxCu1 (x=0, 1, 2, 3) alloys, and its effect on high temperature magnetic properties are reported. The alloys were preapred by melt spinning technique and followed by annealing under an argon atmosphere to induce nanocrystallization structure. With increasing Nb, crystallization temperatures (Tx1 and Tx2), thermal stability and activation energy for secondary crystallization are increased. It slows down dissolution of α-FeCo crystallites for secondary phase formation [1,2]. The temperature dependence of magnetic moment shows minimum values at three temperatures, Tc1, Tc2 and Tc3, corresponding to formation of α-FeCo nanocrystals from as-quenched amorphous phase and residual amorphous phase, and (FeCo)23B6 phase, respectively (Fig.1). X-Ray diffraction pattern (XRD) and transmission electron microscopy (TEM) confirm α-FeCo nanocrystallites in amorphous matrix for samples annealed at 450οC and secondary phase (FeCo)23B6 along with α-FeCo nanocrystallites for 550οC annealed samples. The TEM images show the generation of 20nm crystallites for 3at% Nb alloy compared to 32nm crystallites for 2 at% Nb alloy, representing the hindrance of grain growth behavior with increasing Nb content.
References
1. A. Hsiao, M. E. McHenry, D. E. Laughlin, M. J. Kramer, C. Ashe, and T. Okubo, IEEE Trans. Mag. 38 (2002) 2946 . 2. J.S. Blázquez, C.F. Conde, A. Conde, Appl. Phys. Lett., 79 (2001) 2898.
10:42 AM
CB-10. Microstructure and magnetic properties of nanostructured FeCo alloys prepared by severe plastic deformation
Narayan Poudyal1, Chuanbing Rong1, Ying Zhang2, Dapeng Wang1, Matthew J. Kramer2 and J. Ping Liu1
1Physics, University of Texas at Arlington, Arlington, TX; 2Materials Science and Engineering, Ames Laboratory, Iowa State University, Ames, IA
Nanocystalline Fe100-xCox ( x = 20, 35, 50, 60) alloys have been prepared by mechanical alloying of the Fe and Co powders via high energy ball milling. The alloy formation process and microstructure evolution of the samples have been investigated by using energy filtered transmission microscopy (EF-TEM) and X-ray diffraction (XRD). EF-TEM observations showed strip formation of the Fe and Co phases at an initial stage of milling (fig.1).The final grain size of the equi-axis grains in the obtained alloys reached 8 nm upon 20 hours milling. The saturation magnetization of the mixtures of Fe and Co increases with milling, indicating the formation of FeCo phase and an increasing compositional homogeneity with milling time. It is found that the saturation magnetization is also dependant on the Co content, which reaches the highest value of 240 emu/g at Fe65Co35. The phase transformation of the FeCo alloys was also studied using DSC and thermomagnetic curves (M(T)).
10:54 AM
CB-11. Nanoheteromicrostructure and soft magnetic properties of Co and Ni substituted FeSiBCuP nanocrystalline alloys
Nicoleta Lupu1, 2, Sorin Corodeanu1, Y. Zhang2, Akihiro Makino2 and Horia Chiriac1
1Magnetic Materials and Devices, National Institute of Research and Development for Technical Physics, Iasi, Romania; 2Institute for Materials Research, Tohoku University, Sendai, Japan
The most reduced power losses for relatively high magnetic induction (Bs) values are characteristics of amorphous and nanocrystalline Fe-based alloy systems [1]. Despite of their lower Bs compared with grain oriented silicon steels [4], Fe-based amorphous and nanocrystalline materials have reduced effective magnetocrystalline anisotropy. The increase of Bs is achieved if Fe is partially substituted with Co and the amount of early transition metal (Nb, Zr or Hf) is reduced [2]. Very recently, it was reported the synthesis of a Nb-free nanocrystalline Fe-based material with high Bs and excellent magnetic softness [3]. In this paper, we are presenting our recent results on the effect of small additions (1-2 at.%) of Co and/or Ni on the microstructure and soft magnetic behaviour of Nb-free Fe83.3÷84.3Si4B8P3÷4Cu0.7 nanocrystalline alloys. The replacement of Fe by small amounts of Co or Ni enhances the nanoheteromicrostructure of as-quenched nanocrystalline melt-spun ribbons, refines the alpha-(Fe,Co) nanograins down to a few nm, and homogenize their distribution into the residual amorphous matrix. The nanocrystallized alloys show high Bs of 1.8-2 T, depending on the amount of Co or Ni, coercive fields below 10 A/m, core losses of 1-3 W/kg a t 50 and 100 Hz, and 5-8 W/kg at frequencies of 300-400 Hz, comparable with the ones of Fe-3.5wt.%Si [4]. The relative magnetic permeability reaches values up to 4x104 below 100 Hz and decreases slightly to 3x104 at 300-400 Hz, the largest values being obtained after annealing in the temperature range 425-5000C. The torroids made of wounded melt-spun ribbons exhibit lower magnetic permeabilities and lower core losses compared with the initial ribbons, as a consequence of the induced stresses. The magnetic characteristics are strongly dependent on the microstructure features, too, their relationship being studied in detail.
References
[1] M. Ohta and Y. Yoshizawa, J. Phys. D: Appl. Phys. 44 (2011), 064004. [2] K.J. Miller, A. Leary, S.J. Kernion, A. Wise, D.E. Laughlin, M.E. McHenry, V. Keylin, J. Huth, J. Appl. Phys. 107 (2010), 09A316 [3] A. Makino, H. Men, T. Kubota, K. Yubuta, A. Inoue, J. Appl. Phys. 105 (2009), 07A308. [4] A. Makino, K. Suzuki, A. Inoue and T. Masumoto. Mater. Trans., JIM 32 (1991), 551.
11:06 AM
CB-12. Vector magnetic properties of Fe-based amorphous sheets under alternating flux condition
Shohei Ueno, Takashi Todaka and Masato Enokizono
Faculty of Engineering, Oita University, Oita, Japan
It is well known that the iron loss of amorphous materials is about one-third of that of the grain-oriented electrical steel sheet [1]. A lot of applications of amorphous materials to electrical machinery such as a transformer have so far been examined although the saturation magnetization is smaller than that of the electrical steel sheets and the amorphous materials are fragile and need high manufacturing costs. However the detailed magnetic properties such as vector magnetic properties have not yet been examined fully. In this research, we have measured the iron loss of amorphous sheets by using the two-dimensional vector magnetic property measurement system [2]. Fig. 1 shows the measurement apparatus of the vector magnetic properties. The measurement samples were cut to be a 80 × 80mm sheet and placed at the center of the magnetic measurement apparatus. We can generate arbitrary magnetic flux waveform in the sample sheet by two-directional excitation. Fig. 2 shows the iron loss of the amorphous material depending on the maximum flux density. Because the magnetic properties of amorphous sheets can be improved by annealing in magnetic field applying in one direction, we annealed the sample sheets in magnetic field applying in transverse direction (θB = 90 deg.). As shown in this figure, the iron loss increased with increasing the maximum flux density. In the case of the low induction, the iron loss of θB = 45 deg. was between those of θB = 0 deg. and θB = 90 deg. However this relation was not satisfied when the maximum flux density was over 1.3 T. In the full version of papers, the iron losses as a vector magnetic property will be clarified under various alternating flux conditions.
References
[1] F.E. Luborsky, H. H. Liebermann, J. J. Becker, and J. L. Walter, Rapidly Quenched Metals 3, Vol. 2, p.188, 1978. [2] M. Enokizono, G. Shirakawa, T. Suzuki and J.Siervert, Two-Dimensional Magnetic Properties of Silicon-steel sheet, Journal of the Magnetics Society of Japan, Vol. 15, No. 4, pp.265-269, 1991.
11:18 AM
CB-13. Analysis of heating effects (magnetic hyperthermia) in FeCrSiBCuNb nanocrystalline wires
Cristina Gomez-Polo, Silvia Larumbe, Jose Ignacio Pérez-Landazábal and José Mart ín Pastor
Universidad Publica de Navarra, Pamplona, Spain
Amorphous wires with nominal composition Fe73.5-xCrxSi13.5Cu1B9Nb3 (x = 3, 7 and 10) were prepared by in-rotating-water quenching technique (diameter ≈ 130 μm) [1]. The magnetic properties of the alloys in the as-cast amorphous state (Curie temperature, Tc) depend on the Cr content of the sample, x [2]: a nearly linear decrease in Tc is detected as function of x. The enrichment in Cr atoms of the residual amorphous phase upon crystallization enables the control of the magnetic behavior of the alloys in the nanocrystalline state [3]. Minimum permeability values and maximum coercitive fields are associated to the magnetic decoupling of ferromagnetic grains at temperatures above the Curie temperature of the residual amorphous phase, Tca [3]. The heating effects on the wires was analyzed under the application of a ac magnetic field, Hac (amplitude and frequency, 50 Oe and 320 kHz, respectively), employing a home-made hyperthermia set-up. A single piece of wire (mass, m = 2 mg) was immersed in a glass capillary filled with water and subjected to the ac magnetic field. The temperature increase (T) was registered as a function of time, t, and the effective Specific Absorption Rate (SAR) was estimated through the initial slope dT/dt for the as-cast amorphous and nanocrystalline states. Moreover, an induction method was employed to determine the complex magnetic susceptibility, χ. Maximum SAR values are obtained in those samples displaying high susceptibility values (i.e. amorphous or nanocrystalline states with high Tca). In these cases, the eddy-current contribution dominates the heating effects in the wires. However, minimum SAR values are obtained for the amorphous wire with Tc ≈ 300 K (x = 10) in spite of the relatively large values of χ. This result should be interpreted as a consequence of the sharp decrease χ with a slight increase in temperature (Hopkinson effect).
References
[1] C. Gómez-Polo, Y. Li, J.I. Pérez-Landazábal, V. Recarte and M. Vázquez, Sensors and Actuators A 1-3 (2003) 230. [2] C. Gómez-Polo, J.I. Pérez-Landazábal, V. Recarte, IEEE Trans. Magn. 39 (2003) 3019. [3] C. Gómez-Polo, J.I. Pérez-Landazábal, V. Recarte, M. Vázquez and A. Hernando, Phys. Rev. B 70 (2004) 094412.
8:30 AM - 11:30 AM, Grand Canyon 8
Chair: Dimitri Houssammedine, Everspin Technologies
8:30 AM
CC-01. Elucidation of transport behavior in bcc-FeCo/MgO(001)/FeCo magnetic tunnel junction by spin-resolved photoemission
Stéphane Andrieu1, Frédéric Bonell1, Thomas Hauet1, Francois Montaigne1, Francois Bertran2, Parick Lefevre2, Amina Taleb2 and Lionel Calmels3
1Nancy University / CNRS, Insitut Jean Lamour, Vandoeuvre, France; 2CASSIOPEE, SOLEIL synchrotron, Saclay, France; 3CEMES, Toulouse, France
Spin polarized transport in magnetic tunnel junctions (MTJ) using an MgO barrier is explained by considering the particular electronic structure along the BCC(001) structure and the symmetry filtering of the MgO barrier. Huge TMR were thus predicted using FeCo electrodes [1]. FeCox / MgO / FeCox (001) layers were thus grown and patterned to examine their transport properties. We actually observed that the TMR is increasing up to 530% (20K) for 25% of Co but surprisingly, the TMR is decreas ing above 30% of Co to reach 220% at 70% of Co. A similar behavior is observed in FeCoB/MgO MTJ [2]. To understand the tunneling in these MTJs, it is necessary to obtain information both spin and symmetry resolved [3]. The transport experiments contain such information, particularly on the conductances as a function of voltage, like in tunneling spectroscopy, but so intricate. The CASSIOPEE beamline offers however remarkable opportunities for this type of study: (i) spin resolution through the MOTT detector, (ii) opportunity to work at very low energy (from 10eV), where the Fe and Co cross sections are large, (iii) low angular aperture of the detector, allowing only looking at Δ symmetry, (iv) use of the light polarization to distinguish between Δ1 et Δ5 state symmetry, due to symmetry reasons [4]. Since photoemission is a surface sensitive technique, Fe1-xCox(001) epitaxial layers were prepared in a MBE coupled to the beamline. The films were grown on Fe(001) buffer layers for the whole x range 0-100% The x stoichiometry was accurately controlled by using RHEED oscillations. The PES experiments were performed on both free and MgO covered FeCo surfaces. SR-PES allows us to show that a new state appears near the Fermi level for x>50% and still exists when FeCo is covered by MgO. ARPES experiments allows us to show that this new state is an interface state with a Δ1 symmetry but surprisingly not spin polarized, whereas ab initio calculations predict a fully minority spin polarized surface state. The half metallic behavior of the Δ1 symmetry specific to the (001) BCC structure is thus broken, explaining the TMR decrease. The special conductance curves are thus completely explained by the occurrence of this interfacial state.
References
[1] X. G. Zhang and W. H. Butler, PRB 70, 172407 (2004) [2] S. Ikeda & al, APL 90, 212507 (2007) [3] F. Bonell et al, Phys. Rev. B, 82, 092405 (2010) [4] L. N. Tong & al, PRB 77, 064421 (2008)
9:06 AM
CC-02. Enhancement of perpendicular magnetic anisotropy in FeB free layers using a thin MgO cap layer
Hitoshi Kubota1, Shota Ishibashi2, Takeshi Saruya1, Takayuki Nozaki1, Akio Fukushima1, Kay Yakushiji1, Koji Ando1, Yoshishige Suzuki2, 1 and Shinji Yuasa1
1National Institute of Advance Industrial Science and Technology (AIST), Tsukuba, Japan; 2Graduate School of Engineering Science, Osaka University, Toyonaka, Japan
We reported the perpendicular magnetic anisotropy (PMA) at the MgO barrier/Co-Fe-B free layer interface in MgO-based magnetic tunnel junctions (MTJs).1 The spin-torque switching current in the MTJs decreased with increasing Fe concentration in Co-Fe-B free layers. Since the interface PMA in the MTJs was relatively weak, the Fe-B free layer was magnetized in in-plane. Recently S. Ikeda et al. fabricated perpendicularly magnetized MTJs using a large PMA at MgO/CoFeB interface.2 To enhance the PMA further we investigated the effect of a thin MgO cap layer on top of the FeB free layer. We prepared PtMn/CoFe/Ru/CoFeB/MgO barrier (1 nm)/FeB free (2 nm)/MgO cap (dcap)/Ta multilayer and microfabricated small MTJs. dcap was varied between 0 and about 2.5 nm. Magnetization curves and magnetoresistive properties were measured after annealing (330C). When dcap=0, the magnetization remained in in-plane because the PMA at MgO barrier/FeB free was relatively weak. With increasing dcap, the plane films’magnetizations became easily saturated along perpendicular fields. The saturation filed (Hs) decreased rema rkably from 4 (dcap=0) to 0.4 kOe (dcap=2.5 nm), indicating the PMA at the FeB free/MgO cap interface was greatly enhanced. Actually in the microfabricated MTJs with dcap =2~2.5nm, the FeB free layer was perpendicularly magnetized under a zero field. From a viewpoint of the parasitic resistance, thin dcap is required; otherwise MR ratios would be decreased greatly. Fortunately we have found that around dcap=1.5 nm the PMA at FeB free/MgO cap interface is enhanced while the parasitic resistance is kept very low. Moreover, the MR ratio at dcap=1.5 nm (~ 90%) was larger than the one at dcap=0 nm (~70%). The improved MR ratio would be due to sharp interface of FeB free/MgO cap than that of FeB free/Ta cap. We will report the effect of the MgO cap on the spin-torque switching properties.
References
1S. Yakata, H. Kubota, Y. Suzuki, K. Yakushiji, A. Fukushima, S. Yuasa, and K. Ando, J. Appl. Phys. 105, 07D131 (2009). 2S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno, Nat. Mater. 9, 721 (2010).
9:18 AM
CC-03. Co-tunneling, Kondo Effect and Impurity-Caused Spin-Flips in CoPt Discontinuous Magnetic Tunnel Junctions
David Ciudad1, 2, Zhen C. Wen3, Aidan T. Hindmarch1, Ezana Negusse4, Dario A. Arena4, Xiu F. Han3 and Christopher H. Marrows1
1School of Physics and Astronomy, University of Leeds, Leeds, United Kingdom; 2Francis Bitter Magnet Lab, Massachusetts Institute of Technology (MIT), Cambridge, MA; 3State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Science, Beijing, China; 4National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY
Both Coulomb blockade (CB) and the Kondo effect can arise in quantum dots or nanocluster-based systems, such as double magnetic tunnel junctions (DMTJs). The former effect can give rise to an enhancement of the tunnel magnetoresistance (TMR) when achieving the co-tunneling conduction regime, whereas the Kondo effect can suppress it [1]. Studying the transition between both effects is important to determine the limitations of future single electron spintronic devices on the one hand, and to clarify the physical foundations of the co-existence of ferromagnetism and the Kondo effect on the other. Here, the transition between Kondo and co-tunneling enhancement of the TMR effects in DMTJs has been studied as a function of the size of CoPt magnetic clusters embedded in an AlOx barrier. Previous works based on CoFe clusters found that the transition was a sharp crossover [2]. However, our CoPt-based DMTJs show a gradual competition between co-tunneling enhancement of the TMR and its suppression due to Kondo effect. Furthermore, both effects have been found to coexist even in the same sample, which we are able to tune between by changing the temperature. Interestingly the transition temperature was far below the superparamagnetic blocking temperature of the CoPt. That means: 1) that the transition is not correlated with the suppression of the fluctuations of the magnetic moment in the clusters as previously thought [2]; and 2) that we made direct observation of the coexistence of a ferromagnetic phase and the Kondo effect [3]. When further decreasing the size of the clusters, a second transition between Kondo effect and spin flip processes due to impurities in the barrier [4] was also found. We suggest a model for the TMR behavior in this limit.
Ref erences
[1] K. J. Dempsey, D. Ciudad, and C. H. Marrows, Phil. Trans. Roy. Soc A 369, 3150 (2011). [2] H. Yang, S.-H. Yang, and S. S. P. Parkin, Nano Lett. 8, 340 (2008). [3] H. Yang, S.-H. Yang, G. Ilnicki, J. Martinek, and S. S. P. Parkin, Phys. Rev. B 83, 174437 (2011). [4] R. Jansen and J. S. Moodera, J. Appl. Phys. 83, 6682 (1998).
9:30 AM
CC-04. Spin Related Quantum Well Effect in Fully Epitaxial Cr/ultrathin-Fe/MgO/Fe Magnetic Tunnel Junctions
Peng Sheng, Takayuki Nozaki and Yoshishige Suzuki
Graduate School of Engineering Science, Osaka University, Toyonaka, Japan
Up to now, spin-dependent resonant tunneling effects have been successfully observed in the epitaxial MTJ systems, for example in Cr(001)/ultrathin Fe(001)/insulator/Fe(Co) [1,2,3]. Furthermore, it is predicted that the TMR and spin-transfer torque can be enhanced at certain biases due to resonant tunneling via quantum well states (QWs) [4,5]. In this study, we systemically investigated an influence of annealing temperature for the ultrathin Fe layer formation on its resonant tunneling property. Fully epitaxial MTJs consisting of Cr(001)/ wedge shaped ultrathin-Fe(001) (2-12ML)/MgO(001) (2.0 nm)/Fe(001) were grown on MgO(001) substrates. Here, the Cr(001) buffer layer is employed as a majority spin reflection layer for the ultrathin Fe layer. By measuring the first derivative conductance (dI/dV) using the lock-in technique, clear peaks in the conductance, which signify the existence of QW states in the ultrathin Fe electrode, were observed depending on the Fe thickness. We also found that the amplitude of dI/dV is strongly affected by the annealing temperature. If the sample was annealed at 200°C, the oscillation component is strongly enhanced compared with the sample without annealing. Interestingly, the conductance at around zero bias exhibits clear oscillatory behavior as shown in Fig. 1. This behavior also can be explained by the QW interference effect. We investigate the annealing temperature effect on the quantum resonant position. The effect of MgO thickness, FeO formation upon the ultrathin Fe layer and interface structures will be discussed. Acknowledgements: This work is supported by Global-COE Program of Osaka University.
References
[1] T. Nagahama et al., J. Appl. Phys. 91, 7035 (2002). [2] F. Greullet et al., Phys. Rev. Lett.99, 187202 (2007). [3] T. Niizeki et al., Phys. Rev. Lett. 100, 047207 (2008). [4]. Z. Y. Lu et al., Phys. Rev. Lett. 94, 207210 (2005). [5]. I. Theodonis et al., Phys. Rev. B 76, 224406 (2007).
9:42 AM
CC-05. Ab-initio justification of correlation between perpendicular magnetic anisotropy and Bloch states spin filtering in MgO-based tunnel junctions
Hongxin Yang, Mairbek Chshiev and Bernard Dieny
SPINTEC, UMR CEA/CNRS/UJF-Grenoble 1/Grenoble-INP, INAC, 38054, Grenoble, France
Perpendicular magnetic anisotropy (PMA) at the interface between ferromagnetic (FM) and non-magnetic layers has become of major interest in a view of next generations of spintronic devices with increased density and thermal stability. This interest has been strongly revived by observation of large PMA values reported for Co|MgO [1] and CoFeB|MgO magnetic tunnel junctions (MTJ) [2]. The latter have been extensively studied because of Bloch state symmetry based spin filtering phenomenon leading to high tunnel magnetoresistance (TMR) values [3] making them ideally suitable for implementation in hard dis k read heads or in magnetic random access memories (MRAM). Here we report systematic ab initio studies of PMA at Fe(Co)|MgO MTJs and reveal a strong correlation between underlying mechanisms responsible for PMA and Bloch states symmetry based spin filtering. The total energy calculations including spin-orbit interaction (SOI) have been performed for fully relaxed in shape and volume supercells comprising various numbers of Fe(Co) and MgO layers. Very large PMA values up to 3 erg/cm2 are found in case MTJs with 5 monolayers (ML) of Fe and 3 ML of MgO. The PMA value is slightly increased up to 3.2 erg/cm2 in case of MTJ with 7 ML of Fe and 11 ML of MgO, indicating that the PMA indeed originates from FM|MgO interfaces, in agreement with recent experiment [2,4]. Furthermore, the PMA weakens for Co|MgO compared to Fe|MgO structures in agreement with recent experiments [4]. In order to elucidate the PMA origin, we performed detailed analysis of the impact of SOI on band structure in case of out-of-plane and in-plane orientation of Fe(Co) moments. We found that degeneracy lift appeared mainly around Γ point and show that influence of SOI on Bloch states around the Fermi level along Γ-H direction plays a key role for PMA origin. Spin filtering of the same Bloch states is also responsible for huge TMR leading to correlation between the TMR and PMA. We show that both PMA and TMR weaken in case of over- and underoxidized interfaces for Fe|MgO junction indicating that the PMA correlates with TMR in agreement with experiment [5]. This work has been supported by Grenoble Nanosciences Foundation and ERC Advanced Grant Hymagine.
References
[1] L. E. Nistor, B. Rodmacq, S. Auffret and B. Dieny, Appl. Phys. Lett. 94, 012512 (2009). [2] S. Ikeda et al, Nature Mater. 9, 271 (2010). [3] W. H. Butler, X.-G. Zhang, T. C. Schulthess, J. M. MacLaren, Phys. Rev. B 63, 054416 (2001). [4] M. Yamanouchi et al, J. Appl. Phys. 109, 09C712 (2011) and references therein. [5] L. E. Nistor et al, IEEE Transactions on Magnetics, 46, 1412 (2010).
9:54 AM
CC-06. Tunnel Magnetoresistance in Magnetic Tunnel Junctions with Low Energy Mg1-xZnxO Barriers
Yosuke Kurosaki, Masaki Yamada, Daisuke Sato, Akinori Nishide, Hiroyuki Yamamoto and Jun Hayakawa
Central Research Laboratory, Hitachi Ltd., Tokyo, Japan
Magnetic tunnel junctions (MTJs) with high tunnel magnetoresistance (TMR) ratio and very low resistance are required for read sensors of ultrahigh density hard disk drives. Since Mg1-xZnxO has lower band gap than MgO with a rock salt (RS) structure, Mg1-xZnxO has attracted much attention as a promising candidate for satisfying these requirements [1]. Our previous study clarified that the crystal structures of Mg0.7Zn0.3O barriers are strongly related to the area-resistance (RA) and TMR ratio, which is about 100 % at RA of 3 kΩ μm2 [2]. In the present study, the Zn composition dependence and the effect of crystalline structures of the MTJs on TMR properties were investigated in detail. All films in this study were deposited on thermally oxidized Si substrates by RF magnetron sputtering with a base pressure of 10-7 Pa. The stacking structure of MTJs is as follows: substrate/buffer-layer/Co20Fe60B20(3) /Co50Fe50(t)/Mg1-xZnxO (t’)/ Co50Fe50(t’’)/Co20Fe60B20(2.4)/cap-layer (thickness in nm). The films were annealed at 280 °C in a vacuum of 10-5 Pa under a magnetic field of 6 kOe for 1 hour. Their electrical properties were measured by current in-plane tunneling method. The crystal structures of barriers were evaluated by XRD measurements with Ta buffer and cap: substrate/Ta(5)/ferromagnetic-layer/barrier(15)/Ta(5). In the MTJs with Mg0.9Zn0.1O barriers, we achieved about double TMR ratio of those with Mg0.7Zn0.3O, reaching 150 % at RA of 40 Ω μm2. XRD profiles suggest that this enhancement is due to the improvements of the crystal orientation of tunnel barriers into RS (001). On the other hand, the low TMR ratio of Mg0.9Zn0.1O compared to that of MgO may originate from imperfect crystallization of MTJs into bcc (001)/RS (001)/bcc (001). This crystal structure can be controlled by inserting Co50Fe50 between Co20Fe60B20 and Mg1-xZnxO, which will also affect TMR properties of MTJs with Mg1-xZnxO.
References
[1] I. V. Maznichenko, et al., Phys. Rev. B 80, 144101 (2009). [2] Y. Kurosaki, et al., ICAUMS2010 EA04.
10:06 AM
CC-07. Effect of crystalline structures on perpendicular anisotropy of CoFeB in MgO based magnetic tunnel junction
Takao Ochiai, Young Min Lee, Chikako Yoshida, Koji Tsunoda, Masaki Aoki and Toshihiro Sugii
Ultra-Low Voltage Device Project, Low-power Electronics Association & Project(LEAP), Tsukuba-shi, Japan
In this study, we show that crystalline structure of CoFeB plays an important role for the interfacial perpendicular anisotropy, and consequently affects the Jc. Subs./BEL/Ru (8)/Ta (0 or 0.2)/CoFeB (1-2)/MgO (1)/CoFeB (2)/CoFe (0.5)/Ru (0.75)/CoFe (2.5)/IrMn (8)/TEL (units nm) were deposited on Si/SiO2 by using magnetron sputtering. Jc of MTJs with 0.2 nm Ta (MTJ1) on Ru buffer layer was about one half of MTJs without Ta layer (MTJ2). On the other hand, retention of these two types of MTJs was almost the same (Fig. 1). CoFeB in MTJ1 showed perpendicular anisotropy when the thickness of free layer was below 1 nm, while that in MTJ2 were not indicate perpendicular anisotropy at the same free layer thickness. By XRD measurements, we found that crystalline structure of CoFeB with only 0.2 nm-Ta buffer was amorphous and weak bcc (001) textured, while with Ru buffer was bcc (110) textured after annealing. Therefore we believe that the texture of CoFeB free layer plays an important role on interfacial perpendicular anisotropy and STT property. This work was supported by the Ministry of Economy, Trade and Industry in the program for Ultra-low voltage device project for low carbon society.
References
[1] M. Pakala et Al., J. Appl. Phys., 98, 056107 (2005).
10:18 AM
CC-08. Composition dependence of tunnel magnetoresistance effect using high-perpendicular magnetic anisotropy Mn-Ga ordered alloys
Takahide Kubota1, Masaaki Araidai1, Shigemi Mizukami1, Xianmin Zhang1, Hiroshi Naganuma2, Mikihiko Oogane2, Yasuo Ando2, Masaru Tsukada1 and Terunobu Miyazaki1
1WPI Advanced Institute for Materials Research, Tohoku university, Sendai, Japan; 2Graduate School of Engineering, Tohoku University, Sendai, Japan
A Material with high perpendicular magnetic anisotropy (PMA) is necessary for the developments of giga-bit-class spin transfer torque magnetoresistive random access memories[1]. L10-Mn-Ga[2] and D022-Mn3-δGa[3] alloys have attractive features of high PMA, extremely small magnetic damping[4], and theoretically predicted high spin polarization (especially for D022-Mn3Ga[5]). We have investigated tunnel magnetoresistance (TMR) effect of magnetic tunnel junctions (MTJs) with D022-Mn71Ga29 electrode and observed 22%-TMR ratio at 10 K[6]. It was a problem that the TMR ratio was quite smaller than the value expected from a first principles calculation of TMR ratio for a stoichiometric D022-Mn3Ga-MTJ[6]. The off-stoichiometry is one of the possibilities; therefore we investigated the composition dependence of TMR ratio in MTJs with Mn-Ga alloys electrode to understand deeply the correlations between off-stoichiometry and TMR ratio. We used three kinds of Mn-Ga compositions; that was L10-Mn54Ga46, L10-Mn62Ga38, and D022-Mn71Ga29. The MTJ stacking films were prepared by magnetron sputtering technique with MgO(100) sub./Cr(40)/Mn-Ga(30)/Mg(0.4)/MgO(2.2)/CoFe(2.5)/cap structures (unit: nm). Epitaxial growth and ordered phase of Mn-Ga layers were confirmed by x-ray diffractometer for all. TMR effect was measured at 10 K and 300 K. TMR ratios of 18%, 23%, and 20% were observed at 10 K for the MTJs with Mn54Ga46, Mn62Ga38, and Mn71Ga29 electrodes, respectively. The TMR ratio does not change so much against composition. One possibile reason for the small difference is that spin polarization of transport electrons is insensitive to the off-stoichiometry. Concerning the small magnitude of TMR ratio, other possible factor is structural imperfections at the MnGa/MgO interface due to a large lattice mismatch (~6%). One way to solve this problem is insertion of ultra-thin another ferromagnetic layer with smaller mismatch to the MgO, such as Iron. Calculated band dispersions of Mn-Ga alloys and the experimental results of MTJs with Iron insertion will be also discussed in the presentation. This work was partly supported by NEDO grant for Young Scientists, ASPIMATT program from JST, and KAKENHI from JSPS.
References
[1] H. Yoda, et al., Curr. Appl. Phys. 10, e87 (2010). [2] M. Tanaka, et al., Appl. Phys. Lett. 62, 1565 (1993). [3] F. Wu, et al., Appl. Phys. Lett. 94, 122503 (2009). [4] S. Mizukami, et al., Phys. Rev. Lett. 106, 117201 (2011). [5] B. Balke et al., Appl. Phys. Lett. 90, 152504 (2007). [6] T. Kubota, et al., Appl. Phys. Express 4, 043002 (2011).
10:30 AM
CC-09. Finite tunnel magnetoresistance at the compensation point of Sm1-xGdxAl2, A ferromagnetic electrode with zero magnetization
Karine Dumesnil, Mathieu Da Silva, Catherine Dufour, Michel Hehn, Danielle Pierre, Daniel Lacour, François Montaigne, Gwladys Lengaigne and Sylvie Robert
Institut Jean Lamour, Vandoeuvre les Nancy, France
Since the discovery of spin dependent tunneling across a Ge barrier by Jullière most efforts have been expensed to discover new tunnel barrier materials. Generally, the electrodes were made mainly using conventional magnetic 3d transition metals. The next challenges consist in using new materials as electrodes: epitaxial materials, materials with magnetization perpendicular to the film plane, Heusler compounds with predicted half-metallicity... An intense research activity also aims at developing new mate rials with exciting properties that would permit to get a perfectly stable spin-polarized electrode in a junction device. The ideal compound would be a half-metallic antiferromagnet or ferrimagnet, exhibiting both 100% spin-polarization and very mall or zero magnetization. Classical ferrimagnetic Rare Earth/Transition Metal alloys have been already seed as electrode in magnetic tunnel junctions and give rise to interesting properties [1]. Although these ferrimagnets can give rise to spin-polarized current in their magnetic compensated state, the zero magnetization state results from opposite spin orders. Beyond RE/TM ferrimagnets, interesting and original materials are developed to achieve simultaneously a zero magnetization state and a true long range ferromagnetic order where all spin contributions point in the same direction. We will present results on the successful epitaxial growth of Sm1-xGdxAl2 (SGA) [2], a zero magnetization ferromagnet [3-4], as well as on its use as an electrode in a magnetic tunnel junction [5]. A tunnel magneto-resistance effect has been observed even when the SGA ferromagnetic electrode is in a magnetic compensated state (zero-magnetization) in which spin and orbital contributions cancel each other. This constitutes the proof that SGA can simultaneously exhibit zero-magnetization and significant polarization of conduction electrons, being thus able to give rise to a spin-polarized current in the magnetic compensated state. The temperature dependence of tunnel magneto resistance reveals that this effect is related to the relative orientation of spin contributions to magnetic moment in both electrodes and that the tunnel polarisation of the Sm1-xGdxAl2 electrode is negative.
References
[1] C. Kaiser, A. F. Panchula and S. S. P. Parkin, Phys. Rev. Lett. 95, 047202 (2005). [2] A. Avisou et al. J. of Crystal Growth 297, 239 (2006); [3] H. Adachi, H. Ino, Nature 401, 148 (1999) [4] A. Avisou, C. Dufour, K. Dumesnil, A. Rogalev, F. Wilhelm, E. Snoeck, J. Phys. : Condens. Matter. 20, 265001 (2008) [5] M. Da Silva, K. Dumesnil, C. Dufour, M. Hehn, D. Pierre, D. Lacour, F. Montaigne, G. Lengaigne, S. Robert, Appl. Phys. Lett. 98, 232504 (2011)
10:42 AM
CC-10. Room-temperature magnetoresistance in CoFeB/polycrystalline SrTiO3/CoFeB magnetic tunnel junctions deposited by ion beam sputtering
Emeline Hassen1, Bernard Viala1, Marie-Claire Cyrille1, Mathilde Cartier1, Olivier Redon1 and Paulo Lima2
1CEA-Léti, Minatec Campus, 17 rue des Martyrs, 38054 Grenoble, France; 2SPTS, Process Technology Systems, Newport, NP18 2TA, United Kingdom
MgO magnetic tunnel junctions (MTJs) are widely used in spintronic applications: magnetic recording, random access memories, spin torque oscillators... However MgO reaches its physical limits as tunnel barrier because the required device impedance has led to continuously decrease the MTJ Resistance x Area (RA) product upon scaling. Therefore, there is a need for a new crystalline barrier with lower band gap to pursue the RA decrease. SrTiO3 theoretically meets this requirement as its band gap is ~2 times lower than MgO’s. Most publications on SrTiO3 based MTJs concern epitaxial growth using pulsed laser deposition or molecular beam epitaxy. In 2003, Bowen[1] reported a TMR of 1800% at 4K in fully epitaxial LSMO/SrTiO3/LSMO. Only one paper deals with polycrystalline SrTiO3 MTJs. In 2009, Coey et al.[2] showed a TMR of 2.25% with an RA of a few kΩ.µm2 at room temperature in sputtered CoFeB/SrTiO3/CoFeB junctions. No TMR was reported under a barrier thickness of 2.5nm . We report on transport measurements in polycrystalline SrTiO3 based MTJs deposited using a SPTS ion beam sputtering system, with standard CoFeB electrodes and a barrier thickness varying between 1.0 and 1.5nm. RA values of 3 to 150Ω.µm2 with TMR varying from 3.1 to 9.5% are reported at room temperature which is a large improvement compared to existing results. For barrier thicknesses smaller than 0.9nm a ferromagnetic coupling between electrodes is observed which indicates barrier defects (pinholes). To improve barrier stoechiometry and integrity, an oxidation step (natural or radical) is performed post barrier deposition. Dependence of RA and TMR with oxydation conditions and GMR ratio improvement are reported.
References
[1] M. Bowen et al, Appl. Phys. Lett. 82, 233(2003) [2] J.M.D. Coey et al, Journal of Magnetism and Magnetic Materials, 321, (2009), 1009-1011
10:54 AM
CC-11. Interfacial oxidation effects on the inverse tunneling magnetoresistance and abnormal bias dependence of Fe4N/Fe3O4/AlOx/Fe junctions
Hua Xiang1, Fengyuan Shi1, Mark S. Rzchowski2, Paul M. Voyles1 and Y. Austin Chang1
1Materials Science, University of Wisconsin Madison, Madison, WI; 2Department of Physics, University of Wisconsin Madison, Madison, WI
We have fabricated magnetic tunnel junctions (MTJs) with the structure Si(001)/TiN(10 nm)/Fe4N(20 nm)/AlOx(1.5 nm)/Fe(6 nm)/IrMn(40 nm)/Ag(100 nm) by dc and rf sputtering deposition and studied their tunneling magnetoresistance (TMR) and interface structure. Both Fe4N and Fe3O4 have been predicted by theoretical calculations to be half metals with negative spin polarization (SP) at room temperature (RT) [1,2]. Our as-prepared junction shows normal TMR of +3.3% at V= 0.02 V. After annealing at 210 oC and field cooling, the TMR at 0.02 V changes sign to negative. The TMR of the 210 oC annealed junctions depends strongly on the bias voltage, changing sign again from negative to positive with increasing positive applied voltage, with +0.7% TMR at 0.4 V. STEM and EELS characterization shows that the bottom epitaxial Fe4N electrode is oxidized during the AlOx barrier fabrication to form Fe3O4 at the Fe4N/AlOx interface. The inverse TMR and the abnormal bias dependence is the result of the imperfect Fe3O4/AlOx interface after the annealing. Annealing at 250 oC improves the Fe3O4/AlOx interface quality and results in all inverse TMR within -0.5~0.5 V measurement range. These results provide experimental facts for the understanding of the interfacial effects on the complicated Fe3O4 based spintronic devices and the abnormal bias dependence. Junctions with TaOx barriers will also be discussed.
References
[1] Z. Zhang and S. Satpatahy, Phys. Rev. B 44, 13319 (1991). [2] S. Kokado, N. Fujima, K. Harigaya, H. Shimizu, and A. Sakuma, Phys. Rev. B 73, 172410 (2006).
11:06 AM
CC-12. Shot noise studies of individual and series arrays of magnetic tunnel junctions
Ryan Stearrett1, Aisha Gokce1, X. Kou1, J. Q. Xiao1, E. R. Nowak1 and C. Nordman2
1Physics and Astronomy, University of Delaware, Newark, DE; 2Nonvolatile Electronics, Eden Prairie, MN
Charge-current shot noise is investigated in sputtered-deposited magnetic tunnel junctions (MTJs) having eithe r AlOx or MgO tunnel barriers. Shot noise data is reported for single MTJs and devices having multiple junctions in series. The ratio of the measured shot noise in single junctions to the expected Poisson value, namely the Fano factor, F, is observed to vary from above unity to well below 0.5, depending on the post-deposition thermal annealing treatment and the voltage bias conditions. Individual MTJs having AlOx barriers or MgO barriers and high TMR (> 225%) show full shot noise indicative of direct tunneling, whereas shot noise suppression is found in ones that were annealed below 350 °C. Fano factors ranging from 0.7 to 0.9 are attributed to electron interactions and/or sequential tunneling processes involving localized states in the barrier, such as oxygen vacancy defects that are not removed during the lower temperature anneal. When F < 1/2, correlated transport involving defect bands in the band gap of MgO may be present. Also, the evolution of shot noise and differential conductance in MgO-based MTJs were studied as a function of annealing time at 380 °C. For short annealing times on the order of minutes, we observe shot noise enhancement for both bias polarities with F=1.2 ± 0.1 and the presence of additional conductance channels as seen in the nonlinear I-V characteristics. With increased annealing time, the shot noise is found to drop down to the full value and is independent of voltage bias out to ±300 mV. Concurrently, the conductance exhibits a double minimum around 300 mV, which illustrates increased crystallinity at the CoFeB/MgO interface [1]. For series arrays, F scales inversely with the number (1 ≤ N ≤ 30) of junctions in series, even for junctions exhibiting suppressed shot noise. The 1/N scaling is consistent with the incoherent tunneling of electrons across junctions and indicates that each junction behaves as an individual noise source. The advantages of incorporating series arrays of magnetic tunnel junctions into devices for magnetic field sensing are discussed.
References
[1] C. Tiusan et al., J. Phys.: Condens. Matter 19, 165201 (2007).
11:18 AM
CC-13. Spin filter functionality in magnetic oxides on silicon: Electronic structure and spin transport
Martina Müller1, Christian Caspers1, Hatice Doganay1, Alexander X. Gray2, Alexander M. Kaiser1, 2, Martina Luysberg1, Andrej Gloskovskii3, Wolfgang Drube4, Charles S. Fadley2 and Claus M. Schneider1
1Peter Grünberg Institute, Research Center Jülich, Jülich, Germany; 2Department of Physics, University of California Davis, Davis, CA; 3Analytic and Anorganic Chemistry, Johannes Gutenberg University, Mainz, Germany; 4DESY Photon Science, DESY, Hamburg, Germany
Spin filtering through magnetic oxides is envisioned as a highly promising route for the efficient electrical spin injection and -detection in semiconductors. In our work, focus is put on establishing spin filter functionality in ultrathin films of the magnetic insulators EuO and EuS grown directly on silicon, thereby opening up the pathway for the integration of these functional magnetic oxides into silicon-based spintronic devices. We present a detailed electronic structure study of high-quality stoichiometric EuO and O-rich EuO thin films grown directly on silicon with no buffer layer using hard x-ray photoemission spectroscopy (HAXPES). We determine the initial state EuO valency from a consistent quantitative peak analysis of 4f valence-state and 3d core-level spectra. The results prove that nearly ideal, stoichiometric and homogeneous EuO thin films can be grown on silicon, with a uniform depth distribution of divalent Eu cations in the bulk and interface regions. Furthermore, we give experimental proof for spin filtering in EuS magnetic tunnel contacts to silicon. Sizeable spin filter efficiencies are found in quasi-magnetic tunnel junctions of the type FM/EuS/Si, in analogy to the conventional FM/SC configuration employed for spin injection. Spin filter magnetoresistance (MR) is observed for different Si doping concentrations, with the maximum MR being located at enhanced bias voltages. Our results interconnect electronic structure and spin transport properties of this exciting class of magnetic materials, and highlight their potential as spin injectors/detectors in semiconductors.
8:30 AM - 11:30 AM, Grand Canyon 9-11
Chair: Steve Russek, NIST Boulder
8:30 AM
CD-01. Breaking the diffraction limit dynamically: Optical observation of single nanomagnet dynamics in dense arrays
Zhigang Liu1, Rebekah Brandt1, Yu Yahagi1, Brett Hansen2, Bruce D. Harteneck3, Jeff Bokor3, Aaron R. Hawkins2 and Holger Schmidt1
1School of Engineering, UC Santa Cruz, Santa Cruz, CA; 2ECEn, Brigham Young University, Provo, UT; 3Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA
As areal bit density increases, characterizing individual magnetic bits within dense arrays becomes difficult with diffraction-limited optics. We demonstrate that dynamic magneto-optical detection breaks this diffraction limit if the characteristic behavior of a nanomagnet is sufficiently different from its neighbors’. We review the use of far-field time-resolved Kerr microscopy to resolve the high-frequency magnetization dynamics of a single, small (150 nm diameter) nanomagnet within a low-frequency background from an array of large (500 nm diameter) magnets. We use this technique to observe and quantify the effects of magnetostatic interactions on the single magnet dynamics as the intermagnet spacing is varied [1]. Fig. 1: (a) Top left: SEM image of a 150 nm Ni nanomagnet embedded within an array of 500 nm magnets. The dashed white line corresponds to the probe beam spot size. Top right: frequency spectrum of nanomagnet dynamics showing distinct frequencies for large (blue) and single small (red) nanomagnet. Bottom: Bias field dependence of the peak frequencies of a 150 nm nanomagnet (circles) embedded within an array of 500 nm magnets (squares). The open symbols represent the measured data and the dashed lines are micromagnetic simulation of the average magnetization of the array. (b) SEM images of sample with two 500 nm Ni magnets on either side of a 150 nm Ni nanomagnet for edge-to-edge spacings of 200-100 nm. (c) FFT spectra of the three magnet system with variable spacing at Hz=4500 Oe. The solid arrow points to the peak frequency of the edge mode of the 150 nm magnet, and the dashed arrow corresponds to the simulated value, showing the influence of magnetostatic interactions from the large magnets on the dynamics of the small one.
References
[1] Z. Liu et al., Appl. Phys. Lett. 98, 052502 (2011).
9:06 AM
CD-02. Vortex dynamics and core reversal by spin waves in metallic double point contact nanopillars
Gino Hrkac1, Lalita Saharan1, Joo-Von Kim3, Thibaut Devolder3, Claude Chappert3, Maurico Manfrini2 and Thomas Schrefl4
1Department of Engineering Materials, University of Sheffield, Sheffield, United Kingdom; 2IMEC, Leuven, Belgium; 3University of Applied Science, St Poelten, Austria; 42Institut d’Electronique Fondamentale, Université Paris-Sud, Paris, France
We conduct a micromagnetic study of vortex dynamics in double point contact metallic nano pillars[1]: CoFe(20nm)/Cu(5nm)/NiFe(5nm) with two Cu contacts (40nm diameter) with a centre-to-centre distance of 500nm. First we nucleate a vortex by applying a high out of plane field and a current of 9mA through both contacts (PC1 and PC2). After an initial phase of nucleation and annihilation of vortices only one remains. Then we apply a current of 9mA through PC1. The vortex gets pulled to PC1 (attraction potential of the Oersted field) and starts orbiting it with a frequency of 50 Hz (fig.1a). Then we apply a current of 9mA at PC2 and the vortex changes its trajectory to an orbital precession around PC2 with f=50 Hz. Then the current is applied through both PCs, the vortex travels from one PC to the other. Once it reaches the physical edge of the PC it starts to interact with the emitted spin waves and changes its precession behaviour (fig. 1b). The Fourier analysis of the magnetization yields two precession frequencies, fy=42Hz and fz=82Hz. A detail analysis of the spatial magnetization distribution shows that once the vortex core starts interacting with the emitted spinwaves, a core reversal of the vortex takes place. A similar effect was found by Kammerer et al. [2].
References
[1]A.Ruotolo,V.Cros,B.Georges,A.Dussaux,J.Grollier,C.Deranlot, R. Guillemet, K. Bouzehouane, S. Fusil, and A. Fert, Nature Nanotech. 4, 528 (2009) [2] M. Kammerer et al., Nature Commun. 2, 279 (2011)
9:18 AM
CD-03. Vortex mode dispersion relations in a 2-D array of interacting disks
Federico Montoncello and Loris Giovannini
Department of Physics-CNISM, University of Ferrara, Ferrara, Italy
Spin-wave propagation in bidimensional arrays of interacting magnetic elements has recently received increased interest in the field of magnonics. Actually, the possibility of tuning the propagating properties, speed and allowed/forbidden band width of magnetic collective excitations with an external field has attracted special attention on these systems. However, while in the saturated case the band diagram has been extensively investigated [1], only a few incomplete studies on collective modes in the vortex state have been reported [e.g. Ref. 2]. Here we present a thorough investigation on this subject: employing the dynamical matrix method [3], we performed calculations on a squared 2D lattice of dots in the vortex state, as a function of the in-plane wavevector, to investigate the first Brillouin zone. We computed the dispersion relations for gyrotropic, azimuthal and radial modes. Dynamics in vortex states is a complex matter, since the coupling is mainly due to the fluctuations of the magnetization, which is a second order effect, but is more interesting because a slight change of the external field can have dramatic consequences on the information carriers (“magnons”), which can be slowed down even to zero speed: in this way information could be stored or delivered with little energy effort within the same device, which operates either as a memory or a waveguide. We discuss the dynamical coupling of modes with different cell wavefunctions, the corresponding mode dispersion and bandwidth, the effects of interdot coupling on the circular polarization on the modes, the Brillouin Light Scattering cross section of the principal modes. The investigation extends also to vortex states in presence of a nonzero applied field, where the vortex core is no more in the center of the disk, and to the corresponding symmetry breaking in the dispersion relations. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°233552 (DYNAMAG).
References
[1] S. Tacchi, F. Montoncello, M. Madami, G. Gubbiotti, G. Carlotti, L. Giovannini, R. Zivieri , F. Nizzoli, S. Jain, A. O. Adeyeye, and N. Singh, Physical Review Letters, in press (2011). [2] A. Yu. Galkin, B. A. Ivanov and C. E. Zaspel, Physical Review B, 74, 144419 (2006). [3] L. Giovannini, F. Montoncello, and F. Nizzoli, Physical Review B 75, 024416 (2007).
9:30 AM
CD-04. Topological and uniform applied field induced magnonic band gaps in zigzag shaped magnonic waveguides
Mykola Dvornik and Volodymyr V. Kruglyak
School of Physics, University of Exeter, Exeter, United Kingdom
Magnonic crystals are central to magnonics - the study of spin waves in magnetic nanostructures [1]. The special role of magnonic crystals is determined by the possibility of opening band gaps in their spin wave spectrum. Magnonic band gaps have been observed in samples with compositional [2] and structural [3] modulation of their properties, where a uniform magnetic field could only change the frequency position of the band gaps. We use OOMMF [4] simulations to show that, in contrast, zigzag shaped magnonic waveguides can act as magnonic crystals in which the width rather than position of magnonic band gaps can be varied using a uniform applied magnetic field. We report the dispersion of spin waves in zigzag shaped Permalloy nanostripes with the period (width) of 50 (15) and 140 (30) nm respectively. First, the structures have been saturated along their long axes by applying external magnetic field of H=1000 Oe and then relaxed to remanence, in which case the magnetization tends to follow the tangent of the stripes’ curvature. In this configuration, short spin waves propagating along the structures are not expected to experience any modulation of either static or dynamic internal magnetic fields, and so, no formation of magnonic bandgaps is expected. However, our simulations show formation of magnonic bandgaps even in this case, which we relate to topological effects. A uniform magnetic field applied perpendicular to the zigzags’ length creates additional magnonic band gaps due to periodic variations of the projection of the field upon the direction of the static magnetization and, at larger field values, due to periodic variations of the angle between the static magnetization and the wave vector of propagating spin waves. The research is funded by EC's 7th Framework Programme under GA 233552 DYNAMAG and the EPSRC of UK.
References
[1] V. V. Kruglyak, S. O. Demokritov, and D. Grundler, J. Phys. D - Appl. Phys. 43, 264001 (2010). [2] Z. K. Wang, V. L. Zhang, H. S. Lim, S. C. Ng, M. H. Kuok, S. Jain, and A. O. Adeyeye, Appl. Phys. Lett. 94, 083112 (2009). [3] K.-S. Lee, D.-S. Han, and S.-K. Kim, Phys. Rev. Lett. 102, 127202 (2009). [4] M. Donahue and D. G. Porter, OOMMF User’s guide, Version 1.0, Interagency Report NISTIR 6376, NIST, Gaithersburg, MD, 1999: URL: http://math.nist.gov/oommf.
9:42 AM
CD-05. Hot Spin-Wave Resonators and Scatterers.
César L. Ordóñez-Romero1, Oleg Kolok oltsev2 and Naser Qureshi2
1Solid State Department, IFUNAM, Mexico City, Mexico; 2Centro de Ciencias Aplicadas y Desarrollo Tecnologico, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico
Thin film spin wave resonators and delay lines are building blocks for classical tunable microwave elements such as oscillators, filters, signal-to-noise enhancers, etc. Recent studies have shown that spin excitations are a very attractive candidate for a new class of computing systems, where they can be used for interconnections between processing logics [1], or even as logic elements, taking into account that a spin wave resonator can possess properties of an artificial atomic particle [2]. In this work we present experimental results on laser-induced spin wave resonators and scatterers in Yttrium Iron Garnet (YIG) thin films. Here, magnetic non-uniformities are optically induced through local thermal demagnetization of YIG. Our experiments have revealed that a focused laser beam creates: (1) A deep 2D potential well for Magnetostatic Surface Spin Waves (MSSWs) leading to the formation of a high-Q MSSW resonator; and (2) a high potential barrier for Magnetostatic Backward Volume Waves (MSBVWs) that efficiently reflects or scatters MSBVW. The reflection spectra shown in figure 1, which was obtained by focusing the laser exactly above the micro-strip line excitation antenna, can be interpreted as an strong confinement of MSSWs with a certain discrete wavenumbers within a micrometer size area of the YIG film irradiated by the laser. Figure 2 shows the behavior of the amplitud frequency characteristic in refletion mode for the MSBVWs for: (a) Laser off, (b) laser spot exactly above the antenna, (c) 1mm away from the antena, and (d) 2mm away from the antenna. Showing clearly in (c) and (d) the characteristic interference of the MSBVWs modes excited by the antenna and reflected by the non uniformity created by the laser spot. The effects observed here are quite interesting because they can be used for the formation of easily reconfigurable spin-wave resonators, magnonic crystal structures, or waveguide channels in thin film magnetic materials.
References
[1] A. Khitun, et. al., Superlattices and Microstructures, 47, 464,(2010). [2] E. Kamenetskii, Phys. Rev. E. 63, 066612 (2001).
9:54 AM
CD-06. Increasing efficiency of microwave to propagating spin wave conversion at nanoscale
Ehsan Ahmad, Yat-Yin Au, Oleksandr Dmytriiev, Toby Davison and Volodymyr V. Kruglyak
School of Physics, University of Exeter, Exeter, United Kingdom
There has been a revival of using spin waves in magnetic materials for data processing in higher standard communication systems [1]. This is partly due to the difficulties addressing the current demand for higher operation frequency by acoustic wave based devices, which are extensively utilized nowadays [2]. The recently demonstrated effect of direct conversion of free space microwaves into propagating spin waves [3] offers unique opportunities to bring spin waves to the centre of stage as means of next generation data processing. The efficiency of microwave to propagating spin wave conversion has immediately become an issue of central importance. We have therefore experimentally fabricated, measured, and numerically simulated the conversion efficiency of several designs, in particular including the “patch” and “crossed-wire” antennas. To quantify the conversion efficiency in the various designs, we introduce figures of merits such as “spin wave susceptibility” and “uniform precession suscep tibility”. We argue that “crossed wire” antenna demonstrate superior performance in exciting spin waves with very short wavelength in the exchange dominated region. This design can therefore lay out the core of future ultimate magnonic information processing at deep sub-micron length scales [4]. The research leading to these results has received funding from the EC's Seventh Framework Programme (FP7/2007-2013) under GAs 233552 (DYNAMAG) and 228673 (MAGNONICS) and from the EPSRC of the UK.
References
[1] V. V. Kruglyak, S. O. Demokritov, and D. Grundler, J. Phys. D - Appl. Phys. 43, 264001 (2010). [2] S. A. Manuilov, R. Fors, S. I. Khartsev, and A. M. Grishin, J. Appl. Phys. 105, 033917 (2009). [3] Y. Au, T. Davison, E. Ahmad, P. S. Keatley, R. J. Hicken, and V. V. Kruglyak, Appl. Phys. Lett. 98, 122506 (2011). [4] S. K. Kim, K. S. Lee and D. S. Han, Appl. Phys. Lett. 95, 082507 (2009).
10:06 AM
CD-07. Static and dynamic properties of cobalt nanocylinders
Yves Roussigne1, Salim Mourad Cherif1, Khalid Bouziane2, Andrei Stashkevich1, Manuel Vasquez Villalabeitia3, Mohammed Reda Britel4 and Mohammed Cherkaoui5
1LSPM (CNRS-UPR 3407), Université Paris 13, 99 avenue Jean-Baptiste Clément, 93430, Villetaneuse, France; 2UIR, Technopolis Rabat-Shore, Rocade Rabat-Salé, 11100 Sala el Jadida, Rabat, Morocco; 3ICMM, CSIC, Campus de Cantoblanco, 28049, Madrid, Spain; 4LTI-National School of Applied Sciences, Tangier, Morocco; 5Georgia Institute of Technology, 225 North Avenue NW, GA 30332, Atlanta, GA
We studied a quasi hexagonal array of cobalt nanocylinders (NCs) of 2 μm length and 30 nm in diameter deposited in an alumina porous template using electrodeposition technique. The packing density was estimated to be P = 0.16. X-ray diffraction revealed the excellent structural quality of the NCs which exhibit a hexagonal c-axis parallel to their axis. The static and dynamic magnetic properties were studied by vibrating sample magnetometry (VSM) and Brillouin light scattering (BLS) techniques, respectively. The VSM measurements showed that the magnetization saturation is achieved more rapidly when the applied magnetic field H lies along the NCs axis than when it is perpendicular to this axis. In both configurations, BLS spectra exhibit two well separated peaks. The measured eigen frequencies varies linearly in the first case and are hardly shifted by the magnetic field H in the second one, as long as H is not high enough to overcome the effective anisotropy of the NCs. The nature of the magnetic modes is discussed and the spin waves behavior is analyzed within the frame of an approach which takes into account the dipolar coupling between the NCs through an effective anisotropy depending on the packing density of the cylinders. This approach enables to give account of the measured frequencies.
10:18 AM
CD-08. Frequency tuning of ultrafast magnetization oscillations by varying the iron content of FePt alloys
Rebekah Brandt1, Fabian Ganss2, Tobias Senn3, Manfred Albrecht2 and Holger Schmidt1
1School of Engineering, UC Santa Cruz, Santa Cruz, CA; 2Institute of Physics, Chemnitz University of Technology, Chemnitz, Germany; 3Institute of Nanometer Optics and Technology, Helmholtz Center Berlin for Materials and Energy, Berlin, Germany
The small angle magnetization precession is an important quantity in magnetic recording design since it is related to the writing speed of the device. Being able to manipulate the magnetization oscillation frequency could be especially useful in microwave-assisted recording devices [1]. We demonstrate that the magnetization dynamics can be tuned by varying the iron content in 20 nm thick fcc disordered FePt alloy films. FePt alloys with iron content ranging from 40 at. % to 100 at. % were sputter deposited onto planar substrates and electron beam patterned polymer masks. The magnetization dynamics were measured using an all-optical time-resolved MOKE microscope [2] using a probe beam with a spot size of 1 μm. Figure 1 shows bias field dependence data on (a) continuous films and (b) single patterned d = 300 nm dots. The dashed lines correspond to Kittel calculations (films) and micromagnetic simulations (dots). It is seen that as the Fe content is increased, the magnetic oscillation frequency increases. We observe an increase of ~8 GHz in both the films and dots as the Fe content increases from 40 at. % to 100 at. %. The change in precession frequency is due to the modification of the saturation magnetization, which is plotted as a function of the Fe content in the inset of Fig. 1(c). The patterning of the films lowers the precession frequency due to the additional demagnetization term present in the dots, but we still observe a clear change in frequency as the Fe content is increased (Fig. 1(c)). This work was supported by the NSF and the W.M. Keck Center for Nanoscale Optofluidics.
References
[1] C. Thirion et al., Nat. Materials 2, 524 (2003). [2] A. Barman et al., Nano Lett. 6, 2939 (2006).
10:30 AM
CD-09. Photo-Magnonics in Spin-Wave Meta Materials
Benjamin Lenk1, Fabian Garbs1, Jelena Panke1, Henning Ulrichs2 and Markus Münzenberg1
1I. Institute of Physics, Georg-August-University of Goettingen, Goettingen, Germany; 2Institute for Applied Physics, University of Münster, Münster, Gabon
On the way towards spin-wave logic devices an understanding of novel mechanisms for the manipulation and guiding of spin waves is important. The present study concentrates on 2D magnonic crystals and the magnetic excitations therein. Femtosecond laser pulses are used to excite and subsequently measure the magnetization dynamics on continuous and structured Ni or CoFeB films [1]. The creation of a periodic magnetic “potential”, namely arrays of micron-sized antidots, induces additional dynamic modes. The respective characteristics depend on the material under examination, more precisely on the dipolar field strength around the individual structural unit. We focus on the physics of these modes and resolve the influence of the in-plane angle between external field and antidot-lattice orientation. On nickel, non-dispersive magnetic modes localize at the antidot edges. Contrastingly, we can verify the magnonic origin in the CoFeB case, where the spin-wave length is determined by the periodicity of the antidot lattice. In particular, spin-wave Bloch states at the Brillouin zone boundary are populated [2]. By changing the magnonic crystals’ symmetry from quadratic to rectangular, hexagonal or honeycomb the wave-vector selection can be tuned in analogy to resonances as observed in photonics.
References
[1] B. Lenk, G. Eilers, J. Hamrle and M. Münzenberg, Phys. Rev. B 82, 134443 (2010). [2] for a review, see B. Lenk, H. Ulrichs, F. Garbs and M. Münzenberg, arXiv:1101.0479. (accepted for publication)
10:42 AM
CD-10. Spatial Coherence and Vortices in Magnon Bose-Einstein Condensate
Patryk Nowik-Boltyk1, Oleksandr Dzyapko1, Vladislav E. Demidov1, Sergej O. Demokritov1 and Natalia G. Berloff2
1Institute for Applied Physics, University of Muenster, Muenster, Germany; 2Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, United Kingdom
The most fascinating manifestation of Bose-Einstein condensation (BEC) is its macroscopic spatial and temporal coherence, which cause such unique phenomena of the BEC systems as interference, superfluidity, and Josephson effect. Spatial coherence also results in macroscopic topological defects, quantised vortices being the most prominent example among them. Here we report on direct experimental observation of coherence in Bose-Einstein condensate of magnons in yttrium-iron garnet (YIG). Magnon BEC in tangentially magnetized films [1] differs from all other known BEC systems by the fact that condensation occurs at non-zero value of wave vector |kBEC|≠0, which results in double degeneracy of the quantum state of the condensate (due to symmetry reasons magnons condensate at two points of phase space corresponding to ±kBEC). In our experiments we create magnon condensate by means of parametric pumping. Condensate properties are investigated by space and time resolved micro-focus Brillouin light scattering technique. In the experiment we directly observe interference pattern, which results from the interference between two components of magnon condensate from different points of phase space. The pattern reveals existence of topological defects in a form of stationary vortices. The observed results are explained by the model involving coupled generalized Ginzburg-Landau equations for two components of the magnon condensate.
References
[1] S. O. Demokritov, V. E. Demidov, O. Dzyapko, G. A. Melkov, A. A. Serga, B. Hillebrands, and A. N. Slavin, Nature 443, 430 (2006).
10:54 AM
CD-11. Observation of spin wave cooling effect
Toshu An1, 2, Ken-ichi Uchida1, 2, Kazuya Harii1, 2, Yosuke Kajiwara1, 2, Kazuya Yamaguchi1, 2 and Eiji Saitoh1, 2
1The Institute for Materials Research, Tohoku University, Sendai, Japan; 2CREST, Japan Science and Technology Agency, Tokyo, Japan
Since the discovery of the spin Seebeck effect (SSE) in 2008, a generation of spin voltage from heat current flowing in a ferromagnet [1], unraveling its nature and its application to the variety of materials has been investigated [2, 3]. Meanwhile, spin Peltier effect, reciprocal effect of the SSE has been expected. If one can realize this cooling effect the problem of heat generation in the miniaturized devices can be solved, and a system based on a new principle is highly demanded. In this study, we focused on utilizing spin wave, collective excitations of spin lattice as a carrier of heat. By exciting a spin wave the energy can be spatially transported by spin wave, and the excited spin wave travelled certain distances decays at the scattering center such as defect or edge of the sample releasing its energy into heat. More interestingly, the place where the spin wave is initially excited can be cooled by the heat transfer to the spin wave, here we call spin wave cooling effect. Yttrium iron garnet (YIG) was chosen as an ideal sample to realize the spin wave cooling effect, because the spin waves excited in the YIG is known to have a long coherence length propagating distances even a few millimeters, and thus is good candidate of heat transporter. Spin wave excitation on the YIG sample was performed by using a microstrip line microwave antenna by monitoring the temperature, and the cooling effect was observed just after the spin wave excitation introducing a new cooling principle.
References
[1] K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa, and E.Saitoh, Nature 455, 778 (2008). [2] K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E. W. Bauer, S. Maekawa, and E. Saitoh, Nature Materials 9, 894 (2010). [3] H. Adachi, J. Ohe, S.Takahashi, and S. Maekawa, Phys. rev. B. 83, 094410 (2011).
8:30 AM - 11:30 AM, Grand Canyon 2-3
Chair: Andrew Pratt, University of York
8:30 AM
CE-01. Room-temperature tunnel magnetoresistance in self-assembled chemically-prepared nanoparticles superlattices.
Julien Dugay, Reasmey Tan, Anca Meffre, Thomas Blon, Lise-marie Lacroix, Julian Carrey, Pier F. Fazzini, Sebastien Lachaize, Bruno Chaudret and Marc Respaud
LPCNO, Toulouse, France
Chemical synthesis is a powerful way to control the size, shape and anisotropy of metallic magnetic nanoparticles (MNPs), but also to elaborate a large variety of organic tunnel barriers [1]. Using such MNPs may open new opportunities in spintronics. Since the first observation of the tunnel magnetoresistance (TMR) on metallic Co MNPs by Black et al. in 2000 [2], the only other metallic systems studied (large CoFe MNPs superlattices) has evidenced exotic MR properties but no clear spin-dependent tunnelling [3,4,5,6]. In all these experiments, MR disappeared above 20 K. Here, we report tunnel magnetoresistance (TMR) up to room temperature in networks of chemically-synthesized Fe metallic MNPs surrounded by two types of organic barriers (hexadecylamine / hexadecylammonium for cubic MNPs: sample I, and hexadecylamine / palmitic acid for spherical MNPs :sample II). We used dielectrophoresis [6] to trap the MNPs between finger-like electrodes displaying a 5-µm gap. To prevent the oxidation of the MNPs, this deposit was made in a glove box under an inert atmosphere. Then, the devices were capped with amorphous alumina layers (100 nm or 200 nm), which avoids any subsequent oxidation. The temperature dependence of the coercitive field µ0HP (± 150 mT for sample I) is compared with µ0HC measured on cubic MNPs thin films using VSM, with the magnetic field applied in the plane of the sample. At low temperature, µ0HP is higher than µ0HC, a well-known phenomenon probably due to the fact that, in the Coulomb blockade regime, current flows preferentially through the large MNPs, which display both a smaller charging energy and a larger coercive field. At T = 300 K, both µ0HP and µ0HC vanishes to zero, evidencing the superparamagnetic behavior of the nanocubes. The TMR decreases sligthly between low temperature and room temperature (1% to 0.3% for sample I and 0.6% to 0.1% for sample II), similarly to what is measured on Co-Al-O nanogranular material [7]. This evidences that the organic ligands used to stabilize the nanoparticles can be efficient spin-conservative tunnel barrier up to room temperature.
References
[1] Chaudret, Organometallic approach to nanoparticles synthesi s and self-organization, C. R Physique 6 (1), 117-131 (2005). [2] Black, C. T., Murray, C. B., Sandstrom, R. L. & Sun, S., Science 290, 1131-1134 (2000). [3] Tan, R. P. et al. Transport in superlattices of magnetic nanoparticles: Coulomb blockade, hysteresis, and switching induced by a magnetic field. Phys. Rev. Lett. 99, 176805 (2007). [4] Tan, R. P. et al., 3000% high-field magnetoresistance in super-lattices of CoFe nanoparticles. J. Magn. Magn. Mater. 320, L55-L59 (2008). [5] Tan, R. P. Carrey, J. & Respaud, M. Voltage and temperature dependence of high-field magnetoresistance in arrays of magnetic nanoparticles, J. Appl. Phys. 104, 023908 (2008). [6] Tan, R. P. et al. J. Magnetoresistance and collective Coulomb blockade in superlattices of ferromagnetic CoFe nanoparticles. Phys. Rev. B 79, 174428 (2009). [7] Kumar, S., Seo, Y. K. & Kim, G. H., Appl. Phys. Lett. 94, 153104 (2009). [8] S. Mitani, S. Takahashi, K. Takanashi, K. Yakushiji, S. Maekawa, and H. Fujimori, Phys. Rev. Lett. 81, 2799 (1998).
8:42 AM
CE-02. Effective Energy Barrier Distribution for Mixed Oxide Magnetic Nanoparticles: Isolated Particles and Periodic 3-dimensional Arrays
Mitsuhiro Okuda, Jean-Charles Eloi, Andrei Sarua and Walther Schwarzacher
H H Wills Physics Lab, University of Bristol, Bristol, United Kingdom
Mixed Fe/Co oxide nanoparticles, diameter 8nm, were prepared using the protein ferritin as a template and characterized by TEM and Raman spectroscopy. We show that the latter effectively distinguishes between magnetite (Fe3O4) and maghemite (γ-Fe2O3). Zero field-cooled susceptibility measurements show isolated magnetite nanoparticles have blocking temperature Tb ~18K, but that adding less than 3% Co raises Tb to >60K (see figure). Data for thermal relaxation from saturation obey T ln (t/τ0) scaling, enabling us to determine the effective energy barrier distributions for the isolated nanoparticles. For Fe oxide the distribution has a single broad peak. However, with the addition of only 1% Co a second component is observed that decreases rapidly with increasing energy. This component could derive from an additional easy axis as remanence measurements for the Co-containing nanoparticles show a deviation from the value of 0.5 Ms expected for random uniaxial anisotropy. Our data show the remarkably strong influence of small quantities of Co on the magnetic properties of Fe oxide nanoparticles. To study how dipolar interactions affect the magnetic properties of the nanoparticles, we have assembled them into 3-dimensional periodic arrays using protein crystallization. X-ray data shows that these arrays are exceptionally perfect. Magnetic data show that interactions raise the blocking temperature by typically 5K. This work was supported by the EU 7th Framework Project Grant Agreement n°228673 (MAGNONICS).
8:54 AM
CE-03. Mossbauer and X-ray Spectromicroscopy Studies of Hematite (α-Fe2O3) Nanocubes
Jeevan Jalli1, Yang-Ki Hong1, Chul-Sung Kim2, Chin-Mo Kim2, Jihoon Park1, Jaejin Lee1, Gavin S. Abo1, Alberto Romero-Herreros3 and Arantxa F. Rodriguez3
1Department of Electrical and Computer Engineering and MINT Center, The University of Alabama, Tuscaloosa, AL; 2Department of Nano and Electronic Physics, Kookmin University, Seoul, Republic of Korea; 3Departament de Física Fonamental and Institut de Nanociència i, Universitat de Barcelona, Barcelo na, Spain
The hematite becomes weakly ferromagnetic in the range of 961 K (TN) to 265 K (TM for bulk hematite) and is antiferromagnetic below 265 K. The Morin transition temperature (TM) is dependent on particle size and shape [1, 2]. We have reported that the Fe3+ spin in the 40 nm spherical hematite particle flips from 90o (in-plane) to 28o (the angle between [110] rhombohedral and c-axis) at 220 K (TM) [3]. In this paper, we report the Mossbauer and x-ray photoemission electron microscopy (PEEM) results of 20 nm hematite nanocubes. Fig. 1(a) shows TEM image of the 20 nm hematite nanocubes. Mossbauer spectra were taken in the range of 4.2 K to 290 K. A sudden change in the quadrupole splitting was not observed like spherical shaped hematite [3], but the site 2 spectrum completely disappears at about 230 K in Fig. 1(b). This is in good agreement with FC and ZFC magnetization curves, which meet at 230 K. This temperature is taken as the TM in this study. Therefore, it is suggested that the hematite particle shape does not have a key role in determining TM at nanoscale. Furthermore, we have studied element-specific electronic properties of single hematite nanocube using a combination of PEEM and synchrotron x-ray absorption spectroscopy. In order to obtain local x-ray absorption spectra of single nanocubes, a series of PEEM images around the Fe L2,3-edges were recorded. Our preliminary results show that the isotropic spectra of the 20 nm nanocubes cannot be superimposed, after multiplication for appropriate scaling factors, to reference spectra of α-Fe2O3 or any other known iron oxide species. This indicates a possible coexistence of different surface for different core stoichiometries of the nanocubes.
References
[1] M. Sorescu, et al., J. of Appl. Phys., Vol. 85, 5546 (1999) [2] R. D. Zysler, et al., Phys. Rev. B, Vol. 68, 212408 (2003) [3] S. H. Gee, et al., IEEE Trans. on Magn., Vol. 40, 2691 (2004)
9:06 AM
CE-04. Optical and Electrical Investigation of Bismuth Telluride Nanoplates
Mustafa Eginligil1, Weqing Zhang2, Viet Giang Truong1, Alan Kalitsov1, Xianmao Lu2 and Hyunsoo Yang1
1Department of Electrical and Computer Engineering, NUSNNI-Nanocore, National University of Singapore, Singapore, Singapore; 2Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore
Topological insulators (TI) are materials which are similar to band insulators in their bulk but have topologically protected metallic edge or surface states for two dimension (2D) or three dimension (3D), respectively, with spin helicity. Particularly, 3D TI with single Dirac cone at the Γ point is promising for magneto-electric applications. As recently shown by angle resolved photoemission spectroscopy (ARPES), Bi2Te3 is a 3D TI [1] and exfoliated Bi2Te3 films show A1u peak which is a Raman spectrum inactive in bulk [2]. Although there is a direct evidence of topological surface states from ARPES, there is no clear demonstration of these states by electrical transport measurements. Here we show our results of micro-Raman and electrical measurements on ensembles of Bi2Te3 nanoplates synthesized by solvent-based chemical method. We observe A1u peak which is Raman active due to crystal symmetry breaking at 120 cm-1 with laser excitation of 488 nm measured excitation power as low as 70 µW. This peak is comparable to bulk A1g2 and Eg2 in magnitude . We fabricated devices consisting of Bi2Te3 nanoplates with non-magnetic electrodes and studied the tunnelling behavior. Current voltage (I-V) characteristics exhibit a symmetric tunneling behavior in most of the devices which is consistent with our I-V calculations based on a simple Hamiltonian. One device shows symmetric, while the other shows a two-step tunneling behavior. We discuss the results, particularly the latter in terms of randomly distributed impurities. We also investigate the effect of magnetic electrodes where magneto-electric effects play a crucial role.
References
[1] Y.L. Chen, et al. Science 325 78 (2009). [2] K. M. F. Shahil, et al. Appl. Phys. Lett. 96 153103 (2010).
9:18 AM
CE-05. Evidence for Highly Suppressed Magnetostructural Transition Temperature in Nanostructured FeRh
Radhika Barua1, Felix Jimenez-Villacorta1, Hellen Jiang3, J. E. Shield3, D. Heiman2 and L. H. Lewis1
1Department of Chemical Engineering, Northeastern University, Boston, MA; 2Department of Physics, Northeastern University, Boston, MA; 3Department of Mechanical Engineering, University of Nebraska, Lincoln, NE
Magnetostructural transitions, comprising simultaneous magnetic and structural phase changes, have the potential to deliver extremely large functional effects in response to small excursions in magnetic field, temperature and strain. In this study, the effects of nanostructuring on magnetostructural transitions are of interest. To this end, a phase-separated system of (FeRh)5Cu95 was synthesized via melt quenching under Ar atmosphere (wheel speed: 50 m/sec) with the goal of precipitating nanoscaled FeRh in a Cu matrix upon isochronal vacuum annealing for 30 minutes in the temperature range 100 K ≤ T ≤ 800 K. The magnetic and structural properties of the (FeRh)5Cu95 ribbons were compared with bulk FeRh which exhibits an abrupt antiferromagnetic - ferromagnetic transition accompanied by a unit cell volume change of 1% at T ~ 370 K. SQUID magnetometry, transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to determine the magnetic behavior and phases of the (FeRh)5Cu95 ribbons. While the annealed ribbons are a complex mixture of spin glass, ferromagnetic and paramagnetic phases, removal of the background Pauli paramagnetic susceptibility signal in the annealed ribbons reveals the presence of a magnetic phase transition at Tt = 130 K. This phase transition becomes more pronounced upon annealing at higher temperatures and is highly reminiscent of the bulk FeRh magnetostructural transition at 370 K. XRD data indicates presence of only Cu with a slightly expanded lattice parameter; however, TEM imaging confirms the presence of coherent nanoscaled precipitates (~10-15 nm diameter) that are hypothesized to consist of FeRh. If indeed the noted unusual magnetic response arises from the presence of FeRh nanoparticles, this result represents an unprecedented reduction in magnetic transition temperatures due to nanoscaling. Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0005250.
9:30 AM
CE-06. Magnetic anisotropy in Nanomagnets
Fabrizio Moro1, Joris van Slageren2, Jonathan McMaster1, Rocio de Miguel3, Carlos G. Moreno3, Anabel Lostao3, Fernando Luis3, S elina Tang1, Edward Lester1, Theocharis Stamatatos4, Anastasios Tasiopoulos5, George Christou6, Yulia Krupskaya7, Vladislav Kataev7, Daniel Sells8, Floriana Tuna8, Eric McInnes8, David P. Mills1, William Lewis1, Alexander J. Blake1, Stephen T. Liddle1, Maria G. Lopez1, Alessandro La Torre1, Carlos G. Garcia9 and Andrei N. Khlobystov1
1University of Nottingham, Nottingham, United Kingdom; 2University of Stuttgart, Stuttgart, Germany; 3University of Zaragoza, Zaragoza, Spain; 4University of Patras, Patras, Greece; 5University of Cyprus, Nicosia, Greece; 6University of Florida, Gainesville, FL; 7IFW, Dresden, Germany; 8University of Manchester, Manchester, United Kingdom; 9University of Valencia, Valencia, Spain
Nanomagnets have been in the centre of intense research because they display a rich variety of phenomena straddling the classical and quantum world, and also because they are expected to enter the market soon as fundamental units in data storage devices and quantum computers. The key property in Nanomagnets is the magnetic anisotropy which allows the stabilisation of the magnetization state and hence the storage of information. Magnetic nanoparticles and molecular nanomagnets are two interesting examples of nanomagnets where quantum and classical properties coexist. An in-depth understanding of the key factors that determine the magnetic anisotropy in both types of nanomagnets will permit the development of improved systems for ultra high-density magnetic data storage devices that have been hindered by the low blocking temperatures of the magnetisation because of superparamagnetism arising at nanoscale. I will present an Electron Magnetic Resonance investigation at GHz and THz frequencies of nanomagnets with high magnetic anisotropies and with different sizes and shapes (cubic, spherical). The study of the temperature and angle dependencies of the magnetic resonance fields and linewidths allowed an accurate determination of the properties of the nanomagnets (damping parameters, magnetic anisotropy constants, sizes and magnetic moments). The experimental spectra are modelled on the basis of classical and quantum theories in order to obtain a common theoretical framework for the description of the nanomagnets [1]. In addition, a delocalised diuranium molecular magnet [2] will be presented along with the first encapsulation of a molecular magnet in carbon nanotubes [3] which isolates these magnets from the environment that may lead to an enhanced spin relaxation time. This property is of fundamental importance for the study of the decoherence time for quantum computing applications and also for the understanding of how Nature evolves from the quantum to the classical world.
References
[1] F. Moro, J. van Slageren et al. In preparation. [2] D.P. Mills, F. Moro, J. McMaster, J. van Slageren, W. Lewis, A.J. Blake, S.T. Liddle, Nature Chemistry 3 (2011) 454-460. [3] M. G. López, F. Moro, A. La Torre, C. J. G. García, P. D. Brown, J. van Slageren, A. N. Khlobystov Nat. Comm. Accepted in June 2011.
9:42 AM
CE-07. Manipulation of magnetic domain walls in nanowires and nanoparticles
Elena Y. Vedmedenko
IAP, University of Hamburg, Hamburg, Germany
Novel theoretical concepts of domain wall manipulation by tuning of exchange interactions (Nature Physics 6,187(2010), Phys. Rev. Lett. 137202 (20 09)) as well as by the spin-polarized tunnel current of a scanning tunneling microscope (Phys. Rev. Lett. 106, 067204 (2011), Phys. Rev. Lett. (2011) accepted ) will be introduced and compared with recent experiments.
10:18 AM
CE-08. Nano-particle Magnetism with a Dispersion of Particle Sizes
Mohammad El-Hilo1 and Roy W. Chantrell2
1Physics, University of Bahrain, Sakhir, Bahrain; 2Physics, University of York, York, United Kingdom
In this paper, different definitions of the distribution functions that can be used to account for the magnetization of dispersed nano-particle systems are discussed. Both the volume and number fraction distributions are found to be equally valid in introducing particle size effects onto the magnetization curve of the system. In addition, the temperature variation of the initial susceptibility for a dispersion of particle sizes is also examined. Due to the rapid variation of the relaxation time (τ) with the anisotropy energy, the exact variation of the function exp(-tm/τ) is approximated by a step-like function with a critical anisotropy energy at tm=τ where tm is the time interval for the measurement. This picture was criticized in some recent studies [1,2] who considered it as “erroneous”, claiming it is an abrupt crude two state model. The χ(T) behavior is calculated using both the exact and the approximated behaviors of the function exp(-tm/τ) and it was found that both approaches give an adequate description to the initial susceptibility of the system. The discrepancy between both models is less than 3% and it is only in the region around the peak in the initial susceptibility. Both approaches describe the same phenomena which is a progressive unblocking process of the particle’s magnetic moments when the temperature is increased. Thus, terms; “two state model” and “progressive crossover model” that are used in those recent studies [1,2] to describe the temperature variation of the initial susceptibility of a dispersed nano-particle system are misleading. In this study it is also found that it is far more important to use a proper value for the frequency factor f0 rather than just use any constant value between 109-1011Hz. The error in calculating the χ(T) values around the peak in the initial susceptibility can reach 20% if an improper value for f0 is used. The f0 value has to be calculated using the physical parameters of the system and not just taken as a constant of value between 109-1011Hz.
References
[1] F. Tournus and A.Tamion, J. Magn. Magn. Mater., 323,(2011)1118. [2] F. Tournus, E. Bonet, J. Magn. Magn. Mater., 323 (2011)1109
10:30 AM
CE-09. Size and surface effects on the magnetic properties of NiO nanoparticles
Mariana P. Proenca1, 2, Celia T. Sousa1, Andre M. Pereira1, Pedro B. Tavares3, Joao Ventura1, Manuel Vazquez2 and Joao P. Araujo1
1Dep. Física e Astronomia, IFIMUP and IN - Institute of Nanoscience and Nanotechnology, Porto, Portugal; 2Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain; 3Dep. Química, CQ-VR, Univ. Trás-os-Montes e Alto Douro, Vila Real, Portugal
NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel para meters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (Keff) with decreasing particle size [1]. The magnetic moment per particle was calculated for all samples using the susceptibility value at T=300 K. The number of uncompensated spins per NP was found to be proportional to nS1/3 ( nS = total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of Keff with particle size [2]. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (~6.5 kOe in the field axis and ~0.18 emu g-1 vertically), coercive field enhancement (HC~4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.
References
[1] M. P. Proenca, C. T. Sousa, A. M. Pereira, P. B. Tavares, J. Ventura, M. Vazquez and J. P. Araujo, Phys. Chem. Chem. Phys. 13 (2011) 9561; [2] D. Tobia, E. De Biasi, M. Granada, H. E. Troiani, G. Zampieri, E. Winckler and R. D. Zysler, J. Appl. Phys. 108 (2010) 104303.
10:42 AM
CE-10. Superferromagnetism: magnetic order from structural disorder
Yuri G. Pogorelov1, Gleb N. Kakazei1, 2, Niyaz I. Nurgazizov3 and Hugo G. Silva4
1IFIMUP/IN, Physics Department, University of Porto, Porto, Portugal; 2Institute of Magnetism, National Academy of Sciences of Ukraine, Kiev, Ukraine; 3Physics and Surface Chemistry Laboratory, Kazan Physico-Technical Institute, Kazan, Russian Federation; 4Centro de Geofísica, University of Évora, Évora, Portugal
Formation of long range ferromagnetic order is considered in a granular layer, consisting of magnetic nanoparticles with no direct exchange coupling between them, only due to the specific coupling by magnetostatic stray fields [1,2]. The random planar array of elongated magnetic islands, as typically observed in real granular films, is modeled by an array of ellipsoidal particles. Each particle has the same saturation magnetization M, volume v, and the (smallest) demagnetizing factor n, and they fill the whole area with a filling factor f < 1. The principal magnetic coupling between particles results from the fact that common demagnetizing field within continuous magnetic bodies can become a "magnetizing" field within discontinuous arrays [3]. This magnetizing effect is described within a mean-field framework, where the magnetic energy per particle (placed in an effective "void" within uniform background of parallel particles) is written as E(x) = − Kx2 − Jx, in function of x = cosθ, where θ is the angle of the particle magnetization with the local anisotropy axis. The parameters of effective anisotropy K = (1 − 3n)ω and coupling J = fω<x> involve the characteristic energy scale ω = M2v. For given inverse temperature β, the average <x> = ∂ lnZ/∂(βJ) results from the explicit partition function Z = ∫ −11 exp[−βE(x)]dx = exp(−βJ2/4K){erfi[(K + J)(β/4K)½] + erfi[(K − J)(β/4K)½]}/2, involving the imaginary error function, erfi. Analysis of the mean-field equation for <x> leads to the estimate for the critical temperature of superferromagnetic transition: kBTc ≈ (2f/3)2ω/(1 − 3n), and with typical values M = 1200 G, v = 2×10−19 cm3, f = 0.6 we evaluate Tc ≈ 550 K, in a fair agreement with the experimental observations [2]. The mean-field treatment is further extended to the case when the neighbor particles are not fully parallel and the local anisotropy axis can vary (slowly enough). This leads to softening of local coupling in the regions where the anisotropy direction is aligned with its gradient and thus to formation of a "shallow" domain structure, possibly explaining our observations by magnetic force microscopy in FeCo/AlO granular multilayers.
References
1. G.N. Kakazei et al, J. Appl. Phys. 90. 4044 (2001). 2. W. Kleemann et al, Phys. Rev. B, 63, 134423 (2002). 3. H.G. Silva et al, Phys. Rev. B 82, 144432 (2010).
8:30 AM - 11:30 AM, Grand Canyon 4-5
Chair: Christopher Morrison, Univ. of Manchester
8:30 AM
CF-01. FePt graded media obtained by ion irradiation
Franca Albertini1, Alessandro di Bona2, Paola Luches2, Sergio D'Addato2, 3, Gian Carlo Gazzadi2, Francesca Casoli1, Pierpaolo Lupo1 and Sergio Valeri2, 3
1IMEM-CNR, Parma, Italy; 2CNR - Istituto di Nanoscienze, Centro di ricerca S3, Modena, Italy; 3Dipartimento di Fisica, Università di Modena e Reggio Emilia, Modena, Italy
Exchange-coupled graded media, i.e. showing a depth grading magnetic anisotropy along the film thickness, have been recently proposed to respond to the insatiable demand of storage density in magnetic recording. Infact, they would assure high thermal stability, while maintaining a low enough coercivity to allow data writing [1]. On the experimental side, to obtain the required anisotropy gradient is a challenging task. Few attempts have been done mainly by exploiting growth/annealing parameters (e.g. a variable deposition temperature during growth [2]). In the present paper we have obtained graded perpendicular media by ion irradiation of FePt L10 ordered films. FePt films of 5 - 20 nm thickness have been epitaxially grown on MgO (001) by RF sputtering. The resulting material is chemically ordered (L10 phase) and shows high uniaxial perpendicular anisotropy (K >1.5 MJ/m3) and coercivity (µ0Hc > 0.38 T). Ion irradiation at low doses has been found to be effective in turning the chemically ordered, magnetically hard, L10 phase into the cubic A1 phase [3, 4], the latter being magnetically soft with in-plane magnetic anisotropy. Depth graded, hard-to-soft FePt magnetic films have been produced by uniform irradiation with broad Ar+ beams. Multiple exposures at different energies and doses allows, to some extent, the control of the depth profile of the chemical order, i.e. the L10/A1 fraction. In this case the magnetic coupling between the magnetically hard and soft phases occurs in stacked geometry. The magnetic properties of the resulting composite systems have been determined by MOKE, MFM and AGFM techniques, and the coupling regimes between the two magnetic phases has been investigated. Different regimes (from rigid magnet to exchange-spring magnet) has been obtained as a function of exposure, with HC reduction up to 50%.
References
[1] D. Suess et al., Appl. Phys. Lett. 92 (2008) 173111. [2] J. Lee et al., Appl. Phys. Lett. 98 (2011) 222501. [3] C. Chappert et al., Science 280 (1998) 1919; C. Vieu et al., J. Appl. Phys. 91 (2002) 3103. [4] F. Albertini et al., J. Appl. Phys. 104 (2008) 053907
8:42 AM
CF-02. Columnar grain growth of L10-FePt thin films
En Yang1, 2, Hoan Ho2, 3, David E. Laughlin2, 3 and Jian-Gang Zhu1, 2
1Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA; 2Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA; 3Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
FePt (L10)is considered to be the most promising medium material for ultra-high recording density applications. For achieving the purpose, the film medium needs to be granular in nature of small and uniform sized grains with non-magnetic grain boundaries. Well defined carbon grain boundaries with small grain sizes have been achieved in FePt (L10)-C films, however, non-columnar film microstructure in these films leads to secondary grain growth with undesired magnetic properties[1]. The difficulty in obtaining columnar growth of L10 FePt grains with large height to diameter ratio lies in perhaps high surface energy associated with the material choice. In this work, we present an experimental method for obtaining perpendicular FePt-SiO2 thin film with large height to diameter ratio FePt(L10) columnar grains. By tuning the thickness of the magnetic layer and the volume fraction of oxide in the film, at 475 C, a ~20nm thick perpendicular FePt film with ~8nm diameter of FePt grains were obtained. The height to diameter ratio of the FePt grains are as large as 2.5. Further increasing the thickness of the magnetic layer leads to cracks in some of the FePt columnar grains. Eventually, multilayer structure of FePt grains will form due to the large thickness of the magnetic film. Increasing the volume fraction of the oxide in the film yields smaller diameter FePt grains . However, it also decreases the maximal thickness required for maintaining the columnar shape without growth disruption. With ~50% of SiO2 in the film, the FePt grain diameter was reduced to ~3nm. However, the thickness of FePt/oxide magnetic films was reduced to ~5nm to keep the grains columnar. Further optimizing of magnetic layer thickness and oxide volume fraction ratio is needed to achieve very small diameter(~3nm) long columnar grains. Ordering at lower temperature can be achieved by introducing an Ag sacrificial layer.
References
[1] A. Perumal, Y. K. Takahashi, and K. Hono, "FePt-C nanogranular films for perpendicular magnetic recording," Journal of Applied Physics, vol. 105, pp.07B732-1 -07B732-3, Apr 1 2009.
8:54 AM
CF-03. Investigation of lattice dynamics and nanoscale thermal transport in FePt/Ag heat assisted magnetic recording (HAMR) media films using psec time-resolved x-ray diffraction
Dongbin Xu1, 2, Chengjun Sun1, Dale L. Brewe1, Sang-Wook Han3, Jingsheng Chen2, Steve M. Heald1 and Gan-Moog Chow2
1Advanced Photon Source, Argonne Nat'l Lab, Argonne, IL; 2Department of Materials Science and Engineering, National Un iversity of Singapore, Singapore, Singapore; 3Department of Physics Education, Chonbuk National University, Jeonju, Republic of Korea
HAMR is one of the proposed approaches towards multiple Tbit/inch2 recording by introducing temperature as another degree of freedom into the writing process1. Thus, the writability can be improved in high anisotropy materials such as L10 FePt, which is the most promising candidate to realize HAMR potential due to its high anisotropy and high chemical stability. Since temperature is involved in the writing process, the thermal design will be a crucial element to be considered in HAMR. In particular, Ag was used as heat sink layer to tune the thermal properties such as fast writing rate and low lateral thermal diffusion since it not only has high thermal conductivity but also can induce the (001) texture in L10 FePt. Former studies on the FePt/Ag system only focused on static properties such as thermal conductivity 2 and epitaxial growth3. However, the real writing process can involve lattice dynamics and nanoscale thermal transport. These may play a dominant role for realizing HAMR as a final product, and have not been investigated in detail. Hence the dynamical mechanism is still unclear. In this study, Ta/FePt/Ag, Ta/FePt and Ta/Ag films were grown on MgO(001) single crystal substrates using sputtering at elevated temperatures. The θ-2θ scans of x-ray diffraction (XRD) results show that both Ag and L10 FePt have (001) texture in the perpendicular direction to the substrate surface. Consistently, the VSM results also imply a highly anisotropic L10 FePt layer at room temperature. Lattice dynamics and nanoscale thermal transport in FePt/Ag HAMR media films were investigated using psec time-resolved XRD as a function of the delay of x-ray beam to the laser (800 nm femtosecond Ti/sapphire pulsed laser) in the range from about 1000 ps to -200 ps at 20-ID-C Advanced Photon Source. The results show that FePt with an Ag heat sink layer shows a faster recovery of lattice expansion by a factor of 2, compared to that of FePt without the Ag later, indicating a fast cooling rate with a Ag heat sink layer. The expansion of Ag (002) and FePt (002) as a function of the delay of x-ray beam to the laser, and the nanoscale thermal transport in FePt/Ag films are also addressed.
References
1. M H Kryder et al. Heat Assisted Magnetic Recording. Proc. IEEE 96(11), 1810-1835 (2008). 2. Robert Fernandez et al. A comparative analysis of Ag and Cu heat sink layers in L10-FePt films for heat-assisted magnetic recording. J. Appl. Phys. 109, 07B763 (2011). 3. J S Chen et al. Structure and magnetic properties of L10 FePt film with Ag heat sink layer. J. Appl. Phys. 105, 07B724 (2009).
9:06 AM
CF-04. Fine control of nanogranular microstructure of FePtAg-C films for perpendicular magnetic recording
Perumal Alagarsamy1, 2, Yukiko K. Takahashi1 and Kazuhiro Hono1
1Magnetic Materials Center, National Institute for Materials Science, Tsukubha, Japan; 2Department of Physics, Indian Institute of Technology Guwahati, Guwahati, India
Granular-type L10 ordered FePt based alloy thin films are considered to be one of the promising candidates for (heat assisted) magnetic recording media with an areal density beyond Tbits/in2. Recently, we reported perpendicular anisotropic nanogranular films with a narrow size distribution and 550 Gbit/in2 TRAR static tester results [1-3]. In this paper, we report the effect of Ar gas pressure (PAr) on fine control of nanogranular microstructure of FePtAg-C films prepared by sputtering at 350oC with PAr between 0.3 and 0.75Pa on SiO2 substrate with a 10nm MgO underlayer and the resulting magnetic properties. With increasing PAr from 0.3 to 0.6Pa, the microstructure (see Figure) changed from interconnected grains to well-separated grains with an average grain size of 4.8nm along with an increase in degree of L10 ordering. On further increasing PAr to 0.75Pa, the particle shape is changed to needle like particles. Room temperature M-H loop of the films prepared at 0.6Pa showed a HC⊥of 18kOe, 100 % remanance and a negative nucleation field of 6kOe suitable for media applications. A detailed investigation on the effect of PAr on the microstructure, degree of L10 order, particle size, and magnetic properties of FePtAg-C films would be presented.
References
[1] A. Perumal, Li Zhang, Y.K. Takahashi, K. Hono, J. Appl. Phys. 108, 083907 (2010). [2] L. Zhang, Y.K. Takahashi, A. Perumal, and K. Hono, J. Magn. Magn. Mater. 322, 2658 (2010). [3] L. Zhang, Y.K. Takahashi, K. Hono, B.C. Stipe, J.-Y. Juang, and M. Grobis, Intermag, Taipei, 2011.
9:18 AM
CF-05. High density temperature assisted recording on granular FePtAgC media
Oleksandr Mosendz, Simone Pisana, James Reiner, Barry Stype and Dieter Weller
Hitachi San Jose Research Center, San Jose, CA
Thermally assisted magnetic recording (TAR), a promising approach to extend data storage densities beyond 1 Tb/in2, requires high anisotropy granular magnetic media with small grains and a tight grain size distribution. We have studied highly L10-ordered FePtAg-C nano-granular thin films as potential TAR media. These films were fabricated in a flexible sputter system allowing deposition of appropriate adhesion, heat sink and seed layers before growing FePtAg-X at elevated temperature to optimize grain size, distribution, texture and chemical L10 ordering. We demonstrate sputtered chemically ordered granular L10 FePt media using carbon segregant on glass substrates, which is suitable for high density recording using TAR. X-ray diffraction and transmission electron microscopy reveal high chemical ordering, an average grain size of <D> =7.2 nm and a size distribution as low as σD/<D>=16%. Magnetic properties studied with a vibrating sample magnetometer show Hc=4.8 T, Hk > 9 T and Ku>4.5*107 erg/cm3. Drag testing of this media shows recording areal densities of 620 Gb/in2.
9:30 AM
CF-06. The Impact of Deposition Temperature on the A1 to L10 Transformation in FePt Films
Bincheng Wang and Katayun Barmak
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
FePt continues to be of interest for development of HAMR and BPMR media with recording densities ≥ 4 Tb/in2. In previous work, we reported the impact of binary FePt composition and that of ternary additions on the A1 to L10 transformation in room temperature deposited films.[1-3] The present study reports the impact of deposition temperature, in the range of 100-300°C in intervals of 50°C, on the transformation of binary FePt films with 42.4-54.0 at.% Pt prepared by sputtering and examined by differential scanning calo rimetry (DSC) following deposition. For films deposited at 100, 150 and 200°C, an exothermic peak for the transformation was observed in the DSC traces. The transformation enthalpy decreased as deposition temperature increased, indicating partial transformation or direct formation of L10 during deposition. Films deposited at 250 and 300°C were fully L10 and thus no DSC peak was observed. With regards to the transformation kinetics, the DSC peak temperatures were only higher for films with 42.4 to 45.6 at.% Pt deposited between 100 and 200°C when compared with films deposited at room temperature (Fig. 1). Thus, it is important to choose a deposition temperature >200°C and a suitable composition to achieve fully ordered films. The transformation activation energy and the Curie temperature of the L10 phase were not impacted by the deposition temperature.
References
1. B. Wang, K. Barmak, J. Appl. Phys. 109, 123916:1-7 (2011). 2. B. Wang, K. Barmak, T. J. Klemmer, J. Appl. Phys. 109, 07B739-1:3 (2011). 3. B. Wang, K. Barmak, T. J. Klemmer, IEEE Trans. Magn. 46, 1773-1776 (2010).
9:42 AM
CF-07. The Ultimate Limit of Magnetic Recording
Hans Juergen Richter1, Andreas Lyberatos2, Ulrich Nowak3, Richard F. Evans4 and Roy W. Chantrell4
1Research, HitachiGST, San Jose, CA; 2Materials Science, University of Crete, Heraklion, Greece; 3Physics, University Konstanz, Konstanz, Germany; 4Physics, University of York, York, United Kingdom
The thermal stability of the recorded information is generally thought to set the limit of the maximum possible density in magnetic recording. We show here that basic thermodynamics always cause errors in the write process, which imposes another, so far unrecognized limit for the maximum possible storage density. For the case of the ultimate recording system where one grain stores one bit of information, this effect can directly be converted into an error rate as BER≈exp(-2mB/kT)] where m is the magnetic moment of the particle, B the applied field and kT the thermal energy. This estimate constitutes an upper bound for the storage density, because it assumes that thermal equilibrium can be achieved during the write process. For standard recording the resulting error rates are negligibly small, but if the writing is accomplished with some form of a write assist, the error rates can increase drastically. If the write assist is achieved with an increased recording temperature, the ratio mB is of the order of a few kT or even less and accordingly the BER becomes very poor. We estimate that the maximum achievable density is 20Tbit/inch2 at a BER of 0.01 for thermally assisted recording for a recording system with one bit-per-grain using L10 FePt as recording material.
10:18 AM
CF-08. Inversion of the induced anisotropy gradient in FePtCu films
Randy K. Dumas1, Brian J. Kirby2, Yeyu Fang3, Chaolin Zha3, Valentina Bonanni3, Josep Nogués3, 4 and Johan Åkerman1, 3
1Department of Physics, University of Gothenburg, Gothenburg, Sweden; 2Center for Neutron Research, NIST, Gaithersburg, MD; 3Materials Physics, Royal Institute of Technology (KTH), Stockholm-Kista, Sweden; 4ICREA and CIN2(ICN-CSIC), Universitat Autònoma de Barcelona, Bellaterra (Barcelona), Spain
Graded anisotropy [1] materials are currently being aggressively researched in order to address the magnetic recording trilemma. Our approach relies on continuously varying the Cu content during co-sputtering to first create a compositionally graded (Fe53Pt47)100-xCux (x=0-30) film [2]. The anisotropy gradient is then achieved after post-annealing, exploiting the strong dependence of the A1→L10 ordering temperature on the Cu content. The critical role annealing temperature (TA) has on the depth dependence of the induced anisotropy gradient is studied using a combination of surface sensitive magneto-optical Kerr effect (MOKE), first-order reversal curve (FORC), and field-dependent polarized neutron reflectivity (PNR) analyses. For a low TA of 300○C, PNR [Fig. 1(a)] reveals a significant soft phase component towards the top, Cu-free region of the film, also confirmed by MOKE measurements [3]. Surprisingly, for an intermediate TA of 400○C, an inverted anisotropy profile is now observed, Fig. 1(b). This anisotropy inversion is due to a combination of a better established L10 phase, due to the elevated TA, and the compositional Cu gradient [4]. *Work supported by SSF, VR, Göran Gustafsson Foundation, and the Knut and Alice Wallenberg Foundation.
References
[1] D. Suess, APL, 89, 113105 (2006). [2] C.L. Zha, et al., APL, 97, 182504 (2010). [3] V. Bonanni, et al., APL, 97, 202501 (2010). [4] R.K. Dumas, et al., PRB, in press.
10:30 AM
CF-09. Design of recording system for heat assisted magnetic recording
Vitaliy Lomakin, Qian Ding, Marco A. Escobar, Marko V. Lubarda, Yeshaiahu Fainman, Eric E. Fullerton, Shaojing Li and Ruinan Chang
Center for Magnetic Recording Research, Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA
Heat Assisted Magnetic Recording (HAMR) is envisioned as the next-generation technology for increasing magnetic recording areal densities. HAMR requires an integrated system of a magnetic head, optical source, and optical near-field transducer. We propose an integrated optical-magnetic head design. In this design, a new metal-coated laser type [1-3] is directly attached to the main pole of the write head. The laser is unique in that it has a low threshold gain to lase at room temperature and it is isolated in terms of unwanted radiation. We further demonstrate a new design of the near-field transducer, which is integrated with the pole tip to result in a “hot spot” in the media layer, which has a controlled potion with respect to the magnetic head-field spot. The near field transducer is driven by the laser and the field strength is enhanced by several electromagnetic phenomena, such as transverse electromagnetic mode enhancement, plasmonic enhancement, and sharp edge localization. The resulting resistive heating hot spot has the size of 30 nm. We present a system analysis of HAMR with the introduced hybrid head concept, including the electromagnetic (optical) and micromagnetic analysis of the hybrid head and media as well as the analysis of the recorded patterns in the granular media layer.
References
1. A. Mizrahi, V. Lomakin, B. A. Slutsky, M. P. Nezhad, L. Feng, and Y. Fainman, “Low threshold gain metal coated laser nanoresonators,” Optics Letters, vol. 33, no. 11, pp. 1261-1263, 2008. 2. M. P. Nezhad, A. Simic, O. Bondarenko, B. Slutsky, A. Mizrahi, L. Feng, V. Lomakin, and Y. Fainman, “Room-temp erature subwavelength metallo-dielectric lasers,” Nature Photonics, vol. 2, no. 6, pp. 395-399, 2010. 3. Q. Ding, A. Mizrahi, Y. Fainman, and V. Lomakin, “Dielectric shielded nanoscale patch resonators”, Optics Letters, vol. 36., no. 10, pp. 1812-1814, 2011.
10:42 AM
CF-10. New Computational Approach for Heat Assisted Magnetic Recording
Pin-Wei Huang and R. H. Victora
Electrical and Computer Engineering Department, University of Minnesota, Minneapolis, MN
One of the most challenging tasks in simulating heat assisted magnetic recording is including the media magnetization dynamics at temperatures close to the Curie temperature, Tc. Micromagnetic simulation with length scales at the angstrom regime are too computationally intensive for treating the entire magnetic recording system. Fortunately, it is known from renormalization group theory [1] that for a multi-spin system with exchange interactions between neighboring spins, the correlation length of spin fluctuations will lengthen as T approaches Tc. This feature makes feasible the micromagnetic simulation of magnetic recording systems at elevated temperatures. In this paper, the stochastic Landau-Lifshitz-Gilbert equation is used to model an FePt alloy in the L10 phase. We calculate the effective anisotropies K through torques exerted on magnetic moments [2]. Then we mimic an atomistic system (length scale 0.3 nm) with a renormalized system at a 2 nm length scale. By equating predicted macroscopic properties such as Ms, K, and anisotropy [3] fluctuations δK of these two systems at temperatures ranging from 500K to 675K, we obtain a specific combination of input parameters Ms, K, exchange A, and damping α suitable for input to the micromagnetic simulations. The success of this approach is demonstrated in Figure 1 where predictions at 2 different length scales can be condensed to one line subject to the usual time renormalization near Tc: H2nm(0.93t)=H1.2nm(t). Therefore, these input parameters can be used for future simulations of large recording systems.
References
[1] Shang-Keng Ma, Modern Theory of Critical Phenomena, W. A. Benjamin, Inc, 1976. [2] P. Asselin et al, Phys. Rev. B 82, 054415 (2010). [3] R.H. Victora and X. Chen, Digest F1, 2010 TMRC.
10:54 AM
CF-11. Application of the Grain Flipping Probability (GFP) model to Heat Assisted Magnetic Recording simulations
Sari Shafidah Shafiee1, Kheong Sann Chan1, Moulay Rachid Elidrissi1, Kwaku Eason1, Rathnakumar Radhakrishnan1 and Yong Liang Guan2
1Data Storage Institute, A*STAR, Singapore, Singapore; 2Nanyang Technological University, Singapore, Singapore
Heat Assisted Magnetic Recording (HAMR), a novel technology that magnetically records data on a media with the assistance of a laser is expected to significantly increase storage areal densities [1]. Micromagnetic simulations and the statistical Grain Flipping Probability (GFP) models have been proposed and implemented for the reproduction of HAMR signals. The GFP model was designed to replicate the behaviour of micromagnetic simulations with a substantially less computational time [2]. In this work, HAMR LLG micromagnetic simulations were actualized to enable the characterization of the GFP model. Channel simulations were then performed to identify the gain of HAMR over conventional recording. Although being computationally intensive, micromagnetic modeling provides a detailed and accurate analysis of the magnetic evolution of magnetized grains with time, given an applied and thermal field [3]. The GFP model mimics the accuracy and realism of micromagnetic simulations in a computationally efficient manner by populating a probability look-up table (LUT) based on the results of the micromagnetic simulations. The LUT describes the likelihood of a grain flipping given the circumstances per bit interval. This model permits the generation of sufficient bits in a reasonable time for channel simulations, while giving a more realistic channel readback signal. The Voronoi granular media with an average grain diameter of 4nm and average grain pitch of 4.5nm grain pitch were generated for the simulations. A Gaussian thermal profile with FWHM of 25nm and a uniform head field were used. Investigations were done to understand how some of the micromag dependent parameters affect the results. Statistics from each simulation were then used to characterize the probabilities in a multi-dimensional GFP LUT. Channel simulations are performed next to study the variation of frame error rates at a variety of code rates. Our preliminary results show the gain of HAMR recording over conventional recording by an approximate factor of 3.2.
References
[1]R. Wood et al, “The Feasibility of Magnetic Recording at 10 Terabits per Square Inch on Conventional Media”, IEEE Trans. Magn. Vol. 45, No. 2, pp917-923, Feb 2009. [2]K.S. Chan, et al. “Comparison of One and Two Dimensional Detectors on Simulated and Spinstand Readback Waveforms”, J.Magn. Magn. Mater, doi:10.1016/j.jmmm.2010.12.022 [3]Manfred E. Schabes, "Micromagnetic simulations for Tbpsi head/media systems”, IEEE Trans. on Magn. vol 44, No.11 November 2008 pp3423-3429.
11:06 AM
CF-12. Application of Landau-Lifshitz-Bloch dynamics to grain switching in HAMR
Terry McDaniel
Model_Physics, Volcano, CA
Magnetization dynamics in heat-assisted magnetic recording [HAMR] involves magnetization collapse and re-formation under rapid local temperature excursion in a temporally varying applied magnetic field, with temperature likely moving above and below the medium Curie point on nanosecond timescales. Traditional micromagnetic simulation of the writing process in magnetic data storage has been restricted to isothermal processes in which magnetization dynamics are handled with the well-established Landau-Lifshitz-Gilbert [LLG] algorithm or close variants. Classical LLG treats the magnitude of material magnetization M as fixed (at zero Kelvin), so initial attempts at micromagnetic analysis of HAMR have typically dealt with the effects of temperature variation in HAMR in a somewhat ad hoc manner [e.g. insertion of M(T), A(T), K(T)], and have not been rigorous. Extension of LLG to non-zero temperature using stochastic H-fields is justifiable1, but the assumption of constant M persists. A much improved treatment of this problem substitutes the Landau-Lifshitz-Bloch [LLB] algorithm in which thermally driven magnetization variation is treated more correctly in an extended semi-classical Landau-Lifshitz framework. The LLB approach was shown to provide an accurate description of the magnetics in a HAMR situation2. We illustrate single-grain switching with this method under application of time-varying temperature and H-field. For a given medium material, we map phase diagrams of grain switching probability over the space of important HAMR parameters such as peak temperature elevation, applied H-field strength, and the synchronization of temperature and field. This algorithm provides an essential building block for a full reco rding simulation of HAMR systems reflecting rigorous recording physics.
References
1 D. Garanin and O. Chubykalo-Fesenko, Phys. Rev. B 70, 212409 (2004). 2 O. Chubykalo-Fesenko, U. Nowak, R.W. Chantrell, and D. Garanin, Phys. Rev. B 74, 094436 (2006).
11:18 AM
CF-13. Critical peak temperature and minimum reversal field in heat assisted magnetic recording (HAMR)
Sonali Mukherjee
Carnegie Mellon University, Pittsburgh, PA
HAMR involves reversal at elevated temperatures. It has the potential of reversing magnetic grains of ultra-high anisotropy as needed for densities much over 1TB/(inch)^2. At high temperatures thermal fluctuations reduce the anisotropy of the magnetic grains. However, this advantage comes at the cost of increased probability of un-reversal of the already reversed grains. The frequency of super-paramagnetic fluctuations increases at elevated temperatures. Here I present results of this interplay of reduced anisotropy and increased super-paramagnetic fluctuations on the reversal field Hr, and the temperature to which the grain has to be heated Tp. Using Monte-Carlo simulations I show the existence of a critical peak temperature T*p<Tc at which the reversal field Hr* is minimum for a given cooling rate. If grain is heated to higher peak temperatures, Tp>T*p, the reversal field Hr does not reduce further because frequency of super-paramagnetic fluctuations increases. When grain is heated to lower peak temperatures, Tp<T*p, reversal field Hr increases due to higher anisotropy at lower temperatures. Moreover, I show the quantities T*p and Hr* depend on grain size. In particular, T*p and Hr* of grain length l=(l_dw)/3 and l=l_dw is compared as it is cooled from peak temperature to room temperature. Here l_dw= πsqrt(A/k) is the domain-wall length, A is exchange constant and k is the anisotropy. For grain of length l=(l_dw)/3, at the critical peak temperature T*p_c the grain is single domain and the frequency of super-paramagnetic fluctuations is comparable to the cooling rate. From T*p_c the grain cools down before super-paramagnetic fluctuations can un-reverse it. The grain reversal is by coherent rotation with Hr*=Hr*_c. For a longer grain of length l=l_dw, at the critical peak temperature T*p_dw the grain is multi-domain and reversal is by domain-wall propagation with Hr*=Hr*_dw. The wall propagation field is found to be smaller than the coherent rotation field, Hr*_dw<Hr*_c and the critical peak temperature T*p_dw for longer grain is found to be higher than the critical peak temperature for the shorter grain, T*p_dw>T*p_c.
8:30 AM - 11:30 AM, Grand Canyon 12-13
Chair: Ivan Skorvanek, Institute of Experimental Physics, Slovakia Academy of Science
8:30 AM
CG-01. From first-order magneto-elastic to magneto-structural transition in (Mn,Fe)1.95P0.50Si0.50 compounds
Huu Dung Nguyen, Lian Zhang, Zhiqiang Ou and Ekkes Brück
Fundamental Aspects of Materials and Energy, Faculty of Applied Sciences, Delft University of Technology, Delft, Netherlands
We report on the behavior of magnetic and structural transitions in MnxFe1.95-xP0.50Si0.50 (x ≥ 1.10) compounds when changing the Mn/Fe ratio. At room temperature we observe the hexagonal Fe2P-type structure (space group P-62m) for all the samples studied. The compounds with x < 1.40 exhibit the hexagonal structure through a first-order magneto-elastic transition, where the lattice parameters a and c experience a steep change in opposite sense, resulting in a smaller change of cell volume (< 0.2 %). The thermal and magnetic hysteresis, which are intrinsic to a first order transition but detrimental to the cyclical applications, decrease with increasing x. Small thermal hysteresis (<1 K) and negligible magnetic hysteresis can be obtained while maintaining a giant magnetocaloric effect. Further replacing Fe with Mn induces a second-order magnetic transition as confirmed from Arrot plots for x ≥ 1.40. Interestingly we observe an additional structural transition for x ≥ 1.90. On cooling below room temperature we find that the paramagnetic hexagonal phase is transformed into a paramagnetic body-centered orthorhombic (space group Imm2) phase. At Curie temperature this orthorhombic structure transforms back into the hexagonal structure in a first-order magneto-structural transition. This observation implies that the ferromagnetism prefers the hexagonal to the orthorhombic structure.
8:42 AM
CG-02. Self-Similarity in (dM/dT)H Curves for Magnetocaloric Materials with Ferro-to-Paramagnetic Phase Transitions
Yi Jin1, Shuo Gu1, Lawrence H. Bennett1, Edward Della Torre1, Virgil Provenzano2 and Qianyi Zhao1
1Electrical and Computer Engineering, George Washington University, District of Columbia, DC; 2National Institute of Standards and Technology, Gaithersburg, MD
We introduce a new mixed-phase theory for magnetocaloric (MCE) materials exhibiting 1st order ferro-to-paramagnetic transitions. This theory replaces the undefined rescaling constants of Franco’s transformation [1-4] with physical constants: ferromagnetic Curie temperature, TFM, paramagnetic Curie temperature, TPM, and the temperature of equal percentage of the two phases, TPeak. The three constants can be defined by the inverse susceptibility curve [5]. This theory is developed based on the co-existence of FM and PM clusters near the magnetic transition [6,7]. Maxwell’s relation, (dM/dT)H curves are an essential indicator of a material’s MCE. If these curves are rescaled with parameters from the mixed-phase theory, then all of the (dM/dT)H curves for a material with a 1st-order transition, such as Gd5Si2Ge2, will collapse onto a single curve with dispersion of less than 5% (Fig. 1). We have extended Franco’s theory by showing that the (dM/dT)H curves have self-similarity near the ferro-to-paramagnetic phase transition. Therefore, it is possible to carry out inexpensive performance screening of MCE materials by utilizing affine projection of the universal curve.
References
[1] V. Franco, J.M. Borrego, C.F. Conde, et. al., J. Appl. Phys. 100, 083903 (2006). [2] V. Franco, J.S. Blazquez, M. Millan, et. al., J. Appl. Phys. 101, 09C503 (2007). [3] V. Franco, C.F. Conde, J.S. Blazquez, et. al., J. Appl. Phys. 101, 093903 (2007). [4] V. Franco, A. Conde, M.D. Kuz’min, et. al., J. Appl. Phys. 105, 07A917 (2009). [5] C. Magen, P.A. Algarabel, L. Morellon, et. al., Phys. Rev. Lett. 96, 167201 (2006). [6] J.D. Moore, K. Morrison, G.K. Perkins, et. al., Adv. Mater. 21, 3780 (2009). [7] Y. Jin, S. Gu, L.H. Bennett, et. al., to be published in Physica B (2012).
8:54 AM
CG-03. Gd5Ge2(Si,Sn)2: giant isothermal variation of the entropy and small adiabatic variation of the temperature
Alexandre M. Carvalho1, Max E. Soffner2, Antonio M. Mansanares2, Adelino A. Coelho2, Julio Cesar G. Tedesco2, Manoel Jose M. Pires2, Sergio Gama3 and André O. Guimarães4
1Materials Metrology, INMETRO, Duque de Caxias, Brazil; 2Applied Physics, UNICAMP, Campinas, Brazil; 3UNIFESP, Diadema, Brazil; 4UENF, Campos dos Goytacazes, Brazil
In spite of a few interesting works about Ge and Si replacement by Sn in the Gd5Ge4-xSix system [1,2,3,4,5,6,7], no systematic study about magnetic and magnetocaloric properties on Gd5Ge2Si2-xSnx series has been reported so far. In the present work, we report the magnetocaloric properties, isothermal variation of the entropy (ΔST) and adiabatic variation of the temperature (ΔTS), for several alloys of Gd5Ge2Si2-xSnx system. As shown in Table 1, as-cast alloys of this system present giant ΔST even for a magnetic field variation of 20 kOe. For x = 0.1, ΔST maximum is around 10 J.kg-1.K-1, which is higher than that reported for metallic Gd (~ 6 J.kg-1.K-1) [8] and smaller than that firstly reported for Gd5Ge2Si2 (~ 14 J.kg-1.K-1)[9]. For x = 0.2, ΔST maximum is around 15 J.kg-1.K-1. Despites the high ΔST values, the as-cast alloys Gd5Ge2Si1.9Sn0.1 and Gd5Ge2Si1.8Sn0.2 present small ΔTS values, obtained as reported in Ref. [10], even when compared to metallic Gd, whose maximum ΔTS for the same magnetic field variation is larger than 3 K [10].
References
[1] H. B. Wang, Z. Altounian, D. H. Ryan, Phys. Rev. B 66, 214413 (2002) [2] D. H. Ryan, M. Elouneg-Jamróz, J. van Lierop, Z. Altounian, H. B. Wang, Phys. Rev. Lett. 90 (11), 117202 (2003). [3] J. C. P. Campoy, E. J. R. Plaza, A. Magnus G. Carvalho, A. A. Coelho, S. Gama, P. J. von Ranke, J. Magn. Magn. Mat. 272, 2375 (2004). [4] H. B. Wang, Z. Altounian, D. H. Ryan, J. Phys.: Condens. Matter 16, 3053 (2004). [5] T. Zhang, Y. Chen, Y. Tang, M. Tu, J. Alloys Compd. 422, 25 (2006). [6] J. C. P. Campoy, E. J. R. Plaza, F. C. Nascimento, A. A. Coelho, M. C. Pereira, J. D. Fabris, M. T. Raposo, L. P. Cardoso, A. I. C. Persiano, S. Gama, J. Magn. Magn. Mat. 316, 368 (2007). [7] V. Provenzano, T. Zhang, A. Shapiro, Y. G. Chen, R. D. Shull, IEEE Trans. Magn. 44 (11), 3048 (2008). [8] S. Yu. Dan’kov, A. M. Tishin, V. K. Pecharsky, K. A. Gschneidner Jr., Phys. Rev. B 57 (6), 3478 (1998). [9] V. K. Pecharsky, K. A. Gschneidner Jr., Phys. Rev. Lett. 78 (23), 4494 (1997). [10] A. O. Guimarães, M. E. Soffner, A. M. Mansanares, A. A. Coelho, A. Magnus G. Carvalho, M. J. M. Pires, S. Gama, E. C. da Silva, Phys. Rev. B 80, 134406 (2009).
9:06 AM
CG-04. Magnetocaloric materials with first-order transition: a comprehensive study of thermal and magnetic hystereses
Konstantin P. Skokov, James D. Moore, Jian Liu, Vladimir V. Khovaylo, Karl H. Müller and Oliver Gutfleisch
Institute for Metallic Materials, Leibniz Institute for Solid State and Materials Research, Dresden, Germany
Compounds with first-order ferromagnetic-paramagnetic transitions are very promising for magnetic-cooling due to very high values of the magnetic entropy change. However, in comparison with the second-order transition, the first-order transition is usually accompanied by field- and temperature-hystereses as well as latent heat evolution. A comprehensive study of the magnetic hysteresis from isothermal and adiabatic M-H loops, the magnetic entropy change, the adiabatic temperature change ΔTad, the heat capacity and the volume change of bulk La(Fe,Si)13 and Ni2+xMn1-xGa alloys undergoing a first-order magnetic transition has been carried out. The reason for choosing these materials for presented study is that the La(Fe,Si)13 alloys have a thermal hysteresis much smaller than the ΔTad, while in the Heusler alloys, the width of the thermal hysteresis surpasses the ΔTad. By building the entropy-temperature (S-T) diagram, a thermodynamic analysis of thermal and field hysteresis in these alloys with first-order transitions has been performed.
9:42 AM
CG-05. Table-like magnetocaloric effect and enhanced refrigerant capacity in clathrate-based composite materials
A. Chaturvedi, S. Stefanoski, M. H. Phan, G. S. Nolas and Hariharan Srikanth
Department of Physics, University of South Florida, Tampa, FL
Efforts in magnetic refrigeration based on the magnetocaloric effect (MCE) is to produce magnetocaloric materials that possess a large magnetic entropy change (ΔSM) over a wide temperature range, i.e. large refrigerant capacity (RC). For ideal Ericsson-cycle based magnetic refrigeration, a magnetocaloric material is required to possess a constant ΔSM in the refrigeration temperature range (known as the table-like MCE). Our recent discoveries of the large and reversible MCEs in Eu8Ga16Ge30 type-I and type-VIII clathrates [1,2], which are better known for their excellent thermoelectric properties, reveal that they can be used as a host material to fabricate novel composite materials with enhanced RC. We report on the large reversible MCE and enhanced RC in multiphase composite materials composed of Eu8Ga16Ge30 type I clathrate undergoing two successive ferromagnetic transitions at 10 K and 35 K and EuO exhibiting a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K has been achieved in the Eu8Ga16Ge30-EuO (40%-60%) composite. This is the largest value ever achieved among the existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K - 100 K. The fine tuning of the proportion between Eu8Ga16Ge30 and EuO is shown to produce composites with table-like MCE desirable for an ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of the Eu8Ga16Ge30-EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.
References
[1] M. H. Phan, G.T. Woods, A. Chaturvedi, S. Stefanoski, G.S. Nolas, and H. Srikanth, Appl. Phys. Lett. 93, 252505 (2008). [2] M. H. Phan, V. Franco, A. Chaturvedi, S. Stefanoski, G.T. Woods, G.S. Nolas, and H. Srikanth, J. Appl. Phys. 107, 09A910 (2010).
9:54 AM
CG-06. Enhancement of the magnetocaloric effect in composites: Experimental validation
Stephanie C. Paticopoulos1, Rafael Caballero-Flores1, Victorino Franco1, Javier S. Blázquez1, Alejandro Conde1, Keith E. Knipling2 and Matthew A. Willard2
1Condensed Matter Physics, Sevilla University, Sevilla, Spain; 2Multifunctional Materials Branch, U.S. Naval Research Laboratory, Washington, DC
Recent calculations have shown [1] that multiphase materials have enhanced magnetic refrigerant capacity (RC) as compared with their constituent materials. We present a systematic experimental validation of the calculated RC improvement (RCI) of a layered composite magnetic material consisting of two Fe88-2yCoyNiyZr7B4Cu1 amorphous alloys. The two alloy compositions investigated correspond to y = 8.25 (constituent A, with a lower Curie temperature) and y = 11 (constituent B, with a higher Curie temperature). The predicted RCI of the composite has complicated dependence on the fraction, x, of constituent B (Figure 1). For small x (x ≤ 0.45), the composite exhibits a reduced RC as compared with the pure constituent A. RC is enhanced for x ≥ 0.45, reaching a maximum value of RCI=34% at x = 0.65. These predictions are in excellent agreement with experimentally measured RCI values for 12 different composite specimens (Figure 1). These results are for an applied magnetic field of H = 1.5 T; the dependence of RC on x (and hence the optimum value of x) is influenced by the applied field, which is also discussed in this work.
References
[1] R. Caballero-Flores, V. Franco, A. Conde, K. E. Knipling and M. A. Willard, Appl. Phys. Lett. 98, 102505 (2011).
10:06 AM
CG-07. Magnetocaloric effect in thin film La(0.56)Sr(0.44)MnO3 alloy and superlattice structures
Dustin D. Belyea1, Tiffany S. Santos2 and Casey W. Miller1
1Physics, University of South Florida, Tampa, FL; 2Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL
In order to understand the impact of nanostructuring on the magnetocaloric effect (MCE), we have investigated the MCE of two compositionally identical but structurally dissimilar LaSrMnO3 thin film. The samples were grown by ozone-assisted oxide molecular beam epitaxy, and have been discussed previously [1]. One sample was a single 30nm layer La0.56Sr0.44MnO3 film grown by codeposition in order to create an alloy with La and Sr randomly distributed on the A-sites of the ABO3 perovskite structure. A second sample was a 30nm thick digitally synthesized superlattice in which the La and Sr atoms are ordered along the growth direction in superlattices of LaMnO3 and SrMnO3: [(SMO LMO)4/LMO]9. M(H,T) data were obtained for each sample via SQUID magnetometry. The magnetic entropy change, ΔS, of the second order phase transitions were determined via the appropriate Maxwell Relation. As with the MCE in other thin films [2,3], this system shows a reduction of the MCE relative to the bulk. Nanostructuring the film as a superlattice reduces the MCE below even that of the alloy film. For a 2 T field change, the peak -ΔS of the superlattice is 0.6 J/kgK, notably lower than the 0.9 J/kgK exhibited by the alloy film (Fig. 1). For the relative cooling power (RCP), the alloy film’s RCP of ~3.8 mJ/kg/T exceeds the superlattice’s 3.1mJ/kg/T (Fig. 2). These results suggest that nanostructuring materials into thin films and heterostructures may be detrimental to their magnetocaloric prope rties. Supported by NSF-CAREER; use of the CNM was supported by the US DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
References
[1] T. S. Santos, S. J. May, J. L. Robertson, and A. Bhattacharya, Phys. Rev. B 80, 155114 (2009) [2] Casey W. Miller, D. V. Williams, N. S. Bingham, and H. Srikanth, JOURNAL OF APPLIED PHYSICS 107, 09A903 (2010) [3] B. J. Kirby,1, J. W. Lau, D. V. Williams,3 C. A. Bauer, and Casey W. Miller3 JOURNAL OF APPLIED PHYSICS 109, 063905 (2011)
10:18 AM
CG-08. Magnetic properties of Y0.9 Gd0.1Fe2 D4.2 compound under continuous magnetic field up to 31 tesla.
Valérie Paul Boncour1, Maurice Guillot2 and Thomas Mazet3
1CNRS- Paris XII University, Thiais, France; 2CNRS- Joseoh Fourier UnivUniversity, Grenoble, France; 3Nancy University, Nancy, France
V. Paul-Boncour1, M. Guillot2, T. Mazet3 YFe2(H,D)4.2 compounds have evidenced a giant (H,D) isotope effect on the transition between a ferromagnetic (FM) and an antiferromagnetic (AFM) structure on the Fe sublattice with an increase of the transition temperature TM0 from 84K (YFe2D4.2) to 131K (YFe2H4.2) [1]. Magnetocaloric effect at the FM-AFM transition the influence of R (Gd; Er; Tb) for Y substitution has been investigated [2; 3]. In this work are presented new results on the structural, magnetic and magnetocaloric properties of Y0.9Gd0.1Fe2D4.2 compound in which TM0 is shifted from 84K to 110K due to Gd influence. Magnetization measurements have been performed with a magnetic field up to 31 T in the 4.2-300K temperature range with a special attention paid near TM0. The spontaneous magnetization at 4.2 K (3.2 µB/mol) is smaller than for YFe2D4.2 (3.7 µB/mol), showing the contribution of Gd moments. In addition, at low T no saturation is observed in Gd compound up to 31 T. Above 110K metamagnetic field -induced transitions are observed: the transition field HTR increases linearly with T. These transitions exist up to 170K. The roles of both cell volume change and Gd magnetic contribution are finally discussed in comparison with other R elements.
References
[1] V. Paul-Boncour, M. Guillot, et al. Phys. Rev. B72 (2005) 174430 [2] V. Paul-Boncour, T. Mazet, J. Appl. Phys., 105 (2009) 013914. [3] V. Paul-Boncour, T. Mazet, et al.,, Proceeding of Fourth IIF-IIR International Conference on Magnetic Refrigeration at Room Temperature, Baotou, China, (2010).
10:30 AM
CG-09. Direct calorimetric measurements of isothermal entropy change on single crystal W-type BaCoxZn2-xFe16O27 hexaferrites at the spin reorientation transition
Martino LoBue1, Frédéric Mazaleyrat1, Vincent Loyau1, Aleksandre Pasko1, Vittorio Basso2, Carlo P. Sasso2 and Michaela Küpferling2
1SATIE, ENS de Cachan, CNRS, Cachan, France; 2INRiM, Torino, Italy
Hexagonal ferrites were intensely studied for permanent magnets and microwave absorber applications. The former related to their easy axis anisotropy configuration (e.g. in M-ferrites) the latter to an easy plane one (e.g. in Y-ferrites) [1]. Moreover, W-type ferrites undergo spin reorientation transitions (SRT) between states of different anisotropy on varying temperature and applied magnetic field [2] . These transitions can be applied to magnetic refrigeration because the associated entropy change can be exploited under application of a rotating magnetic field [3,4]. With respect to other alloys [5,6], the main advantage of ferrites is that they contain no critical rare-earth elements. In addition the SRT temperature can be tuned in the room temperature region by changing the proportion between the Co and Zn substitutions [3]. A further interest relies on the possibility to prepare nanocrystalline materials [7] opening further possible enhancements by exchange coupling with other magnetic phases. SRT transition in W-type ferrites has been studied by magnetic measurements, Mössbauer spectroscopy and neutron diffraction [2,3,8] however the discrepancies present in the literature between MCE values [2,3,9] demands a clarification of the issues. Here we present measurements of the magnetic field induced entropy change Δs(Ha,T) by direct calorimetry [10]. Single crystals of composition BaCoxZn2-xFe16O27 (0.7 ≤ x ≤ 2), were prepared by a flux method. With x=0.62, at 296 K the entropy change due to the rotation of the magnetization from easy axis to hard plane is 0.11 J kg-1K-1. The entropy change data are interpreted using a model taking into account the anisotropy energy, the magnetic field energy and the energy of demagnetizing field [6]. At 296 K one deduces a value of K1=8.7 104 J m-3 for the first anisotropy constant, in good agreement with [2]. The SRT temperature is estimated to be about 205 K. The absence of any latent heat in DSC scans at that temperature indicates that the SRT transition between easy plane and easy axis occurs through an intermediate easy cone state as in [3]. This research has received funding from the European Community’s Seventh Framework Programme under grant Agreement No. 214864.
References
[1] J. Smit and H.P. Wijn, Ferrites, Philips Technical Library, Eindhoven (1959) [2] G. Asti, F. Bolzoni, F. Licci, and M. Canali, IEEE Trans. Magn. 14, 883 (1978) [3] E.P. Naiden, and S.M. Zhilyakov, Russ. Phys. J. 40, 869-74 (1997) [4] M. D. Kuz’min and A. M. Tishin. J. Phys. D: Appl. Phys. 24, 2039 (1991). [5] S. A. Nikitin, K.P. Skokov, Yu.S. Koshkid’ko, Yu.G. Pastushenkov, and T.I. Ivanova, Phys. Rev. Lett. 105, 137205 (2010) [6] V.Basso, C.P.Sasso, M.Kuepferling, K.Skokov, O.Gutfleisch, J. Appl. Phys. 109, 083910 (2011). [7] A. Pasko, F. Mazaleyrat, M. LoBue, V. Loyau, V. Basso, M Kuepferling, C. P. Sasso and L. Bessais, J. of Physics: Conference series, in press (2011) [8] A. Paoluzi, F. Licci, O. Moze, G. Turilli, A. Deriu, G. Albanese, and E. Calabrese, J. Appl. Phys. 63, 5074 (1988) [9] V.Basso, C.P.Sasso, M.Kuepferling, Mater. Res. Soc. Symp. Proc. Vol. 1310, mrsf10-1310-ff03-16 (2011). [10] V. Basso, C. P. Sasso, and M. Kupferling, Rev. Sci. Instrum. 81, 113904 (2010)
10:42 AM
CG-10. Magnetocaloric Effect of NiFeCoCrPdx High Entropy Alloys
Dustin D. Belyea1, C. A. Bauer1, M. Lucas2, E. Michel2, 3, J. Horwath2, 4 and Casey W. Miller1
1Physics, University of South Florida, Tampa, FL; 2Air Force Research Laboratory, Wright-Patterson AFB, OH; 3UTC Inc., Dayton, OH; 4Wright State University, Dayton, OH
FeCoCrNi is one of many “high entropy alloys” (HEA), which are multicomponent alloys (with ~equimolar constituents) with a high entropy of mixing. These materials often have high hardness, and resistance to wear and corrosion, making them attractive for applications [1]. Here, we report on the magnetic entropy change and mag netocaloric effect of the FeCoCrNiPdx system. The addition of Pd to FeCoCrNi enables the critical temperature to be tuned from 130 K for x=0 to 500 K for x=2 [2]. Isothermal magnetization measurements were made on samples with x = 0.00, 0.25, and 0.50 as functions of temperature. The magnetic entropy change (ΔS) was then calculated using the appropriate thermodynamic Maxwell Relation, which is acceptable for second order phase transitions. We find that Pd additions tune the peak ΔS temperature from 130 to 300K, while modestly increasing the peak ΔS magnitude (see Fig. 1). Interestingly, the Pd leads to a doubling of the relative cooling power (RCP), which also appears to be temperature independent. The RCP of ~35 J/kg for a 1T field change puts the Pd-containing HEAs in competition with other magnetocaloric materials in the 100-200 K operating range. This alloy system’s combination of durability and a tunable Curie temperature without appreciable change in cooling power may make this system interesting for magnetocaloric applications.
References
[1] J.-W. Yeh et al., Adv. Eng. Mater. 6, 299 (2004). [2] Lucas et al., J. Appl. Phys. 109, 07E307 (2011)
10:54 AM
CG-11. Influence of Ni and Mn Additions on Magnetocaloric Response in γ-(Fe70-XNi30+X)89-YMnYZr7B4 alloys
Jhon J. Ipus and Michael M. McHenry
Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
(Fe70-XNi30+X)89-YMnYZr7B4 (X=0,5 and Y=0,10) powders prepared by mechanical alloy were solution annealing treatment at 973 K to produce the metastable γ-phase. Spontaneous magnetization curves, Fig 1a, show that the substitution of Fe by Ni increases the Curie temperature, TC, while the addition of Mn reduces it. This agrees with values reported for Ni addition1 but not for Mn2, although consistent with the often reported antiferromagnetic exchange interactions for Mn substitutents3. From isothermal magnetization curves the magnetocaloric response, ΔSM, for a maximum applied field 0.5 T was calculated, Fig. 1b. It can be seen that ΔSM increases with Ni content as has been observed4, while the addition of Mn does not show a significant influence on ΔSM. The refrigerant capacity does not change considerably with Mn but shows an important increase with Ni content from ∼18 J/kg to ~36 J/kg for alloys with 30 and 35 at.% Ni, respectively. Peak magnetic entropy change can be expressed as5: ΔSMpk=aHn. Normalizing this expression a dimensionless relation is obtained6: δs=hn, where the exponent could be composition dependence. Figure 1c shows that the exponent has two values depending on composition: n=1.08 and 1.06 for Mn free and Mn containing alloys. This difference suggest that the Ni content does not change the magnetic behavior of these alloys around their transition temperatures but the Mn addition has influence on this behavior.
References
1 L.J. Swartzendruber, V.P. Itkin, C.B. Alcock, J. Phase Equilibria 12, 288 (1991). 2 W. Huang, Calphad, 13, 243 (1989). 3 K.L. McNerny, Y. Kim, D.E. Laughlin, M.E. McHenry, J Appl. Phys. 107, 09A312 (2010). 4 M. Balli, D. Fruchart, D. Gignoux, J. Tobola, E.K. Hlil, P. Wolfers, R. Zach, J. Mag. Mag. Mater. 316, 358 (2007). 5 V. Franco, J.S. Blázquez, A. Conde, Appl. Phys. Lett. 89, 222512 (2006). 6 R. Caballero-Flores, V. Franco, A. Conde, K.E. Knipling, M.A. Willard, Appl. Phys. Lett. 96, 1 82506 (2010).
11:06 AM
CG-12. Spin and Lattice Contributions to the Isothermal Entropy Change
Tathagata Mukherjee, R. Skomski, S. Michalski, D. J. Sellmyer and Ch. Binek
Physics and Astronomy, University of Nebraska, Lincoln, Linolcn, NE
The quest for advanced magnetocaloric (MC) materials has intensified in recent years, but despite the growing number of publications and improved insight into many problems, some fundamental aspects of the involved thermodynamics and statistical mechanics have largely been overlooked in the literature. Statistical considerations reveal that the maximum isothermal entropy change per mole of magnetic atoms having an individual angular momentum J originating from of spin multiplicity is equal to Rln(2J+1). However, in materials with giant magnetocaloric effect, the isothermal field-induced entropy change goes beyond the spin-multiplicity limit. This isothermal surplus entropy originates from elastic degrees of freedom and gave rise to some indistinctness in the literature regarding the applicability of fundamental thermodynamic relations in data analysis [1]. Specifically, we show that the Maxwell relation incorporates contributions from both the spin and elastic degrees of freedom into the isothermal entropy change. A minimalist model involving pairs of exchange-coupled Ising spins with variable real-space positions is investigated. It is explicitly shown that elastic degrees of freedom can be activated via applied magnetic fields and that the integrated Maxwell relation contains this lattice contribution in both the classical and quantum-mechanical cases. A simple and intuitive analytic expression for the isothermal entropy change in the presence of field-activated elastic degrees of freedom is provided. Acknowledgement: This work is supported by NRI and NSF through EPSCoR, Career DMR-0547887, and MRSEC Grant No. 0820521.
References
T. Mukherjee, S. Michalski, R. Skomski, D. J. Sellmyer and Ch. Binek, Phys. Rev. B 83, 214413 (2011).
11:18 AM
CG-13. Energy conversion efficiency analysis using thermomagnetic properties of ferromagnetic materials
Chin-Jui Hsu, Samuel M. Sandoval, Kyle P. Wetzlar and Gregory P. Carman
Mechanical and Aerospace Engineering, UCLA, Los Angeles, CA
Thermomagnetics and thermoelectrics are two approaches to directly convert thermal energy into electrical energy. While the efficiency of thermoelectric generation is experimentally reported to be 10-20% of Carnot [1], the theoretical prediction for thermomagnetic generation are on the order of 50% of Carnot [2] (i.e. 3-5 times higher). However, experimental results supporting the analytical predictions of thermomagnetic efficiency is lacking in the literature. In this study, the theory of thermomagnetic generation is reviewed and the experimental data of efficiency using the magneto-thermal properties of 3d transitional and 4f rare earth ferromagnetic elements is presented. By using the area method to calculate the net available magnetic energy in a thermomagnetic cycle, we compared the performance of each element in terms of thermomagnetic efficiency. The results indicate that efficiencies are significantly below the theoretical prediction (i.e. range 2-25% of Carnot). In general, the efficiencies of rare earth elements with Tc in the cryogenic range are substantially superior when compared to the 3d transitional elements, which also require large magnetic fields for marginal increases in efficiencies. Furthermore, we also report that the higher susceptibilities along the magnetocrystalline easy axis in single crystals produce higher efficiencies as compared to polycrystals. These results are used to analytically suggest that single domain ferromagnetic elements will have significant higher efficiencies due to spontaneous remanent magnetizations. Our calculation shows that a threefold increase on efficiency for a given cycle is possible using single domain. Therefore, ferromagnetic materials engineered to be single domain will increase the thermomagnetic efficiencies closer to those predicted analytically.
References
1. D. M. Rowe, Thermoelectrics handbook: marco to nano: CRC press, 2006. 2. L. Brillouin and H. Iskenderian, "Thermomagnetic generator," Electrical Communication, vol. 25, pp. 300-311, 1948.
8:30 AM - 11:30 AM, Grand Canyon 1
Chair: Ranko Heindl, NIST-Boulder
8:30 AM
CH-01. Enabling highly accurate magnetoelastic resonance sensors by substantially reducing the influence of external magnetic fields by an anti-symmetric bias-field
Bernhard Bergmair1, 2, Thomas Huber1, 2, Florian Bruckner2, Christoph Vogler2 and Dieter Suess2
1Institute of Analysis and Scientific Computing, Vienna University of Technology, Vienna, Austria; 2Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria
Magnetoelastic sensors are used in a wide field of wireless sensing applications. [1] They comprise a low-cost amorphous ferromagnetic ribbon whose resonant frequency reflects changes of the measured quantity. The accuracy of magnetoelastic sensors is restricted as the resonant frequency is also influenced by the earth’s magnetic field or other stray fields. Some interesting concepts to reduce this source of errors have been published by Jain et al. [2] and Ong et al. [3], but they imply major restriction on the sensors’ applicability. We present a technique to minimize the influence of disturbing external fields by biasing the ribbon with an anti-symmetric magnetic field, whereby the usability of the sensor was only marginally compromised. The response of a thus biased ribbon to external fields was measured and compared to a conventionally biased one. Fig. 1. shows the sensors’ frequency characteristics and their spreading due to the earth’s magnetic field. It could be seen that the influence of the earth’s magnetic field is reduced by 74% to 99%. Such a substantial increase in accuracy opens a whole range of new applications and markets.
References
[1] C. A. Grimes, S. C. Roy, S. Rani, Q. Cai, Theory, instrumentation and applications of magnetoelastic resonance sensors: A review, Sensors 11 (3) (2011) 2809-2844. [2] M. K. Jain, S. Schmidt, K. G. Ong, C. Mungle, C. A. Grimes, Magnetoacoustic remote query temperature and humidity sensors, Smart Mater. Struct. 9 (4) (2000) 502-510. [3] K. Ong, E. Tan, C. Grimes, R. Shao, Removal of temperature and earth’s field effects of a magnetoelastic pH sensor, IEEE Sens. J. 8 (4) (2008) 341-346.
8:42 AM
CH-02. Reducing the effect of 1/f noise in MgO magnetic tunnel junctions
Huanan Duan1, Amol Gupta1, Hsinwei Tseng2, Yun Li2 and Robert B. van Dover1
1Materials Science and Engineering, Cornell University, Ithaca, NY; 2School of Applied and Engineering Physics, Cornell University, Ithaca, NY
Magnetic tunnel junctions (MTJs) can serve as ultrasensitive low-frequency magnetic sensors but their low-frequency performance is limited by low-frequency noise, i.e. 1/f noise. Reducing the effect of 1/f noise can be achieved by either understanding the noise origins and engineering them out or down through optimizing the sensor design, or by chopping and modulating the input magnetic signal at certain oscillating frequency so that the sensor can be operated in a high frequency region where the 1/f noise is much lower. This talk will cover our endeavor in both directions. In part I, we investigate the 1/f noise in MgO magnetic tunnel junctions with tunnel magnetoresistance of 160%, and examine the influence of annealing, MTJ size, and MTJ shapes. In part II, we test our innovative chopping design by both finite-element analysis and experiment using anisotropic magnetoresistance (AMR) sensors. The results show that the annealing process can not only improve the TMR dramatically, but also strongly decrease the MTJ noise. The effect is discussed in terms of the structure of MgO barriers and tunneling probabilities. Increasing the MTJ area decreases the noise, and a 6400 μm2 rectangle device exhibits voltage spectral density of 11 nV/Hz1/2 at 1000 Hz [1]. Possible reasons for the area dependence are discussed. Finite-element analysis shows that our chopping efficiency can reach above 80% by judiciously selecting dimension parameters and this high efficiency will be verified by experiment.
References
1) H. Duan, H. W. Tseng, Y. Li and R. B. van Dover, “Improvement of the low-frequency sensitivity of MgO-based magnetic tunnel junctions by annealing”, Journal of Applied Physics 109 (2011) 113917.
8:54 AM
CH-03. The Effect of Interfacial Stresses on the out-of-plane anisotropy of continuous CoCrPt Thin Films
Nicholas J. Jones1, Cassandra L. Ondeck4, Vincent Sokalski1, Michael E. McHenry1, 3 and David E. Laughlin1, 2
1Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA; 2Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA; 3Biomedical Engineering, Carnegie Mellon University, Pittsburgh, PA; 4Biomedical Engineering, Duke University, Durham, NC
CoCrPt thin films are a source of much interest for perpendicular magnetic recording media [1], where the shape of each grain and the crystallographic preferences of Co make it easy to achieve a perpendicular easy magnetic direction. Therefore, continuous films have been proposed as a replacement for hard magnetic pillars in portable AGFM sensors [2,3]. When sputtering thicker films, which are continuous, however this out-of-plane preference becomes more difficult to control. These films exhibit good crystalline texture at all thickness, but as this quality improves with thickness, the easy axis becomes harder to saturate. The high remnant magnetization seen in thin films is greatly reduced, decreasing the possible signal-to-noise ratio for these small magnetometers. CoCrPt films have been sputtered at thicknesses from 10 nm to 500 nm to inspect this change in anisotropy. It was found that at thicknesses up until 40 nm, the direction perpendicular to the film was the easy direction. While the magnetic hysteresis loops all had roughly the same shape at these thinner values, the slope around the origin continually decreas ed from almost vertical at 10 nm, with anisotropy fields ranging from 500 Oe at 10 nm and eventually plateauing around 5,700 Oe at 500 nm. After 40 nm the shapes of the hysteresis loops begin to change approaching magnetic saturation, due to exchange coupling of the continuous film, with further shearing of the loops as thickness increases until 500 nm. This shearing is attributed to the interfacial stresses provided by the (00.1) Ru underlayer which leaves the negatively magnetostrictive CoCrPt layer in tension near the interface. The effect of these epitaxial stresses decreases with thickness, first being observed above 10 nm. This interfacial effect is verified by XRD and TEM analyses, along with a Stoner-Wohlfarth Model with added magnetoelastic anisotropy terms.
References
[1] V. Sokalski et al., Applied Physics Letters 95, 102507 (2009). [2] N. J. Jones et al., Journal of Applied Physics 109, 07E512 (2011). [3] I. Lucas et al., Sensor Letters 7, No. 4, 563-570 (2009).
9:06 AM
CH-04. Submicron Size Epitaxial Graphene Devices for Magnetosensing Applications
Vishal Panchal1, Olga Kazakova1, Alexander Tzalenchuk1, Karin Cedergren2, Sergey Kubatkin2 and Rositza Yakimova3
1NPL, Teddington, United Kingdom; 2CTH, Göteborg, Sweden; 3Linköping University, Linköping, Sweden
Submicron Hall crosses are widely accepted for detection of superparamagnetic nanobeads in biomedical applications. The aim is to develop a sensor with the smallest lateral dimension possible, providing the highest spatial resolution and coupling coefficient with a nanobead. Improvement of the coupling can be achieved by reducing the vertical distance between the active channel and nanoobject. Recent developments in graphene allowed incorporating this material with outstanding electronic properties and tunable photoactive gating for magnetosensing applications. Graphene Hall sensors with the width range from 0.5 to 20.0 µm were fabricated using techniques described in Ref.1-2 (Fig. 1). The devices were studied at room temperature using the Hall coefficient and noise spectrum measurements. For a microscopic device the Hall coefficient is RH ≈ 790 Ω/T, carrier density is ≈ 8 x 1015 m-2 and Josephson noise is ≈ 20 nV/√Hz being limited by a relatively high resistance of the device, yielding to Bmin ≈ 1 µT/√Hz. Further improvement of the device performance can be achieved by photoactivation of the capping resist layer that provides electron acceptors under UV light exposure leading to significant decrease of the carrier density and minimum detectable field [2]. Thus, we demonstrated epitaxial graphene magnetometers with room-temperature Hall coeffcient rivaling that of state-of-the-art semiconductor devices. Such devices are readily available and very attractive for chemical and biosensing as well as for magnetic storage applications. Photoactive resist provides a soft electrical gating of the devices enabling tunable and non-invasive control and optimization of their electrical parameters.
References
[1] A. Tzalenchuk et al., Nature Nanotechnology 2010, 5, 186. [2] S. Lara-Avila et al., Advanced Materials 2011, 23, 878.
9:18 AM
CH-05. Array of 12 Coils to Measure the Position, Alignment, and Sensitivity of Magnetic Sensors over Temperature
Hendrik Husstedt1, Udo Ausserlechner2 and Manfred Kaltenbacher1
1Applied Mechatronics, Alps-Adriatic University Klagenfurt, Klagenfurt, Austria; 2Sense and Control, Infineon Technologies Austria AG, Villach, Austria
During the testing of magnetic sensors for automotive applications, a 3D reference sensor is required that can be precisely positioned, is capable of temperatures from -40° to 180°C, has a measurement range from 10µT to 200mT, and is highly reliable. Since there are no fully integrated sensors available fulfilling these requirements, a system of three Hall plates [1] orientated 90° to each other has been constructed (Fig.1). The mechanic frame of this system includes reamed holes providing an accurate positioning, and three coils for self-calibration guaranteeing high reliability. However, the position and alignment of the Hall plates vary inside the frame due to assembly tolerances. Furthermore, over temperature and lifetime, material fatigue and thermal expansion may cause changes of the position and alignment of the Hall plates, and of the coil constants required for self-calibration. Therefore, a measurement setup is developed that allows analyzing these impacts over temperature and lifetime. To this end, an array of 12 coils is used where the number of coils is larger than the number of unknowns to increase accuracy, and to ensure an adequate measurement signal for arbitrary orientations of the Hall plates. The 12 coils are attached on three sides of a cube so that the inner volume is accessible from the other sides. First, the magnetic field is scanned with a special measurement system [2,3] so that the magnetic field generated by each of the 12 coils is known. Then, a magnetic sensor can be positioned in the volume where the field has been scanned, and the output signal caused by each of the 12 coils is measured. This output signal can also be calculated with the scanned field values as a function of the position, alignment and sensitivity of the Hall plate. Finally, an optimization problem is formulated which leads to the desired parameters.
References
[1] Infineon Technologies AG, Am Campeon 1-12, 85579 Neubiberg, Germany, “TLE4998S3C: Linear Hall sensor, Data Sheet Rev1.0”, 2009. [2] H. Husstedt, U. Ausserlechner, M. Kaltenbacher, "Accurate Measurement Setup for Strongly Inhomogeneous Magnetic Fields", Proceedings of Sensors+Test 2011, p.343-348, 2011 [3] K. Iniewski, “Sensors Technologies: Optical, Acoustic, Magnetic and Mechanical Sensors”, CRC Press, to be published
9:30 AM
CH-06. Withdrawn
9:42 AM
CH-07. Steel Stress Monitoring Sensors Based on Elasto-Magnetic Effect and Using Magneto-Electric Laminated Composites
Yuan-Feng Duan1, Ru Zhang1, Yang Zhao1, Siu Wing Or2 and Ke-Qing Fan3
1College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, China; 2Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong; 3School of Information Engineering, Wuyi University, Jiangmen, China
Steel structural components are widely used in Mechanical and Civil Engineering structures. Their stress state is essential for the structural safety and health evaluation, while the stress monitoring of in-service structures is challenging. The Elasto-Magnetic (EM) sensor is promising for stress monitoring of steel components, due to its actual-stress measurement, nondestructive monitoring and long service-life. Nevertheless, the high magne tic susceptibility, low sensitivity and slow response of the EM sensor, resulted from using a secondary coil as the sensing unit, keep from its engineering applications. We present a novel Steel Stress Monitoring Sensor (SSMS) using a magneto-electric (ME) sensing unit to take the place of the secondary coil of the EM sensor for the first time. The ME sensing unit is made of ME laminated composites, which can measure the magnetic induction simply and precisely. They possess ultrahigh ME voltage coefficient due to the product effect of the piezoclectric effect and magnetostrictive effect. Their induced ME voltage shows a good linear relationship to the applied magnetic induction. The theoretical analysis and characterization experiments are first conducted on the ME sensing unit, showing its high sensitivity and real-time response characteristics for induction measurement. Then the SSMS deploying the ME sensing unit is experimentally characterized for prestress monitoring of a steel cable. The pulse excitation of nagnetization is applied to the SSMS for reducing the magnetization heat and facilitating the implementation. The tension test of the steel cable is conducted to characterize our smart EME sensor, demonstrating 100 times faster response, and 100 times higher sensitivity than the common EM sensor. Therefore, the ME sensing unit makes our novel SSMS feasible for real-time and precise stress monitoring of steel structural components.
9:54 AM
CH-08. Biosensing Based on Magnetically Induced Motion of Superparamagnetic Beads
Semir Gessesse1, Ioanna Giouroudi2 and Jürgen Kosel1
1Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia; 2Institute of Sensor and Actuator Systems, Vienna University of Technology, Vienna, Austria
A method for in-vitro detection of biomolecules and biological cells using conductive microstructures to accelerate superparamagnetic beads (SPB) in an integrated microfluidic device is presented. The innovative aspect of the proposed system is that it utilizes the volumetric change of SPBs when biomolecules or cells are attached to their functionalized surface and thus their drastic decrease in velocity, when accelerated by an external magnetic force. The magnetic force is produced by current carrying gold microstructures, which are arranged in such a manner that they provide the required magnetic field gradient for the SPBs to travel from the inlet to the outlet of a microfluidic channel. A current is applied sequentially to the conducting microstructures so that the SPBs travel through the microfluidic channel without stopping or reversing direction. The advantage of using conducting microstructures over an external permanent magnet is that it ensures a better control of the magnetic field gradient hence allowing uniformity regarding the acceleration of the SPBs. In addition to that, conducting microstructures are easier to integrate in a complete microsystem than a permanent magnet. The motion of the SPBs through the channel is detected using an optical microscope and then a digital image processing method is utilized in order to estimate the velocity of the SPBs. The SPBs with the attached biomolecules show a significant decrease in velocity compared to the unattached SPBs. The presented biosensing system is a step towards an integrated solution for an on-chip analysis system, which yields much faster results and saves costs significantly.
10:06 AM
CH-09. Integration of Thin Film Giant Magneto Impedance Sensor and Surface Acoustic Wave Transponder
N. Pelin M . H. Salem1, Bodong Li1, Ioanna Giouroudi2 and Jürgen Kosel1
1Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia; 2Institute of Sensor and Actuator Systems, Vienna University of Technology, Vienna, Austria
An integrated device consisting of a multi-layer Giant Magneto-Impedance (GMI) sensor and a Surface Acoustic Wave (SAW) transponder is designed and fabricated. The GMI sensor acts as the load impedance for the SAW transponder. When the two-port SAW is wirelessly interrogated, one of its interdigital transducers (IDT) acts as reference reflection compared to the reflection of the IDT with the GMI sensor load. The measured difference in the phase and magnitude [1] of the two reflections correlates with the external magnetic field applied to the GMI sensor. The device is designed for three different frequencies: 80 MHz, 433.92 MHz and 2.45 GHz whereby the bandwidths provided by ISM regulations, which are 10 MHz for 80 MHz operation frequency, 1.75 MHz for 433.92 MHz and 80 MHz for 2.45 GHz, are taken into consideration. The SAW transponder is designed to have a passive impedance matching circuit integrated on the same wafer. The SAW structures are made of 200 nm thick Au deposited on a LiNbO3 wafer using standard fabrication steps and reactive ion etching. The thin film GMI sensor (NiFe/Cu/NiFe) is deposited on the same wafer using sputtering, and patterned using reactive ion etching and wet etching. A constant uni-axial magnetic field of 200mT was applied during the sputtering in order to induce a transverse magnetic anisotropy to the magnetic layers of the GMI sensor. This integrated device is proposed for passive and remote magnetic sensing applications such as bio-implants or harsh environments offering the advantages of mass fabrication.
References
[1] L. Reindl, "Unwired SAW Systems", LFNM 2005, Ukraine, p. 189-198.
10:18 AM
CH-10. Planar Hall effect sensors with shape-induced effective single domain behavior
Vladislav Mor1, Omer Sinwani1, Moty Schultz1, Asaf Grosz2, Eugene Paperno2 and Lior Klein1
1Department of Physics, Nano-magnetism Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan, Israel; 2Electrical & Computer Engineering Department, Ben-Gurion University of the Negev, Beer Sheva, Israel
We show that shape anisotropy induces effective single domain behavior in elliptical structures of thin permalloy films with long axis ranging between several microns to several millimeters, provided that the ratio of the film long and short axes is large enough. We also show that the thin film elliptical structures exhibit a wide range of effective anisotropy fields, from less than 10 Oe up to more than 100 Oe. We use elliptical structures to fabricate planar Hall effect (PHE) sensors with nanotesla resolution and discuss the advantage of ellipses (instead of rectangles) and shape anisotropy (instead of exchange-bias or induced uniaxial anisotropy). The figure shows the normalized transverse voltage measured across a permalloy ellipse as a function of the angle between the applied field and the electric current. The ellipse has a 2-mm length, 0.25-mm width, and 60-nm thickness, and the electric current is applied along its long axis. For each angle, the voltage is measured twice: with a field of 100 Oe (full symbols) and with the field switched off (empty symbols). The voltage measured in the latter case indicate s that for each angle the magnetization direction fully returns to the easy axis, and the variations in the zero-field signals are consistent with the expected effect of a small ambient field.
10:30 AM
CH-11. Thermally assisted switching on intermediate timescales in magnetic tunnel junctions
Leoni Breth1, 2, Dieter Suess2, Rudolf Heer1, Theodoros Dimopoulos1 and Hubert Brückl1
1Health and Environment, Austrian Institute of Technology, Vienna, Austria; 2Solid State Physics, Vienna University of Technology, Vienna, Austria
Thermal fluctuations play an important role for the switching behavior of magnetic elements when the timescale of the field sweep is above a fundamental limit of τ0 ~ 10-10 s which is related to intrinsic material properties. They assist the switching randomly and are therefore responsible for the lowering of the coercive field on longer timescales as well as for a switching field distribution which depends on the field sweep rate applied in the measurement. The transition rate at finite temperature between two stable magnetization states separated by an energy barrier ΔE can be described by the Arrhenius-Neél Law τ-1 = τ0-1 exp(-ΔE /kBT). The height of the energy barrier with respect to an external magnetic field obeys a power law, in which the exponent depends on the exact minimum energy path of magnetization reversal. Following a theoretical approach originally developed by J. Kurkijärvi for the fluctuations in a superconducting ring closed with a Josephson junction [1] we derive an expression for the switching field distribution of a Stoner-Wohlfarth particle switched along its magnetic easy axis. We use this model to estimate the limit of detection of an experimental setup for magnetic field sensing consisting of a magnetic tunnel junction with an embedded current line [2]. In the AC Oersted field of the line the free layer of the junction is switched in the kHz range between its saturated states. This way, we aim to measure DC magnetic fields in the sub-nano-Tesla regime by the detection of the second harmonic in the magnetoresistance signal. The switching field distribution after a sequence of magnetic field sweep cycles is identified to be crucial for the detection limit of such a sensor. We simulate the signal using a Monte-Carlo method to include thermal fluctuations and compare the obtained switching field distributions to the theoretical model.
References
[1] Kurkijärvi, J., "Intrinsic fluctuations in a superconducting ring closed with a Josephson Junction", Phys. Rev. B 6, pp.832-835, 1972 [2] Breth, L.; Dimopoulos, T.; Schotter, J.; Rott, K.; Bruckl, H.; Suess, D.; , "Fluxgate Principle Applied to a Magnetic Tunnel Junction for Weak Magnetic Field Sensing", IEEE Transactions on Magnetics 47, no.6, pp.1549-1553, 2011
10:42 AM
CH-12. Planar Hall effect sensors with patterned voltage leads and improved resolution
Vladislav Mor1, Moty Schultz1, Asaf Grosz2, Eugene Paperno2 and Lior Klein1
1Department of Physics, Nano-magnetism Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan, Israel; 2Electrical & Computer Engineering Department, Ben-Gurion University of the Negev, Beer Sheva, Israel
Decreasing the resistance of voltage leads in a planar Hall effect (PHE) sensor reduces its thermal noise and would potentially improve the sensor resolution at frequencies where other noise sources become negligible. However, voltage leads with very low resistance shunt the sensor current and reduce the sensitivity. We show that patterning trenches in the voltage leads significantly reduces the shunting and enhances the sensitivity without substantially increasing the resistance of the voltage leads that is measured across the sensor easy axis. As a result, the sensor resolution improves. We analyze the effect of patterning on the distribution of the sensor current by using numerical simulations and a simple analytical model. Our experiments confirm the theoretical analysis. We also discuss the relevance of the new design of voltage leads to other devices, where a transverse voltage is measured; e.g., ordinary Hall effect sensors. The figure shows the profile of the current along the easy axis of a PHE sensor with conventional and patterned voltage leads. The sensor comprises a 30-nm thick permalloy film and voltage leads made of 30-nm thick tantalum. The resistance between the voltage leads and the contact pads is negligibly small.
10:54 AM
CH-13. A study on the sensitivity of a spin valve with a Conetic-based free layer
Jangyup Son, Sanghoon Kim, Soogil Lee, Jungho Ko and Jongill Hong
Materials Science and Engineering,, Yonsei university, Seoul, Republic of Korea
In the case of bio-sensors detecting the extremely low stray field of magnetic nano-particles, soft magnetic materials with well-defined magnetic anisotropy are an essential ingredient. Though Conetic (Ni77Fe14Cu5Mo4) has known to be very soft magnetic materials, not many studies have reported on a spin valve with Conetic or Conetic-based free layer [1]. Spin valves with a structure of Ru 5/IrMn 6/Co90Fe10 1.5/Ru 0.9/Co90Fe10 2.5/Cu 3/free layer 3.5/Cu 1/ Ru 5 (nm) were deposited by an ultra-high vacuum (UHV) DC sputtering system whose base pressure was less than 2×10-9 Torr. To compare the sensitivity of spin valves with various free layers, 3.5 nm-thick Co90Fe10, Co90Fe10/Conetic, and Conetic layers were deposited as a free layer, respectively. We annealed them at 250°C for 1 hr under 4.5 kOe to induce the uniaxial anisotropy of free layers. A transverse magnetic field in the range of 0 to 20 Oe was applied along the hard axis to measure the sensitivity of spin valves and improve the linear field response. From the MR loops of spin valves with different transverse field as shown in Fig. 1, we found that the spin valve with the Co90Fe10/Conetic free layer has the highest sensitivity of 0.19%/Oe at 14 Oe of a transverse field, which is comparable to or higher than ones reported. In this presentation, we compare the sensitivity of spin valves with various free layers before and after annealing, and discuss the effect of a transverse field to enhance the sensitivity of spin valves. This research was supported in part by Samsung Electronics Co. and Basic Science Research Program through the National Research Foundation of Korea (2011-0003263).
References
[1] Z. Q. Lei et al., IEEE Trans. Magn. 47 (2011), 714-717.
11:06 AM
CH-14. Modelling and optimization of submicron Hall sensors for the detection of superparamagnetic beads
Alessandra Manzin1, Vahid Nabaei2, 1 and Olga Kazakova3
1Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy; 2Dipartimento di Ingegneria Elettrica, Politecnico di Torino, Torino, Italy; 3National Physical Laboratory, Teddington, United Kingdom
Recently, there has been an increasing attention towards submicrometer Hall sensors [1-3] for nanomedicine and molecular biology applications (e.g. detection of magnetic nanoparticles used as biological labels). The design of high-sensitivity devices can be aided by ad-hoc numerical models able to provide information about the influence of probe geometry on Hall response [4, 5]. In this work, semiconductor based Hall sensors are modelled in the presence of a nonuniform magnetic field, where non-uniformity is due to the stray field of a superparamagnetic nanobead. A 2D finite element model is developed to simulate the spatial distribution of the electric potential inside the Hall plate; the bead is described as a magnetic dipole with magnetization obeying Langevin function. The attention is focused on sensors composed of two crosses; the detection is performed by applying dc and ac magnetic fields perpendicular to the sensor plane [2]. Different geometries will be investigated, analysing the role of bar width w and cross shape on the device capability to convert the bead stray field into a measurable Hall voltage. The figure shows the influence of w on the amplitude of the ac Hall voltage due to a bead located above one of the crosses of the sketched sensor.
References
[1] P. Manandhar et al., Nanotechnology 20, 355501 (2009). [2] L. Di Michele, C. Shelly, J. Gallop, and O. Kazakova, J. Appl. Phys. 108, 103918 (2010). [3] G. Mihajlović, K. Aledealat, P. Xiong, S. von Molnár, M. Field, and G. J. Sullivan, Appl. Phys. Lett. 91, 172518 (2007). [4] H. Guillou, A. D. Kent, G. W. Stupian, and M. S. Leung, J. Appl. Phys. 93, 2746 (2003). [5] Y. G. Cornelissens and F. M. Peeters, J. Appl. Phys. 92, 2006 (2002).
11:18 AM
CH-15. Design and testing of piezoelectric energy harvester for powering wireless sensors of electric line monitoring system
Jing Qiu, Yumei Wen, Ping Li and Jin Yang
ChongQing University, ChongQing, China
This paper reports our design and testing results on the electric output performance of a piezoelectric energy harvesting device, which consists of a cantilever beams, a magnetoelectric (ME) transducers and magnets. The geometric parameters of the device are based on optimization design with a targeted resonant frequency of 50 Hz. Experimental results show that an output voltage of 56.1 V and an average power of 0.24 mW are achieved responding to current of 3A at 50 Hz.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Suzanne te Velthuis, Argonne National Lab
CP-01. Critical magnetic fields in the rutheno-cuprate Ru(1-x)NbxSr2Eu1.4Ce0.6Cu2O10
Maria E. Botello1, Oscar E. Ayala-Valenzuela2, Marcelo Jaime2 and Jose Matutes-Aquino1
1CIMAV, Chihuahua, Mexico; 2NHMFL, Los Alamos National Laboratory, Los Alamos, NM
II-type superconducting rutheno-cuprates are both interesting and challenging systems whe re there is a coexistence of superconductivity and magnetism so far poorly understood. The magnetic transitions that occur before the superconducting transition in Ru-1212 are simpler and better understood than those found in Ru-1222. In this work polycrystalline Ru(1-X)NbxSr2Eu1.4Ce0.6Cu2O10 samples (x = 0, 0.2, 0.4, 0.6) were prepared by conventional solid state reaction using stoichiometric amounts of high purity oxide powders: RuO2, Nb2O5, SrCO3, Eu2O3, CeO2 and CuO. The phases were identified by X- ray diffraction. From electrical resistance measurements as a function of the applied magnetic field at different temperatures (4K to 40K), using a Quantum Design PPMS, the Hc2 critical field or a lower limit of this was determined for each composition. At the lower measurement temperatures, from T = 4 K to T=17 K, it was only possible to set a lower limit for the critical magnetic field, Hc2, of 14 T for compositions X= 0 and 0.2, while for compositions x= 0.4 and 0.6 the critical magnetic field, Hc2, were 11 and 8.4 T respectively at T= 4 K. The dependence of these critical magnetic fields, Hc2, with temperature and composition is discussed [1,2].
References
[1]M. T. Escote, V.A. Meza,R. F. Jardim et al (2002)Physical Review B 66144503. [2] G. Fuchs, S-L Drechsler, N. Koslova, V. Grinenko, J. Frudenberger et al (2010)Journal of Physics: Conference Series 234 012013.
CP-02. Thickness dependence of critical current density in GdBCO thin films with BaSnO3 addition
Duc H. Tran1, Witha B. Putri1, Changhwan Wie1, Byeongwon Kang1, Namhoon Lee2, Wonnam Kang2, Dongho Kim3 and Wonkyung Seong4
1Department of Physics, Chungbuk National University, Cheongju, Republic of Korea; 2Department of Physics, Sungkyunkwan University, Suwon, Republic of Korea; 3Department of Physics, Yeongnam University, Gyeongsan, Republic of Korea; 4Convergence Technology Laboratory, Korea Institute of Science and Technology, Seoul, Republic of Korea
Influence of 4% BaSnO3 (BSO) addition on the thickness dependence of critical current density (Jc) of GdBa2Cu3O7-δ (GdBCO) thin films deposited by using the pulsed laser deposition (PLD) technique was investigated. The results measured at 77 K show that Jc of the GdBCO films is significantly enhanced by the addition of BSO. Thickness dependence of self field Jc in the BSO doped GdBCO films is reduced in comparision with that of undoped GdBCO films. The improved field performance of Jc is also observed for the BSO doped GdBCO films up to 1.5 μm thick. The enhancements are attributed to the growth of artificial pinning centers (APCs) in form of BSO nanorods. Effect of BSO nanorod alignment on Jcs of GdBCO films are analysed by using the angular dependent Jc measurements and the cross sectional TEM images.
References
[1]. S. R. Foltyn, Q. X. Jia, P. N. Arendt, L. Kinder, Y. Fan and J. F. Smith, Appl. Phys. Lett., 75,23, 1999. [2]. B. W. Kang, A. Goyal, D. F. Lee, J. E. Mathis, E. D. Specht, P. M. Martin and D. M. Kroeger, J. Mater. Res.,17, 2002,7. [3]. H. Zhou, B. Maiorov, S. A. Baily, P. C. Dowden, J. A. Kennison, L. Stan, T. G. Holesinger, Q. X. Jia, S. R. Foltyn and L. Civale, Supercond. Sci. Technol., 22, 2009, 085013.
CP-03. The scaling analysis on effective activation energy Ueff(T,B,J) in HgBa2CaCu3O8+δ
Bin Lv, Rubin Xie, Huiming Shao and Xiaoshan Wu
Physics, National Lab of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing, China
We review the methods of calculating the effective energy Ueff(T,B,J) for both transport measurements and magnetic decay, and suggest a new resistivity expression that compatible both with vortex glass theory and TAFF theory. Then we apply these methods in our Hg-1223 single phase superconductor to obtain the activation energy. Transport results show the validity of our new expression, shows that the magnetic field and temperature dependence of the Ueff can be well described as U0(J)kBTc(1-T/Tc)/(B/B0)β. Magnetic relaxation shows that the current density dependence of U(J), can be scaled onto a single curve Ueff∝J-μ. μ is about 0.978 in the temperature range 4.2-60 K, close to the value of 7/9 in the regime of flux creep in large vortex bundles predicted by vortex glass theory.
References
[1] M. P. A. Fisher, Phys. Rev. Lett. 62 (1989) 1415. [2] M. V. Feigel’man, V. B. Geshkenbein, A. I. Larkin, V.M. Vinokur, Phys. Rev. Lett. 63(1989) 2303. [3] M.P. Maley, J.O. Wills, H. Lessure, M.E. McHenry, Phys. Rev. B 42 (1990) 2639. [4] E. Zeldov, N.M. Amer, G. Koren, A. Gupta, M.W. McElfresh, R. J. Gambino, Appl. Phys. Lett. 56 (1990) 680. [5]M.R. Bleasley, R. Labusch, W.W. Webb, Phys. Rev. 181 (1969) 682. [6] M. Anderson, A. Rydh, O. Rapp, Phys. Rev. B. 63 (2001) 184511. [7]Q. H. Wang, X.X. Yao, Physics C 249 (1995) 69. [8]C.J. van der Beek, et al. Physica C 197(1992) 320. [9]M. Jirsa, L. Pust, H.G. Schnack, R. Griessen, Physica C 207(1993) 85. [10]P. W. Anderson, Phys. Rev. Lett. 9 (1962) 309.
CP-04. Dynamically induced Fermi arcs and pockets: A model for the pseudogap in underdoped cuprates
Han Yong Choi and Seung Hwan Hong
Physics, SKKU, Suwon, Republic of Korea
We investigate the effects of the dynamic bosonic fluctuations on the Fermi surface reconstruction in two dimensions as a model for the pseudogap in underdoped cuprates. At energies larger than the boson energy ωb, the dynamic nature of the fluctuations is not important and the quasi-particle dispersion exhibits the shadow feature like that induced by a static long range order. At lower energies, however, the shadow feature is pushed away by the finite ωb. The detailed low energy features are determined by the bare dispersion and the coupling of quasi-particles to the dynamic fluctuations. We present how these factors reconstruct the Fermi surface to produce the Fermi arcs or the Fermi pockets, or their coexistence. Our principal result is that the dynamic nature of the fluctuations, without invoking a yet-to-be-established long range order, can produce the Fermi pocket centered away from the (π/2,π/2) towards the zone center which may coexist with the Fermi arcs. This is discussed in comparison with the experimental observations.
References
H.Y. Choi and S.H. Hong, PRB 82, 094509 (2010).
CP-05. A study on the extensive nano-twinning obtained in YBa2Cu3O7-δ Superconductors fabricated by Preform Optimized Infiltration Growth Process
Devendra N. Kumar, Missak P. Swarup Raju and Seshubai Vummethala
School of Physics, University of Hyderabad, Hyderabad, India
< p height="0">High temperature superconductors in bulk form can have a wide range of technological applications as fault current limiters, permanent magnets, flywheel etc. [1, 2]. Bulk YBCO superconductors with high current densities (Jc) maintained to very large magnetic fields, with Jc in excess of 1 kAcm-2 up to fields of 6.5 Tesla at 77 K, have been fabricated employing Preform Optimized Infiltration Growth Process (POIGP) [3]. The final products were found to be free from major macro-defects like shrinkage, pores and cracks. The sample made under optimized conditions showed superior microstructures and Jc(H). A homogeneous distribution of Y-211 inclusions in the matrix of Y-123 was observed causing uniform Jc(H) across the volume of the samples [3]. Extensive twinning on a nano-scale (with twin widths ~ 40-100 nm) was observed in the samples [4]. Crossing twins were observed in the regions where Y-211 particles were in close proximity (FIG.1). In the regions where Y-211 particles were well separated, no crossing twins were observed. The lattice mismatch effects at Y-211/Y-123 interfaces affecting the widths of the twins in the proximity of Y-211 particles were observed. A detailed study is carried out in understanding the origin of these nano-twins in the present samples. The twin widths are measured and are correlated to the twin boundary energies following the twin shape method as suggested by Boyko et al. [5]. The role of twins and twin boundaries affecting Jc(H) at high fields are discussed.
References
1. M. Noe and M. Steurer, Supercond. Sci. Technol. 20 (2007) R15. 2. A.P. Malozemoff, Nature Mater. 6 (2007) 617. 3. N. Devendra Kumar et al. Supercond. Sci. Technol. 23 (2010) 105020 4. N. Devendra Kumar et al. Supercond. Sci. Technol. 24 (2011) 085005 5. V.S. Boyko et al. Phys. Rev. B 63 (2001) 224521.
CP-06. Temperature-dependent Raman scattering of double perovskite Ba2FeReO6 and Sr2CrReO6
Ali F. García-Flores1, Ulisses F. Kaneko1, Eduardo Granado1 and J. Gopalakrishnan2, 3
1Instituto de Física “Gleb Wataghin,” Universidade Estadual de Campinas, Campinas, Brazil; 2Center for Superconductivity Research, University of Maryland, Maryland, MD; 3Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India
Raman scattering measurements on polycrystalline double perovskite Ba2FeReO6 and Sr2CrReO6 over a wide temperature range from 17 to 475 K, and from 30 to 730 K, respectively, were performed. Ba2FeReO6 compound is an interesting material due to its half metallicity, large room temperature magnetoresistance, high Curie temperature of TC ~305 K, and large spin polarization [1,2]. The crystallographic structure of this metallic compound presents cubic symmetry (space group Fm3m) at room temperature where the Fe3+(3d5) and Re5+(4d2) orbitals are in alternating FeO6 and ReO6 octahedra [2,3]. Sr2CrReO6 has a high Curie temperature (TC = 635K). The crystal symmetry is tetragonal I4/mmm at room temperature [4]. The Raman spectra for Ba2FeReO6 showed three Raman active modes out of four at around 396, 497, and 582 cm-1. From temperature dependence of the Raman spectra, we observed a remarkable response of the phonon mode, associated to stretching vibration modes of the BO6 octahedra, at 582 cm-1 to the onset of the magnetic ordering phase at TC = 305 K, where the mode frequency hardens anomalously with decreasing temperature below TC. Interestingly, the phonon mode at 615 cm-1 of Sr2CrReO6, associated to stretching vibration modes, also presented an anomalous hardening below TC. This anomalous deviation of the phonon frequency for both double perovskites can be understood in terms of spin-phonon coupling mechan ism.
References
[1] J. M. De Teresa, et al., J. Magn. Magn. Mater 290-291, 1043 (2005). [2] W. Prellier et al., J. Phys. Condens. Matter 12, 965 (2000). [3] J. Gopalakrishnan et al., Phys. Rev. B 62, 9538 (2000). [4] H. Kato et al., Appl. Phys. Lett. 81, 328-30 (2002).
CP-07. Structural and magnetic phase transition of mixed olivines LixFe1-yNiyPO4 by lithium deintercalation
In Kyu Lee, Chin Mo Kim, Sam Jin Kim and Chul Sung Kim
Department of Physics, Kookmin University, Seoul, Republic of Korea
The structural and magnetic phase transition for LixFe1-yNiyPO4 has been investigated by the x-ray diffraction measurement and Mössbauer spectroscopy. Stoichiometric LixFe1-yNiyPO4 (0.0≤y≤0.6) polycrystalline powders were synthesized by the solid-state reaction method. Chemical oxidation to prepare the fully lithium deintercalated Fe1-yNiyPO4 (0.0≤y≤0.6) was performed by reaction of LixFe1-yNiyPO4 and NO2BF4 in acetonitrile [1]. Rietveld refinement of x-ray diffraction patterns of LixFe1-yNiyPO4 (0.0≤y≤0.6) revealed that the lattice parameter of a0=10.328, b0=6.007, and c0=4.692 Å for LiFePO4 linearly changed to a0=10.154, b0=5.923, and c0=4.687 Å for LiFe0.4Ni0.6PO4 by the substitution of Ni ions. Also, the fully lithium deintercalated Fe1-yNiyPO4 (0.0≤y≤0.6) series has same behavior of the change in lattice parameter with decrease of the unit cell volume by the lithium ion deintercalation. According to the temperature dependent magnetic susceptibility curves, the LixFe1-yNiyPO4 has an antiferromagnetic order with decrease of the magnetic Nèel temperature (TN) from 51 K for LiFePO4 to 36 K for LiFe0.4Ni0.6PO4. Also, the magnetic order of fully deintercalated Fe1-yNiyPO4 (0.0≤y≤0.6) has different antiferromagnetic order with decrease in TN from 114 K for FePO4 to 62 K for Fe0.4Ni0.6PO4 which coming from the spin-transition of Fe2+/Fe3+ and Ni2+/Ni3+ due to the lithium ion vacancy in olivine structure. The Mössbauer spectra below TN for LixFe1-yNiyPO4 (x=0, 1, 0.0≤y≤0.6) were fitted with asymmetrical eight Lorentzian. The electric quadrupole splitting value (ΔEQ) of LixFe1-yNiyPO4 (0.0≤y≤0.6) increases while Fe1-yNiyPO4 (0.0≤y≤0.6) decreases with increasing Ni substitution. This can be explained by the concentration of Ni and Li ion can be affect on the charge distribution FeO6 octahedral site.
References
[1] C. Delmas, M. Maccario, L. Croguennec, F. Le Cras and F. Weill, Nature Mater. 7, 665 (2008)
CP-08. Photo carrier induced effects on the magnetic ground state of La(2)CuO(4)
Andreas Suter1, Elvezio Morenzoni1, Thomas Prokscha1, Zaher Salman1, Bastian M. Wojek2, 1, Evel yn Stilp2, 1, Saikat Das3, Christian Bernhard3, Gennady Logvenov4, 5, Adrian Gozar4 and Ivan Bozovic4
1Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, Villigen PSI, Switzerland; 2Physik-Institut, Universität Zürich, Zürich, Switzerland; 3Physics Department, University of Fribourg, Fribourg, Switzerland; 4Brookhaven National Laboratory, Upton, NY; 5Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany
The undoped mother compounds of the cuprate based high temperature superconductors are charge-transfer insulators (CTI) with a band gap of about 2eV [1-3]. They are model substances for a spin-1/2 quasi 2D antiferromagnetic (AF) Heisenberg Hamiltonian [4]. For many models, trying to explain high temperature superconductivity, the understanding of doping these CTI is prerequisite. However there is typically a difficulty, especially in the La(2-x)Sr(x)CuO(4), namely that doping holes into the CuO(2) planes is accompanied by adding structural and hence electronic disorder due to Sr. If photo generated charge carriers are long-lived excitations (i.e. the electron-hole recombination rate is small enough), this could be a different route towards the understanding of a doped CTI, one in which structural disorder is absent. At the low-energy muon spin rotation (LE-muSR) setup at PSI muons with tunable energy between 1 and 30 keV can be implanted from a few nm depth up to about 300 nm in matter, thus perfectly matching the maximum of the excess carrier concentration induced by light. This offers the unique possibility to study photoinduced phenomena in near-surface regions and thin-film heterostructures with a very sensitive local magnetic probe, which has not been available up to now. We find in undoped La(2)CuO(4) (LCO) thin films the presence of a very robust photo persistent state (relaxation times at T=20K and T=80K of several hours). We irradiated the samples with green (λ=535nm → E=2.3eV) and blue (λ=470nm → E=2.6eV) light, respectively. As source of the light, an array of high power LED's was used. The studied films so far were MBE grown LCO on SrLaAlO(4) (band gap of about 5eV [5]) with a thicknesses t=53nm, and PLD grown LCO on SrTiO(3) (band gap of about 3.2eV [6]) with t=75nm. The magnetic order is fully wiped out at low temperature which means either an effective doping of the CuO(2) planes of x>0.02, or that the photo generated charge carriers act differently than Sr doping would do. There is a very sharp transition at T=167K, where the photo persistent state disappears and the antiferromagnetic state fully recovers.
References
[1] Y.H. Kim et al. PRL67, 2227 (91). [2] T. Thio et al. PRB42, 10800 (90). [3] S. Uchida et al. PRB43, 7942 (91). [4] S. Chakravarty et al. PRB39, 2344 (89). [5] J. Demsar et al. PRB76, 054304 (07). [6] M. Cardona, PR140, A651 (65).
CP-09. Structural, magnetic, and specific heat investigations on polycrystalline MnCr2O4
Zhaorong Yang
Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, China
Geometrically frustrated spin system have been extensively investigated because of the fascinating fundamental physics they display. In cubic spinels ACr2O4 (A = Mn, Co), geometrical frustration results in noncollinear spiral magnetic ordering at temperatures below Ts and concomitant magnetoelectric coupling. Although after being studied for more than several decades, the ground state magnetic structure in these materials is still controversy. In this paper, we investigate the structural, magnetic, and specific heat properties of polycrystalline MnCr2O4, mainly focusing on the ferrimagnetic-spiral transition around Ts. We show that, with increasing magnetic field, the magnetic anomaly around Ts in M-T curve is gradually suppressed. However, external magnetic field up to 5 T has no evident influence on the spiral magnetic transition as revealed by specific heat measurements. Upon cooling below Ts, AC susceptibility displays no frequency dependence, X-ray diffraction results show no structural phase transition. But the coercivity increases abruptly, and the lattice parameter decreases with temperature in the low temperature region. Applying external pressure increases the coercivity at 5 K, suggesting the strengthening of the spiral order. We suggest that with decreasing temperature across Ts, the spiral component develops in the direction perpendicular to the easy axis of the parent collinear ferri-magnetic phase and coexists with the collinear component.
CP-10. Effect of oxygen off-stoichiometry on magnetic and magneto-transport in under-doped LCMO nanomanganites
Yugandhar Bitla and S. N. Kaul
School of Physics, University of Hyderabad, Hyderabad, India
La1-xCaxMnO3 (LCMO) is one of the most exhaustively studied manganite systems and yet no general consensus on how the ferromagnetic insulating state at low doping levels x = 1/8 evolves from the antiferromagnetic insulating state at x = 0 has emerged so far. La0.875Ca0.125MnO3-δ (δ = 0.00 and 0.06, denoted by x1 and x2) samples, prepared by sol gel method, exhibit a single phase with pnma-type orthorhombic structure. The field emission scanning electron microscopy and energy dispersive absorption of x-rays confirm the uniformity of grain sizes (20 nm) and chemical composition while the oxygen off-stoichiometry, δ, was estimated using iodometric titration. The ferromagnetic-to-paramagnetic phase transition temperature drops from 143 K for x1 to 134 K for x2 whereas the saturation moment, measured at 2 K and 9T, is higher in x2 due to the increase in the concentration of Mn4+ ions. The irreversibility in magnetization, calculated from the ‘field-cooled’ (FC) and ‘zero-field-cooled’ (ZFC) magnetizations, is stronger in x2 as a result of the enhancement in magnetic anisotropy with oxygen deficiency. The spin wave stiffness constant, obtained from the spin-wave analysis of the high-field magnetization at low temperatures, is larger in x2. A detailed analysis of ac susceptibility shows that these samples behave as uniaxial dipolar ferromagnets in the asymptotic critical region, reflecting the importance of dipolar interactions and the nature of dominant magnetic anisotropy present. The isothermal change in magnetic entropy is higher in x1. The resistivity data shows a metal-insulator transition at 97 K (77 K) for x2 (x1) and a resistivity minimum (less prominent in x1) at low temperatures. These features get progressively suppressed with field and a field-induced bifurcation in ZFC and FC resistivities is observed at low temperatures. Oxygen off-stoichiometry in nanomanganites affects the magneto-transport drastically while the nature of magnetic order remains essentially unaltered.
CP-11. Ziz-zag Interface and Strain-influenced Ferromagnetism in Epitaxial Mn3O4/La0.7Sr0.3MnO3 Thin Films grown on MgO (100) and SrTiO3 (100) substrates
Devajyoti Mukherjee, Robert Hyde, Nicholas Bingham, Manh-Huong Phan, Hariharan Srikanth, Pritish Mukherjee and Sarath Witanachchi
Department of Physics and Center for Integrated Functional Materials (CIFM), University of Sout h Florida, Tampa, FL
Precipitation of the chemically compatible insulating oxide Mn3O4 on the surface of La0.7Sr0.3MnO3 (LSMO) thin films during their pulsed laser deposition (PLD) has been an emerging issue for fabricating high quality devices [1]. A recent study has shown that the magnetic properties of (La0.7Sr0.3MnO3)0.7:(Mn3O4)0.3 nanocomposite thin films strongly depend on the microstructure and lattice distortion due the presence of different domains and grain boundary structures [2]. Here, we report on the structure and magnetic properties of epitaxial Mn3O4/LSMO bilayer thin films, which were deposited in-situ at 750 oC on single crystal MgO (100) and SrTiO3 (STO) (100) substrates using PLD technique. X-ray diffraction (XRD) confirmed the single crystalline nature and epitaxial relationship between the layers. XRD asymmetric/symmetric scans revealed considerable increase in lattice distortion of LSMO with increasing the thickness of the Mn3O4 top layer. Cross-sectional high resolution transmission electron microscopy (HRTEM) evidenced a zig-zag interface between Mn3O4 and LSMO as opposed to the atomically sharp and flat interface between LSMO and substrates. Magnetization measurements showed a gradual decrease in the saturation magnetization (Ms) of the Mn3O4/LSMO bilayers with increasing the thickness of the Mn3O4 layer, keeping the same LSMO thickness of 100 nm for the samples. The Ms decreased from 436 emu/cm3 for single LSMO-STO films to 240 emu/cm3 and 166 emu/cm3 for Mn3O4/LSMO-STO bilayer films with 30 nm and 50 nm thick Mn3O4 layers, respectively. Due to the larger lattice mismatch between LSMO and MgO (~ 8%), the Ms (372 emu/cm3) of the LSMO film grown on an MgO (100) substrate was found to be smaller than that (436 emu/cm3) of the LSMO film grown on a SrTiO3 (100) substrate (lattice mismatch ~1%). These results revealed that the intrinsic growth morphology of Mn3O4 layers on LSMO thin films significantly changed the lattice strain of LSMO and hence the magnetic properties of the material.
References
[1] T. Higuchi, T. Yajima, L. Fitting Kourkoutis, Y. Hikita, N. Nakagawa, D. A. Muller, and H. Y. Hwang, Appl. Phys. Lett. 95, 043112 (2009). [2] Z. Bi, E. Weal, H. Luo, A. Chen, J. L. MacManus-Driscoll, Q. Jia, and H. Wang, J. Appl. Phys. 109, 054302 (2011).
CP-12. Structure and properties of epitaxial perovskite Pb(Zr0.52Ti0.48)O3/ La0.7Sr0.3MnO3 heterostructures
Cheng Zou1, 2, Yuan-fu Chen1, Bing Peng1, Wen-Xu Zhang1 and Run-Wei Li2
1University of Electronic Science and Technology of China, Chengdu, China; 2Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China
Recently, magnetoelectric effects are attracting much attention. In this article, we focus on the preparation, morphology, crystalline structure, and the magnetoelectric effect of epitaxial peroskite ferroelectric/ferromagnetic heterostructures. Pb(Zr0.52Ti0.48)O3/La0.7Sr0.3MnO3 (PZT/LSMO) structures were grown on single crystalline SrTiO3 substrates by pulse laser deposition. The AFM morphology characterization shows that both LSMO layer and PZT top layer are smooth and homogenous. The crystalline structure measurements indicate th at good epitaxial relationships between LSMO and PZT and STO were obtained, as shown in Figure 1. Interestingly, the ferroelectric behavior of PZT layer was found to be obviously influenced by the magnetic state of LSMO layer and the ferromagnetic property of LSMO layer was also influenced by the ferroelectric polarization state of PZT layer.
CP-13. Size effect on the structural, magnetic and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3
Rini Thomas2, Gangadhar Das2, Rajib Mondal2, Rabindra N. Mahato1, R. Nirmala2, A. V. Morozkin3, Jagat Lamsal4, W. B. Yelon4, 5, A. K. Nigam6 and Satish K. Malik1
1International Institute of Physics (IIP)-UFRN, Natal, Brazil; 2Indian Institute of Technology Madras, Chennai, India; 3Chemistry, Moscow Lomonosov State University, Moscow, Russian Federation; 4University of Missouri-Columbia, Columbia, MO; 5Missouri University of Science and Technology, Rolla, MO; 6Tata Institute of Fundamental Research, Mumbai, India
There is a great deal of interest in understanding the fundamental properties of colossal magnetoresistance manganites as a function of grain size. In fact, systematic studies have been performed on hole-doped R1-xAxMnO3 type oxides (where R = rare earth, A = divalent cation such as Ca, Sr) but electron-doped systems have not been explored in detail. Recent work on one electron-doped system, namely, Ca0.15Ca0.85MnO3 reveals an incomplete crystal structural transition from orthorhombic to monoclinic type at ~170 K concomitant with a paramagnetic to antiferromagnetic (AFM) phase transition [1]. Magnetic structure of this compound undergoes a further change to a different AFM type at ~100 K.[1,2] Neutron diffraction data indicate a competitive coexistence of antiferromagnetic and ferromagnetic (FM) interactions in the structurally phase separated state. The present work aims to understand the grain size dependent magnetic and transport properties of this oxide. Nanocrystalline La0.15Ca0.85MnO3 samples of various grain sizes ranging from ~17 to 35 nm have been prepared by sol-gel technique. Phase purity and composition are verified by room temperature X-ray diffraction and SEM-EDAX analysis. DC magnetization measurements on nanocrystalline La0.15Ca0.85MnO3 of particle size ~17 nm shows a peak at ~170 K (TN) in zero-field-cooled (ZFC) state which is close to the TN of the bulk[1]. Field-cooled magnetization bifurcates from ZFC data around 250 K hinting a weak FM component near room temperature due to surface moments of the nanoparticle sample. AC magnetic susceptibility of this sample shows a peak at TN which shifts to higher temperatures with increasing frequency of the applied ac field indicating glassy behaviour. A maximum magnetoresistance of about -42% in 9 T is observed at 80 K, in the magnetically ordered state. Low temperature neutron diffraction experiments (incident neutron wavelength 1.478961 Å, MURR, USA) suggest that the incomplete structural transition also occurs in the nanoparticle sample as in the bulk. The magnetic entropy change near TN is calculated from the field dependent magnetization data and an inverse magnetocaloric effect is observed signifying the AFM state.
References
1. Tong et al, Appl. Phys. Lett. 93 (2008) 202504 2. Qian et al, Phys. Rev. B 77 (2008) 094423
CP-14. Imp act of Fe doping on radiofrequency magnetotransport in La0.7Sr0.3Mn1-xFexMnO3
Sujit Kumar Barik and Mahendiran Ramanathan
Physics, National university of Singapore, Singapore, Singapore
Mn-based perovskite oxides have been studied extensively for the past fifteen years due to colossal magnetoresistance behavior exhibited by them. In contrast to numerous studies of magnetoresistance measured with direct current, magnetoresistive response of manganites subjected to alternating current or magnetic field received less attention[1,2,3].Ac magnetotransport in manganites will be interesting for practical applications and fundamental studies because ac magnetoresistance of more than 30% in a low magnetic field of H = 600 Oe has been demonstrated whereas a magnetic field of 7 Tesla is needed to induced dc magnetoresistance of same magnitude. A comparison of ac magnetotransport in Ln0.6Sr0.4MnO3 also revealed contrasting behavior between Ln = La and Sm, which underscores different electronic contributions to the ac transport[4]. In this work, we have investigated the effect of Fe doping on the ac impedance (Z(T,H,f)= R(T,Hf)+iX(T,H,f)) as a function of temperature(T), magnetic field(H) and frequency (f) of the ac current (f = 0.1 -20 MHz) in La0.7Sr0.3Mn1-xFexO3. It is found that Z in zero field shows an abrupt jump at a temperature T* which agrees with the ferromagnetic Curie temperature TC in all the compositions. The appearance of the anomaly in Z depends on the frequency which increase with increasing Fe content. The anomaly in Z precedes the insulator-metal transition as x increases. Ac magnetoresistance as high as 40% in a field of 700 Oe has been obtained for x = 0.05 sample. We discuss possible mechanisms of the enhanced ac magnetoresistance at low magnetic fields observed in these compounds.
References
1. C. Y. Hsu, H. Chou, B. Y. Liao and J. C. A. Huang, Appl. Phys. Lett. 89, 262510 (2006). 2. D. Stanescu, P. Xavier, J. Richard and C. Dobourdieu, J. Appl. Phys. 99, 073707 (2009). 3. A. Rebello, C. L. Tan and R. Mahendiran, Solid State Comm. 149, 1204 (2009). 4. V. B. Naik, A. Rebello, and R. Mahendiran, J. Appl. Phys.109, 07C728 (2011)
CP-15. Magnetic properties of 1D-Ising chain CoV2O6
Bongjae Kim, Beom Hyun Kim, Kyoo Kim, Hong Chul Choi, Sang-Yeon Park, Yoon Hee Jung and Byung Il Min
Physics, Pohang University of Science and Technology, Pohang, Republic of Korea
CoV2O6 (CVO) is an interesting material that has quasi-one dimensional Ising ferromagnetic chains of edge-shared CoO6 octahedra. There are two types of CVO: α and γ phase. Recent experiments have shown that α-CVO has peculiar magnetic characteristics. Among the attractions are the one-third plateau in MH-curve, the exceptionally large magnetic moment found (4.5μB) in high-spin Co2+(d7) and the unique magnetic anisotropy differently from other similar systems. We have investigated the magnetic properties and unique anisotropy of this Co-vanadate brannerite system employing the full-potential linearized augmented plane wave (FLAPW) band method. It was found that the cooperative effects of the peculiar crystal field and the spin-orbit interaction play an important role. Also, the experimental specific data indicates a low-dimensional character of the system. Analyzing the system using band calculation and microscopic model calculation, we have found that the strong intrachain interaction freezes the spin degree of freedom and Ising-type chain character can be shown.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Sami El-Khatib, American University of Sharjah
CQ-01. control of magnetic and transport properties in Nd0.45Sr0.55MnO3 films through epitaxial strain
Yuqin Zhang, Hui Meng, Xianwei Wang, Yinlian Zhu and Zhidong Zhang
Institute of metal research, Shenyang, China
The influence of epitaxial strain on the magnetic and transport properties of Nd0.45Sr0.55MnO3 (NSMO) thin films has been investigated. Here a tuned tetragonal c/a distortion is obtained , from 0.965 to 1.034, through the use of different substrates and varying film thicknesses. It is found that the 6nm film grown on SrTiO3 (STO), under the largest tensile strain, and films grown on NdGaO3 (NGO), also under tensile strain, exhibit bulk-like transport properties, namely, metallic behavior appearing below the A-type antiferromagnetic (AF) temperature. Moreover, the former is highly stable with the resistance at 7T being almost unchanged compared to the zero-field case. While thick NSMO films grown on STO under tensile strain and films grown on LaAlO3 (LAO) under compressive strain exhibit insulator behavior in the measured temperature range at zero field, an insulator-metal transition is only observed in the 30nm film grown on LAO under a magnetic field of 7T. This is possibly related to a transition from C-type AF insulator to ferromagnetic metal below the C-type AF temperature of 255K, which is the peak temperature recorded by the magnetic measurement. In addition, ferromagnetic fluctuation is observed above the A-type AF temperature in the 80 nm film grown on LAO. In a word, the correlation between our transport and magnetic measurements indicates that epitaxial strain, depending on the value of c/a, can enhance or decrease the stability of A-type AF so far as to produce the appearance of C-type AF or ferromagnetic fluctuation above the A-type AF temperature TN. In addition, epitaxial strain modulates TN over a large temperature range from 150 K to 280 K, thus expanding the application range of this useful spin structure.
References
1.Y. Tokura and N. Nagaosa, Science 288, 462 (2000). 2.K. Horiba, A. Maniwa, A. Chikamatsu, K. Yoshimatsu, H. Kumigashira, H. Wadati, A. Fujimori, S. Ueda, H. Yoshikawa, E. Ikenaga, J. J. Kim, K. Kobayashi, and M. Oshima, Phys. Rev. B 80, 132406 (2009). 3.R. Kajimoto, H. Yoshizawa, H. Kawano and H. Kuwahara, Phys. Rev. B 60, 9506(1999). 4.T. S. Santos, S. J. May, J. L. Robertson and A. Bhattacharya, Phys. Rev. B 80, 155114 (2009).
CQ-02. Nanometer size effects on magnetic order in La1-xCaxMnO3 (x = 0.5 and 0.6) manganites, probed by ferromagnetic resonance
Alexander I. Shames1, E. Rozenberg1, E. Sominski2 and A. Gedanken2
1Physics, Ben Gurion University of the Negev, Be'er-Sheva, Israel; 2Department of Chemistry and Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan, Israel
It is well documented [1] that in the La1-xCaxMnO3 system the complex magnetic/ electronic ground state with ferromagnetic (FM) and charge ordered antiferromagnetic (CO/AFM) components and pure CO/AFM one are characteristic for x = 0.5 and 0.6 (Ca0.5 and Ca0.6) bulk manganites, respectively. This seems to be a typical example of electron-hole doping asymmetry in La-Ca manganites. To study an influence of the transition from bulk- to nanometer sized samples on the above asymmetry, we have used the low temperature FM resonance (FMR) data, which were analyzed to explore the long-range magnetic/ electronic order in nano-sized half- and electron-doped Ca0.5 and Ca0.6 compounds. Representative Ca0.5 and Ca0.6 samples, having about the same mean grain size of 18 ± 3 nm were chosen for comparison of their FMR data with corresponding results for bulk. These sonochemically prepared [2] nanoparticles show a single FMR line down to ~ 5 K, which evidences on their high homogeneity and decreased magnetic anisotropy as compared to those in bulk. The normalized doubly integrated intensity (DIN) of the FMR signal for nano Ca0.5 is enhanced on about an order of magnitude in regards to corresponding value in bulk. In a contrast, low temperature DIN (proportional to transverse magnetic susceptibility) recorded on nano Ca0.6 is enhanced, but comparable with the bulk value. Mutual analysis of the FMR data and the results of magnetic and neutron diffraction measurements [1, 3], allowed us to conclude the following. The combined effects of dimensional decrease and homogeneity improvement on magnetic order in Ca0.5 and Ca0.6 nano-samples are strongly different. A suppression of bulk-like CO/AFM component and stabilization of frustrated FM-like order were found for Ca0.5. While, the CO/AFM ground state appears to be stable and coexists with an enhanced surface FM spin correlations in nano Ca0.6. The latter result may be connected to the local nature of ferromagnetic double exchange coupling, leading to the localization of carriers, as was shown recently for electron doped Ca0.7 compound [4].
References
[1] M. Pissas et al. PRB 68, 134414 (2003). [2] G. Pang et al. Mater. Res. Bull. 38, 11 (2003). [3] Z. Jirák et al. Phys. Rev. B 81, 024403 (2010). [4] M. Auslender et al. JAP 109, 07D705 (2011).
CQ-03. Electron resonance and magnetic response of low-doped La0.88Ca0.12MnO3 and La0.9Sr0.1MnO3 manganite single crystals
Evgeny Rozenberg1, A. I. Shames1, M. I. Tsindlekht2, I. Felner2 and Ya M. Mukovskii3
1Physics, BGU of the Negev, Beer-Sheva, Israel; 2Racach Institute of Physics, Hebrew University, Jerusalem, Israel; 3Physics, Moscow Steel and Alloys Institute, Moscow, Russian Federation
The La0.88Ca0.12MnO3 and La0.9Sr0.1MnO3 (Ca0.12 and Sr0.1) single crystals belong to so called ‘low-doped’ manganites characterized by magnetic ground state, which comprises coexisting canted antiferromagnetic- and ferromagnetic-insulator (AFMI and FMI) domains [1-3]. To probe this complex ground state in Ca0.12 and Sr0.1 at different time and length scales, we have used the FM resonance (FMR) and ac/dc magnetic measurements. The Ca0.12 and Sr0.1 crystals, grown by floating zone method, show respective Curie temperatures (TC) of about 135 and 141 K. This fact evidences on closeness of the effective concentration of Mn4+ ions and equivalency of the double exchange strength in these samples. While, in the vicinity of TC the real part of ac susceptibility demonstrates characteristic ‘two maxima’ shape, signaling therefore on a complex nature of magnetic transition in both crystals. Their low-temperature (LT) dc magnetization curves are non-linear and non-saturated at the magnetic field up to 5 T. Maxima of doubly integrated intensity (DIN, pr oportional to high frequency magnetic susceptibility) of the FMR signals are observed at ~120 K for both samples and DIN in Ca0.12 is notably enhanced in regards to corresponding value in Sr0.1. In addition, a noticeable maximum of DIN versus T is recorded on Sr0.1 at about 60 K, which is in line with the ac susceptibility data. Analysis of the results of both FMR and magnetic measurements allowed us to conclude the following. The Ca0.12 crystal is slightly under-doped for the transition to LT FMI state [1], so the numerous FM clusters embedded into canted AFM matrix are observed; while, an incomplete transition to the FMI state in Sr0.1 is accompanied by following AFM and orbital ordering upon cooling. The mesoscale chemical disorder and structural imperfections in these crystals [4] may lead to a partial depletion and confinement of holes in some regions. Such disorder seems to be an important prerequisite for the formation of observable complex magnetic/electronic ground state.
References
[1] G. Biotteau et al., Phys. Rev. B 64, 104421 (2001). [2] A. de Andres et al., Phys. Rev. B 79, 014437 (2009). [3] J. Deisenhofer et al., Phys. Rev. Lett. 95, 257202 (2005). [4] E. Rozenberg et al., Appl. Phys. Lett. 92, 222506 (2008).
CQ-04. Magnetic tunability and photovoltaic response in La0.8Hf0.2MnO3/Nb-SrTiO3 heteroepitaxial junctions
Zhenping Wu, Lin Wang and Ju Gao
Physics, The University of Hong Kong, Hong Kong, Hong Kong
The magnetic tunability and optoelectronic response in electron-doped manganites-based heterojunctions has been studied. La0.8Hf0.2MnO3 (LHMO) is a tetravalent cation-doped manganite in which the La3+ ions of the parent compound LaMnO3 are substituted by Hf4+ ions. In our study, heterojunctions were fabricated by depositing LHMO films on 0.7 wt% Nb-doped SrTiO3 single crystal substrates. Good rectifying characteristics were observed in these heterojunctions in a wide temperature range from 40 to 280 K as shown in Fig. 1. It was also found that the transport properties of these heterojunctions could be considerably influenced by external magnetic fields. When a magnetic field was applied, I-V curves were shifted to a lower bias voltage. Both ideality factor and saturation current were also affected by magnetic fields. Special attention has been paid to the diffusion voltage , photovoltage open circuit voltage (Voc) and short circuit current. It is notable that, under illumination of a light with λ = 532 nm, the I-V curve shifts downward with an Voc of 0.72 V at 40 K and 0.21 V at 280 K (insert of Fig. 1). The energy band diagram of such heterojunction is constructed and physics related to the rectifying behaviors as well as the photovoltaic effect is discussed.
CQ-05. Electron spin resonance and magnetization studies on Bi0.5Ca0.5Mn0.95TM0.05O3 (TM = Cr, Fe, Co and Ni)
D. Vijayan, Joji Kurian and Rajender Singh
School of Physics, University of Hyderabad, Hyderabad, India
In the recent years there has been interest in understanding the effect of substitution of 3d TM ion at Mn site on the properties of Bi-manganites1-5. In this work, we present the influence of various 3d TM dopant on the charge ordering (CO) and magnetic properties of Bi-manganite samples Bi0.5Ca0.5Mn0.95TM0.05O3 (TM= Cr, Fe, Co and Ni) synthesized by solid state reaction method. The crystal structure was estimated by XRD studies.The detailed temperature dependent ESR and magnetization measureme nts were carried out in the temperature range 4- 450K. The temperature dependent double integrated intensity (DI) of the ESR signal and the magnetic moment (M) have been used to estimate the charge ordering temperature (TCO), antiferromagnetic ordering (AFM) onset temperature TO and Neel temperature, TN. The TCO values from the ESR data are ~ 317K, 286K and 258K for undoped, Ni and Fe doped samples respectively. Very weak CO transition is observed in the case of Cr doped sample. The TCO value for Co doped sample could not be estimated from ESR studies as no ESR signal is observed for this sample. However, the magnetization data indicate a weak CO transition at ~ 265K for this sample. The 1/DI vs T and 1/M vs T plots show FM correlations in the temperature range T >TCO and coexistence of ferromagnetic (FM) and AFM correlations in the temperature range TCO >T >TO. The magnetization data give TCO values close to the values obtained from ESR data. The TO values from ESR data are higher than obtained from magnetization data. This is because ESR being an atomic probe can sense AFM onset in microregions taking place at higher temperatures. The experimental data are analyzed in view of electronic phase separation (PS) model. The suppression of charge ordering is explained in view of the changes in short range magnetic interactions between Mn ions and the possible change in the band structure as a result of TM ion doping.
References
1.D. Vijayan, Joji Kurian and R. Singh, IEEE Transactions on Magnetics-2011 (in press). 2.Joji Kurian and R. Singh J. Physics: Confer. Series 200, 012100 (2010). 3.Joji Kurian and R. Singh J. Appl. Phys. 107, 09D715 (2010). 4.D. Tzankov, D. Kovacheva, K. Krezhov, R. Puzniak, A. Wisniewski, E. Svab and M. Mikhov J. Phys.: Condens Matter 17, 4319 (2005). 5.O. Toulemonde, I. Skovsen, F. Mesguich and E. Gaudin Solid State Sci. 10, 476 (2008).
CQ-06. Phase coexistence and magnetocaloric effect in Sm1-xSrxMnO3 (x = 0.42, 0.44, 0.46) manganites
N. S. Bingham1, T. L. Phan2, M. H. Phan1, S. C. Yu2 and H. Srikanth1
1Department of Physics, University of South Florida, Tampa, FL; 2Department of Physics, Chungbuk National University, Cheongju, Republic of Korea
Sm1-xSrxMnO3 (SSMO) with 0.3< x <0.6, has been reported to show a multi-critical magnetic phase diagram [1]. SSMO with x~0.44 is most interesting as it undergoes multiple phase transitions, with an antiferromagnetic transition at TN~350 K - 200 K, followed by a charge ordering transition at TCO ~125 K - 170 K, and finally a ferromagnetic transition at TC ~130 K. While the nature of phase coexistence and critical magnetic behavior around TC in this system have been explained to some extent, an anomalous decrease in the magnetization observed around 40 K is yet to be understood. To address this emerging issue, we have systematically studied DC magnetization, radio-frequency transverse susceptibility (TS), and magnetocaloric effect (MCE) in polycrystalline Sm1-xSrxMnO3 (x=0.42, 0.44, 0.46) samples prepared by a standard ceramic method. The magnetic entropy change ΔSM was calculated from a family of the M-H isotherms using the Maxwell relation. X-ray diffraction patterns indicated the high quality of the samples with an orthorhombic structure. Magnetization measurements and Arrot plot analyses reveal a first-order ferromagnetic transition for all samples investigated, with TC = 130 K, 134 K, and 133 K for x = 0.42, 0.44, and 0.46, respectively. The drop in the magnetization is observed at TS = 38 K, 40 K, and 37 K for x = 0.42, 0.44, and 0.46, respectively. Interestingly, TS experiments reveal metamagnetic states around TC and a distinct change in TS spectra at T < TS for all the samples, pointing to a coexistence of ferromagnetic and antiferromagnetic phases at ~TC and that TS is actually the onset of a spin-reorientation of the spontaneous magnetism. A relatively large ΔSM (~3.5 J/kg K for Δμ0H = 5T) has been observed around TC for the samples. Support: DOE-BES (HS) and USF-NRG (MHP).
References
[1] Y. Tomioka, H. Hiraka, Y. Endoh, and Y. Tokura, Phys. Rev. B 74, 104420 (2006).
CQ-07. The magnetic field-induced positive magnetoresistance effect in buffer layer modified manganite-based heterojunctions
Weiwei Gao1, Weiming Lü1, Aidong Wei1, Jirong Sun1, Jing Wang1, Fengxia Hu1, Jun Shen2 and Baogen Shen1
1State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Science, Beijing, China; 2Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing, China
Manganites have been the focus of past decades due to their colossal negative magnetoresistance(MR).1,2 However, the MR of simple manganite junction is quite low, though it is still negative.3,4 After introducing a LMO interface modified layer, here we prepared the new-type La0.67A0.33MnO3/LaMnO3(t)/SrTiO3:0.05wt% Nb (LAMO/LMO(t)/STON) heterojunctions with a LMO layer of the thickness from 0 to 12 nm (A=Ca, Sr). An enhancement in magnitude and a change in sign of the positive MR are observed when the junctions are under forward biased. The magnetic response is found to be strongly dependent on the thickness of the LMO layer and, furthermore, it is significantly stronger in LCMO/LMO(t)/STO than in LSMO/LMO(t)/STO. The LAMO/LMO(t)/STON junctions were fabricated by the pulsed laser ablation technique. Rectifying characteristic analysis shows an excellent linear relation between log J and V. Magnetic field causes a decrease in current, signifying a positive MR. This effect varies with the thickness of LMO. It is weak when t is small, maximizes at t ≈ 4 nm, and disappears when t exceeds 8 nm. A direct calculation gives the maximal MR of ~90% of the LCMO and ~52% of the LSMO junctions, obtained at 50 K under a field of 5 T. Significant MR persists up to the temperature 350 K, and it is ~30% and ~24% for the LCMO and LSMO junctions, respectively. The MR is further found to vary regularly with applied field, growing first rapidly then slowly with the increase of magnetic field, and there is no tendency to saturation up to 5 T. The enhanced MR could be ascribed to the decoration effects of LMO on the LAMO/STON interface. Its presence may awaken the dead layer, making it is easier for the magnetic field to active localized holes at the junction interface. This actually means a field-induced growth in depletion width and interfacial barrier, thus a positive MR. This effect is especially strong when t is small. However, when the buffer layer is thick, the proximity effect is depressed. As for the difference of the MR of the LCMO and LSMO junctions, it could be ascribed to the different magnetic responses of LCMO and LSMO.
References
[1] H. Tanaka, J. Zhang, and T. Kawai, Phys. Rev. Lett. 88, 027204 (2001). [2] J. R. Sun, C. M. Xiong, T. Y. Zhao, S. Y. Zhang, Y. F. Chen, and B. G. Shen, Appl. Phys. Lett. 84,1528 (2004). [3] N. Nakagawa, M. Asai, Y. Mukunoki, T. Susaki, and H. Y. Hwang, Appl. Phys. Lett. 86, 082504 (2005). [4] D. J. Wang, J. R. Sun, Y. W. Xie, W. M. Lü, S. Liang, T. Y. Zhao, and B. G. Shen, Appl. Phys. Lett. 91, 062503 (2007).
CQ-08. Large magnetocaloric effect for magnetic refrigeration from 210 to ~275 K in La0.7Ca0.3Mn1-xCoxO3
YingDe Zhang, The-Long Phan, Peng Zhang and Seong-Cho Yu
Department of Physics, Chungbuk National University, Cheongju, Republic of Korea
We have studied the magnetic properties and magnetocaloric effect in La0.7Ca0.3Mn1-xCoxO3 (0.0 ≤ x ≤ 0.05) prepared by solid-state reaction. Investigations of magnetization versus temperature revealed that both magnetization and the Curie temperature (TC) decreased gradually with increasing Co content. Basing on field dependences of magnetization recorded at various temperatures, curves of magnetic entropy change (ΔSM) for the samples were determined, as shown in Figure 1. Under an applied field of ΔH = 15.0 kOe, the maximum ΔSM achieved around TC for x = 0.0 and 0.05 are about 5.8 and 5.1 J.kg-1.K-1, respectively. While the linewidth of ΔSM curves (δTFWHM) ranges from 12 (for x = 0.0) to 16 K (for x = 0.05), the refrigerant capacity is in between 70 and 83 J/kg. Though an increase in Co content widens δTFWHM (i.e., the magnetic phase transition becomes smoother), the samples still exhibit the first-order phase transition. This is due to the fact that the Co doping does not cause remarkable changes in an orthorhombic structure of La0.7Ca0.3MnO3. If combining these samples for magnetic refrigeration, the temperature range can be used in between 210 and 275 K (with ΔT = 65 K, see Fig. 1), where ΔS is about 3.5 J.kg-1.K-1. Thus RC is about 228 J/kg, and comparable to that of some alloys. Such the features make La0.7Ca0.3Mn1-xCoxO3 become promising magnetocaloric materials.
CQ-09. Strain effect caused by substrates on phase separation and transport properties in Pr0.7(Ca0.8Sr0.2)0.3MnO3 thin films
Ying-ying Zhao, Jing Wang, Feng-xia Hu, Ling Chen, Ji-rong Sun and Bao-gen Shen
State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing, China
Phase separation(PS) has been supposed to be a universal phenomenon in Colossal magnetoresistive(CMR) manganites.[1] Pr1-xCaxMnO3 with narrow eg band(W) exhibits charge-orbital ordering(COO). The chemical replacement of Sr for Ca at A sites widens the W and leads to a metal-insulator(MI) transition in a percolative manner.[2] Here, we report strain effect on PS and transport properties in Pr0.7(Ca0.8Sr0.2)0.3MnO3(PCSMO) thin films. We deposited PCSMO thin films on three kinds of (100)substrates, STO(3.905Å), LAO(3.788Å), and LSAT(3.868Å) by PLD technique. The films of 100nm on STO and LSAT experience tensile strains(-1.28% for the former and -0.08% for the latter) while the one on LAO experiences compressive strain of about 2.08%. Fig.1 displays the temperature dependence of resistance(R) for the films and bulk. The bulk shows hysteresis at MI transition due to percolative mechanism, which is generally indicative of PS.[1] However, the films on tensile strained STO a nd LSAT show insulating behavior in the full temperature range, indicating the COO phase is enhanced and the percolation threshold is not reached. For the film on STO, the large tensile strain (-1.28%) enhances the stability of the long-range COO phase so that a 5T magnetic field can not melt the COO phase, resulting in an insulating behavior even under 5T. In contrast, the COO phase in the film on LSAT is less robust due to small tensile strain(-0.08%), a 5T magnetic field can make it partially melt and bulklike percolative transport with hysteresis was observed. For the compressive strained films on LAO, sharp MI transition with pronounced hysteresis was observed for both films, indicating that the long-range COO is suppressed while the short-rang FM/COO is favored. Upon magnetic field application, R reduces remarkably while the MI transition and hysteresis becomes broader and insignificant, indicating the COO is melted into FM and PS becomes less pronounced. Details about strain effect, competition between FM and COO phase, PS, and CMR are discussed.
References
[1] M. Uehara, et al, Nature 399, 560 (1999). [2] Y. Tomioka, et al, Phys. Rev. B 66, 104416 (2002).
CQ-10. Double Exchange Interaction between Mn3+ and Ru4+ ions in La1-xSrxMn1-xRuxO3
Yao Ying1, 2, Jingwu Zheng1, Liang Qiao1, Shenglei Che1, Liqiang Jiang1, Li Pi2, Langsheng Ling2 and Yuheng Zhang2
1College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, China; 2High Magnetic Field Laboratory, University of Science and Technology of China, Hefei, China
In manganites, the substitution of Ru ion at Mn site induces ferromagnetic metal (FMM) behavior. However, the role of Ru ion in the formation of the FMM state is still in debate. Primarily, the type of interaction between Mn3+ and Ru4+ should be clarified. To study this issue, we designed a system of La1-xSrxMn1-xRuxO3, in which Mn and Ru ions should present only as Mn3+ and Ru4+. All samples are taken out in part annealing in Ar atmosphere to investigate the effect of excessive oxygen. In this work, we confirmed that the interaction between Mn3+ and Ru4+ is double exchange (DE). The magnetic and the transport properties of polycrystalline La1-xSrxMn1-xRuxO3 (0<=x<=0.80) were investigated. For both unannealed and annealed samples, ferromagnetism is enhanced with x increasing for x<=0.30, which indicates that Ru substitution induces and enhances ferromagnetism. With Ru substitution further increasing for x>=0.40, Curie temperature decreases. As for the transport properties, all samples show insulating behaviors, but resistivity decreases with x increasing for both unannealed and annealed samples. Meanwhile a shoulder appears in the /row(T) curves at around corresponding Curie temperature for 0.10<=x<=0.40. Magnetoresistence effect is observed in x=0.40. Thus, it is concluded that the interaction between Mn3+ and Ru4+ is DE interaction. For X<=0.30 , the DE Mn3+-Ru4+ interaction induces and enhances ferromagnetism. Due to the lower Curie temperature corresponding to the Ru4+-Ru4+ FM interaction, the enhanced Ru4+-Ru4+ FM interaction oppositely suppresses Curie temperature for x>=0.40 .
CQ-11. Hall effect in La1-xCaxMnO3 (x=0.23, 0.3)
Yakov M. Mukovskii1, Inna I. Lobanova1, Mikhail A. Anisimov2, Sergey V. Demishev2, Nikolay E. Sluchanko2, Natalia A. Kozlovskaya1 and Vladimir V. Glushkov2
1Theoretical Physics and Quantum Technologies, National Science and Technology University (MISiS), Moscow, Russian Federation; 2A.M.Prokhorov General Physics Institute of RAS, Moscow, Russian Federation
We report the results of magnetoresistance and Hall effect study carried out on the single crystals of La1-xCaxMnO3 (x=0.23, 0.3) in the wide temperature interval (4.2-300K) in magnetic fields up to 7 T. For x=0.23 compound a metal-insulator transition is clearly identified on the temperature dependence of resistivity at T*≈31 K, which is well within the ferromagnetic phase (TC=184.5K). Below T* the resistivity ρ(T) of La0.77Ca0.23MnO3 is found to decrease by more than 20 times saturating at rather low values of ρ(T=4.2K)~2.8Ωcm. Separating of normal and anomalous contributions to Hall effect showed that reduced carrier concentration n/nMn for x=0.23 compound corresponds rather well to the nominal doping while the high values of n/nMn~1-2 for La0.7Ca0.3MnO3 are induced by two (hole and electron) groups of carriers with very low mobilities μH~(0.1-1)cm2/(Vs). The exponential uprise of anomalous Hall coefficient RHA(T)~exp(T/T0) with the characteristic temperature T0=54.4K is observed for the first time in the ferromagnetic state of x=0.3 compound (T<TC=249 K). This extraordinary behavior of RHA(T) can not be explained by neither side-jumping or skew-scattering models nor a Berry phase contribution to be appeared for a carrier moving in a topologically nontrivial spin background1. Support by Federal Programme “Scientific and Educational Human Resources of Innovative Russia” and Joint RFBR-NSF Grant 09-02-92661 is acknowledged.
References
1 J. Ye et al., Phys. Rev. Lett. 83, 3737 (1999)
CQ-12. Influences of leakage currents on the transport properties and photoelectric effects in Pr0.7Sr0.3MnO3/Nb:SrTiO3 heterojunctions
Jianfeng Wang, Zhenping Wu and Ju Gao
Department of Physics, The University of Hong Kong, Hong Kong, China
Despite great importance and common existence, non-ideal factors, e.g. the leakage currents, have been rarely discussed for heterojunctions composed of manganites and titanates. In this work, the effects of leakage currents in Pr0.7Sr0.3MnO3/0.05 wt.% Nb-doped SrTiO3 heterojunctions were studied. These effects was explored by comparing current-voltage curves (both with and without light illumination) for various small-size heterojunctions (~250μm×250μm) on the same substrates. Under reverse bias, the transport in less rectified junctions was dominated by leakage currents. In contrast, small leakage currents had less impact on the current transport under forward bias. Open circuit voltage was found to be sensitive to leakage currents. Thus, it provides a useful tool to detect non-ideal factors in such junctions. These results are helpful for identifying and understanding carrier transport processes in heterojunctions composed of manganites and titanates.
CQ-13. Structural, transport, magnetic properties and band structure calculations of Nd doped two dimensional compound Sr2CoO4
Qiwen Yao1, Hideo Kimura1, Xiaolin Wang2, Kosta Konstantinov2 and Hongyang Zhao1
1Multifunctional Materials Group, Optical and Electronic Materials Unit, National Institute for Materials Science, Tsukuba, Japan; 2Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong 2500, NSW, Australia
Here we report our studies of the two dimensional perovskite compounds Sr2−xNdxCoO4 (x=0.5, 0.75, 1, 1.25). Rietveld refinement confirmed that these compounds crystallize in K2NiF4 type (I4/mmm) structures. Lattice parameter c was found to decrease as the doping level x increases. Paramagnetic to ferromagnetic transition temperatures was found to be about 150, 100K for the x=1 and 0.75 samples respectively. Band structure calculations indicated that these compounds show high spin polarization. The temperature dependence of resistivity for the samples can be well fitted by the 2D Variable Hoping model.
References
[1] X. L. Wang and E. Takayama-Muromachi, Phys. Rev. B 72, 064401 (2005).
[2] J Matsuno, Y. Okimoto, Z. Fang, X. Z. Xu, Y. Matsui, N. Nagaosa, M. Kawasaki, and Y. Tokura, Phys. Rev. Lett. 93, 167202 (2004).
[3] Y. Moritomo, K. Higashi, K. Matsuda, and A. Nakamura, Phys. Rev. B 55, R14725 (1997).
[4] Y. Shimada, S. Miyasaka, R. Kumai, and Y. Tokura, Phys. Rev. B 73, 134424 (2006).
[5] B. A. Hunter, Commission Powder Diffraction Newsletter 20, 21 (1998).
[6] M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark, and M. C. Payne, J. Phys.: Condens. Matter 14, 2717 (2002).
[7] M. Sanchez-Andujar and M. A. Senaris-Rodriguez, Solid State Sci. 6, 21 (2004).
[8] M. Ziese, Rep. Prog. Phys. 65, 143 (2002).
[9] H. Y. Hwang, S.-W. Cheong, N. P. Ong, and B. Batlogg, Phys. Rev. Lett. 77, 2041 (1996).
[10] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, and Y. Tokura, Nature (London) 395, 67 (1998).
CQ-14. Structure and properties of novel cobaltates Ln0.30CoO2 (Ln=La, Pr, and Nd)
K. Knizek1, Z. Jirak1, J. Hejtmanek1, M. Marysko1 and J. Bursik2
1Institute of Physics ASCR, 162 00 Prague 6, Czech Republic; 2Institute of Inorganic Chemistry ASCR, 250 68 Rez near Prague, Czech Republic
Novel layered cobaltates LnxCoO2 (Ln = La, Pr, Nd; x~0.30) with triangular lattice of cobalt ions have been prepared by an ionic exchange from isostructural NaxCoO2 (x~0.90) precursors. Their structural characterization shows that Ln3+ ions occupy one third of available prismatic sites between the CoO2 layers, forming a 2-dimensional superstructure. The deviation from the ideal stoichiometry Ln1/3CoO2 introduces itinerant hole carriers into the diamagnetic LS Co3+ matrix. Their narrow band character is manifested by a significant Pauli paramagnetism and thermopower that increases linearly with temperature and saturates above room temperature at large value of 150-200 μV/K. On the other hand, the conductivity is affected by occupational disorder and follows Mott’s T-1/3 law characteristic for variable range hopping in 2-dimensions. The magnetism of LnxCoO2 is strongly enhanced by insertion of rare-earths ions Pr3+ and Nd3+, which bring about local magnetic moments. By virtue of exchange interactions with itinerant spins in cobalt subsystem, a ferromagnetic state can be stabilized, as found actually for Nd0.30CoO2. The ordering develops at low temperatures of few K only, but an increase of critical point with increased hole doping may be anticipated. The present study thus opens a route to thermoelectric systems which properties will be effectively controlled by magnetic fields.
CQ-15. Magnetic and Magnetotransport Properties of Misfit Cobaltate Ca3Co3.93O9+δ
Jirí Hejtmánek1, Karel Knízek1, Miroslav Maryško1, Zdeněk Jirák1, David Sedmidubský2, Ondrej Jankovský2, Štěpán Huber2, Bertrand Lenoir3 and Philippe Masschelein3
1Magnetics and Superconductors, Institute of Physics of the ASCR, v.v.i, Cukrovarnická 10, 162 00 Praha 6, Prague, Czech Republic; 2Inorganic Chemistry, Institute of Chemical Technology Prague,Technická 5, 166 28 Prague 6, Prague, Czech Republic; 3CP2S, Institut Jean Lamour, UMR 7198, CNRS-Nancy Université-UPVM, Ecole Nationale Supérieure des Mines de Nancy, Parc de Saurupt, 54042 Nancy Cedex, Nancy, France
Misfit Ca3Co3.93O9+δ samples were prepared by various sintering processes including classical ceramic route (CCR) and spark plasma sintering (SPS). The samples were characterized by means of magnetic, electric and thermal measurements in the region of 2-1200 K and the data were analyzed respecting that sample did not exhibit perceptible texturing. The distinct behaviour between oxygen rich samples, represented by CCR, and oxygen deficient samples, represented by SPS, is observed both at low and high temperatures. In general the magnetic properties are explained in coherence with heat capacity measurements supposing the enhanced Pauli susceptibility and the local moments associated with Co species in rock-salt blocks. We presume that the temperature independent susceptibility is associated with CoO2 layers while ferimagnetic order is controlled by Co ions in rock-salt blocks; a simple model corroborates the dominance of IS Co3+. In distinction to oxygen rich CCR samples where the long-range magnetic order detected below ~ 12 K this is absent in SPS samples. Simultaneously, the low temperature negative magnetothermopower observed in oxygen rich CCR samples is not detected in SPS samples, which are highly resistant, but posses similar negative magnetoresistance as CCR ones. The low temperature specific heat is strongly magnetic field dependent; magnetic field induced entropy “transfer” to higher temperature is observed in oxidized CCR sample, whereas specific heat of SPS sample is suppressed with increasing magnetic field. Finally, a distinct magnetic, transport and thermal anomaly observed at 390 K in oxidized sample is not reproduced in oxygen deficient SPS one. The Czech Science Foundation supported this work, grant No. GA 203/09/1036.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Takao Mori, NIMS
CR-01. Magnetic transitions in erbium at high pressures
Sarah A. Thomas1, Georgiy M. Tsoi1, Lowell E. Wenger1, Yogesh K. Vohra1 and Samuel T. Weir2
1Physics, University of Alabama at Birmingham, Birmingham, AL; 2Lawrence Livermore National Laboratory, Livermore, CA
Electrical resistance measurements have been carried out on polycrystalline erbium (Er) at pressures up to 20 GPa in order to study the magnetic transitions as a functio n of pressure. An abrupt change in the slope of the resistance is observed with decreasing temperature below 80 K which is associated with the c-axis modulated antiferromagnetic (ANFM) ordering of the Er moments. With increasing pressure, the temperature of the resistance slope change and the corresponding ANFM ordering temperature decrease until vanishing above 11 GPa. Above 11 GPa, a more gradual change in slope of the resistance is found around 45 K which disappears at pressures near 19 GPa. In addition, a small resistance change is found between 25 K and 20 K which is weakly dependent upon pressure over the entire pressure range measured. The occurrence of these resistance changes at similar pressures and temperatures with crystalline and magnetic structural phase transitions suggest (i) that the disappearance in the c-axis modulated antiferromagnetic ordering of Er moments at 11 GPa is correlated to the transformation of hcp structural phase to a nine-layer α-Sm structural phase and (ii) that the smaller resistance changes around 25 K and 45 K can be related to the cycloidal magnetic structures observed in ambient pressure neutron diffraction results.
CR-02. Electric-currents-induced reemergent metal-insulator transition, step-like resistance jump and negative differential resistance in Nd0.7Sr0.3MnO3 thin films
Jianfeng Wang, Zhenping Wu and Ju Gao
Department of Physics, The University of Hong Kong, Hong Kong, China
Colossal magnetoresistance manganites are susceptible to electric fields/currents as well as magnetic fields. We report emergent properties induced by electric currents in Nd0.7Sr0.3MnO3 thin films. X-ray diffraction revealed single phase and good epitaxy of these films. To study the current induced effect, these samples were patterned into microbridges with widths of ~20 μm by photo lithography and chemical etching. After the films were processed by currents of high density at room temperature, a second peak appeared at low temperatures in R-T curves. This resistance peak was very sensitive to weak current. More salient features were the steps-like resistance jump (R-T curves) and negative differential resistance (I-V curves) in the vicinity of this induced peak. The interfacial effects related to electrodes could be ruled out. As these phenomena resembled those in mesoscopic structure of manganites with large-scale phase separation,1,2 they might be to current enhanced inhomogeneity and the spatial confinement.
References
1T. Wu et al., Appl. Phys. Lett, 86, 252505 (2005). 2 H.Y. Zhai et al., Phys. Rev. Lett. 97, 167201 (2006)
CR-03. FMR study of Ni nanowire arrays with tailored strength of interaction
Abhishek Srivastava1, 2, Jose Vargas1, Jin Hee1, 3, John B. Wiley1, 3 and Spinu Leonard1, 2
1Advanced Material Research Institute, University of New Orleans, New Orleans, LA; 2Physics Department, University of New Orleans, New Orleans, LA; 3Chemistry Department, University of New Orleans, New Orleans, LA
Ni nanowires were fabricated by electro-deposition method in three different geometrical arrangements supported in alumina templates . FMR experiments on these Ni nanowire samples with different strength of dipolar interaction (due to different geometry) were performed at 9.4 GHz (X-Band). The spectra at room temperature and at T = 4.2 K show a main line associated with a uniform mode. The angular variation of this line is characteristic of an easy axis along the wire length for the three samples. At low temperature the dynamic response was different from that at room temperature which is evident from FMR angular variation (Fig.1). This qualitatively different angular variations in the resonant field are explained using a theoretical model that considers the interaction field in nanowire networks (1)
References
(1) I.Dumitru,F.Li,J.B.Wiley,D.Cimpoesu,A.Stancu,L.Spinu,IEEE TRANSACTIONS ON MAGNETICS,41,3361 (2005)
CR-04. Influence of ferroelectric poling induced strain on magnetic and electric properties in tetravalent cation-doped La0.9Hf0.1MnO3 films
Zhenping Wu, Lin Wang, Erjia Guo and Ju Gao
Physics, The University of Hong Kong, Hong Kong, Hong Kong
The conventional method to study the substrate-induced strain is to fabricate the epitaxial films on substrates with certain lattice mismatch. However, it is difficult to control the strain in manganite films continuously and reversely. Recently, C. Thiele et al. and R. K. Zheng et al. reported that the strain state of films can be tuned by applying electric fields on 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) ferroelectric single crystals. While most attention are focused on the hole-doped manganites, In our study, the effects of the strain induced by ferroelectric poling on the magnetic and electric properties of epitaxial La0.9Hf0.1MnO3 (LHMO) films grown on PMN-PT substrates have been investigated.The in-plane tensile strain is found to be reduced after ferroelectric poling,resulting in a decrease of resistance and an enhancement of transition temperature and magnetization in LHMO films (Fig.1).The resistance of strained LHMO films can be modulated greatly by external electric field. This could be explained by the the strain-induced distortion of MnO6 octahedron which modulates the competition between the coexisting phases.
CR-05. Effects of High Pressures on Magnetism in ErCo2
Martin Míšek1, Jan Prokleška1, Vladimír Sechovský1, Anna F. Kusmartseva2, Konstantin V. Kamenev2 and Jirí Kamarád3
1DCMP, Charles University in Prague, Prague, Czech Republic; 2CSEC, The University of Edinburgh, Edinburgh, United Kingdom; 3Institute of Physics ASCR, Prague, Czech Republic
ErCo2 exhibits magnetic ordering below TC = 33K, where the localized Er magnetic moments order ferromagnetically and the itinerant moments in the Co sublattice align antiparallel to the Er moments. The first order transition at TC is characterized by a suddenly emerging spontaneous magnetization, sharp peak in specific heat and AC susceptibility data and a large drop of the electrical resistivity. Recently, a tiny bump in AC susceptibility data observed in paramagnetic state (at 100 K) has been explained in terms of Co moments clustering with the freezing temperature Tf ~ 100K and the onset of “parrimagnetism” [1]. We will present results of our study of ErCo2 with respect to the temperature dependence of a) the electrical resistivity, AC susceptibility and specific heat in hydrostatic pressures up to ~2.5 GPa generated by a piston-cylinder pressure cell implemented in a PPMS apparatus and b) the DC magnetization measured under pressures up to 11 GPa using a diamond anvil cell [2] mounted in MPMS7 (Quantum Design). We have observed the decrease of both, TC and Tf with applied pressure up to 2.5 GPa at a rate dTC/dP ~ -7K/GPa and dTf/dP ~ -4.5K/GPa, respectively. The TC vs. P data are in a nice agreement with data published previously [3], the Tf pressure dependence is reported first by us. Our data also confirm decoupling of the magnetic ordering phenomena in the Er and Co sublattices (manifested as a splitting of the AC susceptibility peak at TC) originally proposed by Hauser [4]. Compared to previously published [3] results, our magnetization measurements at higher pressures above 2.5 GPa revealed small but clearly observable evolution of both critical temperature and a saturated magnetization. The results will be discussed in terms of present understanding of magnetic interactions of intermetallics consisting of localized 4f- and itinerant 3d-electron sublattices.
References
[1] J. Herrero-Albillos et al., Phys. Rev. B 76, 094409 (2007) [2] G. Giriat et al., Rev. Sci. Inst. 81, 073905 (2010) [3] O. Syshchenko et al., Phys. Rev. B 63, 054433 (2001) [4] R. Hauser et al., Phys. Rev. B 63, 1198 (2000)
CR-06. Magnetic and charge ordering properties of Bi0.6-x(RE)xCa0.4MnO3 (0.0≤x≤0.6) perovskite manganites
Kamlesh Yadav1, Mangala P. Singh3, H. K. Singh2, F. S. Razavi3 and Ghanshyam D. Varma1
1Physics, Indian Institute of Technology Roorkee, Roorkee, India; 2National Physical Laboratory, New Delhi, India; 3Physics, Brock University, St Catharines, ON, Canada
In the present paper we have studied structural, magnetic and charge ordering properties of polycrystalline Bi0.6-x (RE) x Ca0.4MnO3 (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) samples synthesized via solid state reaction route by taking RE = La, Pr and Nd, separately. X-ray diffraction results reveal that all samples are single phase and the crystal structure transforms from triclinic (~ monoclinic) to orthorhombic as x increases from 0.0 to 0.6. The temperature dependence magnetization measurement reveals that the x=0.0 compound (BCMO) exhibits COAFM phase with TCO ~289 K and TN ~136 K. The doping of small amount of La in BCMO destroys the CO completely and the samples with La doping concentrations x=0.2 to x=0.6 exhibit paramagnetic to ferromagnetic (PM-FM) transition and PM-FM transition temperature (TC) decreases progressively from 241 K to 203 K as x increases from 0.2 to 0.5. However, the samples with x=0.0 and 0.1 do not show PM-FM transition. On the other hand systematic substitution of Pr/Nd at Bi site induces an interesting interplay between the charge ordering and antiferromagnetism. It has been found that the charge ordering temperature (TCO) decreases with increasing x and the antiferromagnetic (AFM) ordering temperature (TN) increases sharply at both the extremes but remains nearly constant from x=0.2 - 0.4. Furthermore, at temperatures lower than TN a transition to the glassy state is observed. The Pr/Nd doping also leads to enhancement in the magnetic moment. In the present paper the correlation between the structural characteristics and magnetic properties of the samples will be described and discussed.
CR-07. A Theoretical Derivation of Analytic Free Surface Expression for the Magnetic Liquid's Conical Meniscus Phenomenon
Hong-soon Choi
School of Electrical Engineering, Kyungpook national university, Sangju, Republic of Korea
Ferro -hydrodynamics was established and fully surveyed by Rosensweig[1]. In [1], he derived a conical meniscus expression to find the response of an initially flat pool of magnetic liquid when a steady current is passed through a wire which emerges vertically from it. He derived the meniscus expression by using Maxwell stress tensor formulation and Bernoulli equation. He recognized, though, the fact that there is an arbitrariness of the grouping of magnetic terms in his derived magnetic force expressions. Anyway, by using the volume force density, which was concluded as zero, and the surface force density from Maxwell stress, he concluded and suggested a conical meniscus expression. It is noted that the suggested expression has not been proved by any experiments until now. On the other hand, it is also noted that there have been controversies and confusions about the magnetic force distributions in the magnetic materials for over a hundred years. In this paper, another conical meniscus expression is presented by using non-zero volume force density based on virtual air-gap scheme[2]. In this scheme, only volume force density is meaningful. In the above mentioned model, the fields H and B and the force density can be obtained analytically in the space without a numerical analysis. By applying the proposed scheme, the four force methods of Maxwell stress tensor, equivalent magnetic charge, equivalent magnetizing current, and Kelvin’s formula have the same volume force expression. This result is interesting if we think about the different concepts and formulations of the adopted force methods. And then the newly derived meniscus expression can be obtained with combining the above force density and the hydrostatics. The principle, the equivalence of the pressures at any location on the free surface, is deployed. This expression is slightly different to Rosensweig’s one by multiplying (μr+1)/2. Neither Rosensweig’s expression nor my derived one has completed the experimental verifications because of the low values of the magnetic liquid’s relative permeability. The experimental verification is left for the near future work by anyone else.
References
[1] R. E. Rosensweig, Ferrohydrodynamics. New York: Dover, 1997. [2] H. S. Choi, I. H. Park, and S. H. Lee, “Electromagnetic body force calculation based on virtual air gap,” Journal of Applied Physics, 99, 08H903, 2006.
CR-08. Neutron scattering measurements in RbMnF3: a test of spin-wave theories at low temperatures and critical behavior near TN
N. Bykovetz1, A. Hoser2, J. Klein3, c.L. Lin1 and M. Seehra4
1Department of Physics, Temple University, Philadelphia, PA; 2Institute for Complex Magnetic Materials, Helmholtz-Zentrum Berlin (HZB), Berlin, Germany; 3Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA; 4Department of Physics, West Virginia University, Morgantown, WV
From the discovery of magnetic ordering in RbMnF3, this unique antiferromagnetic system was recognized as a prime case for a test of conventional spin-wave theory (CSWT) because of its negligibly small anisotropy and its simple, cubic structure. CSWT predicts a simple T2 fall-off of the sublattice magnetization at low temperatures (up to ~1/4 TN, or so) for antiferromagnets that have a negligible anisotropy. Yet to this day, no stringent tests have been made of this prediction, primarily because of the difficulties in doing NMR in RbMnF3 (severe frequency-pulling effects). However, one of the original investigators of RbMnF3, P. Seiden (Phys Lett 28A, 239, 1968), claimed to deduce a T3 low-temperature behavior on the basis of AFMR measurements (although with some dubious assumptions), alleging inapplicability of CSWT predictions. We have carried out neutron scattering (NS) measurements at HZB (Berlin) on both single-crystal and powdered samples of RbMnF3 in order to test CSWT, as well as two other, more recent, semi-empirical spin-wave schemes. U. Koebler’s scheme predicts a T2 behavior all the way up to at least ~0.75 TN [i.e., much higher than for CSWT], and N. Bykovetz’s either a T2.29 or a T2.67 simple power-law fall-off up to ~0.5 TN. We will present an analysis of these NS measurements. Additionally, we made measurements in the critical regime and found that the critical exponent β had a value of ~1/3 (in agreement with previous other measurements) when done on a pulverized-crystal sample, but about one half this value in the single-crystal sample. We attribute the latter result to presumed severe extinction effects from the highly perfect crystal structure (mosaic spread of only ~0.08 deg).
CR-09. The crystal growth and evolution of magnetism and superconductivity in Pd-doped CeRhIn5 and Ce2RhIn8
Marie Kratochvilova1, Klara Uhlirova1, 2, Jan Prokleska1, Martin Misek1, 3, Alexandra Rudajevova1 and Vladimir Sechovsky1
1Department of Condensed Matter Physics, Charles University, Faculty of Mathematics and Physics, Praha, Czech Republic; 2Magnetic and Superconducting Materials, Leiden Institute of Physics, Leiden, Netherlands; 3Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republic
CeRhIn5 and Ce2RhIn8 are the heavy fermion antiferromagnets in which unconventional superconductivity is induced near the quantum critical point by applying external hydrostatic pressure [1, 2]. The motivation of our work was to learn how the physics of these materials can be affected by the Pd substitution for Rh. By applying the solution growth method we have synthesized the single crystals of CeRh1-xPdxIn5 for x = 0, 0.1 and 0.25 and Ce2Rh1-xPdxIn8 for x = 0, 0.10, 0.15, 0.30, 0.45, 0.85 and 1. For Ce2PdIn8, the growth conditions have been optimized according to results of high-temperature differential thermal analysis (DTA) in order to avoid the preferential growth of the CeIn3 spurious phase. The crystals were subjected to specific heat (in various magnetic fields), electrical resistivity and AC susceptibility measurements. The resistivity and AC susceptibility measurements have also been performed on crystals exerted to hydrostatic pressures up to 3 GPa. We have observed that (similar to the Co and Ir doping) the Néel temperature of CeRh1-xPdxIn5 only slightly decreases with Pd doping; TN = 3.78 K (CeRhIn5) to 3.74 K (CeRh0.75Pd0.25In5). The pressure influence on CeRh0.75Pd0.25In5 does not significantly differ from CeRhIn5 pressure response. On the other hand the critical temperature of superconductivity induced by external hydrostatic pressure increases with increasing x. To explore the low-temperature part of the phase diagram and confirm coexistence of the superconductivity and antiferromagnetic ordering, we have performed the resistivity and AC susceptibility measurements under high pressures using a dilution refrigerator. The Néel temperature of Ce2Rh1-xPdxIn8 dec reases with increasing Pd content more rapidly than in the previous case and the antiferromagnetism vanishes at x ~ 0.35. The results will be discussed in the context of superconductivity and magnetism evolution in other CeTX5 and Ce2TX8 compounds (T - transition metal, X - p-electron element).
References
[1] H. Hegger et al., Phys. Rev. Lett. 84, (2000). [2] M. Nicklas et al., Phys. Rev. B 67, 205061 (2003).
CR-10. Magnetostochastic resonance under colored noise condition.
Marco Trapanese
Dipartimento di Ingegneria Elettrica, Elettronica e delle Telecomunicazioni, Università di Palermo, Palermo, Italy
Stochastic resonance (SR) is generally considered as an amplification of the system response for certain finite values of the noise strength that is pumped into the system [1-3]. In particular the signal to noise ratio (SNR) and the signal amplification show a maximum as a function of the noise intensity. SR has been experimentally observed (for a review see[4]) in many physical systems and also in magnetic systems. Some theoretical approaches have been developed to describe SR, they usually assume that the system is bistable, a white-noise condition and that there is an hysteresis in the switching between one state and the other, but no theoretical approach is able to describe SR in systems that present a magnetic-like hysteresis area (i.e. an entire area of accessible states, that is surrounded by a major loop), in this paper this effect will be called Magnetic Stochastic Resonance (MSR). Moreover, as far as magnetic-like systems are concerned, both the available theoretical and the numerical descriptions of SR do not include the dynamic features of the system and suppose the presence of white noise. However this is clearly a rough approximation. In order to clarify the influence of a colored noise, in this paper MSR in magnetic systems described by Dynamic Preisach Model (DPM) is numerically investigated. The use of DPM allows to study the features of the SR in connection with the dynamic features of the magnetic systems. More particularly, in this paper it is shown that: -magnetic systems described by DPM presents SR; -the resonance in the power amplification disappears if the frequency of the input signal is greater than characteristic frequency of the system introduced by DPM and SR in Signal to Noise Ratio (SNR) is strongly reduced under the same condition; -frequency and phase locking phenomenon is detected under resonance condition.
References
1)R. Benzi et al. J. Phys. A , Vol. 14, pp. 453, 1981. 2)R. Benzi, et al. Tellus, Vol 34, pp.10, 1982. 3)A.N. Grigorenko et al., IEEE Trans. on Magnetics, Vol. 31, pp.2491-2493 1995. 4)L.Gammaitoni et al. Rev. Mod. Phys., Vol. 70, pp.223-287, 1998.
CR-11. The valence electronic structure of multiferroic BiFeO3 from high energy X-ray photo-electron spectroscopy and first principles theory
Ronny Knut1, Sergey Faleev2, Dipanjan Mazumdar2, Oleg N. Mryasov2, Arunava Gupta2 and Olof Karis1
1Physics and Astronomy, Uppsala, Uppsala, Sweden; 2MINT center, MINT Center University of Alabama, Tuscaloosa, AL
BiFeO3 (BFO) is a multi-functional material with high ferroelectric and magnetic ordering temperature. For any application, fundamental understanding of the BFO electronic properties is a prerequisite. Here we have inve stigated the electronic structure of (001) oriented 100nm rhombohedral BFO thin films [1] using high energy X-ray photoelectron spectroscopy (XPS). Additionally we performed ab inito calculations [2]. By making use of the energy dependence of the relative cross sections for different states, we were able to selectively probe the elemental contributions to the valence band (figure 1). At high energies, states with high main quantum number will have a higher relative probability for photo-ionization, i.e., the Bi 6s and 6p contributions in the valence region are enhanced relative to the Fe 3d and O 2p. Our data suggests that the Bi 6p states hybridize strongly with the valence band dominated by the Fe 3d and O 2p states, resulting in a splitting of the 3d states due to bonding and anti-bonding combinations with the Bi 6p. This interaction may have decisive impact on the physical properties of the material. Our results thus suggest that a previously relatively ignored electronic interaction needs to be considered for BFO and related Bi-TMOs. Our ab initio calculations indicate the importance of screened Coulomb correlations to describe Bi and Fe electronic states.
References
1.Vilas Shelke, V. N. Harshan, Sushma Kotru, and Arunava Gupta, J. Appl. Phys 106, 104114 (2009). 2.S. V. Faleev, M. van Schilfgaarde, and T. Kotani, Phys. Rev. Lett. 93, 126406 (2004)
CR-12. Random magnet with competing anisotropies in FexNi1-xF2 alloys
Felio A. Perez, Trent A. Johnson and David Lederman
Phyisics, West virginia University, Morgantown, WV
A series of epitaxial (110) FexNi1-xF2 thin film samples were deposited on (110) MgF2 single crystal substrates and capped with a layer of BaF2 via molecular beam epitaxy. The Fe concentration x was determined from x-ray diffraction measurements of the tetragonal c-axis lattice parameter. The actual thickness of each layer and the roughness of each interface were determined by fitting x-ray reflectivity data. The antiferromagnetic ordering of samples with x = 0.10, 0.20, 0.30, 0.40, 0.45, 0.50, and 0.85 were studied and compared with BaF2/NiF2 (x=0) and BaF2/FeF2 (x=1) bilayers via standard magnetometry measurements. A significant enhancement of the Néel temperature in alloys and evidence of spontaneous magnetization along c-axis were found.
CR-13. Withdrawn
CR-14. Exchange Anisotropy Tuning in Cluster Glass AgMn Alloys
F. Jimenez-Villacorta, T. Sepehrifar, J. L. Marion and L. H. Lewis
Department of Chemical Engineering, Northeastern University, Boston, MA
Exchange-biased materials are proposed as potential candidates to increase the magnetic moment stabilization for a variety of permanent magnet applications, including alternative energy and military technologies. Here, unprecedented exchange bias values, up to 16 kOe at low temperatures, have been recorded in rapidly-solidified AgMn alloys with a high Mn concentration (>25 at.% Mn). Moreover, it is shown that this exchange bias may be tailored with variation of the Mn content. The purpose of extending the Mn content in these alloys beyond that reported in earlier work is to take advantage of the increasing antiferromagnetic character of these high-Mn alloys to explore new perspectives in the creation of exchange-biased systems. Silver manganese alloys of composition Ag100-xMnx, with 25 ≤ x ≤ 40 at.% Mn, were prepar ed by rapid solidification in Ar atmosphere and at a wheel speed of 31 m/s. An exhaustive characterization of their structural, compositional and magnetic features has been carried out by microscopy, x-ray diffraction techniques and SQUID magnetometry. Structural analysis shows the formation of an fcc γ-phase in all the AgMn alloys with lattice parameters and coherently-diffracting sizes on the order of 40-100 nm that are strongly determined by the Mn content. Magnetic analysis shows an increase of the exchange bias field with increasing Mn concentration as well as an increase of the peak temperature in the magnetothermal zero-field-cooling / field-cooling (ZFC/FC) curves, which represents the onset of blocked behavior, up to about Tpeak ~ 105 K. This remarkable magnetic behavior has been considered in the framework of the structural characterization results and statistical fluctuations in the manganese content. This research has been funded by the Office of Naval Research (ONR) under Grant # N00014-10-1-0553.
CR-15. c-axis anisotropic transport study of layered manganite based on micro-fabricated devices.
Arash A. Omrani1, 2, Andras Kis2 and Henrik M. Rønnow1
1Physics, EPFL, Lausanne, Switzerland; 2Microtechnique, EPFL, Lausanne, Switzerland
The perovskite-type manganites are naturally layered and the physics properties significantly depend on the effective dimensionality. Among them, the double-layered perovskite-type manganite, La2-2x Sr1+2x Mn2 O7, demonstrates strong anisotropy of charge transport and magnetic properties along ab-plane and c-axis. However, an STM study [1] shows that, while each bi-layer becomes metallic, local c-axis transport remains gapped, providing something as remarkable as a stack of metallic sheets stacked with 2nm spacing. Furthermore, High-resolution hard X-ray photoemission with different energies confirms that the surface electronic structure of the bilayered LSMO is identical to that of the bulk material [2]. Based on these results, it was assumed that the phase transition in bulk bilayered LSMO, which has been seen in transport measurement along c-axis, is due to the magnetic and crystal domains in the bulk material. To explain the macroscopic c-axis transport we proposed a “meandering” mechanism where electrons traverse the metallic layers to find weak barriers for tunneling to the next layer. Plausible candidates for the weak barriers are magnetic domain walls. To show the validity of this assumption the c-axis electrical transport should be studied in a size smaller than magnetic domain and structural crystal defects. Devices based on the bilayer manganite La1.4Sr1.6Mn2O7 are fabricated and investigated. Our mesoscopic devices have dimensions comparable to a typical domain, allowing us to study structures going from a single domain to several domains. To pass the current through c-axis, devices is fabricated with two island on the polished LSMO surface (001) with the c-axis perpendicular to the surface. The measured resistance of the device does not present any transition below bulk transition temperature, which is the effect of approximately pure insulating c-axis transport that also achieved by finite element simulation in COMSOL multyphysics 4.2.
References
[1] H. M. Rønnow, Ch. Renner, G. Aeppli, T. Kimura and Y. Tokura, Nature, Vol 440, 20 April 2006. [2] S. de Jong, F. Massee, Y. Huang, M. Gorgoi, F. Schaefers, J. Fink, A. T. Boothroyd, D. Prabhakaran, J. B. Goedkoop, and M. S. Golden, PRB 80, 205108, 2009.
8:00 AM - 12:00 PM, Saguaro Ballroom
Chair: Andrei Kirilyuk, Radboud University Nijmegen
CS-01. Magneto-optical Four Wave Mixing in Garnet Thin Films
Marie Barthelemy, Mircea Vomir, Monica Sanches Piaia, Michèle Albrecht and Jean-Yves Bigot
Institut de Physique et Chimie des Matériaux, CNRS- Université de Strasbourg, Strasbourg, France
In the present work we demonstrate a magneto-optical Four Wave Mixing (FWM) emission from garnet thin films excited by femtosecond laser pulses. It allows isolating the coherent dynamics of the magnetization associated to the coupling between the photons and the spins observed recently in magneto-optical pump-probe Kerr and Faraday configurations [1]. Our experiment is performed in a pump-probe degenerate configuration using amplified femtosecond laser pulses (120 fs; 793 nm; 5kHz) and a crossed polarisers technique. The sample is a 7 µm thick (GdThPtBi)3(FeGa)5O12 film deposited on a GGG substrate by liquid phase epitaxy. The figure displays the magneto-optical FWM signal detected in the direction 2k1-k2 for two opposite directions of an applied magnetic field H=±3kOe perpendicular to the sample plane. It shows the high magneto-optical contrast present in the wave mixing emission. We will show a detailed analysis of this coherent magneto-optical wave emission compared to the spins thermalization dynamics that is simultaneously observed in the pump-probe Faraday configuration. This new result is of primary importance for understanding the fundamental interaction between the photons and the spins in magnetic media.
References
[1] J.-Y. Bigot, M. Vomir, E. Beaurepaire, Nature Phys. 5, 515 (2009).
CS-02. Deflagration in Magnetism
Javier Tejada, Saul Velez, Joan Manel Hernandez, Ferran Macia and Alberto Hernandez-Minguez
Department of Fundamental Physics, University of Barcelona, Barcelona, Spain
Deflagration is a subsonic combustion process governed by thermal conductivity. The simplest example is a piece of paper locally heated with a fire source until getting totally burned; one layer burns and heats the next layer and so on. The flame propagates and converts the initial combustible into ashes. Recently, it has been observed that molecular magnets exhibit explosive relaxation toward thermal equilibrium that resembles of a flame -deflagration- in which the role of the chemical energy stored in a flammable substance is played by the Zeeman energy [1]. In this case, both the speed and the ignition of the process are modulated by the quantum tunneling of the relaxation rate [2,3]. It has been also suggested that a deflagration to detonation transition take place for very high sweeping rates of the magnetic field [4]. The occurrence of magnetic deflagration overpasses the case of only molecular magnets. As a matter of fact it is predicted to occur every time there is a barrier height separating different spin configurations and enough energy is deposited into the system in the spin reversal process. Manganese based oxides and intermetallic compounds as Gd5Ge4 showing huge magnetic steps in the magnetization curves at low temperature were good candidates to test this effect. In these cases the deflagration corresponds to the propagation of a phase transition throughout the sample. Additionally, in the case of Manganites, the phase transition corresponds to a transition from an insulating antiferromagnetic phase into a metallic ferromagnetic [5]. This process involves very fast and huge v ariations of the electrical resistance in very short times [6]. On the other hand, in the intermetallic case, the magnetic deflagration is accompanied by a magnetostructural transition [7]. We have verified that the speed change with the crystallographic direction along which the deflagration propagates [8]. The fact that the magnetic deflagration is non destructive, that is, it can be produced in a material as many times as it is desired, brings the possibility to simulate experimentally many different real situations. The aim of this talk is to review all the experiments done in this novel field.
References
[1] Y. Suzuki et al. Phys. Rev. Lett. 95, 147201 (2005). [2] A. Hernàndez-Mínguez et al. Phys. Rev. Lett. 95, 217205 (2005). [3] F. Macià et al. Phys. Rev. B 79, 092403 (2009). [4] W. Decelle et al. Phys. Rev. Lett. 102, 027203 (2009). [5] F. Macià et al. Phys. Rev. B 76, 174424 (2007). [6] F. Macià et al. Phys. Rev. B 77, 012403 (2008). [7] S. Velez et al. Phys. Rev. B 81, 064437 (2010). [8] S. Velez et al. In preparation.
CS-03. Dependence of the damping parameter on Ga concentration in Fe_1−x Ga_x thin films
Michael L. Schneider1, Peter S. Burns1, Adam McClure2 and Yves U. Idzerda2
1Physics and Astronomy, University Montana, Missoula, MT; 2Physics, Montana State University, Bozeman, MT
Single crystal thin films of Fe_1−x Ga_x were deposited on ZnSe buffer layers grown on single crystal GaAs(001) and GaAs(110) substrates in an MBE system with a base pressure of around 5 x 10^−11 Torr. The thickness of the films ranged from 17 nm to 21 nm and did not noticeably influence the Ferromagnetic resonance (FMR) measurements. We use broadband Vector Network Analyzer based FMR with frequencies up to 40 GHz to characterize the damping parameter in Fe_1−x Ga_x thin films. By examining the frequency dependence of the linewidth we are able to separate out the Gilbert damping parameter from the inhomogeneous linewidth contribution. We find that in general the damping parameter increases with increasing Ga concentrations. We also find that the inhomogeneous broadening increases with increasing Ga concentration after an initial drop when compared with a pure Fe film. These results are consistent with earlier single frequency linewidth measurements.(1) Figure 1 shows an example of the linewidth versus frequency measurements, in this case for Fe_57 Ga_43, measured along the 110 direction. The inset of figure 1 shows the variation in the damping parameter as a function of Ga concentration taken along the 110 direction.
References
1. A. McClure et al., Properties of single crystal Fe1-xGax thin films. J. Appl. Phys. 105, (Apr, 2009).
CS-04. Random Magnetization Dynamics at Elevated Temperatures
Isaak D. Mayergoyz1, Giorgio Bertotti2, Claudio Serpico3, Ziyu Liu4 and Andrew Lee4
1Department of Electrical and Computer Engineering, UMIACS and AppEl Center, University of Maryland College Park, College Park, MD; 2INRiM, Torino, Italy; 3Dipartimento di Ingegneria Elettrica, Università di Napoli Federico II, Napoli, Italy; 4Department of Electrical and Computer Engineering, University of Maryland College Park, College Park, MD
The classical Landau-Lifshitz and/or Landau-Lifshitz-Gilbert equations describe magnetization dynamics for which the magnitude of ma gnetization is conserved. This is usually justified well below the Curie temperature because at such temperatures the strong exchange interaction prevails over all other forces at the smallest spatial scale compatible with the continuous media hypothesis. However, this may not be the case for elevated temperatures which may occur in such applications as thermally assisted magnetic recording and all-optical magnetic recording. For this reason, it is very desirable to develop generalizations of the classical Landau-Lifshitz equation which will be applicable at elevated temperatures and which will describe magnetization dynamics when magnetization magnitude is not conserved. In the paper, one such generalization is developed and it is based on the description of thermal bath effects by a jump-noise process. Previously [1], this approach has been used for the description of randomly perturbed Landau-Lifshitz magnetization dynamics which takes place on the sphere |M(t)| = Ms = const. The latter has been achieved by constraining the scattering action of a jump-noise process to the above sphere. It is shown in the paper that by relaxing this constraint on a jump-noise process and by introducing an additional term in the effective magnetic field (or, equivalently, in free magnetic energy) which accounts for changes in magnetization magnitude, a magnetization dynamics equation can be derived that accounts for deviations of |M(t)| from Ms. This equation has the mathematical form which is similar to the so-called "Landau-Lifshitz-Bloch" equation discussed in [2]. The unique feature of the proposed approach is that longitudinal and transverse damping (relaxation) terms emerge directly from the structure of a jump-noise process. Moreover, the corresponding longitudinal and transverse damping parameters are magnetization dependent and the explicit formulas for these damping parameters in terms of free magnetic energy are derived in the paper.
References
1. I. Mayergoyz, G. Bertotti, C. Serpico, Physical Review B, 83, 020402(R) (2011). 2. D. A. Garanin, Physical Review B, 55, 3050-3057 (1997).
CS-05. Spin-torque diode measurements of MgO-based magnetic tunnel junctions with asymmetric electrodes
Rie Matsumoto1, André Chanthbouala1, Julie Grollier1, Vincent Cros1, Albert Fert1, Akio Fukushima2 and Shinji Yuasa2
1Unité Mixte de Physique CNRS-Thales, Palaiseau, France; 2Spintronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
Spin-transfer torque in MgO-based magnetic tunnel junctions (MTJs) is attracting more and more attention because of its potential for device applications. The Spin-transfer torque consists of the classical in-plane torque (TIP) and the out-of-plane field-like torque (TOOP). In MgO-based MTJs, TOOP can reach over 25% of TIP [1]. The TOOP can be useful to control the magnetic momentum of spintronic devices [2]. Back-and-forth magnetization switching requires a linear bias dependence of TOOP. In conventional MgO-based MTJs with symmetric electrodes, however, the bias dependence of TOOP is quadratic and symmetric with respect to polarity of bias. For junctions with asymmetric electrodes, on the other hand, the bias dependence of TOOP is theoretically expected to be asymmetric and linear at low bias [3]. The asymmetric bias-dependence of TOOP has been experimentally investigated with an analysis method based on thermal activation model [4]. In this study, we perform spin-torque diode measurements of the asymmetric MTJs: CoFeB/MgO/CoFe/NiFe whose MR ratio is 60-70% to quantitatively evaluate the dc bias dependence of TIP and TOOP precisely. The external magnetic field is applied along the hard axis of the free layer. The measured bias dependence for the free layer mode exhibits that, as expected by theory, TIP is asymmetric as a function of bias. The sign of TOOP also changes with the current direction. In the low-bias region between ±60 mV, the TOOP reaches up to 40% of the TIP [2]. In the presentation, we will show a magnetic field - dc bias diagram and discuss in detail the physical origin of asymmetric bias dependence of TOOP [3, 4, 5]. We thank Canon ANELVA for preparation of the magnetic films. This work was supported by the European Research Council (ERC Stg 2010 No. 259068) and JSPS Postdoctoral Fellowships.
References
[1] J. C. Sankey et al., Nat. Phys. 4, 67 (2007); H. Kubota et al., ibid. 4, 37 (2007). [2] A. Chanthbouala, R. Matsumoto et al., Nat. Phys., (2011). [3] J. Xiao et al., PRB 77, 224419 (2008). [4] S.-C. Oh et al., Nat. Phys. 5, 898 (2009). [5] A. Manchon et al., PRB 82, 174420 (2010); M. Chshiev et al., APS March Meeting 2009 (invited).
CS-06. Non-thermal excitation and control of magnetization in Fe/GaAs film by ultrafast laser pulses
Yu Gong1, 3, Arjunen R. Kutayiah1, X. H. Zhang2, J. H. Zhao2 and Y. H. Ren1
1Physics and Astronomy, Hunter College of the City University of New York, New York, NY; 2State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China; 3Physics and Astronomy, The Graduate Center of the City University of New York, New York, NY
Magnetic recording is rapidly approaching the nanometer scale as storage densities are projected to increase to a terabit per square inch. High volume of data requires higher data transfer rates. These present new challenges and opportunities in nanometer scale materials engineering and in controlling the magnetization of the material by means other than magnetic fields. We report on our recent study of non-thermally excitation and coherently control the spin reorientation in 10-nm epitaxially grown Fe thin films by way of low-energy femtosecond laser pulses. The magnetization dynamics were recorded by pump-probe differential magnetic Kerr (DMK) and magnetic hysteresis curves using linear polarized femtosecond laser pulses. We show that magnetization excitation and reorientation show strong dependences of the polarization of pump pulses and a clear switching in DMK signal is observed with the rotation of pump polarization. These results indicate a non-thermal origin of magnetization dynamics in Fe films. We reveal that spins can interact coherently with the polarization induced by the pulsed laser field in magnetic metals. Therefore, the pump pulses have a substantial influence on the correlation between the magnetic and optical responses by state excitation and dephasing processes. Such optomagnetic interactions are instantaneous and are only limited in time by the properties of laser pulses. Our results show the feasibility of ultrafast optical control of both the magnetization and the demagnetization responses in magnetic films.
CS-07. Mechanism analysis of ultrafast magnetic switching
Jianming Li, Baoxi Xu, Jing Zhang and Kaidong Ye
Data Storage Institute, (A-STAR) Agency for Science, Technology and Research , Singapore, Singapore
Effective magnetic field and thermal analyses of electronic, spin and lattice temperatures are important for design of ultrafast magnetic switching. The ultrafast magnetic switching below 100 fs has been investigated, which are booming the field of ultrafast magnetization [1-3]. Its challenge is that the underlying switching mechanism is not fully understood though the inverse Faraday effect (IFE) as the mechanism has been proposed. Researchers are keen to learn whether IFE can produce big enough magnetic field for switching, and what role the electronic, spin and lattice temperatures play. The electric fields obtained by finite-difference time-domain (FDTD), instead of incident Gaussian fields, were used to achieve more accurate analysis of effective magnetic field. The thermal spots in Tb25Fe68Co7 magneto-optical media (Coercivity >1k Oe ) were studied using 25fs laser pulses with different powers. By experiments, the maximum applicable laser fluence defined by its minimum value for producing thermal spots could be found. Based on the experimental parameters, a safe and accurate method has been proposed to estimate the maximum possible value of the effective magnetic field, where the 25 fs laser pulses with wavelength of 780nm, NA of 0.6 and circular polarization were used. The results show that the effective magnetic field in the media is very small, about 0.3 kOe with the maximum applicable laser fluence. It implies that the spin flips can be implemented by a small magnetic field. The electronic, spin and lattice temperatures were analyzed to describe spin energy and thermal process with the same pulses. The spin temperature can reach the Curie temperature that reveals a non-thermal mechanism of spin flips under a small magnetic field. Plots of the effective magnetic field and spin temperature versus practical laser parameters were investigated. It provides approach to studying the critical point of spin flips and optimizing laser parameters for ultrafast magnetic switching. Discussions on the characteristics of the effective magnetic field and the three temperatures in the media are also presented.
References
[1] E. Beaurepaire, J.C. Merle, A. Daunois, and J. Y. Bigot, Phys. Rev. Lett, 76, 4250(1996) [2] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, Th. Rasing, Phys. Rev. Lett, 99, 047601 (2007) [3] A. Kirilyuk, A. V. Kimel and Th. Rasing, Reviews of Modern Physics, 82, 2731(2010)
CS-08. Static and Dynamic Magnetic properties of epitaxial Fe1.7Ge thin films grown on Ge(111)
Mohamed Belmeguenai1, Dominique Berling2, Salim Mourad Cherif1 and Philippe Moch1
1LSPM (CNRS-UPR 3407), Université Paris 13, 99 avenue Jean-Baptiste Clément, 93430, Villetaneuse, France; 2ISMM, (CNRS-LRC 7228), 4 rue des frères Lumière, Université de Haute-Alsace, 68093, Mulhouse, France
Alloys obtained from ferromagnetic transition metals and IV-semiconductors attract particular interest, as they are compatible with existing complementary metal oxide semiconductor technologies and thus can be integrated into future spintronic devices. Therefore, we studied the magnetic properties of Fe1.7Ge thin films (5<d<40nm) grown on Ge(111) substrates by molecular beam epitaxy. For all samples, X-ray diffraction revealed an excellent epitaxy of the Fe1.7Ge films, with its axes and in the sample plane. The static magnetic properties were studied by Magneto-optical Kerr Effect (MOKE) at room temperature. The dynamic magnetic properties were investigated by Microstrip Ferromagnetic Resonance (MS-FMR) at room temperature (within the 2- 18 GHz frequency domain sweeping). The variations of the spectra versus the orientation of an in-plane applied magnetic field reveal that the in-plane anisotropy results from the superposition of a two-fold and a six-fold symmetry. The six-fold magnetocrystalline anisotropy field increases with thickness. The angular dependence of the in-plane anisotropy energy and the remanent normalized magnetization, studied by MOKE and analyzed within the frame of a coherent rotation model, are in agreement with the dynamic measurements. A good agreement is observed for the field anisotropy values obtained from MS-FMR, MOKE Transverse Bias Initial Inverse Susceptibility and Torque data.
CS-09. Tunable magnetization relaxation in spin valves
Xuhui Wang and Aurelien Manchon
KAUST, Thuwal, Saudi Arabia
The rising of spin transfer torque [1] promises possibilities to manipulate and control magnetization dynamics in normal metal- ferromagnet heterostructures. Magnetization relaxation holds the key to magnetization dynamics in response to applied fields and current-driven torques, which is well described by the celebrated Landau-Lifshitz-Gilbert (LLG) phenomenology [2] augmented by spin-transfer torque. In spin valves, through spin pumping mechanism, the damping parameters of the free layer are determined non-locally by the entire magnetic configuration [3]. We investigate, in a dual spin valve [4] as depicted in Fig. 1, the tensor-like damping parameters that are tunable by a conscious selection of materials. The detailed form of the damping tensor is derived in terms of contact conductances, spin diffusion length and other material dependent properties. The generalization of the damping tensor in a continuous magnetic texture agrees well with early results [5]. Simulations based on the LLG and the macrospin-model are performed with the tensor-like damping. As briefed in panel (b), the relaxation time (in the unit of nano-second) of the free layer magnetization is found largely dependent on while tunable through the magnetic configuration of the source-drain magnetization.
References
[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996). [2] T. L. Gilbert, IEEE. Trans. Mag. 40, 2443 (2004). [3] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Phys. Rev. Lett. 88, 117601 (2002); Phys. Rev. B 67, 140404(R) (2003). [4] L. Berger, J. Appl. Phys. 93, 7693 (2003). [5] Y. Tserkovnyak and M. Mecklenburg, Phys. Rev. B 77, 134407 (2008); S. Zhang and S. -L. Zhang, Phys. Rev. Lett. 102, 086601(2010).
CS-10. Magnetization reversal in the hundred-nanometer-scaled permalloy hollow cylinders array
Yu-Chen Huang1, Cheng-Yi Kou2, Jia-Hong Shyu2, Lance Horng2, Ching-Ming Lee3 and Jong-Ching Wu2
1Graduate Insititute of photonics, National Changhua University of Education, Changhua, Taiwan; 2Department of Physics, National Changhua University of Education, Changhua, Taiwan; 3Graduate School of Materials Science, National Yunlin University of Scence and Technology, Douliou, Taiwan
Highly anisotropic magnetic properties of nanostructures are scientifically interesting for fundamental physics understanding and potential applications in spintronics. Among them, infinite hollow magnetic cylinders have been studied intensely and the main subjects were focused on the magnetization reversal. However, infinite hollow cylinders are not suitable to be connected with electronic device in reality. The so lution is using finite cylinders to instead of infinite ones. In this report, magnetization reversal in hundred-nanometer-scaled permalloy hollow cylinders arrays have been investigated by using an alternating gradient magnetometer. The dimension of a permalloy hollow cylinder was fabricated as a diameter of 300 nm, a period of 750 nm, and a wall thickness of 20 nm. M-H loops of a finite aspect ratio about 2.3 are shown in Fig. 1(c) and reveal Mr⊥/ Mr∥~ 1.25 and Hc⊥/ Hc∥~ 0.5 owing to a mixed magnetized configurations and rotation modes consisting of two vortex walls on both cylinder ends and an uniform domain in the middle part. With decreasing the period, the coupling effect between cylinders is observed in Fig. 2(a).
CS-11. Magnetization switching behavior of Co/Pt multilayer dot by in-plane nanoseconds pulse field
Yoshihiro Suyama, Nobuaki Kikuchi, Satoshi Okamoto and Osamu Kitakami
IMRAM, tohoku university, Sendai, Japan
Magnetization dynamics of a nanosized magnet becomes an important issue for development of future devices. In the present study, we report on magnetization switching of Co/Pt multilayer dots with perpendicular magnetization by an in-plane pulse field with nanoseconds duration which can produce larger torque compared with a perpendicular pulse field used in previous work [1,2]. A [Co(1.1 nm)/Pt(0.6 nm)]5 multilayer was patterned into a dot array of 100 nm in diameter with a Pt cross shape electrode for anomalous Hall effect measurements. The coersivity Hc of the array measured in static field was 4.6 kOe. A Cu line of 5 µm in width was fabricated on the dots separated by a 400 nm thick insulating layer (Fig.1(a)). Electric pulses transmitted through the Cu line were generated by discharging a coaxial cable, and the pulse field amplitude was controlled by charging voltage V0. The pulse field amplitude was estimated to be 3.7 kOe at V0= 400 V. Magnetization switching experiments were carried out by applying in-plane pulse field along with perpendicular bias field Hdc. In Fig.1(b), the Vsw required voltage for magnetization switching is plotted as a function of Hdc for the pulse duration τp= 10 ns and the pulse rise time τr= 70 ps and 4 ns. Although Vsw increases with decrease in Hdc for both τr, clear difference between τr= 70 ps and 4 ns was observed for Hdc< 2 kOe, indicating that smaller rise time pulse can decrease the switching field due to the dynamic effect. Fig. 1 (a) SEM image of Co/Pt multilayer dot array on Pt Hall cross electrode. (b) Critical voltage (Vsw) for magnetization switching as a function of dc bias field (Hdc) for pulse field rise time τr= 70 ps, 4 ns. The estimated in-plane pulse field (Hp) is also indicated.
References
[1] A. Ito et al., IEEE Trans. Magn. 44, 3446 (2008) [2] N. Kikuchi et al., J. Appl. Phys. 105, 07D506 (2009)
CS-12. Critical slowing down in laser induced demagnetization of Gd
Muhammad Sultan1, 2, Alexey Melnikov2, 3 and Uwe Bovensiepen1
1Faculty of Physics, University of Duisburg Essen, Duisburg, Germany; 2Institute of Experimental Physics, Freie University, Berlin, Germany; 3Fritz-Haber-Institut der Max-Planck Gesellschaft, Berlin, Germany
Re cent advancement in femtosecond (fs) laser and X-ray sources has made it possible to modify and monitor the magnetization on the ultrafast time scale. In recent years, a number of new phenomena have been observed including manipulation and reversal of the magnetic order on the sub picosecond (ps) time scale. Despite its extensive study, the physical mechanism leading to demagnetization after fs laser excitation is still under debate. We investigate the Gd, a model Heisenberg ferromagnet, which can facilitate the disentangling of different microscopic processes responsible for demagnetization. Localized magnetic moment and weak spin-lattice coupling leads to two step demagnetization of Gd after fs laser excitation, i) an ultrafast ∼1 ps at the time scale of electron-phonon equilibration and ii) slower 40 ps which is related to the spin-lattice relaxation [1]. Picosecond magnetization dynamics on Gd was investigated for different equilibrium temperatures using time resolved magneto-optical Kerr effect after fs laser excitation. Demagnetization gets slower as temperature increases. The characteristic demagnetization time increases 10 times when equilibrium temperature increased from 50 K to 290 K, showing divergence near Curie temperature. Enormous increase in the slower demagnetization time as a function of temperature is attributed to the critical slowing down. Moreover, angular momentum transfer rate was calculated using demagnetization amplitude and time. The angular momentum transfer rate per unit magnetization also increases with temperature, which suggests that both phonons and critical fluctuations facilitate the flow of angular momentum from the spin system to other reservoirs.
References
[1] M. Wietstruk et al, Phys. Rev. Lett. 106, 127401 (2011)
CS-13. Insights on all-optical magnetization switching by tailoring optical excitation parameters
Mirko Cinchetti, Sabine Alebrand, Daniel Steil, Alexander Hassdenteufel and Martin Aeschlimann
Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Kaiserslautern, Germany
In 2007 Stanciu et al. [1] discovered that it is possible to switch the magnetization in GdFeCo by using circularly polarized laser pulses without any applied external magnetic field. A phenomenological description of this effect based on the inverse Faraday Effect was presented by Vahaplar et al. in 2009 [2]. Nevertheless until now the microscopic processes leading to all-optical switching are still unclear. In this poster presentation we focus on the investigation of the all-optical switching behaviour by changing the properties of the exciting laser pulse. We present wavelength- and pulse duration-dependent studies and demonstrate that all-optical switching is possible for all wavelengths in the visible range as well as even for pulse durations up to almost 11 ps. In addition we show that the threshold fluence needed for all-optical switching is nearly independent on the pulse duration. We discuss the consequences of the results in the context of existing models. This work was supported by the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement NMP3-SL-2008-214469 (UltraMagnetron).
References
[1] Stanciu et al. PRL 99, 047601 (2007) [2] Vahaplar et al. PRL 103, 117201 (2009)
CS-14. Radial-spin-wave-mode-assisted vortex-core magnetization reversals
Myoung-Woo Yoo1, Jehyun Lee2 and Sang-Koog Kim1
1National Creative Research Initiative Center f or Spin Dynamics & Spin-Wave Devices and Nanospinics Laboratory, and Research Institute of Advanced Materials, Department of Materials Science and Eng., Seoul Natl Univ, Seoul, Republic of Korea; 2Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria
Magnetic vortices in patterned magnetic nanodisks have potential applications as information carriers in future information storage devices owing to their bi-state core orientations of extremely high thermal stability and their low-power-driven vortex-core switching [1]. Not surprisingly, vortex-core reversals, in both their fundamental and technological aspects, have attracted much attention [2]. Until now, such vortex-core switching has been found to be mediated by means of the nucleation and annihilation of vortex-antivortex pairs, which processes drive low-power-consumption vortex-core reversals via resonant vortex-core gyration excitations [2]. In this presentation, we report on an additional possible means of ultrafast, low-power-driven vortex-core switching that involves oscillating fields applied perpendicularly to the disk plane. In this case, the oscillating field frequencies are tuned to radial-spin-wave-mode eigenfrequencies. This mechanism is totally different from the vortex-antivortex-pair-mediated one [3]. In the present study, we performed micromagnetic simulations on a Permalloy (Py) disk of 160 nm diameter and 7 nm thickness. As for the disk’s dimensions and geometry, the eigenfrequencies of the radial modes were determined to be fn = 10.7, 15.2, and 20.7 GHz, as indexed by the quantized numbers n = 1, 2, and 3, respectively. To construct phase diagrams of vortex-core switching events driven by single-harmonic oscillating fields applied perpendicularly to the disk plane, we used field amplitudes H0 and frequencies fH within the H0 = 100 - 1400 Oe and fH = 8 - 21 GHz ranges. On the basis of the phase diagram, we found that the threshold fields are as low as 200 Oe at 10 GHz and 600 Oe at 14 GHz. These values are an order of magnitude smaller than the threshold field strength, ~ 5.3 kOe, of a perpendicularly applied static field, and the switching times are less than 1 ns. The core reversal occurs whenever the exchange energy density highly localized at the core reaches the threshold value, ~ 3.45 × 106 J/m3, which value was obtained for the case of switching under static perpendicular fields.
References
[1] B. Van Waeyenberge et al., Nature 444, 461 (2006). [2] K.-S. Lee et al., Phys. Rev. B 76, 174410 (2007); S.-K. Kim et al., Appl. Phys. Lett. 92, 022509 (2008); S.-K. Kim et al., Appl. Phys. Lett. 91, 082506 (2007). [3] R. Wang et al., International Conference of AUMS 2010, Jeju Island, Korea, 2010, Oral GB-02 (unpublished). [4] This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20110000441).
CS-15. Ferromagnetic resonance in exchange biased ferromagnetic/compensated antiferromagnetic bilayers
Ana L. Dantas1, Leonardo L. Oliveira1, 2, Melquisedec L. Silva3 and Artur S. Carriço2
1Departamento de Física, Universidade do Estado do Rio Grande do Norte, Mossoro, Brazil; 2Departamento de Fisica, Universidade Federal do Rio Grande do Norte, Natal, Brazil; 3Campus Simões Filho, Instituto Federal de Educação, Ciência e Tecnologia da Bahia, Pitanguinhas Simões Filho, Brazil
We present a theoretical study of the ferromagnetic resonance frequency Ω of a compensated antiferromagnetic/ferromagnetic exchange coupled bilayer. Both layers have uniaxial anisotropy, with the easy axis along the same direction. We focus on the field dependence of Ω, with the external field H perpendicular to the easy axis. We show that the strength of the external field required to switch the magnetization of the ferromagnetic layer, leading to Ω = 0 is smaller than the anisotropy field. For external field values larger than the switching field, the spins of both sublattices, in the substrate atomic plane at the interface, are canted towards the field, leading to a liquid moment along the external field direction. The spectrum of long wave-length excitations has been calculated numerically for a model system consisting of two linear chains of exchange coupled spins, using a Jacobi-like algorithm. We have also shown that the down-shift of the switching field may be calculated using a simple model which may be a valuable means of estimating the interface exchange energy. There are two regimes for the field dependence of the ferromagnetic resonance frequency. For H<HA+ Hint⊥, Ω(H) is a monotonically decreasing function of the field strength, starting with Ω0 = γ(HA(HA +4πMS))1/2 at H=0, and vanishing for H=HA+ Hint⊥. For H>HA+ Hint⊥ the resonance frequency is given by Ω(H)=γ((Hint⊥+H+4πMS )(Hint⊥+H-HA))1/2 where Hint⊥ is the effective perpendicular componente of the interface exchange field. The theoretical model has been applied to Fe/FeF2(110) and Fe/MnF2(110) systems. We have found that Fe/MnF2(110) systems display larger interface effects due to the rather small value of the anisotropy. Our results suggest that the value of the effective interface field (Hint) and the blocking temperature may be estimated from external the field dependence of the frequency of the long wave-length excitation of the ferromagnetic layer.
8:00 AM - 12:00 PM, Saguaro Ballroom
Co-Chair: Felix Jimenez Villacorta, Northeastern University; Co-Chair: Oleg Mryasov, University of Alabama
CT-01. Ultrahard Magnets
Pankaj K. Sahota2, 1, Y. Liu1, R. Skomski1, P. Manchanda2, 1, R. Zhang1, G. C. Hadjipanayis3, A. Kashyap2 and D. J. Sellmyer1
1Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE; 2Department of Physics, The LNM Institute of Information Technology, Jaipur, India; 3Department of Physics, University of Delaware, Newark, DE
Permanent magnets are typically judged by the energy product which is essentially the energy per unit volume of magnetic material. This consideration is important for bulk applications, for example in cars, where magnet weight and volume matter. However, the magnet volume is not the main consideration in thin films for MEMS applications and the question arises whether the energy product remains a valid figure of merit. In our presentation, we discuss alternative figures of merit such as the hardness product and answer the question which materials are best suitable for such permanent-magnet applications. As exemplified by our measurements and VASP calculations on CrPt, some antiferromagnets (AFM) have very high anisotropies of magnitude of 3.5 MJ/m3 and even higher for some AFMs. However, using coercivity as a figure of merit yields a well-known but not very useful compensation-point singularity. The hardness product, defined as a product of coercivity and remanence, does not suffer from this shortcoming but is essentially proportional to the first anisotropy constant K1, overestimating the performance of highly coercive magnets with small magnetization. A better approach is to request the stability of the magnetization with respect to stray fields proportional to the magnetization itself. While unrelated to the magnet volume, it yields expressions very similar to the energy product. Based on this finding, the best material available at present is an Fe-Pt bilayer consisting of a L10-type hard phase and an iron-rich cubic soft phase, having a net magnetization of 1.74 T, a coercivity of 1.21 T, and an energy product of 510 kJ/m3. Very hard thin-film L10 magnets have been produced before [1], but the extension of this result to bilayers is nontrivial because free soft-magnetic surfaces are microstructurally less forgiving than other geometries such as embedded soft spheres. This is shown by using analytical micromagnetic calculations using NMAG code. The latter solves the Landau-Lifshitz-Gilbert equation by finite element method. The research is supported by DST (Indo-European DYNAMAG project and Nano Mission (PKS, PM and AK)), NSF MRSEC (RS, PKS), BREM (RS, YL), ARO (RZ), ARPA-E (RS), DOE (DJS, GCH), and NCMN.
References
[1] T. Shima, K. Takanashi, Y. K. Takahashi, and K. Hono, Appl. Phys. Lett. 85, 2571-2573 (2004).
CT-02. Effects of doped nanometer particle on magnetic properties and microstructure of 2:17-type Sm(CobalCu0.09Fe0.09Zr0.03)7.69 magnet
Jun Huang1, Dongtao Zhang1, 2, Ming Yue1, Weiqiang Liu1, Jiuxing Zhang1 and You Qiang2
1College of Materials Science and Engineering, Beijing University of Technology, Beijing, China; 2Physics Department, University of Idaho, Moscow, ID
Effects of doped SmHx nano particle on magnetic properties and microstructure of Sm (CobalCu0.09Fe0.09Zr0.03)7.69 magnet was studied. In this paper, the sintered magnets with doped SmHx nano particle ranged from 0 to 3 wt% were studied. All the samples were prepared by conventional powder metallurgy techniques. With increasing SmHx content, the degree of square of demagnetization curves got better, but coercivity of the sample deteriorated. The intrinsic coercivity decreased approximately 53%, the remanence increased 1.4 times larger, and the maximum energy product increased almost 2.2 times bigger than none doped SmHx magnet. The results indicate that the magnet with 2 wt% SmHx nano particle has the following properties: Br=11.55kGs, Hcj=8.15kOe, (BH)max=30.68MGOe. The large coercivity of 2:17-based permanent magnets is due to pinning of domain walls at the cell boundaries of the pyramidal cell structure, the process leading to the formation of the strong pinning barrier is controversial. It should be noted that the coercive field is not related to the width of the cell wall phase but exclusively to the slope of the transition region between the 2:17 and the 1:5 phase. And it is possible to observe that the volume fraction of Sm2Co17 increases while the SmCo5 decreases, which not according to the binary Sm-Co system. That can be explained why the behaviors of intrinsic coercivity and remanence after doped SmHx nano particle.
References
[1] J. F. Liu, Y. Zhang, D. Dimitrov, and G. C. Hadjipanayis , J. Appl. Phys. 85, 2800 (1999).
CT-03. Structures and magnetic properties of Sm5Fe17 melt-spun ribbons
Tetsuji Saito1, Hiroya Miyoshi1 and Daisuke Nishio-Hamane2
1Chiba Institute of Technology, Chiba, Japan; 2The University of Tokyo, Kashiwa, Japan
The Sm5Fe17 phase is a new ferromagnetic phase in the binary Sm-Fe system [1,2]. It is a metastable phase and its formation has been reported only in sputtered films [3,4]. Recent studies have revealed that the Sm5Fe17 phase can be obtained by annealing amorphous Sm-Fe melt-spun ribbons [5]. Sm5Fe17 melt-spun ribbon shows a large coercivity exceeding 25 kOe, and a remanence around 40 emu/g, which is not comparable with that of Nd-Fe-B magnets. If the annealing condition of the amorphous Sm-Fe melt-spun ribbon is optimized, this optimization would lead to an increase in the magnetization of the Sm5Fe17 phase. The purpose of this study was to seek the possibility of producing the Sm5Fe17 phase by annealing the melt-spun ribbon. Sm5Fe17 alloy ingots were prepared by induction melting under an argon atmosphere. The alloy ingots were induction melted and then ejected through an orifice with argon onto a copper wheel rotating at a surface velocity of 50 ms-1. The melt-spun ribbons were annealed under an argon atmosphere at temperatures of 773-1073 K at heating rates of 0.05-5 K/s. The Sm5Fe17 melt-spun ribbon initially had an amorphous structure and hence exhibited very low coercivity values. Heat treatment of the amorphous melt-spun ribbon resulted in the formation of the hard magnetic Sm5Fe17 phase. Transmission electron microscope (TEM) and thermomagnetic studies revealed that the amount of the Sm5Fe17 phase in the annealed specimen was deeply dependent on both the annealing temperature and the heating rate. The optimally annealed Sm5Fe17 melt-spun ribbon consisted mostly of the Sm5Fe17 phase and exhibited a remanence of 50 emu/g with a high coercivity of 40 kOe.
References
[1] G. C. Hadjipanayis, et al. J. Magn. Magn. Mater. 78, L1 (1989). [2] C. Neiva and F. P. Missell, Proc. of 8th Inter. Symposium on Magnetic Anisotropy and Coercivity, 315 (1984). [3] M. Katter, et al. J. Appl. Phys. 56, 1377 (1990). [4] F. J. Cadieu, et al. Mater. Lett. 11, 284 (1991). [5] T. Saito and M. Ichihara, , Scripta Mater. 57, 457 (2007).
CT-04. Magnetic properties and crystal structure of melt spun Sm(Co, M)7 (M=Al and Si) ribbons
Chih-Chieh Hsieh1, Chih-Wei Shih1, Zhuang Liu1, Wen-Cheng Chang1, Huang-Wei Chang2 and An-Cheng Sun3
1National Chung Cheng University, Chia-Yi, Taiwan; 2Tunghai University, Taichung, Taiwan; 3Yuan Ze University, Taoyuan, Taiwan
To seek novel permanent magnetic materials for high temperature applications, Sm(Co, M)7 alloys with TbCu7-type structure have been intensively studied due to their excellent intrinsic magnetic properties.1,2 However, 1:7 is a metastable structure and is easy to decompose into 1:5 and 2:17 phases, and therefore, either rapid quenching or a third element is requisite to stabilize the Sm-Co 1:7 phase. Luo et al.3 reported that 1:7 phase could be formed in the cast SmCo5.85Si0.9 ingot followed by 1000 oC annealing for 1 week in an evacuated quartz tube, but not for the Al substituted Sm(Co, Al)7 ingots. Our previous studies have shown that, by employing the melt spinning technique, 1:7 phase could be easily formed in Sm(Co, M)7 (M= Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Ge and Sn)4-8 ribbons and exhibit excellent coercivity on them, and the third elements M all prefer to occupy the 2e site. Accordingly, it is of interest to know the occupancy of Si and Al and how Si and Al substitution for Co affect the magnetic properties of the melt spun Sm(Co, M)7 (M= Si and Al) ribbons. The x-ray diffraction analysis shows that Sm(Co, M)7 (M= Al and Si) ribbons spun at a wheel speed of 40 m/s can also crystallize in TbCu7-type structure. The calculated XRD pattern fits well with the observed XRD pattern, and the results show that Si and Al prefer to occupy the 3g site, which is much different from other elements, such as IVB, VB, VIB and Ge elements. However, higher Al or Si content is necessary to have the same effect as IVB, VB, VIB and Ge elements in enhancing the magnetic anisotropy field of 1:7 phase, which normally degrades the saturation magnetization, the remenent magnetization and (BH)max of the ribbons. In this study, the optimal magnetic properties of σ12kOe=49.4 emu/g, σr= 45.3 emu/g, iHc=5.8 kOe, (BH)max=4.2 MGOe could be achieved for SmCo6.3Si0.7 alloys. The influence of Si and Al addition on the magnetic properties melt spun Sm(Co, M)7 will be systematically discussed in this presentation.
References
1 M.Q. Huang, W.E. Wallace, M. McHenry, Q. Chen, and B.M. Ma, J. Appl. Phys. 83, 6718 (1998). 2 J. Zhou, I.A. Al-Omari, J.P. Liu, and D.J. Sellmyer, J. Appl. Phys. 87, 5299 (2000). 3 J. Luo, J.K. Liang, Y.Q. Guo, Q.L. Liu, L.T. Yang, F.S. Liu and G.H. Rao, Appl. Phys. Lett. 84, 3094 (2004) 4. C.C. Hsieh, H.W. Chang, C.W. Chang, Z.H. Guo, C.C. Yang, and W.C. Chang, J. Appl. Phys. 105, 07A705 (2009). 5. Z.H. Guo, H.W.Chang, C.W. Chang, C.C. Hsieh, A.C. Sun, W.C. Chang, W.Pan, and W. Li, J. Appl. Phys. 105, 07A731 (2009). 6. Z.H. Guo, C.C. Hsieh, H.W. Chang, M.G. Zhu, W. Pan, A.H. Li, W.C. Chang, and W. Li, J. Appl. Phys. 107, 09A705 (2010). 7. C.C. Hsieh, H.W. Chang, Z.H. Guo, C.W.Chang, X.G. Zhao, and W.C. Chang, J. Appl. Phys. 107, 09A738 (2010). 8. C.C. Hsieh, H.W. Chang, X.G. Zhao, A.C. Sun, and W.C. Chang, J. Appl. Phys. 109, 07A730 (2011).
CT-05. Fabrication of anisotropic SmCo5-FeNi and SmCo5-CoFe hard-soft nanocomposites by electroless plating.
M. Lamichhane1, Sanjay R. Mishra1, N. V. Vuong2 and J. P. Liu2
1Physics, The University of Memphis, Memphis, TN; 2Physics, University of Texas, Arlington, TX
Exchange spring nanocomposite magnets, with hard-soft magnetic phases, are theoretically predicted to have high (BH)max. These magnets synthesized via rapid quenching have often randomly dispersed hard grains leading to isotoropic anisotropy consequently reducing the (BH)max value of the magnet. In the present paper, hard-phase anisotropy enhancement is achieved by surfactant assisted ball milling and the soft phase is achieved via electroless coating of CoFe and FeNi nanoparticles on the hard-phase powder. A simple and economical route of fabricating CoFe and FeNi nanoparticles deposited on the surface of SmCo5 by electroless plating using alkaline solution of hypophosphite is presented. Crystallographically anisotropic SmCo5 nanoflakes were obtained via balled mil ling micron size powder for 12hrs at 200 rpm in 10% Oleic acid-heptane solution. The SmCo5 nanopowder with an average crystallite size of 95 nm was first cleaned using mild HCl and was electroplated with Co-Fe and Fe-Ni layers. The thickness of the coating was varied by changing electroless plating duration viz. 1/2, 1 and 1.5 hrs. The increase in wt%. of FeNi and CoFe (EDX analysis) and decrease of SmCo5 XRD peaks with the coating time indicates thickening of respective coatings with electroplating time. The XRD patterns show strong peaks of Co70Fe30 and FeNi3 phases while SmCo5 peak decreases sharply with electroplating time. The magnetic hysteresis loops of these samples were obtained at the RT. The coercivity (Hc) and Ms (Ms) value for as ball milled SmCo5 powder was 2.2 kOe and 20 emu/g, respectively. The Hc of SmCo5-CoFe sample reduced with increasing CoFe thickness from 605 Oe (0.5h) to 288 Oe (2.0h), however the Ms value increased from 46 to 53 emu/g for the same. FeNi coated sample also show Hc reduction with electroplating time from 1925 Oe (15 min) to 913 Oe (1hr), however the Ms value increased from 13.5 emu/g to 18 emu/g. The Ms of SmCo5-FeNi samples is lower than SmCo5-CoFe as CoFe has higher Ms than FeNi. The reduction in the Hc in both cases is directly related to the increase in the soft-magnetic component. The greater exchange coupling interaction between hard SmCo5 and soft FeNi or CoFe phase leads to the lowering of the Hc.
CT-06. Magnetic anomalies in single crystalline Tb5Si3
Kartik K. Iyer1, K. Mukherjee1, P. L. Paulose1, E. V. Sampathkumaran1, Y. Xu2 and W. Löser3
1DCMPMS, Tata Institute of Fundamental Research, Mumbai, India; 2State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Shaanxi, China; 3IFW Dresden, Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden, Germany
The polycrystalline form of the compound, Tb5Si3, crystallizing in Mn5Si3-type hexagonal structure and its derivatives, has been recently reported by us to exhibit interesting magnetoresistance (MR) anomalies [1]. For instance, the parent compound undergoes a field-induced magnetic transition in the antiferromagnetically ordered state (<69 K) and the electrical resistance increases dramatically at a critical field (e.g., about 30 kOe at 50 K; 58 kOe at 1.8 K), resulting in positive MR unlike in other metamagnetic systems. We attributed this to ‘inverse metamgnetism’ (in which magnetic fluctuations are induced at the critical field) - a phenomenon not so commonly recognized in metallic magnetism. In order to understand the magnetic anomalies of this compound better, we synthesized [2] single crystals of this compound and subjected to intense magnetization and MR studies. While the results reveal that the magnetic behavior is strongly anisotropic with the easy axis along basal plane, there are multiple magnetic transitions in the close vicinity of 70 K (78K, 69 K, 67K and 62 K), as though several magnetic structures are nearly degenerate. In addition, there are orientation-dependent multiple steps in isothermal magnetization (which could not be resolved in the data for polycrystalline data) - a topic of current theoretical interest. Interestingly, the positive MR anomaly appears for an application of magnetic field (H) parallel to basal plane only, whereas, for the perpendicular orientation (H//c), MR varies rather essentially linearly with H. These results establish that this compound is an interesting magnetic material.
References
1. Niharika Mohapatra et al, Phys . Rev. B 80, 214425 (2009); K. Mukherjee et al, Phys. Rev. B 81, 184434 (2010) and references therein. 2. Y. Xu et al, J. Crystal Growth 321, 45 (2011).
CT-07. Complex magnetism in the intermetallic compound Tb2Mn3Si5: A high magnetic field study
N. M. Xia2, Z. W. Ouyang2, J. Chen2, S. S. Sheng2, Y. Y. Wu2, Z. C. Xia2, L. Li2, G. H. Rao3, A. V. Morozkin4, R. Nirmala1 and S. K. Malik5
1Physics, Indian Institute of Technology Madras, Chennai, India; 2Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, China; 3Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China; 4Chemistry, Moscow Lomonosov State University, Moscow, Russian Federation; 55Departamento de Física Teórica e Experimental, Natal-RN, Brazil
Magnetic materials showing multiple transitions have always attracted the attention of condensed matter physicists. In rare earth-transition metal based compounds, the appearance of multiple magnetic transitions is usually associated with varying temperature dependences of the exchange strengths of rare earth and transition metal sublattices and/or different ordering temperatures of inequivalent magnetic sublattices. The rare earth intermetallic compounds of type R2T3X5: (R = rare earth; T = transition metal; X = Si, Ge) are of considerable interest due to the wealth of interesting magnetic properties that include valence fluctuation, Kondo behaviour, giant magnetoresistance and magnetic ordering temperatures disobeying de Gennes scale. In particular, the R2Mn3Si5: (R = Tb, Dy, Ho and Er) compounds crystallize in tetragonal Sc2Fe3Si5: -type structure (space group P4/mnc) in which R ions occupy single site and Mn ions are located in two distinct crystallographic sites and exhibit multiple magnetic transitions. It was proposed that the layered crystal structure with repeating layeres of R-Si-Mn-Si-R determines the strength of competing R-R and R-Mn magnetic interactions, leading to complex multiple magnetic transitions and favoring giant magentoresistance in the magnetically ordered state. Thus R2Mn3Si5 compounds mimic magnetic multilayers and hence understanding their magnetic properties is of vital importance to design artificial magnetic multilayer structures. The compound Tb2Mn3Si5: shows two transitions, one at ~90 K (TC) and another at ~50 K. The paramagnetic state is Griffiths phase-like suggesting the presence of short-range FM correlations. Magnetization data acquired in pulsed fields up to 28 T at 13 K yield magnetic moment of only 17.9 μB/f.u. suggesting a ferrimagnetic arrangement and/or a strong role of anisotropy. In the magnetically ordered state, two field-induced transitions are observed. While one is reversible, the other is irreversible. These results could be understood in terms of competing Tb-Mn1, Tb-Mn2 and Mn-Mn interactions.
CT-08. Body center tetragonal iron compounds with perpendicular crystalline anisotropy
Nian Ji and Jian-Ping Wang
U of Minnesota, Minneapolis, MN
High energy density permanent magnets are a critical enabling component in the future electric vehicle and wind generation systems. It is of great challenge and global need to develop a new-rare-earth-free high energy density pe rmanent magnet. Iron nitrides were famous for its giant saturation magnetization decades ago [1],[2]. However, its crystalline anisotropy were rarely discussed since most of the iron nitrides are magnetically “soft”. Here We present the epitaxial growth and characterization of body centered tetragonal Fe-N thin films on Fe(001)/GaAs(001). By varying the N concentration, the crystal tetragonality (c/a) can be tuned in a relatively wide range (Fig. 1a). The in-plane M-H loops measured on these samples show a monotonic enhancement of saturation field (Hs, Fig.1b) as the c-lattice lattice constant increases, suggesting the presence of perpendicular crystalline anisotropy (PCA) and is progressively increased as c-lattice constant enlarges. Furthermore, it is found that upon N site ordering, the formed Fe16N2 develops even larger PCA accompanied with a saturation magnetization of up to 2.7T, which offers a path to fabricate rare-earth free magnet with its ideal energy product reaches up to 130MOe.
References
[1] T. K. Kim and M. Takahashi, Appl. Phys. Lett. 20 (12), 492 (1972) [2] Y. Sugita, et al., J. Appl. Phys. 70 (10), 5977 (1991).
CT-09. Structure and magnetic properties of melt-spun Nd(Fe,Mo)12 ribbons and their nitrides
Jingzhi Han, Zhong Lin, Meiying Xing, Yunbo Yang, Jinbo Yang, Qing Xu and Yingchang Yang
Peking University, Beijing, China
ThMn12-type (1:12) nitrides have been extensively studied due to their excellent intrinsic magnetic properties and potential applications as permanent magnet. To date, the reported coercivity of 1:12 nitrides is still low and only 1/10 of its anisotropy field. It is well known that in addition to the anisotropy field, the microstructure governed by preparation process plays a key role in the determination of the coercivity. Melt-spinning method is a promising method to improve coercivity of 1:12 nitrides because the microstructure of samples can be adjusted conveniently by changing the wheel speed. In this work, a systematical investigation on the phase composition and phase transition of melt-spun Nd(Fe,Mo)12 ribbons with different wheel speeds was executed using X-ray diffraction, Transmission Electron Microscopy and Thermo-magnetic analysis. It is found that the Nd(Fe,Mo)12 phase can be obtained at the wheel speed of 10 m/s, and Nd(Fe,Mo)7 phase was formed with the wheel speed higher than 10 m/s. The amorphous phase was achieved at 65m/s. Moreover, the average grain size (D) of the major phases in the ribbons decreases linearly with increasing wheel speed. D(10m/s)=275nm, D(40m/s)=125nm and D(65m/s)=5nm. The 1:12 nitrides obtained from annealed ribbon quenched at 65m/s shows a higher coercivity (Hc=1.0 T) than the nitrides obtained from the ribbon quenched at 10 m/s (Hc=0.266T) (Figure.1), which is due to the smaller grain size in the former ribbons.
CT-10. Effects of Nitrogen Deficiency on Magnetostructural Properties of Antiperovskite Manganese Nitrides
Daiki Kasugai, Atsushi Ozawa, Tetsuya Inagaki and Koshi Takenaka
Department of Crystalline Materials Science, Nagoya University, Nagoya, Japan
Antiperovskite manganese nitrides Mn3AN (A: transitional metals or semiconducting elements) demonstrate a variety of physical properties due to the strong relationship between magnetism and the crystal lattice [1]. One of the most important properties is giant negative thermal expansion in Ge- or Sn-doped antiperovskites. Here Ge or Sn dopants play a role as a “relaxant” of the sharp volume change at the magnetic transition of pure, stoichiometric Mn3AN. The mechanism as well as chemical procedures of broadening of the volume change is one of the primary concerns in this field. In particular, the recent experiments suggest that nitrogen deficiency also can broaden the volume change [2]. In order to clarify the role of nitrogen deficiency in the manganese antiperovskites, we have studied the effects of nitrogen deficiency on the magnetostructural properties of Mn3GaN1-δ in comparison with that of Mn3Cu1-xGexN1-δ. Although the magnetic transition temperature is increased with increasing nitrogen deficiency for both Mn3GaN1-δ and Mn3Cu1-xGexN1-δ, there is a striking difference in the broadening of the volume change: the sharp volume change is robust against nitrogen deficiency for Mn3GaN1-δ, whereas it becomes broadened as nitrogen deficiency proceeds for Mn3Cu1-xGexN1-δ. The present results suggest that nitrogen deficiency itself may not broaden the volume change, although it assists the broadening in cooperation with the Ge or Sn dopants. We discuss the origin of the broadening in terms of local structural distortion of the MnN6 octahedral. This work was partly supported by MEXT, Japan and by NEDO, Japan.
References
[1] K. Takenaka, K. Asano, M. Misawa, and H. Takagi, Appl. Phys. Lett. 92, 011927 (2008). [2] K. Takenaka and H. Takagi, Appl. Phys. Lett. 94, 131904 (2009).
CT-11. Magnetic anisotropy of diluted Fe and FeCo alloys with 5d atoms
Liqin Ke1, Vladimir Antropov1 and Mark van Schilfgaarde2
1Ames Laboratory, Ames, IA; 2Arizona state university, Tempe, AZ
Using density-functional theory, the magnetic anisotropy of body-centered cubic Fe and FeCo systems is analyzed, when doped with 0.5-12% atoms from the 5d series. Several mechanisms potentially responsible for the anisotropy have been studied, in particular asymmetry in the distribution of 5d atoms, concentration and tetragonal distortions. We find that the magnetic anisotropy is determined mostly by weakly magnetic 5d atoms. The anisotropy depends greatly on the symmetry of nearest 5d neighbors, and their relative orientation. This effect is very long ranged: it was found, for instance, that the magneto-anisotropic interaction between two W impurities along 111 is very significant even at distances larger than three lattice constants. The effect is rather itinerant and strongly depends on the shape of Fermi surface. In conclusion we analyze possible use of these materials as permanent magnets where high anisotropy is needed. This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), under its Vehicle Technologies Program, through the Ames Laboratory. Ames Laboratory is operated by Iowa State University under contract DE-AC02-07CH11358.
CT-12. Structures and Magnetic Properties of Fe_{x}Co_{1-x} and W doped Fe_{x}Co_{1-x} Alloys
Manh Cuong Nguyen1, 2, Xin Zhao1, 2, Min Ji1, 2, Cai-Zhuang Wang1, 2 and Kai-Ming Ho1, 2
1Ames Laboratory, Ames, IA; 2Physics, Iowa State University, Ames, IA
Using genetic algorithm for crystal structure prediction, we performed a systematic search for low energy structures of Fe_{x}Co_{1-x} and W doped Fe_{x}Co_{1-x} Alloys for p ermanent magnet applications using only the compound composition as input information. Atomic structure optimization and energy evaluation were carried out by first-principles density functional calculations within the generalized-gradient approximation. We found stable low energy states for Fe_{x}Co_{1-x} with various compositions in addition to the known FeCo-B2 ground state at x=0.5. The low energy states have BCC underlying lattice for Fe_{x}Co_{1-x} compound with x > 0.25, consistent with experiment. For Fe-rich compounds, the Fe simple cubic (SC) sublattice of FeCo-B2 structure is unchanged and the extra Fe atoms sit on Co SC sublattice in some patterns obeying a sum rule. We also performed a structure search for FeCoW ternary alloys showing new low energy arrangements on the BCC underlying lattice. The magnetic properties of these new alloy structures are also investigated.
CT-13. Combinatorial search of rare-earth-free permanent magnets: magnetic and microstructural properties of Fe-Co-W thin films
Tieren Gao1, Ichiro Takeuchi1, Yaqiao Wu2, Matthew J. Kramer2, Iver E. Anderson2, Bill McCallum2, Kevin W. Dennis2, Ke Wang3 and Leonid Benderksy3
1University of Maryland, College Park, MD; 2Ames Laboratory, Iowa State University, Ames, IA; 3National Institute of Standards and Technology, Gaithersburg, MD
We are using the thin film composition spreads to search for rare-earth free permanent magnetic materials. Ternary co-sputtering is used to generate composition spreads. We have thus far looked at W doped Fe-Co as one of the initial systems to search for possible compounds with enhanced coercive fields. The films were deposited on Si (100) substrates, and they are annealed at different temperatures. The structural properties of films are mapped by synchrotron diffraction. We find that there is a phase transition from a crystalline state to an amorphous state at about 10% atomic W composition in the as deposited state. With increasing annealing temperature, the W concentration where the phase transition occurs increases from 15% for 600 degrees to 20% for 700 degrees. We find that some of the compounds at low W concentration display enhanced coercive field in out-of-plane magnetic hysteresis loops. The microstructure of films is characterized by SEM and high resolution TEM. It was found that the grains of low W content Fe-Co films which show enhanced coercive fields vertically standing platelet-like structures typically 150 nm in length and 50 nm in width in lateral dimensions. The angular dependence of the switching field of Fe-Co-W films indicates that the magnetic reversal process evolves from domain wall displacement to coexistence of domain wall displacement and coherent rotation with increasing W content. This behavior can be explained by the different microstructures of the films.
CT-14. Structural Properties and Magnetic Phase Transition in HoNi2Mn (57Fe)
Jianli Wang1, 2, Stewart J. Campbell3, M. Hofmann4, M. Hoelzel4, 5, Rong Zeng1, Shi X. Dou1 and Shane J. Kennedy2
1Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, NSW, Australia; 2Bragg Institute, ANSTO, Sydney, NSW, Australia; 3School of Physical, Environmental and Mathematical Sciences, UNSW@ADFA, Canberra, ACT, Australia; 4Technische Universität München, FRM II, München, Germany; 5Fachbereich Materialwissenschaften, Technische Universität Darmstadt, Darmstadt, Germany
Structural and magnetic properties of HoNi2Mn(57Fe) have been investigated by x-ray and neutron diffraction, ac susceptibility and dc magnetization and 57Fe Mössbauer spectroscopy. The 57Fe doped HoNi2Mn compound crystallizes in the MgCu2-type structure with Fd-3m space group, and is isostructural with HoNi2 and HoMn2. Bulk magnetization and neutron diffraction measurements demonstrate that HoNi2Mn(57Fe) exhibits ferrimagnetic ordering below the Curie temperature TC~60 K. This TC value is slightly lower than that reported for undoped HoNi2Mn (TC=75 K) [1] but is much higher than the corresponding values for HoNi2 (TC=15 K) [2] and HoMn2 (TC=24 K) [3]. Detailed investigations of the dc magnetization and ac susceptibility of HoNi2Mn(57Fe) indicate that the magnetic phase transition at TC~60 K is second order. Rietveld refinements of x-ray and neutron data indicate that the transition metal atoms occupy two inequivalent crystal sites (8a and 16d). The Mössbauer spectra (Fig. 1) above TC are described well by two sub-spectra representing the 8a and 16d sites. Below TC the spectra have been fitted using a site model comprising three sub-spectra, and a model with a distribution of hyperfine fields, to provide further insight to the magnetic behaviour of HoNi2Mn(57Fe). The spectral fits are consistent with our conclusion that the Mn (57Fe) atoms occupy both 8a and 16d sites in RNi2Mn [4]. The Debye temperature has been derived to be θD=190(20) K by fitting the temperature dependent mean isomer shift.
References
[1] J.L. Wang, C. Marquina, M.R. Ibarra and G.H. Wu, Phys. Rev. B 73, 94436 (2006) [2] M. R. Ibarra, J. I. Arnaudas, P. A. Algarabel, and A. Del Moral, J. Magn. Magn. Mater. 46, 167 (1984). [3] K. Inoue, Y. Nakamura, A. V. Tsvyashchenko, and L. Fomicheva, J. Phys. Soc. Jpn. 64, 2175 (1995). [4] J.L. Wang, S.J. Campbell, S.J. Kennedy, R. Zeng, S.X. Dou and G.H. Wu, J. Phys. Condens. Matter 23, 216002 (2011)
CT-15. Magnetic and Magnetocaloric Properties of the New Rare Earth - Transition Metal Intermetallic Compound Gd3Co29Ge4B10
Margaret P. Hill1, Igor Dubenko2, Tapas Samanta2 and Naushad Ali2
1Physics & Engineering Physics, Southeast Missouri State University, Cape Girardeau, MO; 2Physics, Southern Illinois University-Carbondale, Carbondale, IL
Recently substantial work has been done on the structural properties of the new family of stable rare earth - transition metal compounds R3T29M4B10 (R = rare earth (except Yb, Pm and Eu); T = Co, Ni; and M = Si, Ge, Al)1. These materials are interesting because of their potential for improving our understanding and development of rare earth intermetallic magnets with interesting and promising magnetic and transport properties. In this study Gd3Co29Ge4B10 and (Gd2Y)Co29Ge4B10 were synthesized by arc melting, and their magnetic and transport properties investigated as a function of temperature and applied magnetic field. X-ray measurements showed single phase samples with the tetragonal crystal structure P4/nmm. We found Gd3Co29Ge4B10 orders ferromagnetically at TC = 212 K and shows a compens ation point at 128 K, indicating a ferrimagnetic ordering of the Co and Gd moments. An entropy change ΔS = 0.5 J/kgK was observed in a 5 T field at TC for this sample, while a change in sign for this quantity was observed both at the maximum value of magnetization (around 200 K) and then again at the compensation point. Substitution of Y for Gd in Gd3Co29Ge4B10 does not affect the Curie temperature, but shifts the compensation point to lower temperatures. The relative contributions of d-d, f-d, and f-f exchange interactions to magnetic behavior of these samples are discussed. Acknowledgment: This work was partly supported by U.S. Department of Energy (Grant No. DE-FG02-06ER46291).
References
1H. Zhang, S. J. Campbell, A. V. J. Edge, J. Phys.: Condens. Matter 12 (2000) L159.
8:00 AM - 12:00 PM, Saguaro Ballroom
Co-Chair: Margaret Hill, Southeast Missouri State University; Co-Chair: Hendrik Ohldag, SSR/SLAC
CU-01. Activation Volumes in Epitaxial Co2FeSi Thin films
James Sagar1, Hiroaki Sukegawa2, Atsufumi Hirohata3, 4, Seiji Mitani2 and Kevin O'Grady1
1Physics, The University of York, York, United Kingdom; 2Magnetic Materials Centre, National Institute for Materials Science (NIMS), Tsukuba, Japan; 3Electronics, The University of York, York, United Kingdom; 4PRESTO, Japan Science and Technology Agency, Kawaguchi, Japan
A material that will give 100% spin polarisation of charge carriers is urgently required. Co-based Hesuler alloys are promising candidates for this application because they have large spin moments, ≥5μB/f.u., and high Curie temperatures, ≥1000K [1]. The structural and magnetic properties of these materials are well known but the magnetisation reversal mechanisms within the films and the effect of crystallographic order of the material is unknown. 20 nm epitaxial Co2FeSi thin films were grown using ultrahigh vacuum magnetron sputtering. Measurements of the time dependence of magnetisation (S(H)=dM/dlnt) and the irreversible susceptibility (χirr(H)) have been made. From these measurements the activation volume of reversal Vact has been determined [2] Vactis the smallest entity that can reverse and gives an indication of the degree of domain wall pinning in the system. This is controlled by the degree of disorder and defects in the films which has been studied by HRTEM.
References
[1] I. Galanakis and P. H. Dederichs, Phys Rev B 66, 174429 (2002). [2] E. P. Wohlfarth, J. Phys. F. Met. Phys. 14, L155-L159 (1984).
CU-02. Structural, magnetic and electron transport properties of MnBi:Fe thin films
Parashu R. Kharel1, 2, V. R. Shah1, X. Z. Li1, R. Skomski1, 2 and D. J. Sellmyer1, 2
1Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE; 2Department of Physics and Astronomy, University of Nebraska, Lincoln, NE
MnBi is one of the few ferromagnetic manganese compounds that has appreciable coercivity above room temperature and can be grown in thin films with large perpendicular magnetic anisotropy. Since these properties are promising for permanent magnet applications, MnBi has attracted much attention as a potential non-rare earth material for permanent magnets. Recently, there have been efforts to improve the structural and magnetic properties of this material with the substitution of a third element which has stimulated the current work. We have studied the effect of Fe substitution on the structural, magnetic and electrical transport properties of MnBi films (Mn55-xFexBi45; 0 < x < 15) prepared by e-beam evaporation and annealing. X-ray diffraction shows that the films have the low-temperature MnBi phase and are highly c-axis textured. Interestingly, the texture and ordering of the films have been improved with Fe substitution as evidenced from the width of the rocking curves. We have observed a clear structural transition from low-temperature to high-temperature phase in Mn55-xFexBi45 films with x ≤ 4. However, the films with higher Fe concentration (x ≥ 5) do not show this phase transition before completely losing the c-axis texture. On the other hand, the saturation magnetization, coercivity and anisotropy constants show a significant change with Fe doping. The room temperature values of the saturation magnetization (Ms) and the anisotropy constant (K1) for MnBi are 560 emu/cm3 and 1.1×107 ergs/cm3 but the values decrease significantly to 135 emu/cm3 and 2×106 ergs/cm3 as Fe concentration increases to 15%. The temperature dependence of resistivity between 4K and 300 K shows that the films with low Fe concentration (up to 4%) are metallic (dρ/dT > 0) but the resistivity changes dramatically as the concentration increases beyond 4%. Below 300 K, a monotonic increase in the resistance of the Mn55-xFexBi45 (x ≥ 5) films has been observed with decreasing temperature (dρ/dT < 0). We explain these observations as the consequences of Fe-induced disorder and the antiferromagnetic interaction that occurs between the interstitial and regular Mn atoms. This research is supported by NSF MRSEC, ARPA-E, ARO and NCMN
CU-03. Structure and magnetic properties of MnxBi100-x (x=48, 50, 55, 60) compounds
Wentian Geng1, Dongtao Zhang1, 2, Ming Yue1, Weiqiang Liu1, Jiuxing Zhang1 and You Qiang2
1College of Materials Science and Engineering, Beijing University of Technology, Beijing, China; 2Physics Department, University of Idaho, Moscow, ID
MnBi with NiAs-type hexagonal crystal structure exhibits a positive coercivity temperature coefficient and it can be potentially used as a high temperature permanent magnet. However, pure Low Temperature Phase (LTP) MnBi alloy is very difficult to be obtained due to the huge difference in melting point of Mn and Bi. In this paper, MnxBi100-x(x = 48, 50, 55, 60) alloys were prepared by improving melting technique, melt spinning and subsequent ball milling. XRD shows that the powders were mainly composed of LTP MnBi. Increasing the speed of melt spinning and reducing the time of annealing treatment can restrain the segregation of Mn from MnBi liquid during the peritectic reaction and as a result, it can increase the LTP content. High energy ball milling can increase the coercivity of MnBi powders. With the increase of milling time, the coercivity increases initially and then decreases gradually. After ball milling for 4 hours, the coercivity of the MnxBi100-x powders is 11.4 kOe for x=48 and 14.8 kOe for x=55, respectively. The decrease of bismuth content leads to a higher coercivity in the Mn55Bi 45 magnet. On the other hand, it gives rise to more LTP MnBi in the Mn48Bi52 magnet because of its better combination with Mn, as shown in Figure 1. The optimal composition of Mn48Bi52 with more LTP has a Ms of 49.98 emu/g and a Mr of 33.57 emu/g.
References
J. B. Yang, K. Kamaraju, W. B. Yelon, and W. J. James, Applied Physics Letters, 12:1912(2001).
CU-04. Preparation and magnetic properties of high purity low temperature phase MnBi
Y.b. Yang, Xiaobai Ma, X.g. Chen, Rui Wu, J.z. Wei, G.j. Lian, Yan Zhang, Y.c. Yang and J.b. Yang
Peking University,School of Physics, Beijing, China
Physical properties of MnBi compound has been studied extensively due to its unusual magnetic and magneto-optical properties. The high temperature phase (HTP) shows excellent magneto-optical properties.[1,2] The low temperature phase (LTP) with NiAs-type hexagonal crystal structure exhibits a positive temperature coefficient for the coercivity from 150K to 550K, which is favourable for high temperature applications. However, the single phase LTP MnBi is very difficult to be achieved by using conventional methods such as arc-melting, mechanical milling and sintering. [3] In this paper, nanocrystalline MnBi with size of 20nm-30nm was fabricated by melt-spinning and subsequently annealing. The influence of quenching speeds, compositions and annealing conditions on the formation of MnBi LTP was investigated. The amorphous MnBi ribbons were prepared by melt spinning with a wheel speed of 65 ms-1. It was found that LTP MnBi can be formed in the range of 533-593K with x=1.20-1.25 in the composition of MnxBi. The coercivity of the MnBi can be greatly further improved by grinding, and exhibits a positive temperature coefficient. A room temperature maximum energy product (BH)max of 6.9 MGOe is obtained for the MnBi powders. The magnetization dependence of temperature exhibits the spin reorientation at 90 K. A peak of magnetization appears about 544K is due to growth of LTP MnBi.
References
[1]C. Guillaud, Journal De Physique Et Le Radium 12 (2), 143-143 (1951). [2]J. B. Yang, K. Kamaraju, W. B. Yelon, W. J. James, Q. Cai and A. Bollero, Appl. Phys. Lett. 79 (12), 1846-1848 (2001). [3]X. Guo, A. Zaluska, Z. Altounian and J. O. Stromolsen, J Mater Res 5 (11), 2646-2651 (1990).
CU-05. Spin reorientation transition and hard magnetic properties of MnBi intermetallic compound
Kiyonori Suzuki1, Xiaodong Wu1, Tetsuya Shoji2 and Akira Kato3
1Department of Materials Engineering, Monash University, Clayton, VIC, Australia; 2Metallic & Inorganic Material Engineering Div., Toyota Motor Corporation, Toyota, Japan; 3Advanced Material Engineering Div., Toyota Motor Corporation, Susono, Japan
The low temperature phase (LTP) MnBi exhibits a positive temperature dependence of the magnetocrystalline anisotropy1 because of the low temperature spin reorientation transition (TSR). It has been reported that LTP-MnBi powders processed by ball milling exhibit a large intrinsic coercivity above 1.2 MA/m at room temperature.2 Owing to the unique temperature dependence of the magnetocrystalline anisotropy, the coercivity of this hard magnetic material is enhanced at elevated temperatures1,3 and hence, LTP-MnBi is a potential candidate for permanent magnets used at elevated temperatures. However, while the effe ct of milling on the room-temperature coercivity values has been reported,2 little has been studied on TSR in ball milled LTP-MnBi despite the fact that TSR is often size dependent.4,5 Hence, the temperature dependence of the coercivity in LTP-MnBi may be influenced by the grain refinement upon milling. In this report, we have investigated the effect of mechanical grinding on the grain size, TSR and the coercive field of LTP-MnBi. LTP-MnBi powders were prepared by crystallization of melt-spun amorphous precursors. The LTP-MnBi powders were milled under an argon atmosphere using a vibration ball mill. Although the crystallization of the amorphous precursor resulted in a micrometer-scale microstructure, it was refined significantly by mechanical grinding and a small grain size below 20 nm was obtained after mechanical grinding for 43.2 ks. The spin reorientation transition temperature determined by the temperature dependence of ac-susceptibility for the as prepared LTP-MnBi powders was about 120 K. However, TSR showed a clear tendency to decrease with increasing milling time and TSR after milling for 43.2 ks was suppressed below 100 K. The coercive force of LTP-MnBi showed a tendency to increase with increasing milling time and a large Hc value above 1 MA/m was obtained after milling for 43.2 ks. This dramatic magnetic hardening effect at room temperature may partially be attributable to the reduced spin reorientation transition temperature since the onset of the positive temperature behavior of the magnetocrystalline anisotropy could be lowered by the suppressed TSR.
References
[1] X. Guo, X. Chen, Z. Altounian and J. O. Ström-Olsen, Phys. Rev. B46 (1992) 14578. [2] S. Saha, R. T. Obermyer, B. J. Zande, V. K. Chandhok, S. Simizu, S. G. Sankar, J. A. Horton, J. Appl. Phys. 91 (2002) 8525. [3] J. B. Yang, K. Kamaraju, W. B. Yelon, W. J. James, Q. Cai and A. Bollero, Appl. Phys. Lett. 79 (2001) 1846. [4] N. G. Akdogan, G. C. Hadjipanayis and D. J. Sellmyer, Nanotechnology 21 (2010) 295705. [5] C. B. Rong, N. Poudyal and J. P. Liu, Phys. Lett. A 374 (2010) 3967.
CU-06. Theoretical investigation on the magnetic phase stability of Fe-doped Bi tellurides
Miyoung Kim1 and Jung-Hwan Song2
1Division of Energy System Research, Ajou University, Suwan, Republic of Korea; 2Department of Physics and Astronomy, Northwestern University, Evanston, IL
Bismuth Telluride and its magnetically doped alloys are drawing much attention these days due to their potential application as thermoelectrical materials as well as their unique electronic structures related to the Topological Insulator. In this research, we investigate theoretically the electronic structures and magnetic properties of the Fe-doped Bi tellurides with 8% of doping level employing ab-initio FLAPW [1] method, aiming to understand the magnetic phase stability and its dependence on the doping type and the atomic optimization. Results show that (i) the ferromagnetic phases are energetically more stable over the paramagnetic phases for both substituional and interstitial Fe dopings, (ii) the substituial Fe doping indicates a high magnetic phase with a largely enhanced Fe spin moment of 3.4μB while the interstitial Fe doping shows the magnetic moment similar to the Fe bulk value, and (iii) the atomic optimization affects to reduce the magnetic moments for both doping types where the effect is bigger for the substitutional doping case. Thermoelectric transport peroperties are also investigated to demonstrate the carrier concentration and temperature dependences for both doping types.
References
[1] E. Wimmer, K. Krakauer, M. Wienert, and A.J. Freeman, Phys.Rev B 24, 864 (1981). * mykim.nu@gmail.com
CU-07. Thermoelectric efficiency of topological insulators in a magnetic field
Oleg Tretiakov, Artem Abanov and Jairo Sinova
Texas A&M Univ, College Station, TX
We study the thermoelectric properties of three-dimensional topological insulators in magnetic fields with many holes (or pores) in the bulk. We find that at high density of these holes in the transport direction the thermoelectric figure of merit ZT can be large due to the contribution of the topologically protected conducting surfaces and the suppressed phonon thermal conductivity. By applying an external magnetic field a subgap can be induced in the surface states spectrum. We show that the thermoelectric efficiency can be controlled by this tunable subgap leading to the values of ZT much greater than 1. Such high values of ZT for reasonable system parameters and its tunability by magnetic field make this system a strong candidate for applications in heat management of nanodevices, especially at low temperatures.
CU-08. High Field (14Tesla) Magneto Transport and Heat Capacity of Sm/PrFeAsO and FeTe
Rajendra S. Meena1, 2, Shiva K. Singh1, Anuj Kumar1, R. Jha1, K. V. Rao2 and Veer Pal S. Awana1
1Quantum Phenomena and Applications, National Physical Laboratory, Delhi, India; 2Deparment of Physics, University of Rajasthan, Jaipur, India
We report magneto transport and specific heat study of parent compounds of arsenides (Sm/PrFeAsO) and chalcogenide (FeTe). Below spin density wave, magnetoresistance (MR) of REFeAsO is positive and increasing with decreasing temperature. The MR of SmFeAsO, is found to be as large as 16% whereas it reaches 65% in case of PrFeAsO, at 2.5K and magnetic field of 14 T. Nonlinear variation of MR at low magnetic fields tends to be linear at high magnetic fields in lower temperature range. The anomalous behaviour of MR exhibited in PrFeAsO with field below 40K, originates from Dirac cone states. Heat capacity CP(T) measurement shows an anomaly at around 140K, for both Sm/PrFeAsO, which is due to the SDW ordering of Fe in these compounds. Below 20K, CP(T) is also measured under applied fields of up to 14 Tesla. In SmFeAsO CP(T) shows a peak at around 4.5K , due to the antiferromagnetic ordering of Sm3+ spins. Again the Cp(T)H of PrFeAsO showed anomalous behaviour mimicking the R(T)H with possible ordering of Pr at around 12K. The situation is similar to high Tc cuprates viz. PrBa2Cu3O7. As far as FeTe is concerned, though the possible SDW metallic step is seen at below 70K in zero field R(T), the same does not exhibits any appreciable MR even up to 14 Tesla field. The CP(T) measurements of FeTe showed an anomaly at 70 K , which is invariant with field. This indicates that the anomaly is mainly due to structural change and the magnetic contribution is relatively much less. Our results of high field up to 14 Tesla magneto transport and heat capacity brings about clear distinction between ground state of superconducting arsenides (Sm/PrFeAsO) and chalcogenide (FeTe). Further the anomalous nature of Pr is seen in PrFeAsO, similar to that as for PrBa2Cu3O7.
References
1. Khuong K. Huynh, yoichi Tanabe, Katsumi Tanigaki Rhys. Rev. Lett. 106 217004 (2011).
CU-09. Synthesis, structure, and magnetic analysis of the cubic defect spinel ZnMnO3
Mohindar S. Seehra1, James D. Rall1, S. Thota2 and J. Kumar3
1Department of Physics, West Virginia University, Morgantown, WV; 2Indian Institute of Technology, Guwahati, India; 3Indian Institute of Technology, Kanpur, India
Previous studies on the Zn-Mn-O system have reported the formation and magnetic studies of either tetragonal ZnMn2O4 [1] or hexagonal ZnMnO3 [2]. Here we report the oxalate-based sol-gel synthesis of cubic ZnMnO3 using acetate precursors. Matching the intensities and positions of the XRD Bragg lines with simulated spectra shows the structure to be a cubic defect spinel with lattice constant a = 0.8369 nm, crystallite size D ≈ 25 nm and formula: 4ZnMnO3 = 3[Mn][Mn1/3Zn4/3V1/3]O4 where V = vacancy. SQUID measurements of magnetization M vs. T (2 K - 370 K) and M vs. H (up to 65 kOe) show it to be a ferrimagnet with TC ≈ 20 K and spin reorientation transition at TS ≈ 7 K. The data of χ vs. T for T > TC fit the Neel equation for ferrimagnets with μ = 4.4μB as the magnetic moment per Mn4+ ion. Coercivity Hc and exchange bias Heb for a field-cooled sample decrease to zero as T → TC with a distinct anomaly between Hc(ZFC) and Hc(FC) at TS (see Fig. 1). These first magnetic studies in cubic ZnMnO3 have established it to be a defect spinel with ferrimagnetism resulting from the different number of Mn4+ ions on the A and B sites.
References
[1] S. Åsbriurk, A. Waskowska, L. Gerward, J.S. Olsen, E. Talik, Phys. Rev. B 60, 12651 (1998). [2] B.L. Chamberland, A.W. Sleight, J.F. Weiher, J. Solid St. Chem. 1, 512 (1970).
CU-10. Field-induced Magnetic Transition in Cobalt-Ferrite
Martin Kriegisch1, Weijun Ren2, Reiko Sato-Turtelli1 and Roland Groessinger1
1Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria; 2Shenyang National Laboratory for Materials Science, Magnetism and Magnetic Materials Division, Shenyang, China
We present magnetostriction and magnetization measurements of a cobalt ferrite (Co0.8Fe2.2O4) single crystal. We observe an unusual behaviour in the magnetic hard axis of the single crystal which manifests in a jump of the magnetization curve at a critical field. This first order magnetization process (FOMP) is visible at temperatures lower than 150 K. By analyzing the anisotropy constants we find that the higher order anisotropy constants K2 and K3 dominate the anisotropy energy. Theoretical calculations on the temperature dependent anisotropy in the literature only take the first anisotropy constant K1 into consideration and should be therefore revised. In the magnetostriction measurements the FOMP is clearly visible in a huge jump, which can be explained by means of a geometric model. A detailed discussion and comparison with literature is provided as well.
CU-11. Withdrawn
CU-12. Impacts of electron correlation in anion p-orbitals on electronic structure and magnetism of nitrogen or carbon doped zinc oxide
Yao-fang Zhang1 , Hong Liu2 and Xu Zuo1
1College of Information Technical Science, Nankai University, Tianjin, China; 2Office of International Academic Exchanges, Nankai University, Tianjin, China
It was proposed that substituting N/C for O is a possible approach to magnetizing nonmagnetic oxides without open d-shell transition metals [1]. Previous calculations based on LSDA/GGA predicted a half-metallic state and strong ferromagnetic coupling between dopants [2]. Conversely, recent calculations based on SIC predicted an insulating state and negligible magnetic coupling for N:MgO [3]. In this work, we study N/C:ZnO using GGA+U exchange-correlation, by which the effects of electron correlation in anion p-orbitals are considered. The 3x3x3 supercells are constructed for wurtzite ZnO, where an O atom is substituted by N/C. The supercells are relaxed by using GGA and GGA+U, respectively, before the calculations of electronic structure. In GGA+U calculations, Hubbard Us are applied on Zn-d, O-p and N/C-p orbitals. The magnetic couping versus distance is calculated using the supercells containing two dopants. Compared to GGA, GGA+U obviously enhances the local magnetic moment on dopants. It lowers the local symmetry for dopant (C3v), preserved by GGA, down to C2v. Consistent with the local symmetry, it also distorts the isotropic spin density in the basal plane by GGA to a dumbbell shape [insets of Fig. 1(a)&(b)]. In contrast to GGA, it leads to an insulating state with a band gap of ~0.5 eV [Fig. 1(a)&(b)]. While both GGA and GGA+U lead to an oscillating magnetic coupling versus distance [Fig. 1(c)], the magnitude of the magnetic coupling by GGA+U drastically decreases with increasing distance. This is consistent with the insulating state, where the carrier-mediated long-range ferromagnetic coupling mechanism, double exchange, is unavailable. However, ferromagnetic superexchange between the next nearest neighbors is strong enough to cause room-temperature ferromagnetism at a high doping concentration.
References
[1] I. S. Elfimov et al., Phys. Rev. Lett. 98, 137202 (2007). [2] L. Shen et al., Phys. Rev. B 78, 073306 (2008). [3] A. Droghetti et al., Phys. Rev. B 78, 140404R (2008).
CU-13. Study on rare-earth doped type-I germanium clathrates
Xiaohui Zhu1, Ning Chen1, Lihua Liu2 and Yang Li3
1School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, China; 2Department of Physics, University of Science and Technology Beijing, Beijing, China; 3Department of Engineering Science and Materials, University of Puerto Rico at Mayaguez, Mayaguez
The type-I clathrates with rare-earth doping are hardly to be experimentally prepared except Eu. The synthesis of rare-earth clathrates RExBa8-xGa16Ge30 still keeps challenging. In this paper, the first-principle method based on the density functional theory was used to investigate the rare-earth substitution for Ba on two sites (2a and 6d). The effects of the various RE elements doping on the binding energy and chemical reaction energy are studied by optimizing the phase structure. The calculated results indicate that rare-earth atoms preferentially occupy 2a site of Ba. It is hard to synthesize RE clathrates without Ga doping since Ga can effectively decrease reaction energy of system. The simulation shows that beside Eu, other rare-earth such as Sm, Nd and Yb could enter cage lattice in the considering of energy, which implies that it is possible to synthesize the (RE,Ba)8Ge30Ga16 clathrates. In addition, the simulation also shows the R E clathrates have their own unique magnetic characters.
CU-14. First Principles Study of the Magnetic Properties of BN Graphene Nanoribbon
Jeffrey Rufinus
Science Division, Widener University, Chester, PA
The emerging field of spin electronics (spintronics) has been continuously attracting researchers. Substantial theoretical and experimental efforts have been made in the quest to find the candidates for future spintronics devices. Recently, the search for new spintronics materials has also included graphene-based materials due to the theoretical prediction that this type of material may show the half-metallic property. Here, we present the results of a density functional theory within a generalized gradient approximation study of Boron Nitride (BN) graphene nanoribbon. The objective of this study is to determine whether this type of material will be ferromagnetic or antiferromagnetic. Our results show that the zigzag shaped BN graphene nanoribbon prefers ferromagnetic state. These results are of the scientific interest in exploring the magnetic properties of graphene-based nanoribbon for future spintronics.
CU-15. Magnetic properties of ferromagnetic carbon materials
Tetsuji Saito1, Shunsuke Yoshii2 and Daisuke Nishio-Hamane3
1Chiba Institute of Technology, Chiba, Japan; 2Institute for Materials Research, Tohoku University, Sendai, Japan; 3Institute for Solid State Physics, Tokyo University, Kashiwa, Japan
Recent studies of some organic materials have indicated that their low magnetization is due to the ferromagnetic interaction of spins [1,2]. Recently, it has been reported that the ferromagnetic carbon material prepared by pyrolysis of polyacrylonitrile exhibits a saturation magnetization at room temperature [3]. In this study, magnetic properties of carbon materials prepared by pyrolysis of polyvinyl chloride (PVC) were investigated. Commercially available PVC was used as the starting material. Pyrolysis of the PVC was carried out using an electric furnace at temperatures between 1073 K and 1673 K for 24 h under an argon atmosphere. Although PVC powder exhibited diamagnetic behavior, the carbon material prepared by pyrolysis at 1673 K exhibited ferromagnetic behavior. Chemical analysis combined with TEM-EDX analysis revealed that the carbon material did not contain any transition metals. The metal-free ferromagnetic carbon material exhibited a large saturation magnetization of 2.0 emu/g at room temperature, enough to be attracted by a permanent magnet. (Fig.1) The ferromagnetic behavior of the carbon material prepared by pyrolysis at 1673 K for 24 h is considered to be due to the presence of nanometer-scale graphite-like structures.
References
[1] A. A. Ovchinnikov and V. N. Spector, Syn. Metals 27, B615 (1988). [2] K. Tanaka, S. Yamashita, T. Yamabe, J. Yamauchi, and Y. Deguchi, Solid State Commun. 71, 627 (1989). [3] T. Saito, S. Yoshii, T. Nojima and D. N. Hamane , Appl. Phys. Lett., 052506, 98 (2011).
8:00 AM - 12:00 PM, Saguaro Ballroom
Co-Chair: Mathew Willard, Naval Research Lab; Co-Chair: Oliver Gutfleisch, IFW, Dresden
CV-01. The partitioning of Dy and Tb in NdFeB magnet: a first-principles study
Xubo Liu a nd Zaven Altounian
physics department, McGill University, Montreal, QC, Canada
To meet the recent demand for higher efficiency and cost effective magnets for hybrid/electric vehicles, the reduction of Dy/Tb content in (Nd,Dy/Tb)-Fe-B magnets with high operating temperatures is a high priority. The motivation is to reduce cost and to prevent the strong decrease in the magnetization of (Nd,Dy/Tb)2Fe14B (2:14:1).1 The promising approaches include two-alloy sintering and grain boundary diffusion methods,2,3 which dope Dy/Tb in the grain boundary region and increase the magneto-crystalline anisotropy in the outer regions of the 2:14:1 grains where magnetization reversal starts. To understand and control Dy/Tb redistribution in NdFeB magnet, the partitioning of Dy/Tb between 2:14:1 and Nd-rich phase (cubic NdO) have been studied by a first-principles density functional calculation. The total energy calculations indicate that Dy and Tb prefer to enter 4f sites in the 2:14:1 phase. The substitution energies of Dy and Tb in 2:14:1 are negative and are -0.35 eV/atom and -0.33 eV/atom, respectively, as Nd is replaced with 25% of Dy/Tb, i.e. stabilizing the 2:14:1 structure. As expected, the moments of Dy/Tb order anti-ferromagnetically with that of the Fe sub-lattice in the 2:14:1 phase. However, the substitution energies of Dy and Tb in NdO have large positive values. They are 0.83 eV/atom and 0.73 eV/atom for Dy and Tb, respectively, when 25% of Nd is replaced by Dy/Tb. The results indicate that Dy/Tb prefer to enter the 2:14:1 phase rather than the Nd-rich phase (NdO). This is the thermodynamic origin for the selective occupation of Dy and Tb in the 2:14:1 structure which enhances the magnetic anisotropy field.
References
[1] O Guteisch, M. A. Willard, E. Brck, C. H. Chen, S. G. Sankar, and J. P. Liu, Adv. Mater. 23, 821 (2011) [2] K. Hirota, H. Nakamura, T. Minowa, and M. Honshima, IEEE Trans. Magn. 42, 2909 (2006). [3] A. M. Gabay, M. Marinescu, W. F. Li, J. F. Liu, and G. C. Hadjipanayis, J. Appl. Phys. 109, 083916 (2011).
CV-02. Enhancing the perpendicular anisotropy of NdDyFeB films by Dy diffusion process
Wenjie Gong, Wei Liu, Sheng Guo, Zhenhua Wang, Yuqin Zhang, Junning Feng, Weibin Cui and Zhidong Zhang
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China
NdFeB thin films with perpendicular anisotropy have been widely investigated for the applications in micromechanical devices and magnetic recording media. Significant process has been made recently, such as grain boundary diffusion process using rare earth fluoride[1]. In this work, we report the large coercivity and anisotropy enhancement in NdDyFeB(200nm)/Dy(tDy) films with perpendicular anisotropy prepared by magnetron sputtering and in-situ annealing. The HC and Mr/Ms reach a maximum at tDy = 50nm, and then decreases with increasing tDy further (Fig. 1), whereas roughness and domain sizes increase with increasing tDy. The HC and Mr/Ms enhancement was due to the Dy enrichment in grain boundary and partially diffusing into matrix grain confirmed by Energy-filtered transmission electron microscopy. The coercivity mechanism changes from pinned-type for NdDyFeB films to nucleation-type for NdDyFeB (200nm)/Dy(50nm) observed by Initial magnetization curves, which may be attributed to the particular Dy distributions in the films.
References
[1] W.F. Li, et al., Acta Mater, 59, 3061 (2011).
CV-03. Effect of sintering conditions on the magnetic and microstructural properties of Nd-Fe-B sintered magnets doped with DyF3 powders
Song-E Park1, Tae-Hoon Kim1, Seong-Rae Lee1, Seok Namkung2 and Tae-Suk Jang2
1Materials Science and Engineering, Korea University, Seoul, Republic of Korea; 2Hybrid Engineering, Sunmoon university, Asan, Republic of Korea
The addition of Dy to improve the coercivity of the Nd-Fe-B magnets is the most obvious way. However, the reduction of the remanence is the shortcoming. The core-shell structure could enhance the coercivity without sacrificing the remanence and saving the Dy [1,2]. We investigated the relationship between the microstructural and magnetic property changes of the Nd-Fe-B sintered magnets doped with DyF3 powder as a function of the sintering temperature and time. The Nd26.0Dy6.5Febal.B1.0M2.4 (wt. %, M=Cu, Al, Co, and Nb) powder were mixed with DyF3 powder (2.4 wt. %). The magnets were sintered at 1050, 1060, and 1070°C for 4hrs. The sintering time was varied from 2 to 4hrs at 1070°C. After sintering, the samples were optimally annealed. Microstructural and magnetic properties were analyzed using EPMA, WDS, and SQUID. Fig. 1 shows that the mapping images of the DyF3-doped magnets as a function of sintering temperature. The coercivity increases as the sintering temperature decreases because the core-shell structure is favorably formed. The sintering time also has similar effect. The Nd-Dy-O rich phase grain (RE-PG) and the Nd-Cu-Co-O rich triple junction phase were observed in the magnet. The RE-PG has no effect on the coercivity enhancement but is only consuming the Dy. When the magnets were doped with the DyF3, the formation of the RE-PG could be suppressed [3]. Accordingly, the Dy-saving of the DyF3 doped magnet is due not only to the core-shell distribution of the Dy but the reduction of the RE-PG phase formation.
References
[1] M. H. Ghandehari and J. Fidler, Mater. Lett., 5, 285 (1987) [2] K. Hirota, H. Nakamura, T. Minowa, and M. Honshima, IEEE Trans. Magn., 42, 2909 (2006) [3] S. E. Park, T. H. Kim, S. R. Lee, D. H. Kim, S. Namkung, and T. S. Jang, IEEE. Tran. Magn, in press (2011)
CV-04. Magnetic properties of Dy-diffused Nd-Fe-B powder prepared by crystallization from amorphous state
Hirotoshi Fukunaga, Ikuo Yamamoto, Masaki Nakano and Takeshi Yanai
Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan
It has been reported that the coercivity Hc of anisotropic Nd-Fe-B magnets is enhanced by the diffusion of a heavy RE metal from their surfaces [1-3]. In these methods, a heavy RE metal was diffused by annealing at a high temperature and/or for long time. When such an annealing is applied to isotropic Nd-Fe-B magnets, it would deteriorate their magnetic properties because of grain growth. For isotropic Nd-Fe-B magnets, we proposed a simultaneous diffusion of Dy with crystallization from the amorphous state and confirmed that the proposed method was valid for amorphous Nd-Fe-B flakes [4]. In this contribution, we applied the above method to amorphous Nd-Fe-B powders. Amorphous Nd2.36Fe14B1.05 powders, 180~250 μm in size, were set in a rotating drum, and Dy was coated on their surfaces by the vapor deposition method. Subsequently, they were annealed with an infrared furnace for crystallization and diffusion of Dy. The furnace was heated up to 923 K in 2min. , and then cooled down to room temperature immediately. Hc and the remanence are shown in Fig.1. Hc increased with increasing the amount of coated Dy and reached approximately 1.8 MA/m at 5wt.%, although Ir decreased. This Hc value is comparable with that obtained for Nd2.36Fe14B1.05 flakes without significant deterioration in Ir [4], which suggests that the method of Dy diffusion using crystallization from amorphous state is applicable to Nd-Fe-B amorphous powders as well as flakes.
References
[1] H. Nakamura et al., IEEE Trans. Magn. 42, 2909 (2006). [2] M. Komuro et al., JPN Patent 000005108 (25 Dec. 2003) [3] M. Komuro et al., Proc. REPM’10, 243 (2010). [4]F. Fukunaga et al., J. Appl. Phys. 109, 07A701 (2011)
CV-05. Magnetic properties and microstructure of Nd-Fe-B sintered magnets with DyHx addition
Youhao Liu1, 2, Shuai Guo1, 2, Xingmin Liu1, 2, Don Lee1, 2 and Aru Yan1, 2
1Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 2Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China
In order to improve the coercivity of Nd-Fe-B sintered magnets, Dysprosium (Dy) has been added to Nd-Fe-B based magnets1-4. In this paper, 1wt% DyHx was blent to Nd30Fe69B powders. After molding and cold isostatic pressing, green compacts were vacuum sintered at different temperatures, from 1000 °C to 1050 °C, for 2 hours. The as sintered magnets were annealed at different temperatures, from 500 °C to 650 °C, for various durations. For comparison, Nd30Fe69B magnets without DyHx doping were prepared under the same condition. The magnetic properties of the as sintered and annealed magnets were measured and shown in Fig. 1. It can be seen that Br increases from 12.8 kGs to 14.27 kGs as the sintering temperature increases from 1000 °C to 1050 °C. These increases can be attributed to the increase of density. The coercivity of the magnets has a maximum value at 1010 °C. Higher or lower sintering temperature leads to a decrease of coercivity. The coercivity of annealed magnets increases as the annealing time extends from 0.5 hour to 8 hours. The microstructure and composition was characterized by scanning electron microscope (SEM) with energy dispersive X-ray detector (EDX). The results show that Dy has different distribution as sintering temperature changes and annealing time extends. This different distribution of Dy leads to the changes of magnetic properties.
References
1. G. L. Yan, P. J. McGuiness, J. P. G. Farr, and I. R. Harris, Journal of Alloys and Compounds 491, L20-L24 (2010). 2. W. Q. Liu, H. Sun, X. F. Yi, X. C. Liu, D. T. Zhang, M. Yue, and J. X. Zhang, Journal of Alloys and Compounds 501, 67-69 (2010). 3. F. Xu, L. T. Zhang, X. P. Dong, Q. Z. Liu, and M. Komuro, Scripta Materialia 64, 1137-1140 (2011). 4. F. Xu, J. Wang, X. Dong, L. Zhang, and J. Wu, Journal of Alloys and Compounds 509, 7909-7914 (2011).
CV-06. Recycling sintered Nd-Fe-B magnets using hydrogen proce ssing
Konrad Güth, Ludwig Schultz and Oliver Gutfleisch
IFW Dresden, Dresden, Germany
In the next decades there will be an increasing demand for permanent magnets based on Nd-Fe-B and Sm-Co in applications like hybrid electrical vehicles (HEV), wind turbines, servomotors and electronic devices. These advanced magnets contain a significant amount of rare-earth elements, which are very cost intensive and some of them such as Dy are in deed rare. It is very important that such materials can be recycled in order to improve their economic and environmental efficiency. In the current work, Nd-Fe-B sintered magnets have been recycled using a hydrogen based route in order to recover magnetically anisotropic powder, suitable for the subsequent production of polymer bonded Nd-Fe-B magnets. Two different Nd-Fe-B commercial sintered magnet grades, one with high Dy-contents and one with low Dy-contents were subjected to hydrogen decrepitation under 50 bar of hydrogen at room temperature [1]. The resulting single crystalline powder was then processed by dynamic hydrogenation, disproportionation, desorption and recombination (d-HDDR) [2]. The hydrogen pressure during disproportionation and recombination was varied over the range 10-150 kPa. Polymer bonded magnets were produced by compacting the recombined powder under an applied magnetic field, in order to align the particles. The effect of the hydrogen pressure during processing on the microstructure, degree of texture and magnetic properties of the recycled magnets was then investigated. Comparable properties to those of non-recycled polymer bonded magnets were obtained.
References
[1] O. Gutfleisch, I.R. Harris, J. Phys. D: Appl. Phys. 29 (1996) 2255-2265. [2] O. Gutfleisch, K. Khlopkov, A. Teresiak, K.-H. Müller, G. Drazic, C. Mishima, Y. Honkura, IEEE Trans. Magn. 39 (2003) 2926-2931.
CV-07. Microstructural evaluation for Dy-free Nd-Fe-B sintered magnets with high coercivity
Ryota Goto1, Masashi Matsuura2, Satoshi Sugimoto1, 2, Nobuki Tezuka1, Yasuhiro Une3 and Masato Sagawa3
1New Industry Creation Hatchery Center (NICHe), Tohoku university, Sendai, Japan; 2Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, Japan; 3Interemetallics Co., ltd., Kyoto, Japan
Nd-Fe-B sintered magnets are used for motors of hybrid or electric vehicles due to their high energy products. For working at high temperature environment, Dy is added to Nd-Fe-B sintered magnets. Because Dy addition decrease magnetization of Nd-Fe-B magnets although increase coercivity, decrease in amount of Dy addition is strongly required. Recently, Sagawa and Une [1] have reported that the powder diameter of Nd-Fe-B achieved 1.1 μm by using He jet-milling system, and the coercivity of sintered magnets fabricated from this powder achieved 20 kOe without addition of Dy or other heavy rare earth elements. In our reports, microstructure of their magnets were observed and compared to magnets whose particle diameter was about 3 μm. The coercivity of magnets consisted of larger particles was 17 kOe. Microstructures were observed by the scanning electron microscope and shapes of grains and distribution of Nd-rich phase were evaluated. The observation was promoted two directions. One is the side perpendicular to magnetically aligned direction (c plane side) and the other is the side parallel to the magnetically aligned direction (c axis side). For magnets consisted of smaller particles, the shapes of grains are isotropic for c plane side and an gular for c axis side. The angle distribution of minor axis of grains was measured to evaluate the direction of grains. The angle of minor axis prefers to parallel to magnetically aligned direction. For the magnets with larger particles, the angle distribution of minor axis of grains show similar tendency between c axis and c plane sides. The distribution of Nd-rich phase for magnets was also evaluated for both magnets. The distribution of triple junctions for the magnets with smaller particles becomes homogeneous compared to that for magnets with larger particles. This difference results in achievement of high coercivity for magnets with smaller particles.
References
[1]Y. Une and M. Sagawa, 21st Workshop on Rare-Earth Permanent Magnets and their Applications, Slovenia (2010).
CV-08. Design and Fabrication of high coercivity sintered permanent magnets without heavy rare earth additions
Bicheng Chen1, 2, Xingmin Liu1, 2, Shuai Guo1, 2, Changjiaing Yan1, 2, Renjie Chen1, 2, Youhao Liu1, 2, Don Lee1, 2 and Aru Yan1, 2
1Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 2Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China
Sintered Nd-Fe-B permanent magnet has been used widely due to their superior magnetic properties 1. As the application of Nd-Fe-B magnets in the fields with high coercivity (Hcj), heavy rare earth are used to obtain high Hcj 2,3. The purpose of this study was to save heavy rare earth in Nd-Fe-B production. Traditional powder metallurgy technology 4 was adapted to prepare samples. By way of optimizing preparation process and adding Pr element in the magnet with the nominal composition of Nd6Pr24X3.6Febal(wt%, X=B,Co,Cu,Al) (sample A), and this high Hcj magnets without heavy rare earth additions were successfully prepared, . In order to understand the effect of Pr addition on the performance of the magnet, alloy Nd24Pr6X3.6Febal (wt%, X=B,Co,Cu,Al) (sample B) was prepared by the same preparation process as comparison. Fig.1 shows the demagnetization curves for sample A and B. It can be found that sample A has much higher Hcj than that of sample B, which is due to the difference of the composition and microstructure of the magnets. Microstructure investigation of the grains and grain boundaries with SEM and TEM were carried out, the results indicated that adding Pr element is beneficial to obtain microstructure with small grains, thin and smooth RE-rich film, which associate with higher Hcj.
References
1Y. Matsuura, J. Magn. Magn. Mater. 303 (2006) 344. 2D. R. Gauder, M. H. Froning, R. J. White, A. E. Ray, J. Appl. Phys., 63 (1988) 352 3G. Bai, R. W. Gao, Y. Sun, G. B. Han, B. Wang, J. Magn. Magn. Mater., 308 (2007) 4S.Z.Zhou, Q.F.Dong, Super Permanent Magnets—Permanent Magnetic Material of Rare-earth and Iron System, Metallurgy Industry Publishing,Beijing(2004)
CV-09. Effect of Co addition on the magnetic properties and microstructure of Nd9.5FebalNb2.5Zr0.5CoXB15 (X=0, 10, 20) bulk magnets
Z. Liu1, 2, W. C. Lin1, C. W. Shih1, C. C. Hsieh1< /SUP>, H. W. Chang4, W. C. Chang1 and A. R. Yan2, 3
1Department of Physics, National Chung Cheng University, ChiaYi, Taiwan; 2Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 3Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 4Department of Physics, Tunghai University, Taichung, Taiwan
Directly cast NdFeB-type rod magnets have been developed to overcome the disadvantage of isotropic bonded Nd-Fe-B permanent magnets[1]. The diameter of Nd9.5FebalTi2.5Zr0.5Cr1B14.5C0.5 rod magnets have been increased to 1.3mm, and the (BH)max and iHc can be as high as 6.8MGOe and 7.7KOe. Further increasing the bulk size decreases the magnetic properties dramatically, due to the inhomogeneous and the larger grain size. Adding some elements to improve the glass forming ability (GFA) of the alloy, for the purpose of obtaining finer grain size, is one of the methods to further improve the diameter and (BH)max. Co element has been reported can improve the GFA of the Nd-Fe-B alloy [2], in this article, the magnetic properties and microstructure evaluation of the Nd9.5FebalNb2.5Zr0.5CoXB15 (X=0, 10, 20) rod magnets, without post-annealing, with diameter from 0.9 to 1.5mm are studied systematically, in order to find the possibility of Co in enhancing the GFA and magnetic energy product of the directly cast bulk magnets. Phase analysis show that 2:14:1, α-(Fe,Co) and amorphous phase are observed in the Co added magnets. DSC analysis show that obvious exothermic peaks were observed in magnets with Co addition, which indicate that amorphous phase tend to appeared in the Co contained magnets, especially at the peripheral region. Magnetic measurements (seen in figure 1) show that higher magnetic energy product and coercivity is obtained in magnets with higher Co content and larger diameter. The microstructure of the core region, outside of core region and the peripheral region of the magnets with different diameter exhibits quite different feature, which leads to different magnetic properties in bulk magnets with different size.
References
[1] H. W. Chang, C. C. Hsieh, J. Y. Gan, Y. T. Cheng, M. F. Shih and W. C. Chang. J. Phys. D: Appl. Phys 44, 2011, 064002. [2] W. Zhang and A. Inoue Appl. Phys. Lett. 80, 2002, 1610.
CV-10. Diffusion of Nd-rich phases in the spark plasma sintered and hot deformed nanocrystalline NdFeB magnets
Youlin Huang1, Zhongwu Liu1, Xichun Zhong1, Hongya Yu1, Xuexu Gao2, Jie Zhu2 and Dechang Zeng1
1School of Materials Science and Engineering, South China University of Technology, Guangzhou, China; 2State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, China
Melt spun Nd-rich Nd13.5Fe73.5Co6.7Ga0.5B5.6 and Fe-rich Nd7.7Pr2.6Fe84.1B5.5 alloy powders were mechanically mixed with different ratios. The mixed powders were consolidated into isotropic magnets and anisotropic magnets by spark plasma sintering (SPS) and SPS followed by hot deformation, respectively. The evidence of Nd-rich phase diffusion between two compositions was in vestigated. For isotropic magnets, the gradient distribution of Nd due to the diffusion of liquid rich-Nd phase was verified, as shown in the Fig.1a. Along the line cross the boundary area between RE-rich and Fe-rich compositions (indicated by the arrow), the RE content changed from 14 at.% to 10.78 at.% with respect to the points from A to D. The grain size in the particle boundary areas between two compositions gradually reduced from the Nd-rich composition to Fe-rich composition. The results indicated that low rare earth content was beneficial to the inhabitation of the Nd2Fe14B grain growth. For the anisotropic NdFeB magnets, the diffusion of Nd-rich phase was also demonstrated as shown in Fig.1b. Similarly, from the points E to I along the line cross the boundary area indicated by the arrow, the RE content gradually reduced from 14.15 at.% to 10.08 at.%. The length-width ratio of the grain decreased with decreasing Nd content and there is no aligned grain in the Fe-rich composition without Nd-rich phase, which indicated that Nd-rich phase was essential for c-axis texture alignment. The role of Nd-rich phase has to be carefully considered in order to achieve deformation anisotropy and fine grains in the NdFeB magnet.
CV-11. Investigation of easy axis orientation of Nd-Fe-B melt-spun ribbons produced by hot rolling and influence of Ti-C addition
Yuta Nakanishi1, Masaaki Takezawa1, Yuji Morimoto1, Jiro Yamasaki1 and Masaaki Yagi2
1Department of Applied Science for Integrated System Engineering, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu, Japan; 2Sojo University, Kumamoto, Japan
Easy axis orientation of Nd-Fe-B melt-spun ribbons is required to achieve a high energy product. In this study, c-axis orientation of Nd-Fe-B melt-spun ribbons produced by hot rolling immediately after quenching and the influence of Ti-C addition have been investigated. Alloys of Nd13(Fe0.97Ti0.03)81B5.4C0.6 were melt spun and hot rolled at a pressure of 150 MPa. Magnetic domains at the surface of the ribbons were observed with a Kerr microscope. In the ribbon with Ti-C, most grains exhibit maze domain patterns, as shown in Fig. 1(b). This indicates that the c-axis orients normal to the ribbon plane by hot rolling. Figure 2 shows the dependence of coercivity and residual magnetization on Ti content. It was found that the magnetic properties can be enhanced by substituting 3% Ti. Moreover, addition of C increased coercivity by reducing the TiB2 phase, which causes low coercivity [1]. Hot rolling and addition of Ti-C promotes c-axis orientation and high coercivity of Nd-Fe-B melt-spun ribbons.
References
[1] S. Hirosawa, et al., J. Jpn. Soc. Powder Powder Metallugy, 51, 143 (2004).
CV-12. Magnetic domain observation of Nd-Cu diffusion Nd-Fe-B sub-micron grain sized magnet by Kerr effect microscopy
Masaaki Takezawa1, Yuki Nagashima1, Yuya Kimura1, Yuji Morimoto1, Jiro Yamasaki1, Noriyuki Nozawa2, Takeshi Nishiuchi2 and Satoshi Hirosawa2
1Kyushu Institute of Technology, Kitakyushu, Japan; 2Hitachi Metals, Ltd, Osaka, Japan
The coercivity enhancement of hydrogenation−disproportionation−desorption−recombination (HDDR)-processed Nd−Fe−B powders with submicron grains by using grain boundary diffusion of Nd-Cu alloy s has been recently reported [1]. In this study, we observed the magnetic domain of the Nd-Cu-diffused Nd-Fe-B magnet with a Kerr microscope. Nd80Cu20 powders were mixed with HDDR Nd13.5Fe72.0Co8.0B6.5 powders at a mass fraction of 10%. The mixed powders were hot-pressed at 580 °C for 2 min. The coercivities of the hot-pressed magnets without and with Nd-Cu were 567 kA/m and 1309 kA/m, respectively. Figures 1 and 2 show domain images of the magnets without and with Nd-Cu, respectively. Bright domains spread across an area of a few microns appeared in the magnet without Nd-Cu, as shown in Fig. 1. This is because magnetization reversal in a few grains occurs almost simultaneously in the magnet. On the other hand, magnetization reversal in the Nd-Cu-diffused magnet occurs grain by grain independently of adjacent grains, as shown in Fig. 2. We found that the coercivity of the HDDR magnet depends on the volume of magnetically coupled grains that simultaneously reverse their magnetization. The change in microstructure caused by Nd-Cu leads to a decoupled magnetization reversal. This study was supported by MEXT’s Elements Science and Technology Project.
References
[1] H. Sepehri-Amin, et. al., Scripta Mater., 63, 1124 (2010).
CV-13. Effects of Ga Addition on Structural and Magnetic Properties of Nd-Fe-B-Ti-C Nanocomposite Magnets
Qiong Wu1, 2, Tian Yu3, Shuai Guo1, Xiaochao Feng1, Minxiang Pan2, Pengyue Zhang2, Baoshan Han3, Hongliang Ge2 and Aru Yan1
1Ningbo Institute of Material Technology&Engineering Chinese Academy of Sciences, Ningbo, China; 2College of Materials Science and Engineering, China Jiliang University, Hangzhou, China; 3Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China
Nd-Fe-B-Ti-C nanocomposite alloys in a composition of Nd content from 6-9 atomic percent and B content from 10-15 atomic percent are excellent hard magnets with a large coercivity [1]. However, the cooling rate required for Nd-Fe-B-Ti-C nanocomposite alloys is not so small, it is difficult to mass-produce these alloys by means of the strip casting (SC) process. Further improvement of cooling-rate dependency of the alloys is important. In this paper, Rapidly solidified ribbons of Nd9Fe73B12.6C1.4Ti4-x Gax (x=0, 0.5, 1) were prepared by melt spinning technique with a surface speed(vs) of 5~20m/s. The influence of Gallium doping on the glass-forming ability (GFA), the magnetic properties and crystallization kinetics of alloys have been investigated. The results of DTA and XRD showed the GFA of Nd-Fe-B-Ti-C alloys have been enhanced by Ga addition, leading to significant improvements of magnetic properties in a low cooling-rate regime. The better magnetic properties with a higher coercivity of 1120 kA/m have been obtained in Nd9 Fe73B12.6C1.4Ti3.5Ga0.5 with vs =5 m/s. Also, the microstructure and exchange-coupling interaction of Nd-Fe-B-Ti-C ribbons with different cooling rate after an optimal crystallization was discussed in detail by MFM.
References
[1] S. Hirosawa, H. Kanekiyo, T. Miyoshi, J. Magn. Magn.Mater.,281 (2004) 58.
CV-14. Effects of magnetic solidification on rod-shaped Nd-Fe-Ti-Zr-Cr-C-B magnets with various diameters
Chen Wang1, 2, Wei-Chi Lin1, Chih-Chieh Hsieh1, Wen-Cheng Chang1, Huang-Wei Chang3 and An-Cheng Sun4
1Department of Physics, National Chung Cheng University, Chia-Yi, Taiwan; 2College of Materials Science and Engineering, Fuzhou University, Fuzhou, China; 3Department of Physics, Tunghai University, Taichung, Taiwan; 4Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li, Taiwan
The Nd-Fe-Ti-Zr-Cr-C-B bulk magnets in cylindrical form with various diameters were prepared by injection casting of the molten alloy into copper molds under argon atmosphere. The diameter of rod-shaped samples ranges from 0.9 mm to 1.5 mm, and the length of rods is 15 mm. The effects of the diameter size and the application of magnetic field during the solidification process on the magnetic properties and microstructure of Nd9.5Fe71.5Ti2.5Zr0.5Cr1C0.5B14.5 alloy have been studied. Table 1 shows the dependence of the magnetic properties on the diameter size for the samples solidified with and without magnetic field. The smaller the diameter size is, the better magnetic properties can be obtained. A smaller diameter size means a higher cooling rate of alloy melt. One may also notice that the iHc increases rapidly, while the Br remains almost constant, with decreasing the diameter size. This implies that the iHc of bulk magnet is very sensitive to the cooling rate and the Br is relatively insensitive to the cooling rate. Compared with the samples solidified at zero magnetic field, there are significant improvements in the magnetic properties, especially the coercivity and the energy product, for the samples solidified at a 3.0 kOe magnetic field. The increase of the intrinsic coercivity is much obvious for the samples with larger size. The phase analyses show that the magnetic solidification is helpful to increase the volume fraction of Nd2Fe14B phase, thus increasing the intrinsic coercivity of magnets.
CV-15. Coercivity enhancement in HDDR-processed Nd-Fe-B magnet by Zn-diffusion doping treatment combined with hot press
Takeshi Nishiuchi1, Noriyuki Nozawa1, Satoshi Hirosawa1, Hossein Sepehri-Amin2, Tadakatsu Ohkubo2 and Kazuhiro Hono2
1Magnetic Materials Research Laboratory, Hitachi Metals, Ltd., Osaka, Japan; 2Magnetic Materials Unit, National Institute for Materials Science, Tsukuba, Japan
We succeeded in increasing coercivity of HDDR-processed magnet by Zn diffusion doping treatment combined with hot press. HDDR-processed Nd15Fe75.3Co3B6.5Ga0.2 powder with the coercivity (HcJ) of 1341 kA/m was prepared by conventional HDDR process. Then Zn powder was added to the HDDR powder with the mixing ratio of x mass % (x = 0, 0.5). The powder mixtures were compacted at room temperature without applied magnetic field, subjected to rapid-hot pressing process [1] at 750 °C for 2 min under the pressure of 586 MPa, and subsequently annealed at 500 °C for 60 min. As shown in Fig. 1, HcJ of the x = 0.5 sample after hot pressing increased to 1367 kA/m, which was higher than the x = 0 sample. Furthermore, after annealing at 500 °C, HcJ of the x = 0.5 sample reached to 1423 kA/m. This study revealed that coercivity can be increased by diffusion process without any additional rare-earth elements, which is in a sharp co ntrast to the recently reported method to use Nd-rich eutectic Nd-Cu alloys as the coercivity enhancer [2]. This study was supported by MEXT’s Elements Science and Technology Project.
References
[1] N. Nozawa et. al., J. Magn. Magn. Mater., 323, 115 (2011). [2] H. Sepehri-Amin, et. al., Scripta Mater., 63, 1124 (2010).
8:00 AM - 12:00 PM, Saguaro Ballroom
Co-Chair: Nora Dempsey, CNRS, Grenoble; Co-Chair: Thomas Woodcock, IFW-Dresden
CW-01. Effect of strained state on the magnetic properties of (001)-oriented L10-FePt films on different substrate
Ai-Mei Zhang1, 2, Zhao-Xi Chen1, Jian-Jian Ge1, Mao Yang1, Wen-Qing Zou1, Jun Du1, Xiao-Shan Wu1, Shi-Jie Zhang3 and Shi-Ming Zhou3
1National Laboratory of Solid State Microstructures & Department of Physics, Nanjing University, Nanjing, China; 2College of Science, Hohai University, Nanjing, China; 3Department of Physics and Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai, China
FePt films were prepared by deposting [Fe/Pt]16 multilayer films under the same conditions at temperature of 300oC and 400oC by magnetron sputtering. Films were directly deposited on LaAlO3 (001) (LAO), SrTiO3 (001) (STO) and MgO (200) single crystal substrates without any seed layer. The lattice constant a of bulk L10-FePt is 3.852Å. The in-plane lattice constants of LAO (001), STO (001) and MgO (001) substrates are 3.789Å, 3.905Å and 4.21Å, respectively. The mismatch ratioes between L10-FePt and LAO (001), STO (001) and MgO (001) substrates are -1.64%, 1.38% and 9.29%, respectively. Effects of compressive and tensive strain on the microstructures and the magnetic properties of the films were studied in detail. Completely ordering (001)-oriented L10-FePt films were obtained at 400oC at different substrate. All films show good ordering parameters S and magnetic squareness ratio (Mr/Ms), which are all above 0.95. Moreover, the coercive field (Hc) of the FePt films on the LAO substrate is about 5693Oe which is much larger than those of the FePt films on the STO (2592.7Oe) and MgO (3243.7Oe) substrates. The saturated Ms of the film on STO and LAO is largely increased in comparation with that of the film on MgO. Results of the grazing incidence X-ray diffraction (GIXRD) show that L10-FePt (001)-oriented films on LAO are under strained condition, while those on STO and MgO substrate are strain-relaxed. X-ray reflectivity (XRR) investigation showed that films at different substrate have different surface and interface microstructures. The different magnetic properties were ascribed to the different strained state in the whole films and the different surface/ineterface microstructures of films on LAO, STO and MgO substrates. It is worthwhile to note in present work that (001)-oriented L10-FePt films with improved magnetic properties can be obtained under compressive strain on LAO substrate.
CW-02. Promotion of perpendicular anisotropy for L10-FePt by rapid thermal processing
Liang-Wei Wang and Chih-Huang Lai
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
FePt nanoparticles (NPs) and granular films with good perpendicular anisotropy had been reported that were grown on SiO2 underlayers [1]-[3]. In this work, the influence of heating rate upon rapid thermal processing on the perpendicular anisotropy of L10-FePt was investigated; furthermore, the mechanism for the promotion of perpendicular anisotropy was also presented. The atomic-scale [Fe/Pt/SiO2]18 multilayers (MLs) were deposited on SiO2 underlayers and subsequently annealed at 400 °C for 10 s with varying heating rates upon rapid thermal processing. The corresponding thickness of Fe, Pt, and SiO2 in each layer was 0.16 nm, 0.18 nm, and 0.28 nm, respectively. Fig. 1 shows the hysteresis loops of [Fe/Pt/SiO2]18 MLs annealed at the heating rate of 40, 10, and 1 °C/s, respectively, exhibiting that the out-of-plane coercivity (Hc,⊥) decreases from 8600 Oe (Fig. 1(a)) to only 100 Oe (Fig. 1(c)) with lowering the heating rate. Since the magnetically hard phase is contributed by the L10-FePt, the larger Hc,⊥ for high heating rate case implies the better ordering of FePt; most important of all, the perpendicular magnetic anisotropy is promoted in the high heating rate condition (Fig. 1(a)), which is attributed to the tensile stress caused by the difference of thermal expansion coefficient between Si and SiO2 upon rapid thermal processing. The detained mechanism about the promotion of perpendicular anisotropy for L10-FePt by rapid thermal processing will be presented.
References
[1] Y. C. Wu, et. al., Appl. Phys. Lett., 91, 072502 (2007) [2] Y. C. Wu, et. al., J. Appl. Phys., 103, 07E126 (2008) [3] Y. C. Wu, et. al., Appl. Phys. Lett., 93, 242501 (2008)
CW-03. Reducing the Switching Field of L10-FePt by Graded Order Parameter
Zhihong Lu1, Junbo Guo1, Shishou Kang2, Rui Xiong3, Gary J. Mankey4 and William H. Butler4
1School of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan, China; 2School of Physics, Shandong University, Jinan, China; 3School of Physics and Technology, Wuhan University, Wuhan, China; 4Department of Physics, University of Alabama, Tuscaloosa, AL
According to the theoretical studies [1-4], the switching field of a fully ordered equiatomic FePt film can be drastically decreased if its anisotropy decreases gradually along the thickness. An important technical challenge for enabling this concept is developing the ability to control the magnetic anisotropy of each magnetic layer so that the anisotropy profile of the multilayered structure can be designed. Here, we investigated the dependence of the magnetic properties of epitaxial FePt films on order parameter S, and demonstrated that the magnetic anisotropy could be tuned by controlling S which can be controlled by the growth temperature. Based on this result, we built two kind of multilayered structures with a 13 nm fully ordered equiatiomic FePt layer as the bottom layer: 1) 4-layered structure with S decreasing layer by layer from bottom to top; 2) multilayer with S changing more continuously along the thickness. The magnetic properties of the films were characterized using VSM. It was found that both structures have easy axis perpendicular to the film; the switching fields of structure 1 and structure 2 are 45 kOe and 37 kOe, respectively, which are much lower than 70 kOe-the switching field of the fully ordered FePt mono-layered film with thickness of 13 nm. The results suggest that the switching field of FePt can be efficiently reduced by gradually changing order parameter.
References
1)D. Suess, Appl. Phys. Lett. 89, 113105 (2006). 2)Z. Lu, P. B. Visscher and W. H. Butler, IEEE Trans. Magn. 43, 2941, (2007). 3)A. Goncharov, T. Schrefl, G. Hrkac, J. Dean, S. Bance, D. Suess, O. Ertl, F. Dorfbauer, and J. Fidler, Appl. Phys. Lett, 91, 222502, (2007). 4)D. Hahn, M. A. Bashir, T. Schrefl, A. Cazau, M. A. Gubbins, and D. Suess, IEEE Trans. Magn, 46, 1866, (2010).
CW-04. Magnetic properties and microstructure of perpendicular FePt (B-Ag) granular films
Jai-Lin Tsai, Jian-Chiang Huang, Li-Hung Chen and Chiun-Shiuan Lin
Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan
The granular FePt(B-Ag) film with perpendicular magnetization was prepared on glass substrate. The immiscible (B, Ag) elements were co-doped to isolate FePt grains and maintain c-axis alignment. The adatom with different mobility was diffused inbetween grains surface and boundaries during annealing. When the Ag was replaced B with ratio ranges (BxAg1-x) (x=0.3-0.7), the maximum ordering degree, increased out-of-plane coercivity, improved c-axis orientation and refined FePt grains were achieved. Figure 1 shows plane-view TEM images of granular FePt(B, Ag) films. In Fig. 1(a), the FePt grains with quite different size are attributed randomly and the amorphous B appears in the grain boundaries or alloys to Fe-B cluster. Fig. 1(b-c) shows the images of FePt(BxAg1-x) (x= 0.7, 0.4). The FePt grains were separated uniformly with thin (B, Ag) grains boundaries and the average grains size is 10.8nm, 11.9 nm, respectively. Figure 1(d) shows the image of FePtAg film. The over growth of FePt grains are attributed continuously that is due to the high diffusive Ag.
CW-05. Effect of intrinsic tensile stress on (001) orientation of single-layered FePt thin films on glass substrates
Shih-Nan Hsiao1, Shi-Hsien Liu2, Shi-Kun Chen2, Fu-Te Yuan3 and Hsin-Yi Lee1
1National Synchrotron Radiation Research Center, Hsinchu, Taiwan; 2Materials science and engineering, Feng Chia University, Taichung, Taiwan; 3Institute of Applied Physics and Center for Nanostoage, National Taiwan University, Taipei, Taiwan
L10 FePt has been drawn attention for application of high-density recording due to its outstanding magnetocrystalline anisotropy. Typically, FePt thin films directly deposited on non-textured underlayers or substrates prefer to grow with a (111) texture. The extrinsic tensile stress (coherency1 or phase transformation stress2) has been reported to enhance the FePt (001) orientation. In this paper, we investigated the effect of intrinsic tensile stress on (001) orientation in FePt thin films, which has not been reported yet. The Lotgering orientation factor (LOF) was -0.39 for the as-deposited film, indicating a fully (111) texture (Fig. 1a). When sample was annealed at 800oC and hold for 0 s, the film exhibited a fully ordered structure with LOF of -0.11. With increase of annealing time (t) from 0 to 100 s, the LOF raised from -0.11 to around 0.99 indicating a fully (001) orientation, and then decreased to 0.82 for t = 3600 s. Fig. 2 shows that the in-plane squareness was larger than that of out-of-plane for t < 67 s. With increasing of t from 67 to 900 s, the in-plane squareness declined to a minimum value of 0.43, whereas the out-of-plane squareness inclined to maximum value of 0.93. The out-of-plane squareness decreases and in-plane squareness increase for t > 900 s. The in-plane residual str ess shows a strong correlation to the LOF (Fig. 1b). The accumulation of intrinsic tensile is mainly contributed by the densification3, since the ordering parameter is fully ordered for t = 0 s. The relief of the residual stress was attributed to the microstructural evolution (from continuous to particulate) and dynamic relaxation. We deduced that the intrinsic tensile stress prompted the (001) texture of FePt thin films due to strain-induced secondary grain growth4.
References
1Y. Hsu et al., J. Mag. Mag. Mater. 260, 282 (2003). 2J. S. Kim et al., J. Appl. Phys. 99, 053906 (2006). 3C. V. Thompson and R. Carel, J. Mech. Phys. Solid 44 657 (1996) 4C. V. Thompson and R. Carel, Mater. Sci. Eng. B 32 211 (1995)
CW-06. Effect of oxygen stoichiometry on microstructural and magnetic properties of FePt/TaOx bilayer
Guijun Li1, Chi Wah Leung2, Ying Ching Chen3, Ko Wei Lin3 and Philip W. T. Pong1
1Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, Hong Kong; 2Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong, Hong Kong; 3Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan
Previously it was found that FePt [1,2] grain size and magnetic properties can be modified by varying oxygen contents in the capping nonmagnetic metallic oxide layer [3]. The effect of oxygen contents in TaOx capping layer on FePt thin films was systemically studied here. A 10 nm thick stoichiometric FePt layer was co-sputtered on thermally oxidized silicon substrate and the capping TaOx layer was sputtered using the dual ion beam deposition by a Kaufman source with 2.3 sccm argon and an End-Hall source with oxygen ranging from 0% to 41%. All the samples were annealed at 550oC for 10 min. FePt/TaOx bilayer sputtered with 15% O2/Ar exhibited the highest order parameter up to 0.557, indicating the most favorable condition for L10 phase FePt formation among all different oxygen contents. The XPS results indicated that there was diffusion from TaOx into FePt after annealing, and the 15% O2/Ar sample exhibited the strongest diffusion. The 15% O2/Ar possessed the smallest grain sizes as confirmed by the AFM characterization. The strong diffusion and small grain size resulted in a large coercivity in FePt layer. The largest coercivity of 4.2 kOe in the out-of-plane direction was obtained under the 15% O2/Ar ratio which is larger than that in single-layer FePt.
References
[1] Y. Inaba, K.L. Torres, A. Cole, R. Vanfleet, R. Ott, T. Klemmer, J.W. Harrell, G.B. Thompson, Thermal annealing of FePt thin films by millisecond plasma arc pulses, J Magn Magn Mater, 321 (2009) 2451. [2] N. Murayama, S. Soeya, Y. Takahashi, M. Futamoto, Ordering and grain growth of FePt thin films by annealing, J Magn Magn Mater, 320 (2008) 3057. [3] K.W. Lin, Y.L. Chiu, A.C. Sun, J.H. Hsu, J. van Lierop, T. Suzuki, Ion-Beam Bombarded SiO2 Layer Effects on the Microstructure and Magnetism in FePt/SiO2 Bilayers, Jpn J Appl Phys, 48 (2009) 073002
CW-07. Enhancement in coercivity of PLD-fabricated Fe-Pt thick film magnets by reducing droplets
Masaki Nakano, Daisuke Urakawa, Takeshi Yanai and Hirotoshi Fukunaga
Graduate school of E ngineering, Nagasaki University, Nagasaki 852-8521, Japan
Fe-Pt thick film magnet is one of hopeful candidates applied to a medical MEMS because of their good corrosion resistance. A high-speed PLD (Pulsed Laser Deposition) method with the deposition rate of several ten microns per hour [1] followed by a post-annealing process enabled us to obtain isotropic Fe-Pt thick film magnets with the L10 order phase. This contribution reports an investigation on the relationship between coercivity of the post-annealing films and droplets. Two kinds of films with different numbers of droplets were fabricated by taking advantage of controlling the focus distance between an optical lens and a Fe-Pt target [2]. An estimation in the difference was carried out by measuring the center line average roughness, Ra. The figure shows the values of coercivity as a function of the post annealing temperature. It was found that the reduction in the numbers of droplets enabled us to increase the coercivity although the average deposition rate decreased 40 to 15 microns per hour. The thickness range and the average composition of all the films were from approximately 15 to 40 microns and Fe50Pt50, respectively. In the samples with Ra less than 0.5 microns, the coercivity exceeded 450 kA/m at the annealing temperature of 673 K. A SEM-EDX analysis of droplets revealed that they tended to have iron-rich composition compared with the average one of Fe50Pt50, which agrees with previously reported result in PLD-made Fe-Pt thin films [3].
References
[1] M. Nakano et al, J. Appl. Phys. 109, 07A723(2011) [2] H. Fukunaga et al, J. Appl. Phys. 109, 07A758 (2011) [3] M. Weisheit et al, J. Magn. Magn,Mater., 290-291, 570(2005).
CW-08. Effect of laser wavelength and magnetic field on phase structure and magnetic properties of pulse-laser-deposited FePt films
H. W. Chang1, C. R. Wang1, C. W. Yuan1, C. W. Shih2, F. T. Yuan3 and W. C. Chang2
1Department of Physics, Tunghai University, Taichung, Taiwan; 2Department of Physics, National Chung Cheng University, Chia-Yi, Taiwan; 3Department of Physics, National Taiwan University, Taipei, Taiwan
Due to high anisotropy constant (K u = 7× 10 7 erg/cm 3 ), coercivity, thermal stability, and also good corrosion resistance, FePt alloy with Face-Center-Tetragonal structure is promising as a next generation high-density magnetic recording media and a permanent magnetic material. [1-4] Over the decades, magnetic properties and disorder-order transformation in the FePt alloys were widely studied. [2-3] In recent years, much attention has focused on the FePt alloys in thin film form, and the main method to fabricate FePt thin films is magnetron sputtering technique.[4] However, very limited papers report on the study of FePt thin films fabricated by pulse laser deposition (PLD) which is also an effective approach to grow high quality films. In this study, we report on PLD-prepared FePt thin films. The samples were deposited on Corning glass substrates at ambient temperature following post-annealing using rapid thermal annealing (RTA). Effect of laser wavelength (λ) and external magnetic field during deposition were investigated. Different energy of the incident laser results in different deposition mode. The films made with longer wavelength of 1064 nm are Pt-rich. The compositional deviation from the equi-atimic target is due to the thermal evaporation-like deposition. The off-stoichiometry results in poor ordering and magnetic properties. On the other hand, for ablation deposition-like PLD with λ of 266 nm, the film composition reflects from the target, resulting in highly-ordered L1 0 -FePt phase. In addition, the introduction of external magnetic field of 3.6 kOe perpendicular to the film surface during the deposition is observed not only to decrease the composition deviation for the sample prepared at λ = 1064 nm but to enhance the squareness of demagnetization curve for films at λ = 266 nm. This suggests that Fe atoms or clusters may possess magnetic moment during FePt deposition, and thus it may suffer the Lorentz force from the magnetic field gradient between the substrate and target by placing the NdFeB magnet under the substrate.
References
[1]K. Watanabe et al. , IEEE Trans. Magn. 23, 3196 (1987). [2]S. C. Chen et al. , J. Appl. Phys. 97, 10N107 (2005). [3]Z. L. Zhao et al. , Appl. Phys. Lett. 88, 052503 (2006). [4]M. Weisheit et al. , J. Magn. Magn. Mat. 290, 570 (2005).
CW-09. Stabilization of perpendicular magnetic anisotropy L11 CoPtCu thin film on glass substrate by Pt(111) underlayer
An-Cheng Sun1, Long-Jie Li1, An-Cheng Sun1, Fu-Te Yuan2 and Jen-Hwa Hsu2
1Department of Chemical Engineering and Materials Science, Yuan-Ze university, Taoyuan, Taiwan; 2Department of Physics, National Taiwan University, Taipei, Taiwan
Rhombohedral L11 CoPtCu film which consists of CoPt matrix and random Cu-rich nano-pinning sites had fabricated on MgO(111) substrate in our previous investigations [1,2]. The film shows perpendicular magnetic anisotropy and high coercivity at low working temperature, suggesting the potential for future recording application. However, the L11 CoPtCu films are usually grown on single crystal substrate such as MgO(111), which needs high cost and becomes a drawback for its application. Therefore, to fabricate L11 CoPtCu on amorphous glass substrate is imperative. In this study, CoPtCu films with L11 structure were successfully sputtered on Pt(111)/glass substrate. When the Cu content was 23~26 at.%, the films showed perpendicular magnetic anisotropy and higher coercivity. The SEM image indicated that the film had a uniform morphology on Pt(111)/glass. On the contrary, the film divided equally into two regions, one with a smooth and the other with a bumpy surface as the film is on MgO(111). This implied different interface, metal to metal and metal to oxide, stress affected the film growth during deposition. In order to study the interface effect, the thickness of the CoPtCu film was gradually reduced to 2 nm, thinnest case in this work. It was of interest that the Hc was increased with decreasing the thickness as the film was on Pt(111)/glass. This was opposite to that on the MgO(111) substrate. When the film was deposited on MgO(111), the Hc was decreased drastically with decreasing the thickness. In this study, the maximum Hc of 2.6 kOe was found at 4 nm-thick CoPtCu film on Pt(111)/glass. The MgO(111) substrate can be replaced by glass substrate and consequently reduces the cost, which largely increases the potential of L11 CoPtCu film in recording applications.
References
[1]F.T. Yuan, A.C. Sun, J.H. Hsu, C.S. Tan, P.C. Kuo, W.M. Liao, H.Y. Lee, J. Appl. Phys. 108 (2010) 113909. [2]F.T. Yuan, A.C. Sun, J.H. Hsu, C.S. Tan, P.C. Kuo, W.M. Liao, H.Y. Lee, J. Appl. Phys. 109 (2011) 07B714.
CW-10. Effect of high tempera ture annealing on ion-irradiation induced magnetization in FeRh thin films
Atsushi Tohki1, Shinya Kosugi1, Kazuma Aikoh1, Kyo Kume2, T. Batchuluun2, Ryoya Ishigami2, Toshiyuki Matsui3 and Akihiro Iwase1
1Department of Materials Science, Osaka Prefecture University, Sakai-shi, Japan; 2The Wakasa Wan Energy Research Center, Tsuruga, Fukui, Japan; 3Research Organization of the 21st Century, Osaka Prefecture University,, Sakai, Osaka, Japan
Equiatomic ordered FeRh alloy with the B2 (CsCl type) structure is of much interest due to the first order anti-ferromagnetic (AF) - ferromagnetic (FM) phase transition near the room temperature. In our previous studies, we have reported that ion irradiation induces FM state in FeRh film samples even at 20 K that is originally AF 1). A high fluence irradiation changes B2 structure into A1 structure, and decreases the magnetization at 20K. Here, A1 structure is known to be a nonmagnetic fcc structure with a random distribution of iron and rhodium atoms. In the present study, we show that the annealing at high temperature can reproduce the magnetization on the FeRh films which have become nonmagnetic by high fluence ion-irradiation. Thin FeRh film we used in the present experiments was deposited on MgO(001) substrate by using an ion beam sputtering from Fe-50at.%Rh target. The film was irradiated with 50 keV Ar ion to the fluence of 1×1016 ions/cm2 at room temperature. The irradiation changed the lattice structure from B2 structure into A1 structure and decreased the magnetization of the film. The film was isochronally annealed up to 800°C with the time interval of 60 min. Before and after the irradiation and annealing, the change in magnetization and the lattice structure were measured by using a superconducting quantum interference device (SQUID) and an X-ray diffractometer (XRD), respectively. On annealing above 400°C, the magnetic spins of the samples repolarized, which meant that the films exhibited FM nature. With increasing the annealing temperature, the magnetization gradually increased. Hereafter the largest magnetization value could be obtained on annealing at 700°C, which is almost same as that of the ferromagnetic sample induced by the ion-beam irradiation. However, a different tendency was observed on 800°C annealing of the sample: the phase transition temperature shifted toward the high temperature side. The XRD measurement shows that the B2 phase was reproduced for the samples the magnetization of which was observed. These results imply that the combination of the process of the ion-beam irradiation and annealing of the samples make the magnetic state significantly controlled.
References
1) Nao. Fujita, et al., J. Appl Phys., 107, 09E302 (2010)
CW-11. Origins of Axial Gradient Performance of Hot Deformed Nd-Fe-B Ring Magnets
Wen-Zong Yin1, 2, Ren-Jie Chen1, 2, Xu Tang1, 2, Min Lin1, 2, Don Lee1, 2 and Aru Yan1, 2
1Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 2Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China
Hot-deformed Nd-Fe-B ring magnets have wide applications especially in DC motors [1, 2]. These ring magnets, however, exhibit axial gradient performance partly due to the diffe rence of texture and grain size [3]. In this work, ring magnets were prepared by back extrusion method. Axial magnetic properties were measured on Vibrating Sample Magnetometer. The effects of density, terminal pressure, deformation temperature and strain rate on axial magnetic performance were investigated, respectively. The textures of different positions of the ring magnets were evaluated by the ratio of remanences along easy and hard directions. It was demonstrated that the texture accounted for the magnetic performance variation along axial direction (Fig.1). Furthermore, morphology and microstructures of the ring magnets were examined with SEM (Fig.1, inset), which revealed two different origins of axial gradient performance. The deformation degree of Nd-Fe-B grains plays a critical role in the performance difference between the top and middle part of ring magnet. But that between the middle and bottom part mainly results from different alignment orientations of platelet Nd-Fe-B grains. It is both deformation degree and alignment orientation that determine the axial texture of hot-deformed ring magnets.
References
[1] A.H. Li, W. Li, B. Lai, H.J. Wang, M.G. Zhu, and W. Pan. J. Appl. Phys., 107 (2010) 09A725. [2] W. Lipiec, H.A. Davies. J. Alloys Compd. 491 (2010) 694-697. [3] H.T. Kim, Y.B. Kin. Phys. Stat. Sol. (a), 201 (2004) 1926-1929.
CW-12. Improvement in magnetic properties of Nd-Fe-B/α-Fe multi-layered thick film magnets prepared by PLD method
Hirotoshi Fukunaga1, Takashi Kamikawatoko1, Masaki Nakano1, Takeshi Yanai1 and Fumithoshi Yamashita2
1Nagasaki University, Nagasaki 852-8521, Japan; 2Rotary Corponent Thechnology Development Division, Minebea Ltd., Shizuoka 437-1193, Japan
Nd-Fe-B/α-Fe multi-layered nanocomposite film magnets are one of candidates for magnets used in milli-size motors because of their high remanence [1]. Previously, we reported magnetic properties of Nd-Fe-B/α-Fe multi-layered nanocomposite film-magnets prepared by the pulse laser deposition (PLD) method. The prepared film magnets were composed of approximately 800 Nd-Fe-B/α-Fe layers, and exhibited Ir of 1 T and Hc of 430 kA/m. However, the obtained properties scattered from a sample to a sample, and the improvement in reproducibility of magnetic properties is strongly required. On the other hand, we have already reported that the control of the laser beam conditions improves the reproducibility of magnetic properties of Nd-Fe-B thick film magnets [2]. Therefore, we applied the above method to Nd-Fe-B/α-Fe multi-layered nanocomposite film magnets. We prepared Nd-Fe-B/α-Fe multi-layered nanocomposite film magnets from a Nd2.4Fe14B/α-Fe composite target with controlling the defocus rate, DF Rate, of the laser beam for ablation of a target as well as the distance between a target and substrate, TS Distance. DF Rate is defined as (TS Distance - F Distance)/(F Distance), where F Distance is the focus distance. Table 1 shows magnetic properties of Nd-Fe-B/α-Fe multi-layered nanocomposite film magnets prepared under different conditions. It is clearly seen that Condition II improves the reproducibility as well as magnetic properties. The control of DF Rate enabled us to improve the reproducibility of the Nd contents of the films. The increase in TS Distance decreased the number and size of droplets. These results strongly suggest that we can obtained nanocomposite thick film magnet with good magnetic properties as well as their high reproducibility by controlling the laser beam conditions.
Re ferences
[1] F. Yamashita et al., IEEE Trans. Magn., 46 (2010) 2012 [2] H.Fukunaga et al., J. Appl. Phys. 109 (2011) 07A758
CW-13. Structural and Magnetic Properties of SmCo4-xFexB
Eric S. Krage1, 2, Bhaskar Das1, Balamurugan Balasubramanian1, Xingzhong Li1, Ralph Skomski1, Yung Huh1, 2 and David J. Sellmyer1
1Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE; 2Physics, South Dakota State University, Brookings, SD
Rare-earth cobalt (R-Co) permanent magnets, especially SmCo5 and Sm2Co17, are the first rare-earth permanent magnets developed and continue to be valued for high-temperature applications. Alloying with B, for example SmCo4B, enhances the anisotropic field to be as high as 120 T, but the magnetization (Ms) and Curie temperature (Tc) substantially decrease. There are two reports describing the improvement of Ms and Tc by substituting Co with Fe content ≥ 20 at%.1 It is also worth noting that recent density functional calculations have shown an increase in magnetic anisotropy and Ms in RCo5 alloys with Fe content ≤ 10 at%.2 The main challenge is, however, to create a homogeneous mixture in combination with a high annealing temperature followed by a quenching process to obtain crystal-ordering per the requirement of bulk magnets Thus it is important to investigate the effect of microstructure and phase changes on magnetic properties of SmCo4-xFexB alloys in order to explore them for practical applications. In this study, we have investigated the effect Fe doping on the structural and magnetic properties of SmCo4-xFexB with 0.05≤ x ≤0.2 using XRD, EDX, SEM, TEM and SQUID magnetometer. SmCo4-xFexB was prepared by arc-melting followed by thermal annealing at 1000 °C for 48 hr. XRD results show that SmCo4-xFexB alloys crystalize into the hexagonal CeCo4B-type structure with x = 0.05 to 0.20. However, a detailed microstructure investigation using TEM reveals the presence of a minor Sm-Co phase and these results are in good agreement with the observation of soft and hard phases in the field-dependent magnetization measured at 10 K. A maximum coercivity of 30 kOe with a magnetization 70 emu/g was observed in SmCo4-xFexB at 10 K. The effect of Fe concentrations on microstructure, coercivity and magnetization will be discussed in detail. This research is supported by ARPA-E, ARO, and NCMN
References
1 H. Ido, H. Ogata, and K. Maki, J. Appl. Phys. 73, 6269 (1993). 2 P. Larson, I. I. Mazin, and D. A. Papaconstantopoulos, Phys. Rev. B, 69, 134408 (2004).
CW-14. Coercivity enhancement in anisotropic Pr12.5Fe80.8B6.2Nb0.2Ga0.3 powders
Zhong Lin, Jingzhi Han, Shunquan Liu, Meiying Xing, Jinbo Yang, Yan Zhang and Yingchang Yang
School of Physics, Peking University, Beijing, China
The hydrogenation disproportionation desorption recombination (HDDR) process is an effective way to produce anisotropic Nd2Fe14B-type powders with high performance. Compared to Nd2Fe14B-type alloys, Pr2Fe14B-type alloys have comparable intrinsic magnetic properties. But the obtained magnetic properties of Pr-based materials are still much lower than that of Nd-based materials. For example, the reported maximum energy product of HDDR anisotropic Pr2Fe14B-type magnetic powders is lower than 27MGOe and the coercivity of Dy-free powders is below 10 kOe. Therefore, Pr element is much less used than Nd in rare earth magnets, which makes Pr, the associate mineral of Nd, become surplus and cheaper in price. Therefore, developing Pr2Fe14B-type magnetic materials is of significance for the comprehensive utilization of rare earth resources. As the demand for Dy-free magnets with high coercivity increases, the Pr-based magnets will have promising potential in the future application if a much higher coercivity can be achieved without addition of heavy rare earth elements. In this paper, highly anisotropic Pr12.5Fe80.8B6.2Nb0.2Ga0.3 powders with Br of 1.45T and a record-high (BH)max of 32 MGOe were obtained by a modified HDDR process. It is found that self-organized rod-like disproprotionation microstructure is helpful for the formation of the crystal texture. But its relative low coercivity of 9555 Oe is an obstacle for practical applications. In order to further improve the coercivity, the HDDR treated powders were mixed with Pr68Cu32 powders which have an eutectic point at 472 OC, and then were vacuum annealed. As a result, the coercivity of the powders increased from 9555 Oe to 14025 Oe due to the modification of the grain boundary phase. Microstructure investigations show that after heat treatment the grain boundary phase is thickened and is rich in Pr and Cu element, which shows strong decoupling effect of the Pr2Fe14B grains.
References
1.T. Takashita, and R. Nakayama, Proceedings of the 12th International Workshop on RE Magnets and their Applications, Kyota, Japan, 1989, P. 551 2.O. Gutfleisch, A. Teresiak, B. Gebel, K. H. Muller, N. B. Cannesan, D. Brown and I. R. Harris, IEEE Trans Magn, 37(4), 2471(2001) 3. H. Sepehri-Amin, T. Ohkubo, T. Nishiuchi, S. Hirosawa and K. Hono. Scripta Materialia, 63(11), 1124(2011).
CW-15. Microstructural study of PrFeB-based sintered magnets with alloying elements by transmission electron microscopy
Teófilo Mendes1, Élio A. Périgo2, Conrado R. Afonso3, Suelanny C. Silva1 and Hidetoshi Takiishi1
1Nuclear and Energy Research Institute - IPEN, São Paulo, Brazil; 2Technological Research Institute - IPT, São Paulo, Brazil; 3National Laboratory of Synchrotron Light - LNLS, São Paulo, Brazil
A correlation between microstructure and magnetic properties of PrFeB-based sintered magnets, prepared with a mixture of Pr16Fe76B8 and Pr14FebalCo16B6Nb0.1Ga0.50 alloys, was performed. By scanning electron microscopy, it was verified that the part with Ga addition presented a mean grain size 17% larger than that found for the magnet without such element. Despite this difference, the coercivity (Hc) of the former was 10% higher. Based on this unexpected result and using a high resolution transmission microscope, it was possible to observe a regular and continuous layer (~ 5 nm) between the magnetic grains, which might be considered as the source of this Hc enhancement. The Ga is located mainly in the RE-rich phase, with a concentration of about 10 at.%, although it could also be found in a boride phase (> 4 at.%). The interplanar distance of the RE2(Fe,Co)14B phase is in agreement with previous works. Despite the improved Hc, the Ga-added magnet also presents strain fields and precipitates of nanometric scale. Further information regarding these features will be included in the complete manuscript.
1:30 PM - 4:30 PM, Grand Canyon 6
Chair: Eric Fullerton, UCSD
1:30 PM
DA-01. Tunnel Magnetoresistance and Spin Torque Switching in MgO-based Magnetic Tunnel Junctions with a Co/Ni Multilayer Electrode
Takahiro Moriyama, Theodore J. Gudmundsen, Pinshane Y. Huang, Luqiao Liu, David A. Muller, Daniel C. Ralph and Robert A. Buhrman
Physics, Cornell Univ, Ithaca, NY
MgO-based magnetic tunnel junctions (MTJs) with a large tunneling magnetoresistance (TMR) whose magnetic orientations can be controlled by spin-torque switching [1] are promising candidates for spin-torque magnetic random access memories [2]. However, for widespread application it will be necessary to reduce the switching current density while maintaining thermal stability for the magnetic states. One strategy is to tune the perpendicular anisotropy of the switching layer to reduce the demagnetization field, but to keep the equilibrium orientation of the switching layer in the sample plane [3]. We have fabricated MgO-based magnetic tunnel junctions with a Co/Ni switching layer having a reduced demagnetizing field. The layer structure is Ta(3)/ [CuN(20)/Ta(3)]2/ Cu(2)/ [Co(0.4)/Ni(0.8)]2/ Fe60Co20B20(1.1)/ MgO(t)/Fe60Co20B20(20)/ Ta(8)/ Pt(30). The numbers in the parentheses are the layer thicknesses in nm. The demagnetizing field is 0.2 T, reduced relative to the average saturation magnetization of 1.2 T. Although scanning tunneling electron microscopy images reveal that the crystal orientation of the switching layer is fcc-(111) rather than bcc-(001) required for optimum band matching to MgO, we nevertheless obtain TMR values as large as 106%. For antiparallel-to-parallel switching we measure a zero-thermal-fluctuation critical current density of 5.0 x 106 A/cm2, in agreement with predictions for the effect of the reduced demagnetization field. For parallel-to-antiparallel switching, the critical current density is higher, which micromagnetic simulations suggest is due to a spatially nonuniform reversal mode made more favorable by the small demagnetization field. We also discuss quantitative spin-torque ferromagnetic resonance measurements on these MgO-based Co/Ni MTJs.
References
[1] D. C. Ralph and M. D. Stiles, J. Magn. Magn. Mater. 320, 1190 (2008). [2] J. A. Katine and E. E. Fullerton, J. Magn. Magn. Mater. 320, 1217 (2008). [3] T. Moriyama, T. J. Gudmundsen, P. Y. Huang, L. Liu, D. A. Muller, D. C. Ralph, R. A. Buhrman, Appl. Phys. Lett. 97, 072513 (2010).
2:06 PM
DA-02. Spin-transfer pulse switching in all perpendicular spin-valve nanopillars*
Huanlong Liu1, Daniel Bedau1, Jonathan Z. Sun2, Jordon A. Katine3, Eric E. Fullerton4, Stephane Mangin5 and Andrew D. Kent1
1Physics, New York University, New York, NY; 2IBM T. J. Watson Research Center, Yorktown Heights, NY; 3San Jose Research Center, Hitachi-GST, San Jose, CA; 4CMRR, University of California, San Diego, La Jolla, CA; 5Nancy-Université, Vandoeuvre Cedex, France
The effect of thermal fluctuations on spin-transfer torque (STT) switching has been studied for a broad range of time scales (sub-ns to s) in all perpendicular spin-valv e nanopillars. Such devices are of interest because low quasistatic switching currents (<100 µA) have been achieved for thermally stable elements at room temperature [1]. In addition, the uniaxial anisotropy makes them model systems for the study of spin-transfer driven magnetization dynamics. We studied both the 50% probability boundaries [2] and the full switching probability distributions [3] at room temperature. The data reveals distinct switch processes in two limits of the switching time. In the short time limit (<10ns), the 50% boundary forms a linear relation between the pulse amplitude and the inverse of the pulse duration (Fig. a, b) and the switching probability distributions have a form that only depends on the net charge in the pulse (Fig. c), consistent with the angular momentum conservation. In the long time limit (>1μs), the 50% boundary forms a linear relation between the pulse amplitude and the logarithm of the pulse duration (Fig. d), consistent with a thermal activation over an energy barrier modified by the spin-polarized current. A single domain model containing both STT and thermal fluctuation is in general agreement with the data in both regimes when the applied field is zero. However, the model cannot explain the strong field dependency of the switching probability. These results along with our recent double current pulse studies will be discussed. *Supported by ARO-Grant W911NF-07-1-0643 and NSF-DMR-1006575.
References
[1] S. Mangin, Y. Henry, D. Ravelosona, J. A. Katine, and E. E. Fullerton, “Reducing the critical current for spin-transfer switching of perpendicularly magnetized nanomagnets,” Applied Physics Letters 94, 012502 (2009). [2] D. Bedau, H. Liu, J-J. Bouzaglou, A. D. Kent, J. Z. Sun, J. A. Katine, E. E. Fullerton and S. Mangin, Applied Physics Letters 96, 022514 (2010). [3] D. Bedau, H. Liu, J. Z. Sun, J. A. Katine, E. E. Fullerton, S. Mangin and A. D. Kent, Applied Physics Letters 97, 262502 (2010).
2:42 PM
DA-03. Sub-volume thermal excitation and optimal perpendicular magnetic anisotropy for spin-torque switched magnetic tunnel junctions in memory
Jonathan Sun, Raphael P. Robertazzi, Janusz Nowak, Philip L. Trouilloud, Guohan Hu, David W. Abraham, Michael C. Gaidis, Stephen L. Brown, Eugene J. O'Sullivan, William J. Gallagher and Daniel C. Worledge
IBM-MagIC MRAM Alliance, IBM T. J. Watson Research Center, Yorktown Heights, NY
We examine observations of spin-torque switching behavior in recently developed CoFeB-based magnetic tunnel junctions with full perpendicular magnetization[1-2]. Comparison with a macrospin model [3-5] suggests the presence of sub-volume magnetic excitations which often dominate the switching process in junctions larger than about 50nm in lateral size. Such sub-volume excitations degrade spin-torque switching efficiency as indicated by the ratio of the measured thermal activation barrier height Eb and the corresponding zero-temperature threshold current Ic0, both were estimated from a rate-dependent switching threshold measurement[4-5]. The sub-volume thermal excitation is shown to originate from a competition between the macrospin fluctuation within a simple uniaxial anisotropy potential and that of thermal magnon excitation, common in finite temperature nanomagnet dynamics[6]. In thin film limit, the sub-volume excitation gives an upper bound of Eb that is dictated by the product of film thickness and its exchange-stiffness energy, making Eb insensitive to the strength of perpendicular magnetic anisotropy energy (PMA energy) when the PMA becomes sufficiently large. Further increase of PMA beyond this point would only degrade the spin-torque switching efficiency Eb/Ic0. This for practical optimizati on of tunnel junctions in memory applications suggests an optimal Hk for a given junction size and magnetic layer’s exchange stiffness where best activation barrier Eb and spin-torque efficiency can be achieved simultaneously. For thin film materials development in such applications, the sub-volume excitation issue highlights the importance of improving the magnetic exchange stiffness of the junction free layer, and the reduction of junction lateral size so that spin torque efficiency can be improved as the switching dynamics crosses over to a more macrospin-like process.
References
References [1] S. Ikeda et al. Nature Materials 9, 721 (2010). [2] D. C. Worledge et al. Appl. Phys. Lett. 98, 022501 (2011). [3] J. Z. Sun, Phys. Rev. B 62, 570 (2000). [4] R. H. Koch et al., Phys. Rev. Lett. 92, 088302 (2004). [5] J. Z. Sun et al., SPIE 5359, 445 (2004). [6] S. Rohart et al., Phys. Rev. Lett. 104, 137202 (2010).
3:18 PM
DA-04. Toward ultra low power spintronics nanodevices with perpendicular anisotropy
Dafine Ravelosona, N. Lei, N. Nguyen, S. Ahn, W. Lin, G. Agnus, S. Eimer, N. Vernier, W. Zhao, Jv Kim, T. Devolder, Jo Klein, P. Lecoeur and C. Chappert
Institut d'Electronique Fondamentale, UMR CNRS8622, Orsay, France
Most magnetic nanodevices rely upon the ferromagnetic order parameter and therefore are inherently non-volatile and radiation hard. Furthermore, the dissipation energy of magnetic processes can be orders of magnitude smaller than the CMOS power-delay product of similar size devices. As such, advances in spintronics will be crucial to continue progress in computation and data storage in the face of dramatic increases in power consumption in highly scaled CMOS devices. Until now, spintronics devices are relied on polarized currents, which still generate relatively high dissipation, particularly for nanodevices based on DW motion [1]. Here I will first show that domain walls in magnetic wires based on CoFeB films with perpendicular magnetic anisotropy (PMA)`can be moved at very low current densities of a few 107 A/cm2, one order of magnitude lower than for conventional Co/Pt or Co/Ni films with PMA. This feature originates from the very low density of pinning defects in CoFeB films, which results in unprecedented low propagation fields < 1mT for structures with PMA. A novel solution to further reduce power consumption in spintronics devices is emerging, based on electric field (E) gating to control the magnetic state. Here, I will also describe our recent experiments on assisting the switching process by an electric field in magnetic nanodevices. Particularly, one of our approach is based on CoFeB structures with PMA grown onto an epitaxial piezoelectric layer [2]. The application of a voltage on the piezoelectric layer generates a strain that is transfered to the CoFeB layer. Both the nucleation and depinning magnetic fields show a strong reduction up to 20% as a result of a modification of the perpendicular anisotropy under strain. These results indicate that current induced DW motion assisted by an electric field may be a solution to reduce the power consumption in spintronics devices such as memories or spin logic devices. The authors acknowledge the financial support from European FP7 program through contracts MAGWIRE n°257707 and NAMASTE n°214499
References
[1] Burrowes et al, Nature Physics 6, 17 (2010) [2] Accepted Phys. Rev. B (2011)
3:54 PM
DA-05. Spin-Orbit torques in ferromagnetic thin films
Mihai Miron
1SPINTEC, Grenoble, France; 2Catalan Institute of Nanotechnology, Barcelona, Spain
Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay data storage media, thanks to their ability to retain a stable magnetization state over long periods of time and their compliance with increasing minaturization steps. A major concern is that the same anisotropy properties that make a material attractive for storage also make it hard to write. We address this issue by investigating novel spin torque mechanisms based on spin-orbit effects. It is well known that spin-orbit coupling is ultimately responsible for magnetocrystalline anisotropy and damping. Under certain conditions, however, spin-orbit effects might either induce or enhance specific spin torque mechanisms. The Rashba interaction is predicted to create a torque in the uniformly magnetized domains [1]. Unlike spin-torque, this phenomenon does not rely on injection of spin polarized current from another magnetic layer or adjacent domain. By the intermediate of the Rashba interaction, the conduction electron spin couples to the crystalline electric fields, and as a current is applied, a spin accumulation transverse to both the current direction and electric field is created. Further on, due to the s-d exchange interaction coupling the conduction electron’s spins to the localized magnetic moments, the spin accumulation will act as an effective magnetic field on the magnetization [2]. Here we demonstrate switching of a perpendicularly magnetized Co dot driven by in-plane current injection at room temperature. Our device is composed of a thin Co layer with strong Rashba interaction and perpendicular anisotropy induced by asymmetric Pt and AlOx interfaces. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. This symmetry is consistent with the spin accumulation induced by the Rashba interaction and spin-dependent mobility observed in nonmagnetic semiconductors [3] as well as with the torque induced by the spin Hall effect in the Pt layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the Co layer, suggesting that the Rashba interaction plays a key role in the reversal mechanism.
References
1. A. Manchon and S. Zhang, PRB 78 212405 (2008) 2. I.M. Miron et al. Nature Materials 9, 230-234 (2010) 3. H.-A. Engel, E. I. Rashba, & B.I. Halperin PRL 98, 036602 (2007).
1:30 PM - 4:30 PM, Grand Canyon 7
Chair: Andrei Slavin, Oakland University
1:30 PM
DB-01. Amplification of Surface Spin Waves in Ferrite Thin Films through Interfacial Spin Scattering
Zihui Wang1, Yiyan Sun1, Mingzhong Wu1, Vasil Tiberkevich2 and Andrei Slavin2
1Department of Physics, Colorado State Univeristy, Fort Collins, CO; 2Department of Physics, Oakland University, Rochester, MI
Spin waves in magnetic films have potential applications for microwave signal processing [1,2,3] and logic operations [4,5,6]. These applications, however, are bottlenecked by the da mping of spin waves. One can compensate the spin-wave damping by parametric pumping, but this method requires the use of an external microwave signal with a frequency twice that of the spin wave and a delicate resonator for the delivery of this signal [7,8]. This presentation reports on a new approach for the amplification of spin waves. It has been demonstrated experimentally that a surface spin-wave packet propagating in an in-plane magnetized ferrite thin film placed in contact with a current-carrying Pt thin layer can be either amplified or attenuated depending on the current/field configuration. The experiments used a 4.6 μm-thick yttrium iron garnet (YIG) film strip with a 20 nm- thick Pt capping layer. A dc current pulse was applied to the Pt layer along the direction of spin-wave propagation and produced a spin current along the Pt thickness direction through the spin-Hall effect. As the spin current scatters off the surface of the YIG film, it exerts a torque on the YIG surface spins [9]. Due to the dipolar and exchange interactions, the effect of this torque is extended to other spins across the YIG thickness and, thereby, modifies the effective dissipation of a surface spin-wave packet traveling in the YIG film strip. The net effect of this interfacial spin scattering process depends critically on the relative orientation of (1) the magnetic moments of the electrons in the Pt layer that scatter off the YIG surface and (2) the precession axis of the magnetic moments on the YIG surface. When they are anti-parallel, the spin-wave damping is reduced and the amplitude of the spin-wave packet is increased. In a parallel configuration, the packet experiences an enhanced attenuation. A relative change in spin-wave amplitude of ±15% was observed. The corresponding change in spin-wave decay rate was ±5 MHz. It was also observed that the interfacial spin scattering affected the power level to which high-power spin-wave packets saturate due to nonlinear damping.
References
[1] P. Kabos and V. S. Stalmachov, Magnetostaic Waves and heir Applicaations (Chapman and Hall, London, 1994). [2] D. D. Stancil and A. Prabhakar, Spin Waves: Theory and Applications (Springer, New York, 2009). [3] J. D. Adam et al., IEEE Trans. Microwave Theory Tech. 50, 721 (2002). [4] S. Bance et al., J. Appl. Phys. 103, 07E735 (2008). [5] T. Schneider et al., Appl. Phys. Lett. 92, 022505 (2008). [6] A. Khitun et al., J. Phys. D: Appl. Phys. 43, 264005 (2010). [7] P. A. Kolodin et al., Phys. Rev. Lett. 80, 1976 (1998). [8] A. V. Bagada et al., Phys. Rev. Lett. 79, 2137 (1997). [9] Y. Kajiwara et al., Nature 464, 262 (2010).
1:42 PM
DB-02. Identification and selection rules of the spin-wave eigen-modes in a normally magnetized nano-pillar
V. V. Naletov1, 2, G. de Loubens1, O. Klein1, J. Grollier3, N. Locatelli3 and V. Cros3
1Service de Physique de l'État Condensé, CEA Saclay, 91191 Gif-Sur-Yvette, France; 2Physics Department, Kazan Federal University, 420008 Kazan, Russian Federation; 3Unité Mixte de Physique CNRS/Thales and Université Paris Sud 11, 91767 Palaiseau, France
Using a Magnetic Resonance Force Microscope [1], we report on a spectroscopic study on the spin-wave eigen-modes inside a circular nano-pillar (Py/Cu/Py). We demonstrate that distinct spin-wave spectra are excited by a uniform in-plane radio-frequency (RF) magnetic field and by an RF current flowing perpendicularly through the layers (see Figure). Three indices are required to label the observed eigen-modes: the azimuthal and radial indices for a single disk, plus an index referring to the anti-biding or binding coupling between the t wo magnetic layers. While the spatially uniform RF magnetic field excites only the axially symmetric modes having azimuthal index l=0, the RF current, creating a circular RF Oersted field, excites only the modes having azimuthal index l=+1. Experimental results are compared to theoretical prediction of the nano-pillar spin-wave spectra calculated both analytically and numerically.
References
[1] O. Klein, G. de Loubens, V. V. Naletov, F. Boust, T. Guillet, H. Hurdequint, A. Leksikov, et al. Phys. Rev. B 78, 144410 (2008)
1:54 PM
DB-03. Electric-field-induced spin wave generation using multiferroic magnetoelectric cells
Sergiy Cherepov1, Pedram Khalili Amiri1, Juan G. Alzate1, Kin Wong1, Mark Lewis1, Pramey Upadhyaya1, Jayshankar Nath1, Mingqiang Bao1, Joshua L. Hockel2, Alexandre Bur2, Tao Wu2, Gregory P. Carman2, Alexander Khitun1 and Kang L. Wang1
1Electrical Engineering, UCLA, Los Angeles, CA; 2Mechanical and Aerospace Engineering, UCLA, Los Angeles, CA
Logic elements based on spin waves constitute a promising alternative to CMOS technology for ultra-low power computation [1], where spin wave interference is exploited to perform the logic operation while information is encoded in the amplitude or phase of the spin-wave [2]. However, energy-efficient schemes for spin wave generation are required to meet power requirements for competing against CMOS technology. Previously reported methods for spin-wave excitation include inductive coupling using currents through antennas [3], or spin polarized currents (i.e. spin transfer torque) [4] through the device itself. These methods, however, require relatively large currents and thus are not energy efficient. In this work, we present an experimental and numerical study of the spin-wave generation due to localized strain-induced (i.e. magnetostrictive) change of anisotropy in a micron-size ferromagnetic stripe. A double-layer (Ni/NiFe) microstrip was deposited and patterned on top of a piezoelectric substrate (PMN-PT). The soft NiFe film provides a low loss medium for spin-wave propagation, while Ni (having a larger magnetostriction coefficient) was used as a coupling enhancement layer to increase the strain-mediated magnetoelectric effect. A radio-frequency voltage applied capacitively to a micron-scale confined region of the PMN-PT substrate (magnetoelectric cell) excites spin-waves by rapid anisotropy change due to the local voltage induced strain. We observed spin wave generation and detection in two-port S-parameter measurements using a magnetoelectric cell and an inductive antenna, as well as using two magnetoelectric cells. Control experiments using non-magnetostrictive (NiFe) and non-piezoelectric (Si) materials did not result in spin wave generation, confirming the strain-mediated origin of the effect. Spin-wave propagation as far as 40 um was observed in a 4 um wide stripe. This experiment confirms that high-frequency magnetoelectric cell can be used for voltage induced spin wave generation, serving as a possible building block of magnonic logic devices [1].
References
[1] A. Khitun, M. Bao, K. L. Wang, Y. Souche: J. Phys. D: Appl. Phys. 43 (2010) 264005. [2] Y. Wu et.al.: J. of Nanoelectron. Optoelectron., 4, (2009) 394 [3] A. A. Serga, A. V. Chumak and B. Hillebrands: J. Phys. D: Appl. Phys. 43 (2010) 264002. [4] S. I. Kiselev et.al.: Nature 425 (2003) 380
2:06 PM
DB-04. Nonlinear Amplification and Mixing of Spin Waves: Experiment and Theory
Robert Camley1, Y. Khivintsev1, 2, J. Marsh1, V. Zagarodnii1, 3, I. Harward1, Pavol Krivosik1, J. Lovejoy1 and Z. Celinski1
1University of Colorado at Colorado Springs, Colorado Springs, CO; 2Saratov Branch of Kotel'nikov IRE RAS, Saratov, Russian Federation; 3Taras Shevchenko Nation University of Kyiv, Kyiv, Ukraine
In contrast to typical cavity-based ferromagnetic resonance measurements where the oscillating magnetic field has an amplitude of about 0.1 Oe, ultrasmall waveguide geometries can produce very large oscillating microwave fields - up to about 500 Oe for an input power of about 2 W. Using this, we explore nonlinear ferromagnetic dynamics in ferromagnetic ribbons of Permalloy and Fe. In particular if two microwave signals at different frequencies (Fig 1a) are sent into the waveguide, we can amplify one wave by adding energy to the other wave. We also demonstrate the creation of a number of waves at new frequencies (Fig 1b), and the development of a comb of equally spaced frequencies (Fig 1d). These experimental results are explained using a macrospin model with perturbation theory and through micromagnetics calculations. The perturbation theory shows that the lowest order nonlinear terms in the Landau Lifshitz equations are cubic and that the nonlinearly generated signals are effectively combinations of three waves. It also gives good insight into both the creation of the new frequencies and the amplification of the spin waves. This work was supported by the ARO Grant No. W911NF-10-1-0225 and NSF Grant No. DMR 0907063
References
Yuri Khivintsev et. al. Appl. Phys. Lett. 98, 042505 (2011)
2:42 PM
DB-05. Collective vortex-gyration modes in magnonic cystals
Dong-Soo Han and Sang-Koog Kim
National Creative Research Initiative Center for Spin Dynamics and Spin-Wave Devices, Nanospinics Laboratory, and Research Institute of Advanced Materials, Department of Materials Science and Eng., Seoul Natl Univ, Seoul, Republic of Korea
Mutual energy transfer between separated magnetic disks by stimulated vortex gyrations has been demonstrated theoretically [1], by micromagnetic simulation [2-3], and experimentally [3-6]. Such a robust mechanism allows for a new type of signal transfer along dipolar-coupled magnetic disks of micrometer or smaller size, offering the advantages of lossless energy transfer and low-power signal input. In this presentation, we are going to report on collective vortex-gyration modes in magnonic crystals composed of one-dimensional (1D) disk arrays, which modes were isolated in micromagnetic simulations. We explored the characteristic band structures of those collective modes in two different array models: one consisting of 25 NiMnSb disks of equal 213 nm diameter and 20 nm thickness, with 15 nm edge-to-edge separation, and the other having the same dimensions but consisting of alternating NiMnSb and Permalloy disks. The simulation results for different orderings of the polarization (p) and chirality (c) denoted by [pj,cj] = [1,1] , [1,(-1)j+1] , [(-1)j+1,1] and [(-1)j+1, (-1)j+1] where j = 1, 2, …, 25, were found to be in excellent agreement with the analytically derived dispersion curves. We found that there was only one branch in the single-material array, but two differ ent branches in the array composed of two different materials, which behaviors are analogous to those of the lattice vibration phenomenon. The width of the bands was wider for pjpj+1 = -1 than for pjpj+1 = 1, because the dipolar interaction between the neighboring disks was much stronger [3]. The group velocity of the coupled gyration-mode propagations was found to be expressed in terms of the eigenfrequency of isolated single disks, the interaction strength between neighboring disks, and the p/c combination. The above results suggest that vortex-gyration-based magnonic crystals could be implemented in future information-signal transmission and processing devices, offering the advantages not only of extremely low energy dissipation by the selection of a negligible damping material, but also low-power signal input.
References
*Corresponding author: sangkoog@snu.ac.kr, Phone: +82-2-880-5854, Fax: +82-2-885-1457 [1] J. Shibat et al., Phys. Rev. B 67, 224404 (2003);J. Shibat and Y. Otani, Phys. Rev. B 70, 012404 (2004) ; K.-S. Lee et al. arXiv:1102.0519 [2] S. Barman et al., IEEE Trans. Mag. 46, 1342 (2010) [3] H. Jung et al., Appl. Phys. Lett. 97, 222502 (2010); H. Jung et al., arXiv:1011.6399v1 [4] S. Barman et al., J.Phys.D : Appl. Phys. 43, 42201 (2010) [5] A. Vogel et al. Phys. Rev. Lett. 106, 137201 (2011) [6] S. Sugimoto et al. Phys. Rev. Lett. 106, 197203 (2011). [7] This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20110000441)
2:54 PM
DB-06. Nanoscale spin wave localization using ferromagnetic resonance force microscopy
Han-Jong Chia1, 2, Lyuba M. Belova3 and Robert D. McMichael1
1Center for Nanoscale Science and Technology, NIST, Gaithersburg, MD; 2Maryland Nanocenter, University of Mayland, College Park, MD; 3Department of Materials Science and Engineering, Royal Institute of Technology, Stockhom, Sweden
We utilize ferromagnetic resonance force microscopy (FMRFM) to generate and detect multiple localized spin wave modes with spatial dimensions less than 100 nm in ferromagnetic thin films. Localized spin waves are a potentially powerful tool for imaging local magnetic properties within ferromagnetic nanostructures [1]. We generate localized spin waves by positioning a 1 μm soft magnetic tip in close proximity to an in-plane magnetized Ni80Fe20 (Permalloy) thin film. The dipole field of the tip generates a field well in the film directly below the tip, where spin waves may propagate while being excluded from the extended film. Our experimental results, Figure 1, indicate detection of multiple spin wave modes (i-v) as well as the extended film (EF). A 1-D WKB approximation and micromagnetic modeling were used to interpret the FMRFM spectra. While the WKB approach yields only a qualitative understanding of the trapped modes, micromagnetic modeling of the shape and field-dependence of the spectra are in close agreement with our experimental results. In addition, we use micromagnetic modeling to generate two dimensional spatial maps of the localized spin wave modes. Fitting the lowest frequency spin wave mode to a Gaussian yields dimensions of 45.2 ± 0.3 nm in the applied field direction.
References
[1] I. Lee et al, Nature 466, 845 (2010)
3:06 PM
DB-07. Amplification of Spin Waves by the Spin Seebeck Effect
Eduardo P. Hernandez, Antonio Azevedo and Sergio M. Rezende
Departamento de Fisica, Universidade Federal de Pernambuco, Recife, Brazil
We present experiments which demonstrate that spin waves propagating in films of yttrium iron garnet (YIG) can be amplified by means of the spin Seebeck effect (SSE). Magnetostatic volume spin-wave packets with frequency in the range 1 - 2 GHz are excited and detected in YIG film strips magnetized in the plane along the strip using standard microstrip antennas. Two different schemes were employed to generate a temperature gradient across the YIG film/GGG substrate, one employs a heater on top of the GGG substrate and the other uses resistive Pt or Mo thin layers deposited on either one or on both sides of the YIG film/substrate sample heated by currents in the range of 0 - 30 mA. In both schemes when a sufficiently large temperature gradient is established across the YIG film the detected delayed microwave pulse due to the spin wave is amplified. The amplification is attributed to the action of a spin-transfer thermal torque created by spin currents generated through the spin Seebeck effect recently observed in YIG.1 Amplification occurs if the temperature gradient exceeds a critical value such that the spin-torque produced by the spin current acting on the magnetization overcomes the relaxation. Amplification gains larger than 20 are observed with a current of 20 mA in a simple YIG/Pt bilayer. The experimental data are interpreted with a spin-wave theory that gives an amplification gain in very good agreement with measurements. The results of this investigation open new possibilities for the use of magnetostatic waves for signal processing in spintronic devices. This work was supported by Brazilian agencies FINEP, CNPq, CAPES and FACEPE.
References
1 K. Uchida, H. Adachi, T. Ota, H. Nakayama, S. Maekawa, and E. Saitoh, Appl. Phys. Lett. 97, 172505 (2010).
3:18 PM
DB-08. Excitation of short-wavelength spin waves in tapered magnonic waveguides
Vladislav E. Demidov1, Mikhail P. Kostylev2, Karsten Rott3, Jana Muenchenberger3, Guenter Reiss3 and Sergej O. Demokritov1
1Institute for Applied Physics, University of Muenster, Muenster, Germany; 2University of Western Australia, Crawley, WA, Australia; 3Bielefeld University, Bielefeld, Germany
The implementation of magnetic spin-wave devices with micrometer and sub-micrometer dimensions demands elaboration of advanced approaches enabling efficient excitation of spin waves with sub-micrometer wavelengths. Here we demonstrate an approach based on the phenomenon of the wavelength conversion in tapered Permalloy micro-waveguides. By using phase-resolved micro-focus Brillouin light scattering spectroscopy we map the spatial distributions of the intensity and the phase of the propagating spin waves with the spatial resolution of 250 nm and measure directly their wavelength and spatial attenuation. We show that, propagating in the waveguide with the width decreasing from 2 to 0.5 micrometers, a spin wave can reduce its wavelength by a factor varying between 3 and 15 depending on the frequency and the static magnetic field. As a result, a spin wave with the wavelength of several micrometers, which can be efficiently exited by a standard inductive technique, transforms into a wave with the wavelength below one mi crometer. We measure the tapered waveguides with different shapes and show that the wavelength conversion is accompanied by only moderate conversion losses, which makes the phenomenon promising for technical applications. Our findings enable a further reduction of characteristic dimensions of integrated magnonic devices and an improvement of their performances.
3:30 PM
DB-09. Collective spin wave modes in a two-dimensional array of magnetic nano-dots
Roman V. Verba1, Gennadiy A. Melkov1, Vasil S. Tiberkevich2 and Andrei N. Slavin2
1Faculty of Radiophysics, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine; 2Department of Physics, Oakland University, Rochester, MI
Arrays of magnetic nano-dots are promising candidates for application in microwave signal processing and in magnetic recording technology. Modern patterning techniques allow one to fabricate arrays with sufficiently small inter-dot distances to guarantee effective magneto-dipolar coupling between dots. Investigations of the spin wave (SW) excitation spectra of such coupled arrays are important to determine the stability of different magnetic ground states of an array and to study the array’s properties in external microwave fields. In this paper we present a general theory of collective SW excitations in a two-dimensional array of magnetic nano-dots coupled by a magneto-dipolar interaction. The theory is based on the macrospin approach, but takes into account real shape-dependent demagnetizing field of individual dots. The developed theory provides a simple way to calculate spectra and mode structure of SW excitations in both periodic and non-periodic ground states of an array, and presents a significant improvement over the existing theories [1-3]. We show that all the SW properties in any periodic ground state of a dot array are determined by the same state-independent array's demagnetization tensor Fk, which depends on the SW wave vector k, on the size and shape of a dot, and on the geometry of the array's lattice. The behavior of SW excitations of a dot array under the influence of weak perturbations is investigated, and two important particular cases - weak damping and weak external microwave field - are considered in detail.
References
1. S. Tacchi, M. Madami, et al., Phys. Rev. B 82, 024401 (2010). 2. R. Arias and D. L. Mills, Phys. Rev. B 70, 104425 (2004). 3. P. V. Bondarenko, A. Yu. Galkin, et al., Phys. Rev. B 81, 224415 (2010).
3:42 PM
DB-10. Controlled enhancement of spin-current emission by three-magnon splitting
Hidekazu Kurebayashi1, Oleksandr Dzyapko2, Vladislav E. Demidov2, Dong Fang1, Andrew J. Ferguson1 and Sergej O. Demokritov2
1University of Cambridge, Cambridge, United Kingdom; 2University of Muenster, Muenster, Germany
Pioneering work by Kajiwara et al. [1] redefined the role of magnetic insulators in the context of spintronics. In their report, angular momentum exchange between electrons’ spins (as spin currents) and spin waves was found at the interface of a magnetic insulator (yttrium iron garnet, YIG) and nonmagnetic metal (Pt). This angular momentum exchange scheme allows spin waves to involve into the spin information transfer/processing within spintronic applications. In addition, YIG has a low magnetic d amping property, which makes possible the investigation of physics on spin wave propagations as well as on spin wave interactions. The aim of this presented research is to explore the role of spin wave interactions in the above-mentioned spin current generation. By using an YIG/Pt bi-layer and a microwave stripline, we excited magnetic dynamics in a wide range of frequency. This wide-frequency excitation allows us to turn on and off a particular spin wave interaction, called three-magnon splitting, the existence of which in our sample is confirmed by BLS measurements. We then characterised the spin current emission across the interface for on- and off-states of the three magnon scattering, and found the magnon splitting enhances the spin current emission [2].
References
[1] Y. Kajiwara et al. Nature (2010). [2] H. Kurebayashi et al. Nature Mater. (2011).
3:54 PM
DB-11. Magneto-acoustic pulses in Nickel films generated with a femtosecond laser excitation
Jiwan Kim, Mircea Vomir and Jean-Yves Bigot
Institut de Physique et Chimie des Matériaux de Strasbourg, CNRS - Université de Strasbourg, Strasbourg, France
We report about the existence of magneto-acoustic pulses propagating in a 200-nm-thick Nickel film excited with 120 fs laser pulses. They are detected from both the front and back sides of the film using the pump-probe magneto-optical Kerr technique. They result from the coupling between the magnetization and the longitudinal waves associated to the propagation of the lattice deformation, induced by the femtosecond laser pulses. Our results open a new way of inducing and probing magnetic pulses induced by lasers in ferromagnets. Figure 1a) represents the time resolved differential Kerr rotations Δθ(t)/θs for several angles φ of the static magnetic field H with respect to the normal to the sample plane. The differential reflectivity ΔR(t)/R0 is also displayed, showing two acoustic echoes at the time delays T=98 ps and 2T=196 ps. Correspondingly, the magneto-optical response shows magneto-acoustic pulses, appearing for the same delays T and 2T, for the three angles φ = 25°, 35° and 45°. The contrast of the magneto-acoustic pulses is even larger after propagating only once through the crystal (figure 1b). In that case the magneto-acoustic pulses are measured from the back side of the sample. They show up for T, 2T and 3T. The observed magneto-acoustic pulses correspond to the coupling between spin waves and the lattice deformation propagating with a velocity of 4082 ms-1.
4:06 PM
DB-12. Measurements of spin-wave mode linewidth in individual nanomagnets of varying size
Hans T. Nembach, Justin M. Shaw and Tom J. Silva
Electromagnetics Division, National Institute of Standards and Technology, Boulder, CO
The strength of the damping process in nanomagnets ultimately determines the switching speed and the critical current for the onset of spin-torque driven precession in devices. The size of the nanomagnets themselves might influence the damping due to enhanced spin-wave mode localization with decreasing size. Some spin-torque ferromagnetic resonance experiments showed an increased damping compared to thin film values, Ref. 1. The complex nature of spin-torque devices can make it difficult to determine the exact origin of the increased damping. Measurements of the magnetization dynamics in nanomagnets with a single magnetic layer are an ideal model system to study dissipation in c onfined geometries. Ensemble measurements of nanomagnets by frequency resolved magneto-optical Kerr effect (FR-MOKE) and Brillouin Light Scattering (BLS) showed that shape distortions of the nanomagnets and size distributions give rise to large inhomogeneous linewidth broadening, Ref.2 and 3. This obscures the details of the damping for the different spin-wave modes. The diffraction limit makes it challenging to access the properties of individual nanomagnets by optical means. We developed a heterodyne magneto-optical microwave magnetometer (H-MOMM), which permits the measurement of extremely weak optical scattering signals from a single nanomagnet by mixing the probe beam with a local oscillator. We found that the frequency dependence of the linewidth of the end-mode in an individual ellipse with nominal dimension 240 nm x 220 nm is significantly different from an ensemble measurement. The linewidth of the end-mode is significantly higher at lower frequencies than the linewidth of the center-mode, but both linewidths coincide at higher frequencies. Finally, the damping of the center-mode increases with decreasing diameter of the nanomagnet.
References
Ref.1: J.C. Sankey, P.M. Braganca, A.G.F. Garcia, I.N. Krivorotov, R.A. Buhrman and D.C. Ralph, Phys. Rev. Lett. 96, 227601 (2006) Ref.2 : J.M. Shaw, T.J. Silva, M.L. Schneider and R.D. McMichael, Phys. Rev. B 79, 184404 (2009) Ref.3 : H.T. Nembach, J.T. Shaw, T.J. Silva, W.L. Johnson, S.A. Kim, R.D. McMichael and P. Kabos, Phys. Rev. B 83, 094427 (2011)
4:18 PM
DB-13. Electrical detection of spin wave quantization in ferromagnetic nanowires
Mahdi Jamali, Jae Hyun Kwon, Ajeesh M. Sahadevan, SankhaSubhra Mukherjee and Hyunsoo Yang
Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore
Although there have been many reports of spin wave quantization, all of them have used the Brillouin light scattering technique which is ultimately limited by the light diffraction. We have measured the spin wave quantized modes inside a ferromagnetic nanowire by an electrical inductive technique for the first time. The structure of the device is shown in Fig. 1(a). We have used 20 nm Permalloy for the ferromagnetic nanowire which is isolated from the waveguides by a 30 nm SiO2 layer. The gap between the injection and detection part is about 3.6 μm and the ferromagnetic nanowires havea 850 nm width and the spacing between the nanowires is 400 nm. We have applied bias magnetic fields in the transverse direction and the frequency response to the applied fields was measured by a vector network analyzer (VNA). The propagating spin wave spectraare shown in Fig. 1(b). We can clearly see four distinct modes that correspond to the quantized spin waves. Our results will be compared with the previous BLS measurement data [1].
References
[1]G. Gubbiotti, G. Carlotti, T. Ono, and Y. Roussigne, J. Appl. Phys. 100,023906(2006).
1:30 PM - 4:30 PM, Grand Canyon 8
Chair: Kyung-Jin Lee, Korea University
1:30 PM
DC-01. Domain wall dynamics in an artificial multiferroic under non-uniform stress
Matthew T. Bryan, Julian Dean and Dan A. Allwood
Materials Science and Engineering, University of Sheffield, Sheffield, United Ki ngdom
Artificial multiferroics are hybrid systems which may be formed from magnetostrictive and piezoelectric elements that are integrated to produce strong magneto-electric coupling via mechanical stress. This can result in electrically-controlled magnetization rotation [1]. Recently, micromagnetic simulations of a magnetic nanowire in contact with a piezoelectric have suggested that magnetic domain wall motion may be induced by the voltage configuration applied to electrodes patterned onto the piezoelectric, via the generation of a non-uniform stress profile in the nanowire [2]. This has the potential to be more energy efficient than wall motion driven by a magnetic field or a spin-polarized current, but the mechanism that drives the wall motion is poorly understood. Here, we derive a one-dimensional semi-analytical model of domain wall motion under a non-uniform stress. The model enables the contribution of the normal and shear stress components and their gradients to be analysed separately. At any given position along the nanowire, the effect of the stress is analogous to a modification of the demagnetization factors. The gradient of the normal stress along the wire axis generates an effective axial field, while gradients in the shear stresses involving the wire axis generate effective transverse and out-of-plane fields. Damping suppresses the initial response to the gradient of the normal stress along the wire axis, although ultimately this term would determine the wall velocity if the gradient could be maintained long enough to reach steady-state propagation. However, depending on the stress profile generated, steady-state motion might not be achieved. In this case, the velocity profile may be dominated by the shear stress profile, which has a strong influence on initial motion.
References
[1] Y. H. Chu, et al. Nat. Mater. 7, 478 (2008) [2] J. Dean, et al. J. Appl. Phys. 109, 023915 (2011)
1:42 PM
DC-02. Probing the Non-Adiabaticity of the Spin-Torque via Direct Imaging of Current Induced Vortex Domain Wall Excitations
Andre Bisig1, 2, Jan Rhensius3, 4, Christoforos Moutafis1, 3, Jakoba Heidler1, Gillian Killiani3, Tolek Tyliszczak6, Laura J. Heyderman4, Bartel Van Waeyenberge5, Hermann Stoll2, Gisela Schütz2 and Mathias Kläui1, 7
1SwissFEL, Paul Scherrer Institut, Villigen, Switzerland; 2Moderne Magnetische Materialien, Max-Planck-Institut für Intelligente Systeme, Stuttgart, Germany; 3Fachbereich Physik, Universität Konstanz, Konstanz, Germany; 4Labor für Mikro- und Nanotechnologie, Paul Scherrer Institut, Villigen, Switzerland; 5Department of Solid State Sciences, Ghent University, Ghent, Belgium; 6Advanced Light Source, LBNL, Berkeley, CA; 7Institut für Physik, Johannes Gutenberg-Universität Mainz, Mainz, Germany
Electrons moving in ferromagnets with inhomogeneous magnetization configurations act on the local magnetization via the spin-transfer-torque. In particular, when the current traverses a magnetic domain wall, spin angular momentum is transferred to the local magnetization and induces a torque on the domain wall; this leads to domain wall motion or domain wall excitations [1,2]. This effect allows for the control of magnetic domain walls by electrical currents, opening the possibility to store information in nonvolatile devices [3]. The spin-torque effect comprises an adiabatic and non-adiabatic torque. The adiabatic spin-torque arises from the conduction electron spins that follow the direction of the local magnetization, whereas the non-adiabatic spin-torque arises from a mistracking of the spin direction as a result of non-adiabatic transport or spin relaxation. The relative strength of the non-adiabatic spin-torque is described by the non-adiabaticity constant β [4]. The spin structure dependence and the size of the non-adiabatic term, that governs for instance the domain wall velocity and thus device performance, are still under debate [5]. To determine the non-adiabatic torque spatially resolved, we directly image the results of the interactions of vortex domain walls and microwave spin polarized currents by time-resolved scanning transmission x-ray microscopy (STXM). We describe vortex domain walls as composed of a vortex, where the magnetization curls in the plane of the nanowire around a central vortex core, and two topological edge defects (half-antivortices). From the phase-shift of the response of the vortex and the half-antivortices we determine the strength of the adiabatic and non-adiabatic spin-torque terms acting on both and find significant differences. We compare the experimental results to the theoretical predictions of the dependence of the non-adiabatic torque on the local magnetization gradients.
References
[1] L. Berger, J. App. Phys. 71, 2721 (1992). [2] L. Thomas, et al., Nature 443, 197 (2006). [3] S. S. P. Parkin, et al., Science 320, 190 (2008). [4] S. Zhang, Z. Li, Phys. Rev. Lett. 93, 127204 (2004). [5] M. Eltschka et al., Phys. Rev. Lett. 105, 56601 (2010).
1:54 PM
DC-03. Role of Spin Diffusion on Current-driven Domain Wall Motion
Aurelien Manchon1 and Kyung-Jin Lee2
1Materials Science and Eng., KAUST, Thuwal, Saudi Arabia; 2Materials Science and Eng., Korea University, Seoul, Republic of Korea
Current-driven magnetic domain wall motion is one of the most productive research areas in spintronics and the nature of the spin torque in magnetic textures is under intense investigation. Recent experiments indicate that the ratio between the non-adiabatic torque and the damping depends on the domain wall structure [1]. In particular, they found that this ratio can be one order of magnitude larger in vortex wall than in transverse wall. This difference has been attributed to the sharpness of vortex wall cores compared to the smooth texture of transverse walls. Several theoretical studies have addressed the nature of the non-adiabatic torque [2,3]. It has been shown theoretically that for very narrow domain walls (1-3 nm) a momentum transfer occurs that can gives to a significant non-adiabatic torque that increases when the domain wall gets narrower [3]. However, achieving such narrow domain walls is experimentally difficult. On the other hand, in the case of perpendicularly magnetized domain walls, the large magnetic anisotropy gives rise to domain walls on the order of 5-10nm wide, which in some cases is comparable to the spin diffusion length. We have analytically studied the influence of spin diffusion on the current-driven velocity of both transverse and vortex domain walls. We find that the spin diffusion gives rise to an additional spin torque that is non-local and strongly depends on the domain wall width. This spin torque has been analyzed using Thiele formalism and is shown to significantly contribute to the domain wall velocity for sharp domain walls. This torque is proportional to (λJ2/Δ2), the square of the ratio between the transverse spin diffusion λJ and the domain wall width Δ. Consequently increases when reducing the domain wall width. This contribution is also stronger for weak ferromagnets. We find that whereas the contribution of spin diffu sion can increase the velocity of transverse walls by a factor of 2, it can be even stronger in vortex walls, changing its transverse velocity by up to an order of magnitude. Therefore, the presence of spin diffusion introduces a width-dependent spin torque that vanishes for smooth domain walls, and becomes dominant for sharp domain walls.
References
[1] C. Burrowes et al., Nat. Phys. 6, 17 (2010); M. Eltschka et al., Phys. Rrev. Lett. 105, 056601 (2010); L. Heyne et al., Phys. Rev. Lett. 105, 187203 (2010). [2] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004); A. Thiaville, Y. Nakatani, J. Miltat, and Y. Suzuki, Europhys. Lett. 69, 990 (2005). [3]G. Tatara and H. Kohno, Phys. Rev. Lett. 92, 086601 (2004); G. Tatara et al., Journal of the Physical Society of Japan 76, 054707 (2007); J. Xiao, A. Zangwill and M. D. Stiles, Phys. Rev. B 73, 054428 (2006).
2:06 PM
DC-04. Coupling parameters and selection rules for spin-transfer induced dynamics of two coupled vortices
Nicolas Locatelli1, Alexey V. Khvalkovskiy2, Paolo Bortolotti1, Grisha Avanesyan3, Julie Grollier1, Vincent Cros1, Konstantin A. Zvezdin3, Vladimir V. Naletov4, Grégoire De Loubens4, Olivier Klein4 and Albert Fert1
1Unité Mixte CNRS/Thales/Univ. Paris Sud, Palaiseau, France; 2Grandis, Inc., Milpitas, CA; 3A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, Russian Federation; 4Service de Physique de l’Etat Condensé, CEA Saclay, Gif-sur-Yvette, France
A today's key issue for spin-transfer nano-oscillators (STNO) relies on the improvement of their spectral coherence. Then the recent introduction of vortex based STNOs has allowed the observation of highly coherent oscillations in comparison to quasi-uniform magnetic configuration dynamics. The originality of our study compared to almost all previous investigation with a fixed polarizer, is to take advantage of the capability to excite the gyrotropic motion of a vortex having a time-dependent/spatially non-uniform polarizer [1], our choice being a second vortex. For this purpose, we perform rf measurements on circular spin-valve nano-pillars made from Cu/NiFe(15nm)/Cu(10nm)/NiFe(4nm)/Au (diameters 100 & 200nm) (coll. LPN-CNRS, Fr). We focus on the magnetic configuration with two vortices, one in each magnetic layer. Given the thickness of the thin layer, it is then interesting to consider the influence of the interactions between the two vortices (body to body and/or core to core), that results in both static and dynamical coupling between them. This has two major implications. First, it affects the natural eigen frequencies of the system: the new eigen gyrotropic modes correspond to coupled ones, where the two vortices are moving together [2]. Second, new conditions for observing a rf emission are emerging because of the symmetry of these coupled modes, tunable through the vortices parameters: their polarity and chirality. In particular, we will show that the relative vortex polarities have a strong influence on the efficiency of the spin transfer excitation of these modes [3]. Notably, the reversal of one vortex core polarity can switch on or off the detected magnetization oscillations, presumably by modifying the critical currents needed to counter the vortices damping. Interestingly, we will show through some micromagnetic simulations that the configuration with antiparallel core polarities can be excited at ideally zero current because of the core-core dipolar interactions. At last, we emphasize that the advantage of using coupled dynamics between the polarizing and the free layers allowed us to reach experimentally very narrow linewidth (down to 50 kHz) of the emitted spectra, even at zero magnetic field.
References
[1] A. V. Khvalkovskiy et al. : Nonuniformity of a planar polarizer for spin-transfer-induced vortex oscillations at zero field, Appl. Phys. Lett. 96, 212507 (2010) [2] O.V. Sukhostavets et al. : Magnetic Vortex Excitation Frequencies and Eigenmodes in a Pair of Coupled Circular Dots, Appl. Phys. Express 4 065003 (2011) [3] N. Locatelli et al. : Dynamics of two coupled vortices in a spin valve nanopillar excited by spin transfer torque, Appl. Phys. Lett. 98, 062501 (2011)
2:18 PM
DC-05. Reliable energy-efficient information recording by tailored orthogonal pulse currents in vortex-core cross-point architecture
Young-Sang Yu1, Ki-Suk Lee1, Hyunsung Jung1, Youn-Seok Choi1, Jun-Young Lee1, Myoung-Woo Yoo1, Dong-Soo Han1, Mi-Young Im2, Peter Fischer2 and Sang-Koog Kim1
1Research Center for Spin Dynamics & Spin-Wave Devices, Nanospinics Laboratory, and Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul Natl Univ, Seoul, Republic of Korea; 2Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA
The magnetic vortex structure, which consists of an in-plane-curling magnetization and an out-of-plane magnetization at the core region, typically is observed in ground states in confined magnetic films. Owing to the binary-representability of those two vortex-core magnetizations as well as their high thermal stability, vortex-state magnetic elements have been considered as potential candidates for information storage media [1,2]. Though the technological potentialities are limited by the requirement of strong static magnetic fields to switch the binary state of vortex-core magnetizations, low-power-driven vortex-core switching, by means of resonant vortex-core gyrations, recently has been demonstrated experimentally [1-4]. In this presentation, we are going to report on how vortex-core magnetizations can be reliably manipulated by pulse-type rotating fields generated with two orthogonal Gaussian pulse currents of optimal width σ and time delay Δt. First, we analytically optimized the values of σ = 1/ωD and Δt = π/(2ωD) with the eigenfrequency ωD for energy-efficient vortex-core switching, based on Thiele’s equation [4]. Then, by means of a state-of-the-art probe, we experimentally demonstrated the vortex-core switching [4]. To directly monitor the time-varying vortex-core gyrations in Permalloy disks, we employed time- and space-resolved full-field magnetic transmission soft X-ray microscopy through X-ray magnetic circular dichroism contrast at the Fe L3 edge [5]. The X-ray microscopy, offering 70 ps temporal and 20 nm spatial resolutions, could resolve vortex-core displacements under 110 MHz repeatable gyrations. The experimentally observed pulse width and time delay were in good agreement with the analytical and simulation results. For reliable memory-bit selection and information recording, we fabricated 2 × 2 vortex-state disks and experimentally demonstrated either up-to-down or down-to-up vortex-core switching at the intersection of two crossed electrodes for a certain optimized time delay [4]. This work provides the foundation necessary for realization of vortex random access memory using the existing cross-point architecture.
References
*Corresponding author: sangkoog@snu.ac.kr, Phone: +82-2-880-5854, Fax: +82-2-885-1457 [1] B. Van Waeyenberge et al., Nature (London) 444, 461 (2006). [2] S. -K. Kim et al., Appl. Phys. Lett. 92, 022509 (2008). [3] K.-S. Lee et al., Phys. Rev. B 76, 174410 (2007); K.-S Lee et al., Phys. Rev. Lett. 101, 267206 (2008). [4] Y.-S. Yu et al., Phys. Rev. B 83, 174429 (2011); Y.-S. Yu et al., Appl. Phys. Lett. 98 052507 (2011). [5] P. Fischer et al., Mater. Today 9, 26 (2006). [6] This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (No. 20110000441). Support for the soft X-ray microscopy measurements was provided by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy.
2:30 PM
DC-06. Joule heating effect as additional source of domain wall motion in NiFe nanostrips.
Jacob Torrejon1, Javier Curiale1, 2, Gregory Malinowski1, Andre Thiaville1, Daniel Lacour3, François Montaigne3 and Michel Hehn3
1Laboratoire de Physique des Solides, (Univ Paris Sud, CNRS), Orsay (Paris), France; 2Laboratoire de Photonique et Nanostructures, CNRS, Marcoussis, France; 3Institut Jean Lamour, Univ Nancy I, Vandoeuvre-lés-Nancy, France
The objective of this work is the study domain wall (DW) dynamics under short current pulses. The coupling between the conduction electrons and those involved in the magnetism induces a spin transfer torque (STT) being responsible for the DW motion. Using high resolution magnetic force microscopy with in situ RF system, we investigated experimentally the wall position and structure in permalloy nanostrips (400 nm wide and 17 nm thick) before and after current pulses of nanosecond duration. A strong transverse field was applied for nucleating an asymmetric transverse wall (ATW) in the center of the track, figure 1a. Such metastable structure is transformed in a vortex wall (VW) under current pulse, giving rise to a large displacement known as automotion [1], figure 1b. After that, the smaller VW motion induced by current is only attributed to spin transfer torque, which is proportional to non-adiabatic parameter β (in absence of DW transformation), figure 1c. Most of the works related to STT neglect the Joule heating. However it has been demonstrated that the thermal effects could play an important role [2]. Noticeable gradients of temperature can be induced in Fe20Ni80 nanostrips when a thick SiO2 layer is inserted between sample and substrate. This is revealed by an easier movement of VW towards the center of nanostrip that it is the hotter side. Finally we discuss the role of this additional source of DW motion in STT measurements with the help of micromagnetic simulations.
References
[1] J-Y. Chauleau, R. Weil, A. Thiaville and J. Miltat. Phys Rev B 82 214414 (2010). [2] J. Curiale, A. Lemaître, G. Faini and V. Jeudy. Appl Phys Lett 97 243505 (2010).
2:42 PM
DC-07. Vortex dynamics in interacting ferromagnetic structures
Andreas Vogel
Institute of Applied Physics, University of Hamburg, Hamburg, Germany
Subnanosecond dynamics and potential technological appli cations give rise to a broad scientific interest in the dynamic properties of ferromagnetic microstructures with vortex magnetization configuration [1]. The magnetostatic interaction between spatially separated elements has to be taken into account when the inter-element distance is smaller than their lateral size [2]. Vortex dynamics in stray-field coupled elements are studied by time-resolved scanning transmission X-ray microscopy (STXM) and broadband-ferromagnetic resonance (FMR) measurements. Using STXM, a pair of permalloy squares is observed to behave like coupled harmonic oscillators after excitation of one of the vortices via a short magnetic field pulse [3]. The excitation is transferred between the elements whereas the transfer efficiency strongly depends on their separation and the configuration of the vortex-core polarizations. For alternating core polarizations, a transfer efficiency of about 56% is achieved in a chain of three elements. The chain can be switched back and forth between a transmitting and a locking state [4]. FMR transmission spectra on arrays reveal a relative broadening of the absorption peak due to resonant vortex-core motion which is proportional to the inverse sixth power of the normalized center-to-center distance between the ferromagnetic elements [5]. This kind of distance dependence is a prominent feature of the fundamental van der Waals type interactions (induced dipoles). Contributions to the broadening in the FMR spectra which are independent on the distribution of the core polarization are found to be dominant in 4 x N arrays. The parameter describing the relative broadening for a given normalized center-to-center distance between the elements is shown to depend on the size of the array [6]. Financial support by the Deutsche Forschungsgemeinschaft via the SFB 668 and the GrK 1286 as well as the Forschungs- und Wissenschaftsstiftung Hamburg via the Exzellenzcluster “Nano-Spintronik” is gratefully acknowledged. Operation of the X-ray microscopes is supported by the US Department of Energy under Contract No. DE-AC02-05-CH11231 and the Max-Planck-Institute for Intelligent Systems, Department Schütz.
References
[1] T. Shinjo, T. Okuno, R. Hassdorf, K. Shigeto, and T. Ono, Science 289, 930 (2000); R. P. Cowburn, D. K. Koltsov, A. O. Adeyeye, M. E. Welland, and D. M. Tricker, Phys. Rev. Lett. 83, 1042 (1999). [2] K. Yu. Guslienko, V. Novosad, Y. Otani, H. Shima, and K. Fukamichi, Phys. Rev. B 65, 024414 (2001). [3] A. Vogel, T. Kamionka, M. Martens, A. Drews, K. W. Chou, T. Tyliszczak, H. Stoll, B. Van Waeyenberge, and G. Meier, Phys. Rev. Lett. 106, 137201 (2011). [4] A. Vogel, M. Martens, M. Weigand, and G. Meier, Appl. Phys. Lett., in press (2011). [5] A. Vogel, A. Drews, T. Kamionka, M. Bolte, and G. Meier, Phys. Rev. Lett. 105, 037201 (2010). [6] A. Vogel, A. Drews, M.-Y. Im, P. Fischer, and G. Meier, IEEE Trans. Magn. 47, 1610 (2011).
3:18 PM
DC-08. Domain Wall Motion by the Magnonic Spin Seebeck Effect
Denise Hinzke, Ulrike Ritzmann and Ulrich Nowak
University of Konstanz, Konstanz, Germany
Uchida et al. [1] have demonstrated that in ferromagnetic materials spatial temperature gradients can lead to spin accumulation, which they attribute to the spin-Seebeck effect. Later on, it was shown that this effect occurs also in insulators [2]. In this context, it is important to note that in addition to a spin polarized charge current also a chargeless angular momentum current driven by spin waves can exit. Here we propose the existence of domain wall (DW) dynamics driven by magnonic spin currents due to temperature gradients. To get some insight into this new effect we introduce two different approaches: the stochastic Landau-Lifshitz-Gil bert equation, applied to spin models, and the Landau-Lifshitz-Bloch equation describing the dynamics of the thermally averaged spin polarization on micromagnetic length scales [3]. We show that these approaches describe this new type of DW motion, where chargeless spin currents following from a temperature gradient drag a DW into the hotter region. The figure shows the DW velocity vs. the temperature gradient for two different damping constants. In the limit of low damping constants and large temperature gradients the DW motion is accompanied by precession and a Walker breakdown exists [4]. Furthermore, for a better understanding of the relevant length scales we investigate the propagation of magnons. We acknowledge financial support by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 767.
References
[1] K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa, and E. Satoh, Nature 455, 778 (2008). [2] K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E. W. Bauer, S. Maekawa, and E. Satoh, Nature Materials 9, 894(2010). [3] N. Kazantseva, D. Hinzke, U. Nowak, R. W. Chantrell, U. Atxitia, and O. Chubykalo-Fesenko, Phys. Rev. B 77, 184428 (2008), D. A. Garanin, Phys. Rev. B 55, 3050 (1997). [4] D. Hinzke and U. Nowak, Phys. Rev. Lett., in press
3:30 PM
DC-09. Highly Efficient Spin-Torque-Assisted Domain Wall Depinning in L10FePt
Kuo-Feng Huang1, Tomohiro Koyama2, Kohei Ueda2, Daichi Chiba2, Teruo Ono2 and Chih-Huang Lai1
1Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan; 2Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan
In current induced domain wall motion (CIDM), the non-adiabatic term (β) is one of key parameters to increase the spin torque efficiency (ξSTT). The β is determined by the magnetic domain wall (DW) width and spin relaxation rate. The L10FePt possesses narrow DWs because of high crystalline anisotropy and high spin relaxation rate due to strong spin-orbit interaction; therefore, the L10FePt can be a good candidate to achieve a large β value. In this work, 5 nm L10FePt films with the (001) texture grown on thermally oxidized Si substrates were patterned into 335nm wide nanowires by e-beam lithography and Ar ion-milling. The DC currents with opposite directions were then applied through the nanowire as the spin torque source. Fig.1(a) shows the hysteresis loops measured by the anomalous Hall effect. The DW first nucleated at a low magnetic field, and was then depinned when the field was further increased. The depinning field (Hdep) was significantly modified by the different current directions. Furthermore, as shown in Fig.1(b), a linear relationship exists between the variations of the Hdep from the opposite current directions with the same current density(ΔHdep) and the absolute differences of the current densities(ΔJapp). Based on the 1D model of CIDM[1], the ratio of ΔHdep to ΔJapp can be regarded as the ξSTT. Therefore, the ξSTT can be obtained from the slope of ΔHdep vs. ΔJapp. The estimated value of the ξSTT is 27.6±0.9Oe-cm2/MA, corresponding to 1.56 for β (polarization=0.8 was used). This high ξSTT measured by the depinning field shift is almost identical to the value measured by the thermal activated method[2], and is about 1000 times larger than th e NiFe[3] case.
References
[1] A. Thiaville, Y. Nakatani, J. Miltat, and Y. Suzuki, Europhys. Lett. 69, 990 (2005) [2] C. Burrowes, A. P. Mihai, D. Ravelosona, J.-V. Kim, C. Chappert, L. Vila, A. Marty, Y. Samson, F. Garcia-Sanchez, L. D. Buda-Prejbeanu, I. Tudosa, E. E. Fullerton and J.-P. Attané, NATURE PHYSICS 6, 17 (2009) [3] M. Hickey, D. Ngo, S. Lepadatu, D. Atkinson, D. McGrouther, S. McVitie, and C. Marrows, Appl. Phys. Lett. 97, 202505 (2010)
3:42 PM
DC-10. Toward ultrafast current assisted domain wall motion
Elena Murè, Jeroen H. Franken, Sjors J. Schellekens, Henk J. Swagten and Bert Koopmans
Applied Physics, Tu/e, Eindhoven, Netherlands
The use of magnetic domain walls (DWs) to store information and the possibility to manipulate them via a charge current is a very attracting topic, also due to its potential application to data storage and logic devices. This work aims at exploring DW motion in the limit of ultrashort (<100ps) current pulses (CPs), thanks to the use of a pump-probe optical technique. The use of ultrashort CPs allows for much higher current densities and provides access to detailed dynamics of depinning processes. The measurements are done on a Pt(4nm)/Co(0.5)/Pt(2nm) stack, patterned in a 100x1μm2 wire (see Fig.1 (a)). A low dose focused Ga+ beam is used to irradiate part of the wire, inducing an anisotropy discontinuity which permits a DW to be nucleated and pinned [1]. The magnetic wire is connected in series with an Auston switch, on which is shone a pulsed Ti:Sapphire laser, creating CPs of up to 3 1013A/m2 intensity and about 100ps duration. The DW depinning under both Hext and CP is detected monitoring the Kerr signal. The results show a high efficiency of CPs in varying the depinning field (ΔHdep up to 70%, see Fig.1 (b)) and present a sign inversion for opposite current direction, excluding a relevant role of Joule heating. Moreover, the data evidence intriguing features such as the sign inversion of ΔHdep for inversion of the DW chirality. Finally, we will present detailed 1D simulation of CP assisted, field induced DW depinning from an anisotropy barrier. Results on the efficiency of CP and continuous current for different pinning potentials will be discussed.
References
[1] R. Lavrijsen, J.H. Franken, J.T. Kohlhepp, H.J.M. Swagten and B. Koopmans, Appl. Phys. Lett. 96, 222502 (2010)
3:54 PM
DC-11. Roles of field and current in thermally activated domain wall motion in submicron-wide magnetic strips with perpendicular magnetic anisotropy
Satoru Emori and Geoffrey S. Beach
Materials Science and Engineering, MIT, Cambridge, MA
We have experimentally studied μm-scale domain wall (DW) motion driven by magnetic field and electric current in submicron-wide Co/Pt multilayer strips with perpendicular magnetic anisotropy. The thermal activation energy for DW motion, along with its scaling with driving field and current, has been extracted directly from the temperature dependence of DW velocity. In a symmetric Pt/(Co/Pt)3 strip with a Co layer thickness of 0.6 nm, the injection of DC current results in an enhancement of DW velocity independent of current polarity, but produces no measurable change in the activation energy barrier. Through this analysis, the observe d current-induced DW velocity enhancement in this Co/Pt multilayer strip can be entirely and unambiguously attributed to Joule heating.
4:06 PM
DC-12. Manipulation of domain walls using a spin-polarized scanning tunneling tip
Robert Wieser, Thim Stapelfeldt, Elena Y. Vedmedenko and Roland Wiesendanger
University of Hamburg, Hamburg, Germany
A theoretical concept for the local manipulation of domain walls is introduced [1]. In the described procedure, a transverse domain wall is driven by a spin-polarized current induced by a magnetic tip, as used in a spin polarized scanning tunneling microscope [2]. The tip is placed above the magnetic nanostripe and moves with a constant velocity along the long stripe axis. The current is flowing through the vacuum barrier perpendicular to the stripe plane. The angular momentum from the spin-polarized current exerts a torque on the magnetic moments underneath the tip and leads to a displacement of the domain wall. Particularly, the manipulation of ferromagnetic as well as antiferromagnetic 180° transverse domain walls has been studied by means of spin dynamics simulations (solving the Landau-Lifshitz-Gilbert equation) and Monte Carlo calculations. Different relative orientations of the tip and sample magnetization have been considered. Additionally, the possibility of switching the transverse domain wall component using a spin-polarized scanning tunneling tip will be demonstrated.
References
[1] T. Stapelfeldt, R. Wieser, E. Y. Vedmedenko, and R. Wiesendanger, Phys. Rev. Lett. in Press [2] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009)
4:18 PM
DC-13. Field Frequency Tuning of the Velocity of Geometrically Confined Domain Walls
Madalina Negoita, Thomas J. Hayward and Dan A. Allwood
Department of Material Science and Engineering, University of Sheffield, Sheffield, United Kingdom
Ultra-cold atoms have great potential as “bits” in quantum information processing applications, and hence there has been a great deal of research into methods by which their position and motion can be controlled. To this end, we have previously proposed creating mobile atom “traps” above domain walls in ferromagnetic nanowires by cancelling the stray field they create with an external applied field [1]. For traps formed using domain walls in Permalloy (Ni80Fe20) nanowires one of the main inhibiting factors in achieving the transport of trapped atoms is the high field-induced velocities of the domain walls (e.g. 800 m/s for 50 Oe) [2], which can cause atoms to be lost from the trap. However, by changing the geometry of the nanowire from that of a straight wire to that of a ring and applying a radial magnetic field we confine the walls to a geometrically defined potential. Rotating the field then allows us to rotate this potential well around the ring, and thus achieve sufficiently low domain wall velocities to keep atoms trapped. In this paper we present an experimental study of the influence of the frequency of the rotating applied field on the domain wall velocity. We find that, while in general the domain walls follow the field, due to defects in nanostructure they slightly lag behind it. As the field frequency is increased, the lag also increases until a point is reached where the walls become pinned by defects and can no longer thermally depin within the experiment’s characteristic time-scale. The frequency at which this occurs depends on the amplitude of the applied field.
References
[1] D.A. Allwood et al, Applied Physics Letters 89 (2006) 014102 [2] D. Atkinson et. al, Nature Materials 2 (2003) 85-87
1:30 PM - 4:30 PM, Grand Canyon 9-11
Co-Chair: Chih-Huang Lai, National Tsing Hua University; Co-Chair: Kazuhiro Hono, NIMS
1:30 PM
DD-01. Magnetic Anisotropy in FePt - Effect of Chemical Disorder and Lattice Distortion
Cecilia J. Aas1, Laszlo Szunyogh2 and Roy W. Chantrell1
1Dept of Physics, University of York, York, United Kingdom; 2Dept of Theoretical Physics, Budapest University of Technology and Economics, Budapest, Hungary
Due to its high magnetocrystalline anisotropy (MAE), L10 FePt is of considerable interest to the development of ultrahigh density magnetic recording applications and spintronics devices. The progress relies on devising a complete first principles model of FePt to be used in generating effective spin Hamiltonians for atomistic and multiscale modelling approaches. Amongst many other issues, this requires an understanding of the role of various sources of disorder such as interfacial effects or chemical disorder. The aim of this work is to investigate in detail the effect of chemical order on the MAE of FePt by means of KKR-CPA calculations (cf [1, 2]). The main goal is to shed light on the recent experimental observations [3], which revealed correlations between the MAE and interfacial disorder (quantified by a lattice mismatch parameter of an FePt thin film layer deposited on a number of different substrates). We use KKR-CPA to calculate the MAE at zero-Kelvin temperature for each of the FePt samples in [3]. Our results are in promising agreement with experiment in that the relative difference between samples is correct. We assume that the magnitude discrepancy arises from a combination of temperature- and chemical disorder effects. The former we estimate as a factor of two reduction [4] and the second is introduced by fitting the KKR-CPA MAE to experiment using the chemical order, s, as a fitting parameter. The results are in agreement with s ∼ √(I001/I002), where Ixyz denotes diffraction intensity [3] from the (xyz) plane.
References
[1] S. Okamoto et. al., Phys. Rev. B 66 (2002) 024413
[2] J.B. Staunton et. al., J. Phys.: Condens. Matter 16 (2004) S5623
[3] Y.F. Ding et.al., J. App. Phys. 97 (2005) 10H303
[4] O.N Mryasov et.al., Europhys. Lett. 69 (2005) 805
1:42 PM
DD-02. Electronic structure and magnetic anisotropy in FePt alloys out of L10 ordering
Yohei Kota and Akimasa Sakuma
Department of Applied Physics, Tohoku University, Sendai, Japan
In this work, we investigate the effects of L10 ordering and tetragonal distortion on the magneto-crystalline anisotropy energy of FePt alloys, since the large perpendicular magnetic anisotropy of L10 type FePt is quite sensitive to them in the experimental work [1] and theoretical one [2]. Recently we had developed the computational way to calculate the electronic structure of disordered alloys taking into account the spin-orbit interaction in first-principles, using TB-LMTO method based on LSDA [3]. So we carried out the more detail investigation for the magneto-crystalline anisotropy of FePt to obtain the principles to improve the material property. Figure 1 shows the obtained results of the uniaxial magneto-crystalline anisotropy energy (Ku) of L10 type FePt as a function of the axial ratio (c/a) and the order parameter (S). We find that Ku increases with c/a, which is similar to the L10 ordered FePt we previously reported [4], so the tetragonal distortion is an important factor. On the other hand, Ku drastically decreases with S from 16.3×106 (J/m3) in ordered (S=1.0) one [4]. The obtained results imply that S is more dominant factor than c/a for Ku of L10 type FePt. Also we will show the anisotropy of the orbital magnetic moments and discuss about the relations with Ku.
References
[1] S. Okamoto, et.al., Phys. Rev. B 66, 024413 (2002). [2] J. B. Staunton, et.al., J. Phys.: Condens. Matter 16, S5623 (2004). [3] I. Turek, et.al., Philosophical Magazine 88, 2787 (2008). [4] A. Sakuma, J. Phys. Soc. Jpn 63, 3053 (1994).
1:54 PM
DD-03. Tailoring Magnetocrystalline Anisotropy of FePt by applied strain: first principles calculation
Pavel Lukashev1, 3 and Renat F. Sabirianov2, 3
1Physics and Astronomy, University of Nebraska - Lincoln, Lincoln, NE; 2Physics, University of Nebraska at Omaha, Omaha, NE; 3Nebraska Center for Materials and Nanoscience, University of Nebraska - Lincoln, Lincoln, NE
We report results of first principles calculations on strain assisted reduction in magnetocrystalline anisotropy of FePt. Our calculations show a gradual decrease of magnetocrystalline anisotropy (MAE) with the application of tensile biaxial strain in ab-plane, and increase of anisotropy with application of compressive strain. We find that MAE decreases by 21% with application of 1.5% tensile biaxial strain. The reduction of MAE is primarily due to the change of the c/a ratio and to some extent due to the increase in volume. We propose building bilayers (or heterostructures) of FePt and piezoelectric film. This structure is expected to have control of anisotropy constant by applying electric field to the system. We also propose forming medium using bilayer of FePt and soft magnetic material with the gradient of anisotropy constant related to the strain gradient in the system. These devices could be used as magnetic recording medium and in sensor applications.
2:06 PM
DD-04. Magnetic anisotropy and order parameters for L10 type FePt polycrystalline films with (001) preferred grain orientation
Daisuke Inoue1, 2, Takehito Shimatsu1, Yuki Inaba1, 2, Hajime Aoi1, Satoshi Okamoto3 and Osamu Kitakami3
1RIEC, Tohoku University, Sendai, Japan; 2Fuji Electric Co., Ltd., Matsumoto, Japan; 3IMRAM, Tohoku University, Sendai, Japan
Uniaxial magnetic anisotropy, Ku, for L10 type FePt polycrystalline films with (001) preferred grain orientation was examined in relation to the degree of ordering and lattice deformation, and was compared to the result for single crystalline films. The left figure presents Ku for various series of FePt films (10 nm thickness) that were fabricated using UHV low-temperature sputter film deposition [1] as a function of substrate temperature, Ts. For polycrystalline films deposited with MgO underlayers on glass disks, Ku reached 3.0 × 107 erg/cm3 at Ts=300 °C, which was almost identical to that for the single crystalline film deposited with a Pd(001) underlayer on MgO(001) substrate at Ts=250 °C. The right figure presents values of the order parameter, S, as calculated using diffraction intensities for the L10-(002) and L10-(001) planes [2]. The values of S for polycrystalline films were nearly twice those of single crystalline films. It is noteworthy that S for polycrystalline films was almost saturated to 1.0 for Ts higher than 350 °C. However, Ku was 3.3—3.5 × 107 erg/cm3, which were smaller than those previously reported [3,4]. XRD analysis revealed that the lattice volumes of L10-FePt polycrystalline films were a few percent larger than those for bulk values, suggesting that lattice deformation plays an important role in reducing Ku, as similar to those for hcp-Co films and hcp-Co-Pt alloy films [5]. This work was supported in part by Research and Development for Next-Generation Information Technology of MEXT, and SRC.
References
[1] T. Shimatsu, Y. Inaba, H. Kataoka, J. Sayama, H. Aoi, S. Okamoto, and O. Kitakami, J. Appl. Phys., 109, 07B726(1-3) (2011); [2] B. E. Warren, in X-Ray Diffraction (Addison-Wesley, Reading, MA, 1969) p. 208; [3] R. F. C. Farrow, D. Weller, R. F. Marks, and M. F. Toney, J. Appl. Phys., 79, 5967 (1996); [4] H. Kanazawa, G. Lauhoff, and T. Suzuki, J. Appl. Phys., 87, 6143 (2000); [5] T. Shimatsu, Y. Okazaki, H. Sato, H. Muraoka, H. Aoi, T. Sakurai, S. Okamoto, O. Kitakami, S. Tanii, and A. Sakuma, J. Appl. Phys., 103, 07F524(1-3) (2008).
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DD-05. L10 ordered phase formation in FePt, FePd, CoPt, and CoPd alloy thin films epitaxially grown on MgO(001) single-crystal substrates
Mitsuru Ohtake1, Shouhei Ouchi1, Fumiyoshi Kirino2 and Masaaki Futamoto1
1Faculty of Science and Engineering, Chuo University, Tokyo, Japan; 2Graduate School of Fine Arts, Tokyo National University of Fine Arts and Music, Tokyo, Japan
FePt, FePd, CoPt, and CoPd alloy thin films with high Ku have been investigated for applications like magnetic recording media, MRAM devices, etc [1,2]. Thin film formation with high degree of L10 ordering and controlling of c-axis direction of the crystal structure are the important issues for practical applications of these magnetic thin films. The film structure and the magnetic property vary depending on the magnetic material and the deposition condition. In the present study, FePt, FePd, CoPt, and CoPd epitaxial films were prepared on MgO(001) single-crystal substrates at temperatures ranging between RT and 600 °C under similar experimental conditions by using an UHV RF magnetron sputtering system. The growth process, the structure, and the magnetic properties of these films were compared. FePt, FePd, and CoPt epitaxial films with L10 ordered structure were obtained at temperatures higher than 400 °C, whereas ordered phase formation was not observed for CoPd films. The epitaxial orientation relationships were determined by RHEED and φ-scan XRD analyses. L10-FePd(001) single-crystal films with c-axis normal to the substrate surface were formed, whereas FePt and CoPt epitaxial films included L10(010) and L10(100) crystals whose c-axes were parallel to the substrate surface in addition to L10(001) crystals. Fig. 1 shows the out-of-plane and the in-plane XRD spectra and the summary of crystallographic properties of FePt, FePd, CoPt, and CoPd films grown on MgO substrates at 600 °C. Higher degree of ordering is observed in the order of FePd > FePt > CoPt. The ordering was promoted by introducing thin seedlayers on MgO substrates for FePt, FePd, and CoPt films.
References
[1] Y. K. Takahashi, K. Hono, T. Shima, and K. Takanashi: J. Magn. Magn. Mater., 267, 248 (2003). [2] V. Gehanno, A. Marty, B. Gilles, and Y. Samson: Phys. Rev. B, 55, 12552 (1997).
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DD-06. Granular L10 FePt-SiNx-C nanocomposite films with large coercivity and small isolated grains for perpendicular recording application
Kaifeng Dong1, Hui hui Li1, Ying Guo Peng2, Ganping Ju2, Gan Moog Chow1 and Jingsheng Chen1
1Department of Materials Science and Engineering, National University of Singapore, Singapore, Singapore; 2Seagate Techonology, Fremont, CA
L10 FePt films with high anisotropy have recently received considerable attention due to its potential application for future ultrahigh density magnetic recording. The key challenge for practical application is to obtain well isolated, small FePt grain with (001) texture. In this work, well isolated granular L10 [FePt (4nm)-SiNx 40 vol.%]-C y vol.% (y = 0, 10, 20) nanocomposite films were fabricated on TiN/CrRu/glass substrate by cosputtering FePt, C and Si3N4 targets at 380oC. The XRD spectra showed that FePt (001) texture remained unchanged with various C doping concentrations, and the calculated ordering parameter S increased with increasing C doping. With increasing C doping concentration from 0% to 20%, the out-of-plane coercivity increased from 20.8 kOe to 24.7 kOe and the solpe of hysteresis loop at coercivity decreased (Fig.1a), indicating an improved exchange decoupling. The increase of the coercivity was due to the exchange decoupling of FePt grains and increasing chemical ordering with increasing C doping. Well isolated grains with clear grain boundaries were observed in TEM images. When the doping concentration of C was increased from 0 to 20 vol.%, the grain size was reduced from 11.5 to 5.6 nm and grain size distribution became narrowed (Fig. 1b). Cross-sectional High-Resolution TEM image showed that one-layer structure was formed and all grains were fct-FePt(001) oriented, which implied that the TiN intermediate layer was favored for smaller grain with good (001) texture.
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DD-07. Addition of Au to reduce ordering temperature of very thin Fe/Pt bi-layered films on MgO underlayer
Makoto Tanaka, Koichi Murata and Shigeki Nakagawa
Dept. of Physical Electronics, Tokyo Institute of Technology, Tokyo, Japan
L10 -FePt ordered alloy thin films with (001) preferential orientation a re regarded as hopeful candidates for high density energy assisted magnetic recording media. We have successfully fabricated 2 nm-thick L10 -FePt ordered alloy thin films with (001) preferential orientation by using MgO (001) underlayer on a very thin Fe buffer layer with annealing under hydrogen atmosphere, but its process temperature was as high as 500 °C[2]. In this study, addition of Au to the Fe[1 nm]/Pt[1 nm] bilayerd films were performed to reduce ordering temperature. Au[X nm](X=0.2,0.5)/Fe[1 nm]/Pt[1 nm] trilayerd films deposited on Fe[5 nm]/MgO[80 nm] bilayered underlayer were annealed in order to improve magnetization characteristics and reduce process temperature. Fig.1 shows X-ray diffraction diagram of FePt film and FePtAu films fabricated from Fe[5 nm]/MgO[80 nm]/Au[X nm]/Fe[1 nm]/Pt[1 nm] layers annealed at 600 °C. FePtAu(X=0.2) showed stronger FePt(001) peak than FePt. This result may be attributed to an increase of domain wall pinning site caused by diffusion of Au to the grain boundaries of FePt grains[3]. The out-of-plane hysteresis loops (of Au[0.2 nm]/Fe[1 nm]/Pt[1 nm] film) told that the film consists of FePt-base crystallites with their easy axis perpendicular to the film plane. As a result, addition of Au to FePt film is very effective to decrease ordering temperature of L10 -FePt ordered alloy from 500 °C to 450 °C.
References
[1] M. Tanaka, Y. Ogata and S. Nakagawa, J. Appl. Phys., vol.109, Issue7 (2011). [2] F. T. Yuan, S. K. Chen, W. M. Liao, C. W. Hsu, S. N. Hsiao and W. C. Chang, J. Magn. Magn. Mater., vol. 304, pp. 109-111 (2006).
2:54 PM
DD-08. Accelerating phase transformation of L10 FePt by pre-formation of AgPt meta-stable phase
Wei-Chih Wen and Chih-Huang Lai
MSE, National Tsing Hua University, Taiwan, Hsin-chu, Taiwan
The mean free diffusion length of Pt is null around 350°C [1], therefore, the formation of L10 FePt phase at low temperature is significantly suppressed. In this work, we prepared Fe/AgPt and Fe/Pt films at room temperature and then annealed samples at 350°C. The AgPt phase is a meta-stable phase because of the positive mixing enthalpy (ΔHmix). This meta-stable AgPt phase is decomposed at low temperature, and the Ag atoms quickly move out due to their high mobility, resulting in the Pt layer containing a large number of vacancies inside. These vacancies play two roles during low temperature annealing process; first, to drive Fe atoms in adjacent layers to diffuse into the Pt layer to form FePt bonding; second, to slightly modify the positions of Pt atoms to accommodate the formation of L10 lattice. We used in-situ heating XRD to monitor the phase transformation of Fe/AgPt films, and verified the decomposition of the AgPt phase, accompanying the formation of the FePt L10 Phase. The intermixing of Fe and Pt at 350°C mediated by the decomposition of the AgPt phase is also observed from the TEM analysis (shown in Fig. 1). The sample of (Fe/Pt) still shows distinct two layers after 350°C annealing; on the other hand, only one layer with the FePt L10 phase is observed for the (Fe/AgPt) sample. The out-of-plane coercivity of 13.3 kOe was obtained after annealing [Fe (4.5 Å)/AgPt (6.0 Å)]6 multilayers at 350°C.
References
[1] B. Rellinghaus, E. Mohn, L. Schultz, T. Gemming, M. Acet, A. Kowalik and B. F. Kock, Ieee Transactions on Magnetics 42 (10), 3048-3050 (2006).
3:06 PM
DD-09. Magnetic reversal characteristi cs of L10-FePt dots
Jung-Wei Liao1, Ju-Cheng Hsiao1, Dustin Gilbert2, Yen-Chun Huang1, Hao-Cheng Hou1, Liang-Wei Wang1, I-Yun Liu1, Kai Liu2 and Chih-Huang Lai1
1Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan; 2Physics Department, University of California, Davis, CA
Morphology can significantly influence magnetic reversal behaviors of patterned films. We first prepared 5 nm (001)-L10 FePt films by annealing (Fe/Pt) multilayers at 350 0C [1]. To obtain isolated or interconnected FePt dots, we directly grew an anodized aluminum oxide (AAO) template on top of FePt films. Hysteresis loops of FePt films were unchanged before and after the fabrication of AAO templates. A Ta film was deposited into the pore of the AAO template and was used as an etching mask. FePt dots with an average diameter of 40 nm were obtained, as shown in the SEM image of Fig. 1(a). By adjusting etching time, 5-nm-thick films were first patterned into films with bump dots on top, that is, dots were interconnected, and eventually into isolated dots, as illustrated in Fig. 1(b). The dot-thickness dependence of coercivity shows similar trends for the experimental and OOMMF simulation results, as shown in Fig. 1 (c). For dots with 1-3 nm thickness, the magnetization reversal is dominated by domain nucleation and growth; the decreased thickness of the etched film reduces the coercivity. As the dot thickness is increased from 3 to 5 nm, dots become isolated-like and the lateral exchange coupling is reduced; rotation takes place for magnetization reversal, and hence the coercivity is increased. The coercivity discrepancy of patterned films between simulations and experiments may be partially ascribed to the incomplete ordering of the L10 phase due to low-temperature annealing. Furthermore, the significant increase in experimentally observed coercivity for patterned films may result from induced domain wall pinning sites during the etching process, which were not taken into account in simulations.
References
[1] Y. C. Wu, L. W. Wang, and Chih-Huang Lai, Appl. Phys. Lett. 91, 072502 (2007).
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DD-10. Withdrawn
3:30 PM
DD-11. Core-Shell type L10- FePt/A1-FePt and L10-FePt/Co exchange spring nanocomposites
Dimitris G. Niarchos1, Th. Speliotis1, George Gainnopoulos1 and George Hadjipanayis2
1Institute of Materials Science, NCSR Demokritos, Aghia Paraskevi, Attikis, Greece; 2Physics and Astronomy, University of Delaware,, NEWARK, DE
The exchange-spring principle identifies a route for creating high-performance permanent magnets from existing materials. In an exchange-spring magnet, nano-scale hard and soft magnetic phases are coupled via interfacial exchange interaction such that the soft phase becomes “hardened” and its high magnetization enhances the energy product (BH)max of the composite. The hard/soft multilayer is simultaneously a model system to study and understand the properties of exchange-spring systems, and a promising candidate for the design of thin-film permanent magnets. Many experimental works have followed the theoretical models. The material most extensively employed for the hard phase is SmCo, but also NdFeB and, more recently, L10 materials like CoPt and FePt have been used. As to the soft phase, Fe, Co or Ni80Fe20 are usually employed. In a few recent works, L10 materials have been chosen as the hard phase to prepare hard/soft bilayers (1,2) of the type FePt or CoPt or CoPt/FePt (3). The exchange spring behaviour of CoPt/Co bilayers, where the effect of thermal treatments on the interphase exchange is considered in detail [4]. A core-shell type L10 FePt/Fe exchange coupled nanocomposite was recently studied by (5). In this work, in order to study the exchange coupling of hard/soft -type nanocomposites, we selected as the hard phase the L10-FePt deposited at 500-700 0C with a thickness of 3-5 nm by sputtering on single crystal MgO substrate, thus producing nanoparticles of an average lateral size of 10-20 nm due to surface tension. On top of these nanoparticles we deposited by sputtering single layers of either soft A1-FePt (at room temperature) or Co of thicknesses varying from 5-12 nm. The morphology of the single and bilayers were checked with SEM, AFM, TEM and the magnetic properties with MFM and SQUID measurements. The coercivity of the hard L10-FePt/ Soft (FePt or Co) decreases drastically as the thickness of the cap soft layer increases, consistent with the theory and this is due to the increase of domain wall width in the core-shell type nanocomposites.
References
1. Crew, D. C. et al. J. Magn. Magm. Mat 233, 3, 257-273 (2001) 2. Kim, J. et al. Acta Mater. 51 (2003) 313 3. Alexandrakis V, et al, J. Appl. Phys. 107, 013903 (2010) 4. Lewis L.H et al, J. Phys D: Appl. Phys. 37, 2638 (2004) 5. Ma, B. et al. , J. Appl. Phys. , 109, 083907 (2001)
3:42 PM
DD-12. Crystallographic study of high Ku metastable Co50Pt50 ordered structure
Fu-Te Yuan1, Jen-Hwa Hsu1, Yi-Hung Lin2, S. N. Hsiao3 and C. S. Ku3
1Physics, National Taiwan University, Taipei, Taiwan; 2Materials Science & Engineering, National Tawian University, Taipei, Taiwan; 3National Synchrotron Radiation, Hsin-Chu, Taiwan
A metastable phase of Co50Pt50 alloy with high perpendicular magnetic anisotropy constant (> 1x107 erg/cm3 ) has previously been observed in the 100nm-thick sample that was epitaxially grown on the MgO (111) surface [1]. The unit cell was identified as rhombohedral, comprising alternative stacked close-packed atomic planes of Co and Pt in the (111) normal direction, with the sequence ABCABC. The sputter-deposited samples with reduced thickness yielded significant broadening of superlattice peak [2]. A microstructural investigation revealed the presence of high-density planar defects in the films [3]. Although x-ray diffraction results confirm the superlattice structure of CoPt, the following question arises. Could planar defects modify the stacking sequence to form hexagonal symmetry? This question can be answered directly by characterizing the lattice symmetry. A proposed model wroking on fcc-hcp transformation, called l-scan, can be used [4]. It has been successfully utilized to separate the hexagonal phase from the cubic matrix in Co3Pt [5]. In this study, the l-scans along the (101)hcp diffraction rod were performed on 20nm and 50nm-thick CoPt thin films that were deposited on MgO(111) substrates at various temperatures by sputtering. Fig. 1 displays selected results. The sample prepared at 200°C had a disordered A1 phase and it transformed into a metastable ordered structure at 400°C. In the l-scan profile, diffraction from the hexagonal structure (ABAB stacking) occurs at l = 1, 2, 3, and 4; peaks that are associated with t he fcc phase (ABC stacking) are located at 1±1/3 and 3±1/3. Fig. 1 shows that the stacking sequence of the CoPt metastable phase is ABC, with no detectable hexagonal component. The results confirm the rhombohedral L11 structure. Films with different thicknesses and growth temperatures yielded similar results.
References
[1] S. Iwata, S. Yamashita, and S. Tsumashima, IEEE Trans. Magn. 33, 3670 (1997). [2] F. T. Yuan, A. C. Sun, and J. H. Hsu, Script. Mater. 62, 762 (2010). [3] F. T. Yuan, A.C. Sun, J.H. Hsu, C. S. Tan, P. C. Kuo, W. M. Liao, and H.Y. Lee, J. Appl. Phys. 08, 113909 (2010). [4] M. T. Sebastian and P. Krishna, Phys. State. Sol. a, 101, 329 (1987). [5] G. R. Harp, D. Weller, T. A. Rabedeau, R. F. C. Farrow, and M. F. Toney, Phys. Rev. Lett. 71, 2493 (1993).
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DD-13. Fabrication of L10-MnAl perpendicularly magnetized thin films for perpendicular magnetic tunnel junctions
Masaki Hosoda1, Mikihiko Oogane1, Miho Kubota1, Takahide Kubota2, Haruaki Saruyama1, Satoshi Iihama1, Shigemi Mizukami2, Hiroshi Naganuma1 and Yasuo Ando1
1Department of Applied Physics, Tohoku Univ., Sendai, Japan; 2WPI-AIMR, Tohoku Univ., Sendai, Japan
Magnetic tunnel junctions using perpendicularly magnetized materials (pMTJs) have great potential with respect to the development of ultra-high-density spin transfer switching type of magnetic random access memory (Spin-RAM). According to theoretical expression, a switching current density (Jc0) is directly related to saturation magnetization (Ms) and Gilbert damping constant (α). In order to achieve high density and low power consumption for Spin-RAM, ferromagnetic materials with large perpendicular anisotropy energy (Ku), small Ms and small α are required. Recently, there have been several studies on perpendicularly magnetized materials such as FePt1), [CoPt/Pd]x2)3), CoFeB4). However, all of them have large Ms of over 1000 emu/cc. In addition, FePt shows large α of 0.065) and CoFeB has small anisotropy of 106 erg/cc. Recently, Mn-Ga alloys have been demonstrated to possess very promising properties, a low Ms, high Ku, and low α6)-8). Here, we focus on a MnAl alloy, which exhibits low Ms and high Ku, similar to the Mn-Ga alloys. The purpose of this study is to fabricate perpendicularly magnetized L10-MnAl thin films for pMTJ application.
The sample structure prepared was MgO (001) single crystal substrate / Cr (40 nm) / MnAl (50 nm) / Ta (5 nm) by a magnetron sputtering system with a base pressure of below 1×10-7 Pa. The composition of the sputtering target was Mn48Al52. The substrate temperature (Ts) was varied from RT to 500oC. Post annealing temperature (Ta) was varied from RT to 600oC. Surface morphology, crystal structure, α and magnetic properties were measured by AFM, XRD, FMR and VSM, respectively.
We have succeeded in obtaining perpendicular magnetized MnAl films in the range of Ts=200oC to 500oC and Ta=300oC to 500oC. The optimized fabrication condition is Ts=200oC and Ta=500oC. We have successfully obtained low Ms and high Ku of 580 emu/cc and 8×106 erg/cc, respectively, for the optimized film. In addition, small magnetic damping α of 0.006 was observed for the film with Ts=400oC, Ta=400oC. The developed MnAl film with low Ms, high Ku and low α is one of the ideal candidates for the electrodes of pMTJs.
This study was supported by FIRST program.
References
1) T. Moriyama, et al., JAP 95, 1669212 (2004) 2) K. Yakushiji, et al., APL 97, 232508 (2010) 3) K. Mizunuma, et al., APL 95, 232516 (2010) 4) S. Ikeda, et al., Nature Mat. 9, 952-954 (2010) 5) S. Mizukami, et al., APL 98, 052501 (2011) 6) F. Wu, et al., APL 94, 122503 (2009) 7) S. Mizukami, et al., PRL 106, 117201 (2011) 8) T. Kubota, et al., APEX 4, 043002 (2011)
4:06 PM
DD-14. Thickness-Dependent Magnetic Anisotropy in Ferromagnetic L10 MnAl Thin Films
Yishen Cui1, Wenjing Yin2, Jiwei Lu2 and Stuart A. Wolf1, 2
1Department of Physics, University of Virginia, Charlottesville, VA; 2Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA
This talk reports the epitaxial growth of ferromagnetic τ phase MnAl(L10)1 thin films on MgO(001) using a Biased Target Ion Beam Deposition technique2. To enhance the crystallinity of the τ phase, alternating Al/Mn quasi-monolayer deposition was developed. By optimizing the growth parameters and annealing conditions, single τ phase MnAl films (<10nm) were obtained. Both the fundamental peak (002) and the superlattice peak (001) of τ-MnAl were observed in the XRD 2θ scan. φ scans on the τ-MnAl (011) peak and the MgO (022) peak indicated the epitaxial growth of τ-MnAl on single crystal MgO substrates with an 45o rotation between the (001)τ-MnAl and (002)MgO. MS was improved to 394 emu/cc in comparison with that of co-sputtered thin films (174 emu/cc). The uniaxial anisotropy increased from negative to positive as the film thickness decreased, indicating the crystalline magnetic anisotropy field was rotated from in-plane to perpendicular. The change of the magnetic anisotropy was ascribed to its dependence on the lattice tetragonal distortion3, which varied as a function of film thickness.
References
1. C. Yanar, et al. , Met. and Matls. Trans. A, 33A, 2413 (2002). 2. K. G. West, et al., J. Vac. Sci. Technol. A, 26(1),133(2008). 3. A. Sakuma, J. of the Phys. Soc. of Jpn, 63, 1422(1994).
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DD-15. Ab initio study on magnetic anisotropy of L10-ordered alloy FeNi
Sho Ozaki1, Yasushi Kuwahara1, Masahito Tsujikawa2, Yoshio Miura1, 2, Kazutaka Abe1, 2 and Masafumi Shirai1, 2
1Research Institute of Electrical Communication, Tohoku University, Sendai, Japan; 2Center for Spintronics Integrated Systems, Tohoku University, Sendai, Japan
Ferromagnetic materials with a large uniaxial magneto-crystalline anisotropy have attracted much attention for their application to high density magnetic storage media because of thermal stability of the magnetization even in a nanometer-scale volume. One of possible materials with large magneto-crystalline anisotropy is L10-ordered alloy such as FePt, CoPt and CoCrPt. These alloys, however, contain a rare metal element Pt, and thus it is worth while seeking for alternative materials. A promising candidate is a L10-ordered alloy FeNi, which has been fabricated as thin films successfully by using alternative monatomic layer deposition technique [1, 2]. It has been observed that the degree of order and the magneto-crystalline anisotropy energy (MAE) of the FeNi thin films depends on the sort of buffer layers [1, 2]. We investigated the magneto-crystalline anisotropy of the L10-ordered alloy FeNi theoretically on the basis of first-principles density-functional calculations. First, we have evaluated the uniaxial MAE of L10-FeNi from the total-energy difference between the cases where the magnetization is along [001] and [100] directions. We found that the magnetization of L10-FeNi prefers the [001] direction with the MAE being 5.3×106 erg/cm3 for the equilibrium lattice constants, a = 0.355 nm and c = 0.358 nm. This value is comparable with the experimental observations; i.e. 6.3×106 erg/cm3 [1] and 7.0×106 erg/cm3 [2]. We also examined the dependence of the MAE on the lattice constants of L10-FeNi. The MAE decreases with increasing the in-plane lattice constant and it becomes 2.0×106 erg/cm3 for a = 0.373 nm (c/a = 0.92). On the contrary, the MAE increases with increasing the in-plane lattice constant and it exceeds 1×107 erg/cm3 for a < 0.334 nm (c/a > 1.14). This result suggests that the compressive stress arising from the lattice mismatch with the buffer layer is favorable for achieving large MAE of L10-FeNi alloy. This work was partly supported by a Grant-in-Aid for Scientific Research in Priority Area “Creation and Control of Spin Current” (Grant No. 19048002) from MEXT, and by a Grant-in-Aid for Scientific Research (A) (Grant No. 22246087) from JSPS.
References
[1] T. Shima et al., J. Magn. Magn. Mater. 310, 2213 (2007). [2] M. Mizuguchi et al., J. Appl. Phys. 107, 09A716 (2010). [3] S. Baroni et al., available online: http://www.pwscf.org.
1:30 PM - 4:30 PM, Grand Canyon 2-3
Chair: Paul Crowell, U.Minnesota
1:30 PM
DE-01. Spin-dependent transport properties of fully epitaxial magnetic tunnel junctions of CoFe/MgO/CoFe ultrathin layer/Co2MnSi
Hongxi Liu1, Tomoyuki Taira1, Yusuke Honda1, Ken-ichi Matsuda1, Tetsuya Uemura1, Masafumi Yamamoto1, Yoshio Miura2 and Masafumi Shirai2
1Division of Electronics for Informatics, Hokkaido Univ., Sapporo, Japan; 2Research Institute of Electrical Communication, Tohoku Univ., Sendai, Japan
An epitaxial heterostructure consisting of a half-metallic Heusler-alloy electrode and a MgO barrier is promising as a basic building block of spintronic devices. We recently demonstrated high tunnel magnetoresistance (TMR) ratios of 1135% at 4.2 K and 443% at 293 K for fully epitaxial (from the lower side) Co50Fe50 (CoFe)/MgO/CoFe (1.1 nm)/Co2MnSi (CMS) MTJs with an ultrathin CoFe (1.1 nm) insertion layer;1 these ratios are higher than the 1049% at 4.2 K and 335% at 293K obtained for CoFe/MgO/CMS MTJs1 and the 375% at 4.2 K and 255% at 293 K obtained for CoFe/MgO/CoFe MTJs. Our purpose in the present study was to investigate the spin-dependent transport properties of CoF e/MgO/CoFe (1.1 nm)/CMS MTJs through tunneling spectroscopy. The dI/dV (= G) vs. V characteristics for the parallel (P) magnetization configuration (Gp spectra) showed a pronounced peak structure at V = ~ -0.23 V for negative V at both 4.2 K and 293 K, where the bias voltage V was defined with respect to the lower electrode. In contrast, CoFe/MgO/CMS MTJs showed no such peak structure in the Gp spectra. This peak structure for P for V < 0 can be explained by a ballistic transport of minority-spin, Δ5-valence-band electrons from the CMS upper electrode through the ultrathin CoFe (1.1 nm) inserted layer and subsequent tunneling to the minority-spin, Δ5-conduction-band above the Fermi level of the lower CoFe electrode. According to electronic band structure calculations for bulk Co2MnSi and CoFe, the minority-spin transport path through the Δ5 band in the CMS/CoFe (1.1 nm) upper electrode is available for the energy window from -0.22 eV to -0.30 eV. Thus, this feature in the Gp spectra is evidence of the ballistic transport of electrons from the half-metallic CMS electrode in the ultrathin CoFe (1.1 nm) layer. This picture supports the model that the CMS upper electrode facing a MgO barrier indirectly via a CoFe (1.1 nm) layer works as a half-metallic spin-source in a CoFe/MgO/CoFe (1.1 nm)/CMS MTJ. The enhanced TMR ratios at both 4.2 K and 293 K, compared with those of CoFe/MgO/CMS MTJs, are probably due to improved structural quality of the interfacial region of the CMS upper electrode grown on a CoFe layer rather than on a MgO barrier.
References
[1] H. -x. Liu, T. Taira, Y. Honda, K. -i. Matsuda, T. Uemura, and M. Yamamoto, unpublished.
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DE-02. Large Current-Perpendicular-to-Plane Giant Magnetoresistance Effect Using Half Metallic Co2Fe0.4Mn0.6Si Heusler Alloy
Mikihiko Oogane, Jo Sato, Hiroshi Naganuma and Yasuo Ando
Tohoku Univ., Sendai, Japan
Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device is a promising candidate of magnetic read head for high-density hard-disk-drive (HDD). However, the disadvantage of CPP-GMR device is low magnetoresistance (MR) ratio. Recently, several groups have succeeded to observe relatively large MR ratio in CPP-GMR devices with half-metallic Heusler alloy electrodes. [1, 2] However, the MR ratio is not enough for HDD with areal densities of several Tbit/inch2. Among the many Heusler alloys, Co2Fe0.4Mn0.6Si (CFMS) Heusler alloy exhibited a very high spin-polarization in a CFMS/Al-oxide/CoFe MTJ [3]. Therefore, in this work, we investigated the CPP-GMR device with CFMS electrodes to achieve a large MR ratio. The GMR structure of MgO(001)-sub./Cr(20)/Ag(50)/CFMS(20)/Ag(5)/CFMS(3-10)/Ag(3)/Ru(3) (in nm) were prepared by an ultra-high vacuum magnetron sputtering system. The CFMS layers were deposited using an Co45.2Fe9.7Mn16.8Si28.3 alloyed target to obtain stoichiometric Co2Fe0.4Mn0.6Si films. After the deposition of the bottom and top CFMS layers, the films were annealed at 300-500oC to improve the chemical ordering. The films were patterned into CPP-type four terminal device structures using electron beam lithography and Ar ion milling. The MR characteristics were measured by the DC four-terminal method. We confirmed by XRD and STEM measurements that the prepared CPP-GMR films had a high (001)-orientation and very sharp CFMS/Ag interfaces. In addition, the CFMS layers were expected to possess high spi n-polarization, because the CFMS layers had a high L21-ordered structure. The maximum MR ratio of 74.8% has been observed at RT in CPP-GMR device with a 10-nm-thick CFMS upper electrode annealed at 450oC. This is the highest value to date for CPP-GMR devices. Additionally, resistance-area product (RA) of 171.3 mΩμm2 is suitable for the HDD head application. The observed large MR ratio is originated from the large ΔRA value of 128.1 mΩμm2. The large ΔRA is resulted from the large spin asymmetry of CFMS Heusler alloy. Developed CPP-GMR devices with CFMS Heusler alloys are promising for the high-density HDD head. This work was partly supported by SCOPE and Western Digital Corporation.
References
[1] Y. Sakuraba, K. Izumi, T. Iwase, S. Bosu, K. Saito, K. Takanashi, Y. Miura, K. Futatsukawa, K. Abe and M. Shirai, Phys. Rev. B, 82 094444 (2010). [2] T. M. Nakatani, T. Furubayashi, S. Kasai, H. Sukegawa, Y. K. Takahashi, S. Mitani, and K. Hono, Appl. Phys. Lett., 96 212501 (2010). [3] T. Kubota, S. Tsunegi, M. Oogane, S. Mizukami, T. Miyazaki, H. Naganuma, and Yasuo Ando: Appl. Phys. Lett., 94 122504 (2009).
1:54 PM
DE-03. CPP-GMR using Co2Mn(Ga0.25Ge0.75) Heusler alloy
Naoki Hase1, Bollapragada Varaprasad2, Yukiko K. Takahashi2 and Kazuhiro Hono2, 1
1University of Tsukuba, Tsukuba, Japan; 2National Institute for Materials Science, Tsukuba, Japan
Current-perpendicular-to-plane giant magnetoresistant (CPP-GMR) devices with Co-based Heusler alloys attracts recent research interest for read sensor application of next generation ultra-high density magnetic recording system. Nakatani et al. reported the CPP pseudo spin valve (PSV) with only 2.5 nm-thick Co2Fe(Al0.5Si0.5) Heusler alloy can show ΔRA of 5.1 mΩ(μm)2 because of high interfacial scattering assymetry [1]. More recent work on CPP-PSV with Co2Fe(Ga0.5Ge0.5) Heusler alloy reported ΔRA of 9.5 mΩ(μm)2 [2]. These suggests the importance of the selection of ferromagnetic material for obtaining high MR. Our recent point contact Andreev reflection measurements suggested the Co2Mn(Ga0.25Ge0.75) (CMGG) alloy exhibits the highest spin polarization among Heusler alloys so far studied [3]. In this work, we investigate the MR of CPP-PSV using CMGG Heusler alloy. Figure 1 shows the MR of CPP-PSV using 4 nm-thick CMGG whose composition was Co52.4Mn28.0Ga6.1Ge13.5 (composition in at%). Even though the composition deviates from Co2Mn(Ga0.25Ge0.75), the relatively large MR (ΔRA of 5.7 mΩ(μm)2 and MR ratio of 35%) was obtained. This result suggests CMGG has good potential as a ferromagnetic electrode for CPP-GMR device.
References
[1] T. M. Nakatani et al., Appl. Phys. Lett., 96, 212501 (2010) [2] Y. K. Takahashi et al., Appl. Phys. Lett., 98, 152501 (2011) [3] B. Varaprasad et al., Appl. Phys. Express., 3, 023002 (2010)
2:06 PM
DE-04. Fabrication of epitaxial magnetic tunnel junctions with a Co2MnSi thin film and a MgO barrier on Ge(001) substrates via a MgO interlayer
Gui-fang Li, Tomoyuki Taira, Hong-xi Liu, Ken-ichi Matsuda, Testuya Uemura and Masafumi Yamamoto
Division of Electronics for Informatics, Hokkaido University, Sapporo, Japan
The monolithic integration of magnetic tunnel junctions (MTJs) and MOSFETs would lead to nonvolatile, ultralow-power logic circuits.1 Furthermore, a semiconductor Ge channel is highly favorable for future-generation MOS devices because of the high mobility of electrons and holes.2 We recently prepared epitaxial Heusler alloy Co2MnSi (CMS) thin films on Ge(001) substrates via a MgO interlayer with extremely smooth and abrupt interfaces.3 Our purpose in the present study was to fabricate fully epitaxial MTJs with a half-metallic CMS thin film and a MgO barrier on Ge(001) substrates via a MgO interlayer. The fabricated layer structure was as follows: (from the upper side) Ru cap (5 nm)/IrMn (10 nm)/Co50Fe50 (CoFe) (1 nm)/CMS (3 nm)/CoFe insertion layer (1.1 nm)/MgO barrier (1.4 - 3.2 nm)/CoFe (50 nm)/MgO interlayer (5 nm), grown on a Ge(001) substrate. The layer structure was annealed in situ at 400 ○C just after deposition of the CoFe lower electrode and at 450 ○C just after deposition of the CMS upper electrode. RHEED streak patterns dependent on the electron injection direction, parallel to [100]Ge and [110]Ge, indicated that all the layers in the heterostructure consisting of CMS upper electrode/CoFe insertion layer (1.1 nm)/MgO barrier/CoFe lower electrode/MgO interlayer were grown epitaxially on a Ge(001) substrate. Fabricated MTJs showed clear exchange-biased tunnel magnetoresistance (TMR) characteristics with a relatively high TMR ratio of 265% at 293 K. This ratio is much higher than the 180% at 293 K obtained for CoFe/MgO/CoFe MTJs identically prepared on Ge(001) substrates via a 5-nm-thick MgO interlayer. The enhanced TMR ratio at 293 K indicates that the CMS layer worked as a spin source in a heterostructure with a higher spin polarization than the CoFe electrode. There is much room for further enhancing the TMR ratio by optimizing the annealing temperature for CMS films. This result suggests the promise of the monolithic integration of CMS/MgO-based MTJs and Ge MOSFETs for future-generation nonvolatile logic circuits. It also suggests the promise of the CMS-based MTJ/MgO interlayer/Ge(001) heterostructure as a key device structure for efficient spin injection into a Ge channel from an MTJ.
References
1S. Sugahara and J. Nitta, Proc. of the IEEE 98, 2124 (2010). 2M. L. Lee et al., APL 79, 3344 (2001). 3G.-f. Li et al., APL 98, 262505 (2011).
2:18 PM
DE-05. Spincaloritronic effects in Heusler compound Co2MnSi thin films
Subrojati Bosu, Yuya Sakuraba, Ken-ichi Uchida, Kesami Saito, Eiji Saitoh and Koki Takanashi
Institute for Materials Research, Tohoku University, Sendai, Japan
Recent discovery of spin Seebeck effect (SSE) [1] in Ni80Fe20 (Py) film opened a new door of research regarding thermal aspects of spin transport. However, concerning the long-range spatial distribution of the spin voltage (much higher than metallic spin diffusion length), the origin of SSE remains to be clarified. Therefore, our main aims of this study are to investigate the effect of spin polarization of conduction electrons on SSE using highly spin polarized Heusler compound Co2MnSi (CMS) and clarify the mechanism of SSE. Conventionally, SSE should be much weaker in highly spin polarized material than that of normal ferromagnetic alloy, if the conduction electrons contribute significantly. However, our very recent observation of S SE in highly spin polarized CMS thin film [2] with comparable signals as that of in normal ferromagnetic alloy Py film suggested that there might be no relationship between the spin polarization of conduction electrons and SSE. Moreover, our present results (Fig. 1) of low temperature enhancement of SSE in CMS and Py thin films indicate that phonon drag process is a dominating factor for the observation of SSE as proposed by Adachi et al. [3].
References
[1] Uchida et al., Nature (London) 455, 778 (2008) [2] Bosu et al., Phys. Rev. B 83, 224401 (2011) [3] Adachi et al., Appl. Phys. Lett. 97, 252506 (2010)
2:30 PM
DE-06. Anisotropic magnetoresistance in Heusler compounds epitaxial films:A fingerprint of half-metallcity/non-half-metallicity
Fujun Yang, Yuya Sakuraba and Koki Takanashi
Institute for Materials Research, Tohoku University, Sendai, Japan
Since half-metals can generate a highly spin-polarized current, there are extensive studies about half-metals especially in magnetic tunnel junctions and CPP-GMR devices. According to Campbell-Fert model [1] and that extended by Tsunoda et al. [2], the anisotropic magnetoresistance (AMR), which is the most conventional MR effect, in half-metals should be always negative. In addition, AMR effect is sensitive to the exsistence of d-states at Fermi level, thus a remarkable change of AMR is expected between half-metallic and non-half-metallic Heusler compounds since the edges of half-metallic gap are generally formed by d-states. If AMR effect can be an index to confirm a half-metallicity(HM)/non-half-metallicity(NHM), it is useful to develop a half-metallic material because AMR effect can be measured in a just single film without any special micro-fabrications. In this study, we prepared Co2MnSi (CMS) and Co2FeSi (CFS) films on MgO substrate with different annealing temperature. The AMR ratio was estimated by measuring a resistivity against a relative angle between magnetic field and applied current. All the CMS films annealed from 400-650°C, that posses a highly B2-ordering, showed a small negative AMR ratio, in while the CFS films annealed from 500-600°C exhibited a small positive AMR ratio in spite of a highly B2-ordering as shown in Fig.1. Since HM and NHM were reported in CMS and CFS, respectively[3], this result implies the sign of AMR effect can be a fingerprint of half-metallicity.
References
[1]I. A. Campbell et al., J. Phys. C: Solid State Phys., 3 (1970) S95. [2]M. Tsunoda et al., Appl. Phys. Express, 2 (2009) 083001. [3]T. Kubota et al., Appl. Phys. Lett. 94, (2009) 122504.
2:42 PM
DE-07. Magnetodynamics in Co1-xFexS2: A view from magnetic resonance
Brian Kaster1, Michael Pechan1, M. Manno2, A. Baruth2 and C. Leighton2
1Physics, Miami University, Oxford, OH; 2Materials Science, University of Minnesota, Minneapolis, MN
Many spintronic applications utilize spin injection from ferromagnetic metals. A promising model system for fundamental studies of such processes is CoS2, which has - 55% spin polarization (P) at the Fermi level, a value that is tunable with Fe doping in Co1-xFexS2, reaching + 85% [1]. Of interest is whether the ferromagnetic damping is influenced by P. For example, one might e xpect the relaxation channels to decrease as P → 100% [2], resulting in a decreasing in damping with a concomitant decrease in linewidth. Single crystals of Co1-xFexS2 have been prepared with close to ideal sulfur stoichiometry using chemical vapor transport and ground into powder for ferromagnetic resonance (FMR). We employed variable temperature FMR at 9.2 GHz to investigate the magnetodynamics of this system for x = 0.00, 0.05, 0.13 and 0.17. An analysis of the linewidth as a function of temperature reveals a minimum in damping at TC (see figure), which provides a measure of both TC and the breadth of the FM transition [1]. In contrast to expectations based on NMR [2], the linewidth is observed to increase rather than decrease with increasing P (see figure inset). This suggests that other mechanisms, such as two-magnon scattering or Fe positional inhomogeneity, dominate the damping. Note also the absence of a paramagnetic resonance well above TC, indicating the absence of a localized moment above the ordering temperature. Acknowledgements: Supported by the US DOE at MU, NSF MRSEC at UMN.
References
[1] Wang et al, Phys. Rev. Lett. 94 056602 (2005) ; Leighton et al, J. Phys.: Condens. Matter 19 (2007) 315219. [2] Kuhns et al, Phys. Rev. Lett. 96 167208 (2006).
2:54 PM
DE-08. Exchange stiffness and magnetic anisotropy of Cu2MnAl thin films grown onto sapphire and MgO substrates
Mohamed Belmeguenai1, Salim Mourad Cherif1, Kurt Westerholt2 and Philippe Moch1
1LSPM (CNRS-UPR 3407), Université Paris 13, 99 avenue Jean-Baptiste Clément, 93430, Villetaneuse, France; 2Experimentalphysik/Festkörperphysik, Ruhr-Universität Bochum, 44780, Bochum, Germany
Ferromagnetic Mn-containing Heusler alloys can be used as electrodes for spin-polarized-current injection into semiconductors. Cu2MnAl has a moderate spin polarization (about 30%) at the Fermi level, but appears interesting as a prototype of all Heusler compounds and for applications as buffer layer for the growth of other Heusler compounds. Therefore, Cu2MnAl films of different thicknesses (50, 70 and 100 nm) were grown by ultra high vacuum radio frequency sputtering on a-plane sapphire and MgO (100) substrates. Their structural properties have been studied by X-ray diffraction. The best quality growth of the Cu2MnAl films was obtained on MgO substrates. Dynamic magnetic properties were investigated using micro-strip line ferromagnetic resonance. From the resonance measurements versus the direction and the amplitude of an in-plane and of a out-of-plane perpendicular applied magnetic field, we derived most of the magnetic parameters: effective magnetization, Landé g-factor, spin wave stiffness coefficient and magnetic anisotropy terms. The in-plane anisotropy can be described as resulting from the superposition of two terms respectively showing a two-fold and a four-fold symmetry without necessarily identical principal axes.
3:06 PM
DE-09. Epitaxial Co2FeSi Heusler alloy films on GaAs substrates with different substrate orientation
Jens Herfort, Bernd Jenichen, Thomas Hentschel and Achim Trampert
Paul-Drude-Institute, Berlin, Germany
The Heusler alloy Co2FeSi is a very promising candidate as a ferromagnetic electrode for GaAs-based spintroni c applications due to its predicted half-metallicity, high Curie temperature (>1000 K), and close lattice matching to GaAs (0.08%). Recently, we have achieved a spin injection efficiency of more than 50% utilizing Co2FeSi as spin injector [1]. However, the spin injection of such devices exhibits a strong dependence on the Co2FeSi growth temperature TS. The key limiting factor is the interplay between the strong in-diffusion of Co and Fe into the semiconductor part of the device, the chemical reaction at the interface and the long-range atomic ordering of the Fe and Si atoms within the Co2FeSi layer adjacent to the interface (which all increase with rising TS). Theoretical calculations show that Heusler alloy/semiconductor (111) and (110) interfaces are even superior with respect to maintaining the halfmetallic property at the interface compared to its (001) counterpart [2,3]. In addition, the long spin relaxation time for out-of-plane spin polarization in GaAs(110) [compared to GaAs(001)] is beneficial for the spin detection process within the semiconductor. Hence, non-(001) oriented Co2FeSi/GaAs layers are an alternative approach to further improve the performance of spintronic devices. We present systematic results on the fabrication as well as the structural and magnetic characterization of single-crystalline Co2FeSi films on GaAs(001), GaAs(111)B and GaAs(110) substrates grown by molecular beam epitaxy. In all cases, heterostructures with a high crystal and interface perfection as well as a high degree of long-range atomic ordering can be obtained under distinct growth conditions as evidenced by double crystal X-ray diffraction, transmission electron microscopy and resistivity measurements. Co2FeSi films on GaAs(111)B grow in an island growth mode and have a stable interface up to TS = 275 °C, which is 75 °C higher than that during growth of Co2FeSi on GaAs(001) and GaAs(110). At the same time, the Co and Fe in-diffusion is strongly reduced compared to the GaAs(001) counterpart. In addition, the interface-related magnetic anisotropy of the films strongly depends on the GaAs substrate orientation.
References
[1] M. Ramsteiner, O. Brandt, T. Flissikowski, H. T. Grahn, M. Hashimoto, J. Herfort, and H. Kostial, Phys. Rev. B 78, 121303(R) (2008). [2] J. Attema, G. A. de Wijs, and R. A. de Groot, J. Phys. D 39, 793 (2006). [3] K. Nagao, Y. Miura, and M. Shinai, Phys. Rev. B 73, 104447 (2006).
3:18 PM
DE-10. Atomic resolution structural study of the Heusler electrodes in Co2(Fe,Mn)Si/Ag/Co2(Fe,Mn)Si Spin Valves
Vlado K. Lazarov1, Leonardo Lari1, Joe Sato2, James Sizeland1, Philip J. Hasnip1, Mikihiko Oogane2, Atsufumi Hirohata3 and Y. Ando2
1Physics, University of York, York, United Kingdom; 2Applied Physics, Tohoku University, Sendai, Japan; 3Electronics, University of York, York, United Kingdom
Heusler based current-perpendicular-to-plane spin valves (CPP-SVs) has attracted considerable attention due to its potentials for high density hard disk drive applications. Heusler alloys have been recently used as FM layers in CPP-SVs devices very successfully [1,2]; their low resistance area product compared to standard ferromagnets make them very promising for CPP-SVs based devices. Controlling the structural ordering of the Heusler alloys and their interfaces with metallic spacers are crucial for optimizing the MR of CPP-SVs. The effect of the interface ordering has been shown recently to have a crucial role in the CPP-SV performan ce [3]. In this work we present a scanning transmission electron microscopy study on the single crystal Co2(Fe,Mn)Si/Ag/Co2(Fe,Mn)Si (CFMS-CS-CFMS) CPP-SVs . By employing the Z-contrast sensitivity in high angle annular dark file imaging (HHADF) we were able (on atomic level) to quantify the level of the ordering in the top and bottom electrode as well as on their interfaces with Ag. The intensity profiles scans on atomic resolution HAADF images from bottom electrode show that it is fully B2 ordered. However a significant amount of DO3 ordering (up to 30%) has been also measured. In contrast to the top electrode, the bottom is only partially ordered. The B2 ordering is significantly deteriorated with increasing the DO3 ordering. In addition we found that CFMS/Ag interface(s) are atomically and chemically abrupt. These structural finding correlate well with the transport properties measured for this device; that is a magnetoresistance of 80% (20%) at low (room) temperature.
References
[1] T. Furubayashi et al. Journal of Applied Physics, 107, 113917 (2010) [2] T. Iwase et al., Applied Physics Express 2, 063003 (2009) [3] V.K. Lazarov et al. Applied Physics Letters, 98, 242508 (2011)
3:30 PM
DE-11. Itinerant and localized Mn moments in ferrimagnetic Mn2CoGa thin films identified with x-ray magnetic linear dichroism
Markus Meinert1, Jan M. Schmalhorst1, Christoph Klewe1, Günter Reiss1, Elke Arenholz2, Tim Böhnert3 and Kornelius Nielsch3
1Department of Physics, Bielefeld University, Bielefeld, Germany; 2Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA; 3Institute of Applied Physics, University of Hamburg, Hamburg, Germany
Epitaxial thin films of the half-metallic Xa-compound Mn2CoGa (Hg2CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of 1.95(5)µB per unit cell were obtained. The average Mn magnetic moment and the Co moment are parallel, in agreement with theory. The x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism allows to distinguish between itinerant and localized Mn moments. We show that one of the two Mn moments has localized character.
References
M. Meinert, J. M. Schmalhorst, and G. Reiss, J. Phys.: Condens. Matt. 23, 116005 (2011)
3:42 PM
DE-12. Proximity effects of antiphase boundaries
Sumesh Sofin R. Gopala Pillai, Han-chun Wu and Igor V. Shvets
Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College,University of Dublin, Dublin, Ireland
Antiphase boundaries (APB) are natural growth defects occurring due to the symmetry difference between the thin film and the substrate crystal structures. Studies of epitaxial thin films and hetero-structures containing APBs have attracted considerable attention during the last decade as APBs can significantly alter the physical properties of thin films, which is advantageous for the development of spintronics devices[1,2]. One of the important epitaxial heterostructures for these studies is Fe3O4 thin fil ms grown on MgO substrates. Since the Fe3O4 (Fd-3m)crystal structure is lower in symmetry than MgO (Fm3m) there are several equivalent nucleation sites on the MgO surface, which enforce the formation of APBs at the junction of neighbouring grains. Here we show that shorter spin chains confined within small antiphase boundary (APB) domains can experience proximity effects of antiferromangnetic coupling across APBs. This can lead to a very high coercivity depending on the domain size (D) which is related to thickness (t) of the film. Here we report for the first time a double hysteresis loop and high coercivity (~0.1T) in Fe3O4 (110) films originating from frustrated spin chains in small APBDs. We also provide an analytical model which explicitly predicts the behaviour and calculates the domain size which follows the well know scaling relation,D=C*Sqt(t)[3].This effect can be utilized for the tunability of exchange bias effects in spintronics devices.
References
[1] S. Lee, A. Fursina, J.T. Mayo, C.T. Yavuz, V. L. Colvin, R. G. S. Sofin, I. V. Shvets and D.Natelson, Nat. Mater. 7, 130 (2008). [2] H. C. Wu, M. Abid, B. S. Chun, R. Ramos, O. N. Mryasov and I. V. Shvets, Nano Lett. 10, 1132 (2010). [3] W. Eerenstein, T. T. M. Palstra, T. Hibma and S. Celotto, Phys. Rev. B 66, 201101(R)(2002).
3:54 PM
DE-13. Enhanced Coercivity of Half-metallic La0.7Sr0.3MnO3 Enhanced by Both Ru Doping and Anisotropic Epitaxial Strain
Kei Shigematsu1, Akira Chikamatsu1, Yasushi Hirose1, 2, Tomoteru Fukumura1 and Tetsuya Hasegawa1, 2
1Chemistry, The University of Tokyo, Tokyo, Japan; 2Kanagawa Academy of Science and Technology (KAST), Kawasaki, Japan
Half-metallic La0.7Sr0.3MnO3 (LSMO) films grown on NdGaO3 (110) (NGO) substrates show in-plane uniaxial magnetic anisotropy due to epitaxial strain [1]. As a consequence, LSMO on NGO exhibits higher coercivity (Hc) compared to that on SrTiO3 (100) with isotropic strain [2]. Meanwhile, Ru-substitution for the Mn-sites in LSMO is also known to be an efficient way to enhance Hc [3]. If both Ru-doping and anisotropic epitaxial strain work in a cooperative way, further increase of Hc is expected. In this study, we have fabricated La0.7Sr0.3Mn1-yRuyO3 (LSMRO) thin films on NGO and investigated their magnetic properties. Epitaxial thin films of LSMRO (y = 0 and 0.1) were grown on atomically-flat NGO by pulsed laser deposition method. The typical thickness of the films was ~30 nm. Crystal structures were confirmed by X-ray diffraction (XRD). Magnetization measurements were performed using a superconducting quantum interference devices magnetometer. The out-of-plane lattice constant of LSMRO (y = 0.1) was determined to be 3.923 Å, which is somewhat longer than that of y = 0 (3.902 Å), indicating that Ru was successfully substituted for the Mn-sites of LSMO. The magnetization vs. field curve of LSMO (y = 0), with the direction of magnetization along [1-10], showed Hc of 90 Oe at 100 K. In addition, 10% Ru doping enhanced Hc up to 250 Oe. Furthermore, the easy(hard) axis for LSMRO (y = 0.1) was almost along [1-10] ([001]) of NGO substrate, corresponding to the result of y = 0. These clearly indicate that the Hc of LSMRO can be substantially enhanced by two effects: antiferromagnetic exchange coupling between Mn and Ru and anisotropic epitaxial strain from NGO.
References
[1] H. Boschker et al., Phys. Rev. B 79, 214425 (2009). [2] G. Ovsyannikov et al., J. Exp. Theor. Phys. 108, 48 (2009). [3] H. Yamada et al., Appl. Phys. Lett. 86, 192505 (2005).
4:06 PM
DE-14. Structural and magnetic properties of Mn2TiSn Alloy
Y. Huh1, 3, P. Kharel2, 3, V. R. Shah3, R. Skomski2, 3, E. S. Krage1, 3 and D. J. Sellmyer2, 3
1Physics, South Dakota State University, Brookings, SD; 2Physics and Astronomy, University of Nebraska, Lincoln, NE; 3Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE
Magnetically ordered Heusler compounds with high spin polarization and Curie temperature well above room temperature have drawn much attention because of their potential for spintronic applications. A ternary intermetallic compound Mn2TiSn is one of such compounds predicted to be half metallic with Curie temperature above room temperature1 but has received very little experimental attention. Although there are a few studies on the isostructural compounds Co2TiSn2 and Fe2TiSn3, no reports on the experimental work of Mn2TiSn has been found to date. We have prepared Mn2TiSn compound using arc melting and annealing. The atomic compositions of the compound were determined using EDX analysis. We have studied the structural properties of this compound using X-ray diffraction and the temperature and field-dependent magnetization measurements were carried out to investigate the magnetic properties. X-ray diffraction shows that the samples are polycrystalline and have the cubic Heusler phase plus an additional impurity phase. Magnetization as a function of temperature and magnetic field measured between 5 K and 350 K show that the samples are ferromagnetic at room temperature but the magnetization at 60 kOe (M) and coercivity (Hc) change significantly with temperature. The room temperature M and Hc are 12 emu/g and 220 Oe respectively. Both the M and Hc increase with decreasing temperature and reach 28 emu/g and 760 Oe as temperature reaches 10 K. We note that the M(H) loops are not fully saturated even at 60 kOe. Magnetization as a function of temperature measured between 5 K and 350 K shows that the samples appear to have magnetic transition at around 400 K. We explain these observations in terms of the exchange interactions between the Mn atoms. — This research is supported by NSF MRSEC, DOE(P. K. and D. J. S) and NCMN.
References
1. M. Meinert, J-M Schmalhorst and G Reiss, J. Phys: Condens. Matter 23, 036001 (2011). 2. M. Majumdar et al., Phys. Rev. B 72, 012417 (2005). 3. A. Slebarski et al., Phys. Rev. B 62, 3296 (2000).
4:18 PM
DE-15. CVD synthesis of polycrystalline Fe3O4 thin films by using the cyclohexadiene iron tricarbonyl liquid precursor
Roberto Mantovan, Silvia Vangelista, Simone Cocco, Alessio Lamperti and Olivier Salicio
CNR-IMM MDM Laboratory, Agrate Brianza (MB), Italy
Magnetite (Fe3O4) is predicted to be half metallic with the highest Curie temperature among oxides (860 K), being one of the most interesting material for spintronic applications. We developed a chemical vapour deposition (CVD) process for the synthesis of pure Fe3O4 by using the cyclohexadiene iron tricarbonyl [Fe(C6H8)(CO)3] liquid precursor, which is delivered to the reactor in a multi-pulsed CVD mode with N2 carrier gas. Reactor base pressure is 10-2 mbar, and the Si/SiO2 substrates are held at 400 °C. By combining X-ray diffraction and reflectivity, Raman spectroscopy, time-of-flight secondary ion mass spectrometry, and conversion electron Mössbauer spectroscopy (CEMS), we show that the as deposited films (10-100 nm) are constituted by pure polycrystalline Fe3O4. CEMS is used for measuring the abundance δ of the vacancies that are introduced at the octahedral Fe site upon gradual Fe2+ → Fe3+ oxidation [1,2]. We detect δ down to 0.005, much lower than the values observed for Fe3O4 grown by CVD with different precursor [2]. The high stoichiometric degree of the Fe3O4 layers is confirmed by the low resistivity values ρ =4÷20 mΩ cm (bulk value: 4 mΩ cm). The linear dependence of logρ ∝ T-1/2 demonstrates that the dominating transport mechanism is the intergranular tunnelling of the (hopping) 3d6 electron at the octahedral Fe site [2-4]. Magnetoresistance (MR) of -1.8 % is measured at 295 K (magnetic field H = 0.8 T parallel to the samples plane), corresponding to an electron spin polarization P∼12 %, if the Inoue’s model is adopted [5]. A gradual MR increase is observed at low temperature. The MR= -4.4 % at 130 K indicates ∼80 % increase of P, which we cannot explain with the sharp Verwey transition [6]. Our approach opens the way to an efficient and cost effective method for the synthesis of pure Fe3O4 thin films on large area substrates.
References
[1] F. C. Voogt, T. Fujii, P. J. M. Smulders, L. Niesen, M. A. James, and T. Hibma, Phys. Rev. B 60, 11193 (1999). [2] Mantovan, A Lamperti, M Georgieva, G Tallarida and M Fanciulli, J. Phys. D:Appl. Phys. 43, 065002 (2010). [3] P. Sheng, B. Abels, and Y. Arie, Phys. Rev. Lett. 31, 44 (1973). [4] H. Liu, E. Y. Jiang, and H. L. Bai, Appl. Phys. Lett. 83, 3531 (2003). [4] J. Inoue and S. Maekawa, Phys. Rev. B 53, R11927 (1996). [5] E. J. W. Verwey, Nature 144, 327 (1939).
1:30 PM - 4:30 PM, Grand Canyon 4-5
Chair: Xinyu Liu, Notre Dame
1:30 PM
DF-01. Giant anomalous Hall effect in diluted magnetic topological insulator with carrier independent ferromagnetic order
Ke He1, Cui-Zu Chang1, 2, Jin-Song Zhang2, Zuo-Cheng Zhang2, Min-Hao Liu2, Kang Li1, Xiao Feng1, 2, Li-Li Wang1, Xi Chen2, Xi Dai1, Zhong Fang1, Xiao-Liang Qi3, Shou-Cheng Zhang3, Yayu Wang2, Xu-Cun Ma1 and Qi-Kun Xue2, 1
1State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China; 2State Key Laboratory for Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing, China; 3Deparment of Physics, Stanford University, Stanford, CA
Introducing ferromagnetism and spin-polarized electrons into semiconductors is the first, and maybe the most difficult step to the realization of the electronics based on spin degree of freedom of electrons. An easier approach is through doping magnetic impurities into semiconducto rs, known as diluted magnetic semiconductors (DMSs)[1,2]. Carriers of certain density and type have long been demonstrated as indispensable medium for ferromagnetic coupling in a DMS[3], a factor restricting the appearance of various DMS-based devices. But the situation can be different in a special kind DMSs, diluted magnetic topological insulators (TIs)[4,5], the unique electronic properties of which can support carrier independent ferromagnetism[6]. Furthermore thin films of these diluted magnetic TIs are expected to host quantized anomalous Hall effect (QAH)[6], a kind of quantum Hall effect that could appear even without external magnetic field which has been sought after for years[7]. In this work, we prepared and investigated ferromagnetic Cr-doped ternary TI BixSb2-xTe3[8], the carriers in which can be controlled between n- and p-type with both Bi:Sb ratio and electrical field effect. With drastic change in both type and density of carriers, good ferromagnetism always holds with Curie temperature little influenced. Meanwhile the anomalous Hall effect is significantly enhanced with decreasing carrier density, with anomalous Hall angle reaching an unusually large value 0.2[7], a phenomenon possibly implying the approaching QAH. The finding of carrier independent ferromagnetic order manifests Cr-doped BixSb2-xTe3 as a promising system to finally realize QAH and other TI-related magnetic phenomena, as well as a unique DMS material that could work in various doping regimes.
References
[1] H. Ohno, Science 281, 951 (1998). [2] T. Dietl, Nature Mater. 9, 965 (2010). [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 (2000). [4] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045(2010). [5] X. L. Qi and S. C. Zhang, arXiv:1008.2026 (2010). [6] R. Yu, W. Zhang, H. -J. Zhang, S. C. Zhang, X. Dai, and Z. Fang, Science 329, 61 (2010). [7] N. Nagaosa, J. Sinova, S. Onoda, A. H. MacDonald, and N. P. Ong, Rev. Mod. Phys. 82, 1539 (2010). [8] J. Zhang, et al. arXiv:1106.1755 (2011).
2:06 PM
DF-02. Electric Field Controlled Ferromagnetism in High Curie Temperature Mn0.05Ge0.95 Quantum Dots
Faxian Xiu
Electrical Engineering, Iowa State University, Ames, IA
Electric field manipulation of ferromagnetism has a potential for developing new generation electric devices to resolve power consumptions and variability issues in today’s microelectronics industry. Among various dilute magnetic semiconductors, group IV elements such as Si and Ge are the ideal material candidates because of their excellent compatibility with the conventional CMOS technology. Here we report, for the first time, (1) the successful synthesis of self-assembled dilute magnetic Mn0.05Ge0.95 quantum dots with ferromagnetic order above room temperature, and (2) the demonstration of electric field control of ferromagnetism in metal-oxide-semiconductor (MOS) ferromagnetic capacitors up to 100 K. We found that by applying electric fields to a MOS gate structure, the ferromagnetism of the channel layer can be effectively modulated via the change of hole concentrations inside the quantum dots. Our results are fundamentally important in the understanding and to the realization of high efficiency Ge-based spin field-effect transistors (Spin-FETs).
References
F.X. Xiu et al. Nature Materials 9, 337-344 (2010).
2:18 PM
DF-03. (In,Fe)As: A new Fe-based n-type electron-induced ferromagnetic semiconductor
Pham Nam Hai, Le Duc Anh and Masaaki Tanaka
Department of Electrical Engineering and Information Systems, The University of Tokyo, Tokyo, Japan
Ferromagnetic semiconductors (FMSs) have been intensively studied for decades as they have novel functionalities that cannot be achieved with conventional materials, such as the ability to control magnetism by electrical gating or light irradiation [1]. Prototype FMSs such as (Ga,Mn)As are always p-type, making it difficult to be used in spin devices. Here, we report on fabrication, microstructure analysis and ferromagnetism of (In,Fe)As, a completely new n-type electron-induced FMS with the ability to control ferromagnetism by both Fe and independent carrier doping. The studied (In1-x,Fex)As layers were grown by low-temperature molecular beam epitaxy on semi-insulating GaAs substrates. Electrons in these layers are generated by independent chemical doping of donors. Microstructure analyses by transmission electron microscope and energy dispersive x-ray spectroscopy show good zinc-blende crystal structure and no visible inter-metallic precipitation in (In,Fe)As. The ferromagnetism was investigated by magnetic circular dichroism (MCD), superconducting quantum interference device (SQUID), and anomalous Hall effect (AHE). With increasing the electron density concentration (n = 1.8×1018 cm-3 to 2.7×1019 cm-3) and Fe concentration (x = 5 - 8%), the MCD intensity shows strong enhancement at optical critical point energies E1 (2.61 eV), E1 + Δ1 (2.88 eV), E0’ (4.39 eV) and E2 (4.74 eV) of InAs, indicating that the band structure of (In,Fe)As is spin-split due to sp-d exchange interaction between the localized d states of Fe and the electron sea. SQUID and AHE measurements are also consistent with the MCD results. The Hall and Seebeck effects confirm the n-type conductivity of our samples. The electron effective mass is estimated to be as small as 0.03-0.175m0, depending on the electron concentration. These reveal that the electrons are purely in the InAs conduction band rather than in the impurity band, making it easy to understand (In,Fe)As by conventional Zener-model of carrier-induced ferromagnetism [2]. Our results open the way to implement novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as help understand the mechanism of carrier-mediated ferromagnetism in FMSs [3].
References
[1] H. Ohno et al., Nature 408, 944 (2000). [2] T. Dietl et at., Science 287, 1019 (2000). [3] P.N. Hai, L. D. Anh and M. Tanaka, cond-mat, arXiv:1106.0561v1 (2011).
2:30 PM
DF-04. Rare-Earth Nitrides: Intrinsic Ferromagnetic Semiconductors
Joe Trodahl1, Ben Ruck1, Franck Natali1, Natalie Plank1 and Claire Meyer2
1Victoria University, Wellington, New Zealand; 2Neel Institute, Grenoble, France
The rare-earth series, across which the atomic 4f orbitals are filled, form ionic mononitrides in the simple NaCl structure. Many of these have been recognised for fifty years as ferromagnetic in their ground state, but it is only recently that the growth of sufficiently ordered material has permitted their electronic state to be confidently established. We have now grown films of almost half of the series and investigated their magnetic and electronic properties with a wide range of measurements. The majority of those are narrow-gap semiconductors both above and below their Curie temperature. They have a number of characteristics that offer promise for spintronics structu res. In this presentation we will describe the growth of polycrystalline and epitaxial films, and our electronic and magnetic studies. We will highlight our results on GdN, SmN and EuN, with 5, 6 and 7 electrons, respectively in the 4f shell. The 4f shells then carry L=0 (GdN), M≈0 (SmN) and J=0 (EuN) in the ground state, leading to particularly interesting magnetic behaviours.
2:42 PM
DF-05. Ns-scale magnetization reversal in (Ga,Mn)As using electrical field gating
P. Balestriere1, Thibaut Devolder1, Joo-Von Kim1, D. Ravelosona1, V. Novak2, J. Wunderlich3 and T. Jungwirth2
1Université paris-Sud, ORSAY Cedex, France; 2Institute of Physics, Praha, Czech Republic; 3Hitachi Cambridge Laboratory, Cambridge, United Kingdom
Manipulating magnetization using electric fields is attracting interest because of its low power consumption. Electric fields can change the anisotropies of GaMnAs [1, 2]. Propositions to use a transient change of anisotropy to induce switching has been formulated [3], but the rate at which one can manipulate the magnetization in realistic conditions remains unknown. Here, we show that ns-scale nucleation events leading to magnetization reversal can be induced using pulsed electric fields on GaMnAs. Starting from n+-GaAs/AlGaAs/AlAs/GaMnAs, the in-plane magnetized (Ga1-x,Mnx)As layer (x=2,5%) is patterned as the channel of a high frequency field effect transistor. Electric field pulses of durations 100 ps-10 ns are applied to the channel using the n+-GaAs gate. The channel conductance includes a 3% contribution from crystalline AMR and can be used to monitor the magnetization orientation. Sweeping the magnetic field back from saturation, we stop at variable fields 6-16 mT before the coercivity (20 mT) and apply the electric field pulses. The pulses induce magnetization reversal in a substantial part of the device, as seen from its resistance (Fig.). Our findings are consistent with electric field induced nucleation events followed by subsequent domain wall propagation thanks to the magnetic field. The area reversed increases with the gate voltage, with the assisting field H and saturates when the pulse duration exceeds 3 ns.
References
[1] M. H. S. Owen et al, New Journal of Physics 11, 023008 (2009) [2] D. Chiba, M. Sawicki, Y. Nishitani, Y. Nakatani, F. Matsukura, and H. Ohno, Nature (London) 455, 515 (2008). [3] P. Balestriere, T. Devolder, J. Wunderlich, and C. Chappert, App. Phys. Lett. 96, 142504 (2010)
2:54 PM
DF-06. Enhancing the Curie Temperature of Ferromagnetic Semiconductor (Ga,Mn)As to 200 K via Nanostructure Engineering
Lin Chen1, Xiang Yang1, Fuhua Yang1, Jianhua Zhao1, Jennifer Misuraca2, Peng Xiong2 and Stephan von Molnár2
1State Key Laboratory For Superlattices And Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China; 2Department of Physics, Florida State UniversityFlorida, Tallahassee, FL
The ferromagnetic semiconductor (Ga,Mn)As has stimulated a great deal of interest in the past decade due to its potential application in semiconductor spintronic devices [1,2]. Many important spintronic functionalities have been realized based on this material. However, all of the device operations were demonstrated at temperatures much below room temperat ure. For practical applications, increasing the Curie temperature TC beyond room temperature is a necessity. The highest TC obtained in (Ga,Mn)As to date is 191 K [3], thus exploring various routes to increase this value is of great fundamental and practical significance. In this work, we will introduce our recent work that TC as high as 200 K was obtained after low-temperature annealing a patterned nanowire [4], which was demonstrated through measurements of temperature dependent resistance, Hall effect and Arrott plots.
References
[1] H. Ohno, Science 281, 951 (1998). [2] Spintronics, edited by T. Dietl, D. D. Awschalom, M. Kaminska, and H. Ohno (Academic Press, New York, 2008). [3] L. Chen, S. Yan, P. F. Xu, J. Lu, W. Z. Wang, J. J. Deng, X. Qian, Y. Ji, and J. H. Zhao, Appl. Phys. Lett. 95, 182505 (2009). [4] L. Chen, X. Yang, F.H. Yang, J.H. Zhao, J. Misuraca, P. Xiong, and S. von Molnár, NanoLetter, x.doi.org/10.1021/nl201187m, (2011) in press
3:06 PM
DF-07. MBE growth and magnetic properties of (Ga,Mn)As/GaAs nanowires
Janusz Sadowski1, 2, Aloyzas Siusys2, Piotr Dziawa2, Anna Reszka2, Bogdan J. Kowalski2, Piotr Dluzewski2 and Tomasz Story2
1MAX-Lab, Lund University, Lund, Sweden; 2Institute of Physics, Polish Academy of Sciences, Warsawa, Poland
The nanowire structures combining gallium arsenide - a material widely used in semiconductor technology, with GaMnAs ferromagnetic semiconductor and MnAs ferromagnetic metal were grown by Molecular Beam Epitaxy (MBE). Nanowires implying ferromagnetic materials have been proposed recently to constitute a base of a new type of nano-magnetic memory structures (racetrack magnetic memory [1]). In this context investigations of NWs combining ferromagnetic materials with commercially used semiconductors like silicon, or gallium arsenide are highly desirable. We have studied the nanowires grown by MBE both on Si(100) and GaAs(111)B substrates. In the first case (Si substrates) the NWs were grown in the autocatalytic growth mode. With use of GaAs substrates the NW growth was induced by gold nanodroplets. In both cases core GaAs NWs were first grown at high substrate temperatures (550 C for GaAs substrates and 600 - 630 C for Si substrates), then (Ga,Mn)As shells were grown at low temperatures (250 - 300 C). Structural properties of the core shell GaAs-(Ga,Mn)As NWs were investigated by scanning transmission microscopy and electron transmission electron microscopy (SEM, TEM, respectively), magnetic properties - by SQUID magnetometry and ferromagnetic resonance. The NWs grown with catalyzing Au nanodroplets on GaAs(111)B and in autocatalytic growth mode on Si substrates differ in thicknesses, lengths and densities. The NWs grown on GaAs substrates have lower diameters (about 50 nm), lower lengths (about 3 um) and higher surface densities in comparison to NWs grown on Si substrates. The latter have typical diameters of 100 - 150 nm, lengths of 5 - 15 um and surface densities lower than NWs grown on GaAs substrates. The (Ga,Mn)As and MnAs NW shells were grown on both types of primary GaAs NW cores, their magnetic properties were compared. This work has been partially supported by the EC Network SemiSpinNet (PITN-GA-2008-215368)
References
[1] Stuart S. P. Parkin, Masamitsu Hayashi, and Luc Thomas, Science 320 (2008) 190.
3:18 PM
DF-08. Origin of uniaxial magnetic anisotropy in (Ga,Mn)As
Magdalena B irowska1, Cezary Sliwa2, Karolina Milowska1, Jacek A. Majewski1 and Tomasz Dietl1, 2
1Institute of Theoretical Physics, University of Warsaw, Warsaw, Poland; 2Institute of Physics, Polish Academy of Sciences, Warsaw, Poland
We present an explanation of the long standing problem of uniaxial magnetic anisotropy energy (MAE) in epitaxially grown cubic (Ga,Mn)As [1,2]. We prove that the uniaxial MAE originates from the energetic preference of the [110] direction (over the [1-10] one) for Mn ion pairs incorporated into the (001) surface of a GaAs substrate, leading to lowering of the crystallographic symmetry of the grown (Ga,Mn)As layers from cubic to C2v, and in consequence to the uniaxial MAE. Our findings are based on ab initio calculations in the framework of the local density approximation to the density functional theory with relativistic pseudopotentials employed to account for the spin-orbit interaction non-perturbatively, and with effects of non-collinear magnetism included. Since the MAE is obtained from extremely small total energy differences, we carry calculations with extraordinarily high accuracy. Two numerical packages (SIESTA and QUANTUM ESPRESSO) are employed. The energy of a Mn pair placed along the [110] direction of reconstructed GaAs surface is lower by 0.3 eV in comparison to the one placed along [1-10]. We have also performed calculations for all possible configurations of Mn pair substituted onto cationic sites of the 64 atom supercell. The configuration of the nearest neighbor Mn pair (implicating C2v symmetry) has been energetically favorable by at least 0.025 eV/pair. This strongly suggests that the Mn pair configuration induced by the epitaxial growth can remain chemically stable in the bulk. Then, we have calculated MAE and determined 6 anisotropy parameters for C2v symmetry. It turns out that the easy axis of magnetization is directed along [110] and the hard axis is perpendicular to the (001) plane, exhibiting the energy difference of 42693 J/m3 and in-plane anisotropy energy of 11468 J/m3.
References
[1] D. Chiba, M. Sawicki, and Y.Nishitani et al., Nature Phys. 455, 07318 (2008). [2] M. Sawicki , K.Y. Wang, K. W. Edmonds et al., Phys. Rev. B 71, 121302R (2005).
3:30 PM
DF-09. Anisotropy variations in Mn-graded GaMnAs
Jonathan Leiner1, Brian J. Kirby2, Kritsanu Tivakornsasithorn1, Xinyu Liu1, Jacek K. Furdyna1 and Margaret Dobrowolska1
1University of Notre Dame, Notre Dame, IN; 2National Institute of Standards & Technology, Gaithersburg, MD
Continuing to increase the storage density of magnetic recording media while maintaining other requirements, such as robust thermal stability and low switching field (or current), is a task that requires creative new technologies. Graded media is one such new approach, where thin films or multilayers have a gradually changing magnetic anisotropy strength from top to bottom, in an attempt to optimize the balance between stability and writeability.[1,2] Despite a Curie temperature that is still too low for practical applications, the ferromagnetic semicondcutor GaMnAs is an interesting model system for studying the graded media approach, as the magnetic properties of GaMnAs depend strongly on Mn concentration [3] and electric field. [4] As a first step in developing GaMnAs nanostructures with electrically controllable magnetic anisotropy, we grew a series of epitaxial GaMnAs films with a vertica lly graded Mn concentration. The films were grown on a GaInAs buffer layer to strain the GaMnAs and promote perpendicular magnetic anisotropy. Polarized neutron reflectometry (PNR) measurements were used to determine the magnetic depth profiles of several such films, and clearly demonstrate the correspondence of the magnetization gradients and the variation in the Mn content. SQUID magnetometry revealed that for a 200 nm GaMnAs sample with Mn content decreasing from 6% to 0% with distance away from the GaInAs buffer, perpendicular anisotropy is dominant. Interestingly, for an otherwise identical sample with Mn content increasing from 0% to 6% away from the buffer, in-plane cubic anisotropy is dominant. The fact that we do not observe dominant perpendicular anisotropy when the Mn content is low near the buffer layer suggests that the effects of strain due to the buffer are short-range, and magnetic exchange is required to mediate perpendicular anisotropy throughout the film. Characterization of the anisotropy profiles of graded GaMnAs samples by PNR will be discussed.
References
[1] D. Suess, Appl. Phys. Lett. 89, 113105 (2006). [2] B. J. Kirby, J. E. Davies, Kai Liu, S. M. Watson, G. T. Zimanyi, R. D. Shull, P. A. Kienzle, and J. A. Borchers, Phys. Rev. B 81, 100405(R) (2010). [3] Y. Zhou, Y. Cho, Z. Ge, X. Liu, M. Dobrowolska, and J. K. Furdyna, IEEE Trans. Magn. 43, 3019 (2007). [4] D. Chiba, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 89, 162505 (2006).
3:42 PM
DF-10. Magnetism of dilute Mn in GaN: from paramagnetism to ferromagnetism
Thibaut Devillers1, Wiktor Stefanowicz2, Bogdan Faina1, Andrea Navarro-Quezada1, Tian Li1, Andreas Grois1, Mauro Rovezzi1, 4, Francesco d'Acapito4, Dariusz Sztenkiel2, Rafal Jakiela2, Arno Meingast5, Gerald Kothleitner5, Maciej Sawicki2, Tomasz Dietl2, 3 and Alberta Bonanni1
1Institute for semiconductor and solid state physics, Johannes Kepler University, Linz, Austria; 2Institute of Physics, Polish Academy of Sciences, Warszawa, Poland; 3Institute of Theoretical Physics, University of Warsaw, Warszawa, Poland; 4Italian Collaborating Research Group, BM08 “GILDA” - ESRF, Grenoble, France; 5Institute for Electron Microscopy - FELMI, Graz University of Technology, Graz, Austria
We summarize our recent work on controlling and elucidating the nature of magnetism and exchange interactions in the model dilute magnetic insulator (Ga,Mn)N. Our findings point to the presence of ferromagnetic superexchange, as theoretically predicted [1,2] but to-date not unequivocally demonstrated. The (Ga,Mn)N layers are grown on sapphire substrate by metalorganic vapor phase epitaxy, with Mn concentrations up to 3%. The structural properties have been extensively studied by means of x-ray diffraction (in-house and synchrotron), high resolution transmission electron microscopy, scanning transmission electron microscopy coupled with electron energy loss spectroscopy, and synchrotron extended x-ray absorption fine structure. These multiple structural characterizations let us to conclude unambiguously on the dilute character of Mn in our single-crystal GaN layers: at least up to 3%, Mn is strictly dilute, substitutional of Ga, without any evidence of segregation or precipitation. Furthermore, x-ray absorption near-edge structure measurements demonstrate that the dilute Mn is incorporated in a Mn3+ charge state. From high-sensitivity SQUID magnetometry measurements we can state that: (i) up to xMn= 1%, Mn beha ves paramagnetically, with a magnetic moment and anisotropy characteristic of Mn3+ [3]; (ii) for higher Mn concentrations, but still in the dilute regime, we have the onset of a ferromagnetic coupling between the Mn spins [4], leading to a ferromagnetic ordering below 1.8 K.
References
[1] K. Sato et al., Rev. Mod. Phys. 82, 1633 (2010) and references therein [2] P. Mahadevan and A. Zunger, Appl. Phys. Lett. 85, 2860 (2004) [3] W. Stefanowicz, ...,T. Devillers, et al. , Phys. Rev. B 81, 235210 (2010) [4] A. Bonanni, M. Sawicki, T. Devillers et al., arXiv 1008.2083, Phys Rev B in print
3:54 PM
DF-11. Intrinsic ferromagnetism in (In,Fe)As and its dependence on Fe concentration
Le D. Anh, Pham N. Hai and Tanaka Masaaki
Department of Electrical Engineering and Information Systems, The university of Tokyo, Tokyo, Japan
Despite the general belief that the tetrahedral Fe-As bonding is anti-ferromagnetic [1], we have successfully fabricated a new Fe-based n-type (In,Fe)As ferromagnetic semiconductor [2].Here, we report on the intrinsic ferromagnetism in (In,Fe)As and its dependence on Fe concentration and layer thickness. Figure 1 shows the normalized MCD spectra of a 100 nm-thick (In0.095,Fe0.05)As film with an electron concentration n = 1.9×1019 cm-3, measured at 10 K under 0.2, 0.5 and 1 Tesla. The spectra show perfect agreement over the whole photon-energy range, proving that the MCD spectra of (In,Fe)As come from a single-phase ferromagnetism.The normalized hysteresis measured by MCD at 2.6 eV, anomalous Hall effect, and magnetization show perfect agreement as well, indicating its intrinsic ferromagnetism in this sample (Curie temperature = 34 K). For samples with higher Fe concentration (In0.092,Fe0.08)As, we observe that the MCD shows two phases of ferromagnetism; one is the matrix phase, and the other having enhanced MCD at low photon energy (< 2.0 eV). The latter turned out to be super-paramagnetic zinc-blende (In,Fe)As clusters with high density of Fe due to the well-known spinodal decomposition phenomena. The cluster phase shows higher Curie temperature of 70 K. We will discuss the ferromagnetic properties of (In,Fe)As with various Fe concentrations and thicknesses.
References
[1] R. Moriya et al., Physica E 10, 224 (2001). [2] P.N. Hai, L. D. Anh and M. Tanaka, cond-mat, arXiv:1106.0561v1 (2011).
4:06 PM
DF-12. Room temperature ferromagnetism in transparent conducting Fe-doped In2O3 films
Michael Osofsky, Heungsoo Kim, Michael M. Miller, Syed B. Qadri, Raymond Auyeung and Alberto Pique
code 6364, Naval Research Laboratory/SSD, Washington, DC
Oxide semiconductors have been widely studied as a host compound for spintronic devices since they can be doped with transition metals to realize a higher Curie temperature and can produce high n-type carriers by either doping with Group IV elements or introducing oxygen vacancies. Among various oxide semiconductors, Fe-doped In2O3 is a promising ferromagnetic semiconductor due to the high solubility of Fe-ions into the In2O3 lattice. However, in many cases the feasibility of extrinsic origin of ferromagnetism, such as magnetic impurity segregation, could not be eliminated. These controversial results on the magnetic properties are because the magnetic properties are extremely sensitive to the film growth condit ions and magnetization measurements are insufficient to distinguish intrinsic ferromagnetism from an extrinsic one. Recently, at NRL, In2-xFexO3 thin films have been deposited on MgO, sapphire, and YSZ substrates by pulsed laser deposition. The lattice constant decreases linearly with increasing Fe-doping concentration suggesting the incorporation of Fe ions into the In2O3 lattice matrix. Magneto-transport characteristics including anomalous Hall effect along with structural analysis demonstrate that an intrinsic ferromagnetism is observed for some films grown under optimized conditions. In this presentation, we will discuss our work to date on the growth of In2-xFexO3 thin films grown by pulsed laser deposition with various deposition conditions (oxygen pressure, growth temperature, and Fe-doping) and present the structural, optical, magnetic, and transport properties along with spin-polarization measurements.
4:18 PM
DF-13. Lattice location of transition metals in dilute magnetic semiconductors
Lino M. Pereira1, 2, U. Wahl3, A. Vantomme1 and J. P. Araujo2
1Instituut voor Kern- en Stralingsfysica and INPAC, Katholieke Universiteit Leuven, Leuven, Belgium; 2IFIMUP and IN-Institute of Nanoscience and Nanotechnology, University of Porto, Porto, Portugal; 3Instituto Tecnologico e Nuclear, UFA, Lisbon, Portugal
The magnetism of dilute magnetic semiconductors (DMS) is primarily determined by the lattice site(s) occupied by the magnetic impurities. The canonical and relatively well understood example is the narrow-gap Mn-doped GaAs. Here, Mn can occupy Ga-substitutional sites, where it provides the localized magnetic moment and the itinerant hole that mediates the magnetic coupling, as well as interstitial sites, where it acts as an electrical and magnetic compensating defect. The situation is somewhat different in wide-gap DMS materials like Co- and Mn- doped ZnO and GaN, where it is generally accepted that the transition metal impurities occupy only cation substitutional sites. <p>We present an experimental study on the lattice location of (i) Mn in GaAs (undoped, n+ and p+) and (ii) Mn and Co in ZnO and GaN, using β- emission channeling from the decay of radioactive 56Mn and 61Co implanted at the ISOLDE facility at CERN. <p>(i) In line with our recent report of high thermal stability of interstitial Mn in p+ GaAs [1], we show that interstitial Mn is stable up to 400°C, with an activation energy for diffusion of 1.7-2.3 eV, irrespective of the GaAs polarity (semi-insulating, n+ and p+). Since varying the concentration of potentialy trapping defects has no measurable effect on the migration energy of the interstitial impurities, we conclude that this high thermal stability is characteristic of isolated intersititial Mn. Being difficult to reconcile these results with the general belief that interstitial Mn is the compensating defect that out-diffuses at ∼200°C, we discuss their implications on the prospects for achieving higher Curie temperatures in Mn-doped GaAs. <p>(ii) Surprisingly, in addition to the expected majority in cation sites, we find significant fractions (∼20%) of Co and Mn substituting the anion (O in ZnO and N in GaN), which are virtually unaffected by thermal annealing up to 900°C. We discuss how this anion site configuration, which had never been considered before, may in fact have a low formation energy. We also propose a basic principle controling the majority-cation versus minority-anion substitutional incorporation of transition metal impurities in wide-gap DMS materials.
References
[1] L. M. C. Pereira, U. Wahl, S. Decoster, J. G. Correia, M. R. da Silva, A. Vantomme, and J. P. Araujo, Appl. Phys. Lett. 98, 201905 (2011).
1:30 PM - 4:30 PM, Grand Canyon 12-13
Chair: Benjamin McMorran, NIST
1:30 PM
DG-01. Three-dimensional imaging of magnetic domains
Rudolf Schäfer1, Christian Grünzweig2, Ingo Manke3, Nikolay Kardjilov3, Andre Hilger3, Sunmi Shin4 and Bruno C. De Cooman4
1Institute for Metallic Materials, Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Dresden, Germany; 2Paul Scherrer Institut, CH-Villigen-PSI 5232, Switzerland; 3Institute of Applied Materials, Helmholtz Centre Berlin for Materials and Energy (HZB), Berlin 14109, Germany; 4Materials Design Laboratory, Graduate Institute of Ferrous Technology Pohang University of Science and Technology, Pohang, Republic of Korea
Driven by the necessity to analyze the domains of materials for spintronics and recording, there has been progress in magnetic imaging by pushing methods towards high spatial resolution and the capability for time-resolved imaging. A prominent example are the methods based on X-ray dichroism. Together with other advanced techniques like electron-, magnetic force-, or Kerr microscopy, a number of capable tools exists that allows domain investigation of magnetic films in transmission or on the surface of materials in reflection. In many devices, however, bulk materials are applied, e.g. as sheet stacks in transformers or electromotors, wound cores in inductive devices or permanent magnets in motors or generators. In all these applications the volume domain structure and its behavior in magnetic fields is decisive for the key magnetic properties, be it for susceptibility, energy loss, noise or coercivity. The 3D analysis of the volume domains is therefore important for understanding and improving bulk magnetic materials. Only in rare cases of well-oriented surfaces the volume domains can be reasonably analyzed by surface observation combined with domain theory. In the general case, complex surface domains are formed that give no hint of the hidden volume domains. In recent years, there was considerable progress in experimental volume domain analysis: By applying the neutron grating interferometry technique, the domains and magnetization propagation in bulk FeSi material can be imaged non-destructively [1, 2, 3] and even tomographic analysis became possible [4] with a lateral resolution down to 10 µm. Higher resolution in the µm-range is possible by a special method invented by Libovicky [5]: In an FeSi alloy sub-µm platelets, oriented along the domain magnetization direction, precipitate at 600°C. At room temperature, this texture causes a birefringence effect after etching, which results in a domain contrast in a polarization microscope. This method was recently revisited [6] and demonstrated to be a powerful tool for 3D domain analysis. In this presentation, the advanced 3D methods for domain imaging are reviewed. The results are compared with surface domain observations by Kerr microscopy.
References
1. C. Grünzweig, et al.: Neutron decoherence imaging for visualizing bulk magnetic domain structures, Phys. Rev. Lett. 101, 025504 (2008) 2. C. Grünzweig, et al.: Visualizing the propagation of volume magnetization in bulk ferromagnetic materials by neutron grat ing interferometry (invited). J. Appl. Phys. 107, 09D308 (2010) 3. C. Grünzweig, et al.: Bulk magnetic domain structures visualized by neutron dark-field imaging. Appl. Phys. Lett. 93, 112504 (2008). 4. I. Manke, et al.: Three-dimensional imaging of magnetic domains. Nature Communications, 1:125 doi: 10.1038/ncomms1125 (2010) 5. S. Libovicky: Spatial replica of ferromagnetic domains in iron-silicon alloys. Phys. Status Solidi A 12, 539-547 (1972) 6. S. Shin, R. Schäfer, B.C. De Cooman: Three-dimensional visualization of the magnetic microstructure in bulk FeSi. Submitted
2:06 PM
DG-02. Scanned Magnetic Perturbation Imaging
Vidya P. Bhallamudi1, 2, Andrew J. Berger1, Dominic E. Labanowski2, David Stroud1 and P. Chris Hammel1
1Physics, The Ohio State University, Columbus, OH; 2Electrical and Computer Engineering, The Ohio State University, Columbus, OH
Imaging variations of spin properties is important for understanding and optimizing spintronic devices. We have developed a numerical tool for solving the spin transport equation in spatially varying magnetic environments. We use the insights gained from this to propose a scheme for imaging spin polarization by integrating a scanned micromagnetic probe with globally-averaging spin detection techniques, such as those based on electrical and optical measurements. The magnetic field from the probe perturbs the spin density within the sample to a degree determined by local spin properties. Thus the local magnetic field of the probe alters the contribution of this confined area, thereby encoding spatial information in the overall signal. Experimentally feasible large field gradients can enable imaging below optical resolution.
References
The authors wish to thank Jairo Sinova and Vivek Amin for valuable discussions.
2:18 PM
DG-03. First-Principles study of magnetic exchange interactions in scanning probe microscopy
Cesar Lazo and Stefan Heinze
Institute of Theoretical Physics and Astrophysics, University of Kiel, Kiel, Germany
The miniaturization of spintronic devices towards the limit of single atoms calls for an understanding of magnetic interactions and spin-polarized electron transport in atomic-scale magnetic contacts. Ideally suited for this purpose are spin-sensitive scanning probe microscopy techniques. With the scanning tunneling microscope (STM) one can indirectly measure the magnetic exchange interaction at surfaces by probing the Kondo resonance [1] or by analyzing magnetization curves using a spin-polarized STM [2]. The magnetic exchange force microscope allows measuring directly the magnetic exchange interaction across a vacuum gap. Here, we study these techniques based on density functional theory calculations in the full-potential linear augmented plane wave implementation and compare our results with experimental data. Firstly, we investigate single atom contacts formed by an antiferromagnetic Cr-tip of a STM and magnetic atoms (Co and Cr) adsorbed on a metallic surface (2 monolayers of Fe deposited on W(110)). We analyze the exchange interaction between tip and adatom from the tunneling to the contact regime and explain a spin-valve effect observed with a spin-polarized STM [3]. Furthermore, we characterize the geometric properties of the contact as the tip approaches towards the sample, and we show that the structural relaxations in the system are affected by the magnetic exchange interaction and can be obtained from the measured spin-polarization. Secondly, we consider the antiferromagnetic monolayer of Fe on W(001) and present the magnetic exchange energies and forces calculated for different tips composed of Cr, Fe, and Cr/Fe alloys. We compare our results with experimental data using magnetic exchange force spectroscopy. For stable tips, which can be distinguished from unstable tips by analyzing the dissipation signal, a very good agreement with our calculations is observed [4].
References
[1] P. Wahl, P. Simon, L. Diekhöner, V. S. Stepanyuk, P. Bruno, M. A. Schneider, and K. Kern, Phys. Rev. Lett. 98, 056601 (2007). [2] L. Zhou J. Wiebe, S. Lounis, E. Vedmedenko, F. Meier, S. Blügel, P. H. Dederichs, and R. Wiesendanger, Nature Phys. 6, 187 (2010). [3] M. Ziegler, N. Neel, C. Lazo, P. Ferriani, S. Heinze, J. Kröger, and R. Berndt, New Journal of Physics, (in press). [4] R. Schmidt, C. Lazo, U. Kaiser, A. Schwarz, S. Heinze, and R. Wiesendanger, Phys. Rev. Lett. 106, 257202 (2011).
2:30 PM
DG-04. Vector analysis of static magnetic field by adjusting measuring axis for Near-field magnetic force microscopy
Hitoshi Saito1, Zhenghua Li2, Rhoichi Ito1, Genta Egawa1 and Satoru Yoshimura1
1Graduate School of Engineering and Resource Science, Akita University, Akita, Japan; 2Venture Business Laboratory, Akita University, Akita, Japan
Magnetic force microscopy (MFM) is a powerful tool to investigate microscopic magnetic domain structures due to the high spatial resolution for static magnetic fields. Vector analysis of magnetic field is also useful to know the details of magnetic domain structures. However, changing the measuring direction of magnetic field is not easy in conventional MFM, because the measuring direction is determined by the direction of tip magnetization. In this study, we demonstrate a new method of vector magnetic field analysis by using our recently developed Near-field magnetic force microscopy [1, 2]. Near-field MFM can detect static magnetic field from hard magnetic samples near sample surface with high spatial resolution by using frequency modulated oscillation of a cantilever induced by an off-resonant alternating magnetic force between a hard magnetic sample and a soft magnetic tip driven by AC magnetic field. Near-field MFM deals with vector information composed of in-phase signal and out-of-phase signal of a lock-in amplifier with respect to the voltage of AC magnetic field source. Here the rotating tip magnetization caused by applied AC magnetic field has these orthogonal components. Vector magnetic field analysis with selecting the measuring direction becomes possible by adjusting the phase of these orthogonal lock-in signals with numerical calculation. Fig. 1 shows the signal transformed images for perpendicular and in-plane magnetic field for a perpendicular magnetic recording medium. The present method is effective to analyze vector magnetic field.
References
[1] H. Saito et al., J. Appl, Phys., 109, 07E330(2011). [2] H. Saito et al., INTERMAG 2011, Abstract CD-08, 2011.
2:42 PM
DG-05. Influence of magnetic film composition of FexB100-x coated tip on the spatial resolution of magnetic force microscopy
Mitsuru Ohtake, Kazuki Soneta and Masaaki Futamoto
Faculty of Science and Engineering, Chuo University, Tokyo, Japan
MFM spatial resolutions better than 10 nm are required for the recorded-bit observations in HDD media with areal densities exceeding 1 Tb/in2. MFM tips are generally prepared by coating magnetic materials on non-magnetic sharp tips made of Si, carbon nano-tube, etc. Remnant magnetization (Mr) of coated magnetic material [1] and tip shape [1-3] influence the sensitivity and the spatial resolution of MFM, respectively. Fe-B alloy is a typical soft magnetic material and Fe-B films have high Mr values. The crystallographic structure of Fe-B film varies from crystal to amorphous with increasing the B composition. The surface roughness of amorphous film is smaller than that of crystalline film, which seems desirable for making a sharp MFM tip. In the present study, MFM tips were prepared by coating Si tips of 2.5 nm radius with FexB100-x (x = 0 - 34 at.%) films by RF magnetron sputtering. The crystallographic structure of Fe-B film was determined by electron diffraction and the films with B contents exceeding 18 at.% were confirmed to be amorphous. The tip shapes were observed by high-resolution SEM. Small surface undulations resulting from crystal grain structure were observed for tips coated with crystalline Fe-B films, whereas tips coated with amorphous films showed smoother surfaces. The average tip radius increased from 15 to 21 nm with increasing the coating thickness from 15 to 30 nm for both cases. Fig. 1 shows the MFM images for a perpendicular medium recorded at 1500 kfci (bit length: 17 nm) observed by using MFM tips coated with 20-nm-thick Fe, Fe92B8, and Fe82B18 films and the power spectra of MFM intensity measured along the track direction of medium. Magnetic bits are distinguishable in the MFM images observed using tips coated with Fe92B8 and Fe82B18 films. A resolution of λ/2 = 6.8 nm is obtained with the tip coated with Fe82B18 film.
References
[1] K. Nagano, K. Tobari, M. Ohtake, and M. Futamoto, J. Phys. Conf. Ser. [Proc. JEMS 2010] (in press). [2] G. N. Phillips, M. Siekman, L. Abelmann, and J. C. Lodder, Appl. Phys. Lett. 81, 865 (2002). [3] H. Kuramochi, H. Akinaga, Y. Semba, M. Kijima, T. Uzumaki, M. Yasutake, A. Tanaka, and H. Yokoyama, Jpn. J. Appl. Phys. 44, 2077 (2005).
2:54 PM
DG-06. Engineering the ferromagnetic domain size for optimized imaging of the pinned uncompensated spins in exchange-biased samples by magnetic force microscopy
Sevil Ozer1, Niraj R. Joshi3, Timothy V. Ashworth2, Pablo G. Stickar3, Sara Romer3, Miguel A. Marioni3 and Hans J. Hug1, 3
1Physics, Basel University, Basel, Switzerland; 2NanoScan Ltd., CH-8600 Dubendorf, Switzerland; 3Empa, Swiss Federal Laboratories for Material Science and Technology, CH-8600 Dubendorf, Switzerland
The spatial distribution of pinned uncompensated spins (UCS) in an antiferromagnet (AF) plays a key role in the exchange bias (EB) effect of thin film ferromagnet (F) /AF structures. MFM is able to image stray fields emanating from magnetic moments of the sample with high spatial resolution and submonolayer spin sensitivity. However, MFM can only detect magnetic moment distributions with spatial wavelengths within a certain range. Concerning the detection of pinned UCS with 10 nm lateral resolution, an additional task is, therefore, to engineer the magnetic structures so that the location of the UCS is near the sample surface. Note al so that MFM registers the total stray field derivative, implying that the fields generated by F-moments and by UCS must be somehow disentangled. This can be accomplished by measuring at F-saturation, since a uniformly magnetized layer produces no stray field. To study these requirements in more detail we prepared two exchange-coupled samples. S1 is Pt(5)/AF/F_t/Pt(2) and S2 is Pt(5)/F_b/S_dec/AF/F_t/Pt(2) (thickness in nm). For AF we used 1.5 nm Co(Cr)O. For F_t, we selected a multilayer 4x[Co(0.3)/Pt(0.7)]/Co(0.3), deposited after oxidation of the AF layer. For F_b, we used 10 repeats of the same Co(0.3)/Pt(0.7) basic structure, covered with a Cu(2)-spacer to exchange-decouple F_b from AF. As expected, MFM images of S1 show micron-sized domains generating a detectable magnetic contrast only near their walls (domain wall contrast). After saturation of the F-layer, the imprinted pattern of UCS is not visible in the MFM images although the existence of such a pattern can be derived from the re-occurrence of the initial domain pattern. Simulations of the expected MFM contrast revealed that the contrast near the up/down domain walls of the UCS pattern is obscured by the granularity of UCS pattern. In sample S2 the thick bottom F_b layer governs a smaller domain structure, leading to a more homogenous contrast within the domains (domain contrast). After saturation, the imprinted domain pattern is well visible in the MFM images. It is noteworthy that a similar domain size engineering concept could also be used to generate a suitable domain pattern in systems with in-plane anisotropy.
References
[1] N. Joshi, A.P. Lett., 98 (2011) 082502 [2] P. J. A. Schendel, J. Appl. Phys., 88 (2000) 435 [3] P. Kappenberger et al., Phys. Rev. Lett., 91 (2003) 267202 [4] I. Schmid et al., Europhys. Lett., 81 (2008) 17001 [5] I. Schmid et al., Phys. Rev. Lett., 105 (2010) 197201
3:06 PM
DG-07. Characteristics of MFM Magnetics on High Moment PMR Writers with High Coercivity Probes
Feng Liu, Shaoping Li, Daniel Bai, James Wang, Zhanjie Li, Dehua Han, Tao Pan and Sining Mao
Western Digital Corporation, Fremont, CA
High resolution magnetic force microscopy (MFM) imaging with high coercivity probes on perpendicular magnetic recording (PMR) writers directly characterizes magnetic responses for the PMR write head [1-4]. In this paper we present finite element modeling (FEM) as well as experimental methods of quantifying the magnetic response of the PMR writer pole tip and the trailing shields at air bearing surface (ABS) in both dynamic state and remanent state. By comparing the response of a high coercivity MFM probe over various head elements when current is applied to the writer coil, we effectively measure the magnetic field contour including both the write field and the trailing shield return field and their respective saturation and hysteresis behavior. A comparison of the FEM modeling on PMR write field and field derivatives and the MFM imaged write field contour suggests that the MFM with high coercivity probes correlates better with the write field than with its second derivative. Characteristics of the MFM magnetics via high coercivity probing are studied in detail, including comparison with low coercivity probes, write field contour and its derivatives of different orders, sensitivity of different field components as well as saturation measurements for both the writer pole tip and shields.
References
1. K. Sueoka, K. Okuda, N. Matsubara, and F. Sai, J. Vac. Sci. Technol. B 12, pp. 1618-1621, 1991 2. P. Rice, J. Moreland, A. Wadas, J. Appl. Phys., vol. 75, pp. 6878-6880, 1994 3. S. Li, S. Stokes, Y. Liu, S. Foss -Schroeder, W. Zhu, and D. Palmer, J. Appl. Phys. 91, pp. 7346-7349, 2002 4. Y. Chen, S. Leong, T. Huang, K. Ng, S. Hu, Z. Yuan, V. Ng, Appl. Phys. Lett., vol.92, pp. 162505-162507, 2008
3:18 PM
DG-08. Single-atom magnet feature of thulium adatoms on W (110)
Jose L. Diez-Ferrer1, David Coffey2, 3, Miguel Ciria2, 3 and Jose I. Arnaudas1, 3
1Laboratorio de Microscopías Avanzadas, Instituto de Nanociencia de Aragón, Universidad de Zaragoza, Zaragoza, Spain; 2Instituto de Ciencia de Materiales de Aragón, CSIC - Universidad de Zaragoza, Zaragoza, Spain; 3Dept. Física Materia Condensada, Universidad de Zaragoza, Zaragoza, Spain
Thulium atoms thermally deposited on a clean W(110) substrate under UHV conditions have been investigated at low temperature with scanning tunneling microscopy and spectroscopy for coverage below 0.2 monolayer. The adsorbed Tm forms disordered structures consisting of single adatoms and some trimers with a minimum average separation of 1.7 nm, as determined from the evaluation of the pair distribution function. Differential conductance spectra and maps recorded at 4.5 K with a magnetic tip show reversed magnetic contrast on adjacent Tm atoms (Fig. 1, right), pointing out to a single atom binary magnetic behavior. This is compatible with a preferred normal to the surface orientation of the prolate 4f magnetogenic shell of Tm. Rare Earth (RE) atoms deposited on metallic surfaces present a relatively large electric moment, 4 D for the W(110) surface [1]; so, below the adatoms, there is an enhanced density of negative charge, promoting a perpendicular orientation of the 4f orbitals. The strong spin-orbit coupling of the RE forces the atom‘s magnetic moment to be also normal to the surface and provides the magnetic anisotropy needed to overcome the thermal instability at the measurement temperature (even for bulk Tm, where the symmetry of the crystal electric field is not broken as it happens at the surface, the uniaxial magnetic anisotropy energy per Tm atom at 4.5 K is about 165 K [2], which is larger than the value obtained from the thermal stability criterion 25 T ≈ 112 K).
References
[1] S. A. Shakirova and M. A. Shevchenko, J. Phys. Colloques 50(1989) C8-129. [2] J. J. Rhyne, in Magnetic Properties of Rare Earth Metals, R. J. Elliott, ed. Plenum Press, NY, 1972, p 105.
3:30 PM
DG-09. Magneto-optical Kerr effect with radially-polarized light
Rene Dost1, Baptiste Paul2, Dan A. Allwood1 and Ifan G. Hughes2
1Materials Science and Engineering, University of Sheffield, Sheffield, United Kingdom; 2Physics, Durham University, Durham, United Kingdom
The magneto-optical Kerr effect has been known for over a century and typically uses uniformly polarized light. Here, we propose a new MOKE configuration that uses non-uniformly polarized light at normal incidence for measurement of in-plane magnetization. Radially-polarized light is brought to a tight focus to create an intense axial electric field, EZ, in the focal plane. This full-aperture ‘R-MOKE’ (radial-MOKE) configuration promises to combine high sensitivity with high spatial resolution. The Richards-Wolf angular spectrum formalism is used to simulate in-focus and through-focus field distributions of focused linearly, azimuthally and radially-polarized light (Fig. 1). We consider sensitivity to in-plane magnetization by the EZ component present at focus. The focal plane of linearly-polarized light includes a bipolar EZ distribution, explaining why full-aperture measurement of in-plane magnetization is not possible with this configuration. Using the R-MOKE configuration, the EZ term can dominate at focus, suggesting a greater Kerr sensitivity than is encountered with linearly-polarized beams, and has a narrower width than the overall intensity envelope, suggesting improved resolution with this technique. Simulations of focusing an annular beam show that spatial resolution below 200 nm can be achieved at optical wavelengths (532 nm).
3:42 PM
DG-10. Unique characterization possibilities in the UHV-STXM MAXYMUS using the new surface sensitive TEY-measurement mode and a rotatable magnetic field up to 0.4T
Daniela Nolle, Markus Weigand, Eberhard Goering and Gisela Schütz
MPI for Intelligent Systems, Stuttgart, Germany
Using the X-ray magnetic Circular Dichroism (XMCD) effect, the soft X-ray range provides powerful detection capabilities concerning the element specific structural, chemical, and magnetic properties of a wide range of material systems. Usually, the need for thinned samples due to the short penetration depth of soft X-rays is a limiting factor for microscopic imaging in transmission microscopy. Furthermore, more surface sensitive techniques allow the investigation of bulk samples, though not providing the possibility of high magnetic fields. As both high magnetic fields and bulk samples are important for magnetic imaging, we present total electron yield (TEY, surface sensitive) microscopy results at bulk samples using the total sample current detection performed at the new UHV scanning microscope “MAXYMUS” at HZB/BESSY II in a rotatable magnetic field up to 0.4T. Additionally, surface sensitive TEY mode investigations are combined with simultaneous standard transmission (bulk sensitive) measurements. By this, the outstanding potential of these new investigation possibilities will be demonstrated. Pictures and spectra with high absorption contrast and 3D like edge enhancement are observed in TEY-mode as known for scanning electron microscopy. Moreover the development of the magnetic domain structure concurrently at the surface and in the bulk could be revealed, providing a profound understanding of fundamental mechanisms in coupled magnetic systems.
References
[1] D. Nolle et al., Microscopy and Microanalysis, accepted
3:54 PM
DG-11. Electron Beams with Orbital Angular Momentum and Their Application to Magnetic Imaging
Benjamin McMorran1, 2, Amit Agrawal1, 3, Henri Lezec1, Jabez J. McClelland1 and John Unguris1
1Center for Nanoscale Science and Technology, NIST, Gaithersburg, MD; 2Department of Physics, University of Oregon, Eugene, OR; 3Maryland NanoCenter, University of Maryland, College Park, MD
Nanofabricated holograms can be used in a transmission electron microscope (TEM) to generate free electron vortex beams composed of electron wavefunctions imprinted with a helical phase [1,2]. Such free electron states are remarkable for their quantized orbital angular momentum (OAM) - we demonstrated from 1 h-bar to 100 h-bar per electron - and associated magnetic dipole moment. These beams can interact with surfaces and materials in unique ways. For example, electron vortex beams can transfer quantized OAM to an atom through inelastic scattering, inducing preferred atomic transitions in which the atom’s magnetic quantum number (m) changes - precisely the same phenomenon that is used in X-ray magnetic circular dichroism (XMCD) measurements. This OAM-dependent “dichroic” signal has been experimentally verified in electron energy loss spectra from magnetic samples in a TEM [1]. In an effort to use this effect to provide high resolution magnetic images, we are modifying a scanning TEM (STEM) instrument to use OAM-carrying focused electron probe beams, with the ultimate goal of analyzing magnetic materials with atomic resolution.
References
[1] J. Verbeeck, H. Tian, and P. Schattschneider, Nature 467, 301 (2010). [2] B. J. McMorran, A. Agrawal, I. M. Anderson, A. A. Herzing, H. J. Lezec, J. J. McClelland, and J. Unguris, Science 331, 192 -195 (2011).
1:30 PM - 4:30 PM, Grand Canyon 1
Chair: Miguel Ciria, ICMA
1:30 PM
DH-01. Investigation of the Magnetic and Magnetoelastic Properties of Zn Doped Cobalt Ferrite
Dibakar Das1, Nalla Somaiah1, T. V. Jayaraman2 and P. A. Joy3
1School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad, India; 2Department of Mechanical and Materials Engineering, University of Nebraska, Lincoln, NE; 3Materials Chemistry Division, National Chemical Laboratory, Pune, India
Cobalt ferrite (CoFe2O4) based materials are excellent candidate for magnetomechanical sensor applications owing to the strong sensitivity of their magnetostriction to applied magnetic field (dλ/dH). Zn doped cobalt ferrite, with nominal compositions CoFe2-xZnxO4 (0 ≤ x ≤ 0.3), have been synthesized by a novel autocombustion technique using Co-, Fe-, and Zn- nitrate as the oxidizers and glycine as the fuel. The structural, microstructural, magnetic, and magnetoelastic properties of the Zn substituted ferrites have been characterized using x-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), vibrating sample magnetometer (VSM), and magnetostriction measurements. High resolution TEM (HRTEM) revealed the as-prepared powders to be crystalline having near-spherical morphology with particle sizes 25-30 nm. As prepared powder and solid state sintering (13000 C for 12 hrs) of the rectangular pellets resulted in a single phase cubic spinel structure, as observed and analyzed from XRD spectra. SEM microstructures showed the sintered pellets to be composed of ferrite grains (2-3 μm). The magnitude of coercive field decreases from 539 Oe for x = 0 to 105 Oe for x = 0.30. Saturation magnetization initially increases and peaks to 86.6 emu/gm for x = 0.2 and then decreases. The magnetostriction monotonically decreases with increasing Zn contents in the range 0.0 to 0.3; however dλ/dH reaches a maximum value (105 × 10-9 Oe-1) for x = 0.1. The Zn doped cobalt ferrite (x = 0.1) with very high strain sensitivity could be a potential material for stress sensor applications.
1:42 PM
DH-02. Strain-mediated magnetization rotation in exchange biased antiferromagnetic/ferromagnetic/piezoelectric composites
Gor Lebedev1, 2, Bernard V iala1, Thomas Lafont2, Dmitry Zakharov1, 2, Orphee Cugat2 and Jerome Delamare2
1CEA, LETI, MINATEC Campus, Grenoble, France; 2G2Elab, Grenoble Electrical Engineering Lab, CNRS-UJF-INPG, St Martin d’Hères, France
We demonstrate strain-mediated rotation of magnetization direction in exchange biased antiferromagnetic(AF)/ferromagnetic(FM)/piezoelectric(PE) composites. Large converse magnetoelectric effect (CME) has been reported with composites which may lead to technological impacts on memory, sensors, and RF tunable devices [1]. However, this is usually investigated for uniaxial magnetic anisotropy which is limited to a simple 90° magnetization reversal scheme. In contrast, when considering unidirectional anisotropy in AF/FM systems, controllable strain-mediated rotation of the direction of magnetization in FM can be predicted. However, this case has so far received very little attention because it is difficult to realize [2]. We investigate composites of IrMn/CoFeB/PZT. Fig.1(a) summarizes the E-field dependences of easy axis EA hysteresis loops measured along the pinning direction, with strain applied at 90°. For negative E (σ>0), the EA hysteresis loops become square due to enhanced uniaxial anisotropy in FM with negligible change in Hex. It translates on hard axis HA as tilted linear loops. For positive E(σ<0), the EA hysteresis loops tilt and start to shift towards zero with a decrease in Hex. The complete angular dependence of Hex when applying constant E was measured showing enhanced strain-mediated exchange bias shifts when the measuring direction (H) deviates from the pinning direction. We report a maximum at 45° with 41% change in Hex for 4 MV/m. Fig. 1(b) shows the measurements (H) realized at 45° (or 135°) where the sensitivity Hex/E is high leading to a controllable rotation of +/- 22° by E (in the same way as H usually) which is conclusive for applications.
References
[1] G.A. Lebedev, B. Viala, T. Lafont, D.I. Zakharov, O. Cugat, and J. Delamare, "Converse magneto-electric effect dependence with CoFeB composition in ferromagnetic/piezoelectric composites", presented in this conference. [2] M. Liu, J. Lou, S. Li, and N. X. Sun, Advanced Functional Materials, 21, 2593-2598 (2011).
1:54 PM
DH-03. Observation of large magnetostriction in annealed and quenched Co1-xFex thin films
Dwight D. Hunter1, W. Osborn2, K. Wang2, R. Suchoski1, L. Bendersky2, S. E. Lofland3, M. Wuttig1 and I. Takeuchi1
1Materials Science and Engineering, University of Maryland, College Park, MD; 2Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD; 3Department of Physics and Astronomy, Rowan University, Glassboro, NJ
We present composition-dependent magnetostrictive properties of Co1-xFex binary alloy films, which were fabricated on arrays of micro-machined cantilever substrates using a co-sputtering based composition spread approach. Magnetostriction measurements were performed on individual compositions from: as-deposited, annealed and quenched Co1-xFex films, with compositions x near the (fcc+bcc)/bcc phase boundary. We found that by tuning the presence of coexisting structural phases present in the textured annealed and quenched Co1-xFex films, effective room-temperature magnetostriction (λeff) in excess of 260 ppm can be achieved at low saturation field of < 10 mT. Microstructural analysis of synchrotron microdiffraction and transmission electron microscopy studies of samples at this composition, suggests a strong correlation between the structural and phase transitions with the magnetostriction present in these samples.
2:06 PM
DH-04. Temperature dependence of magnetoelastic properties of Fe100-xSix (5<x<20)
G. Petculescu1, P. K. Lambert2, A. E. Clark3, K. B. Hathaway4, Q. Xing5, T. A. Lograsso5, J. B. Restorff6 and M. Wun-Fogle6
1Physics, Univ Louisiana Lafayette, Lafayette, LA; 2University of Maryland, College Park, MD; 3Clark Associates, Adelphi, MD; 4G. J. Associates, Annapolis, MD; 5Division of Materials Sciences and Engineering, Ames Laboratory, Ames, IA; 6Carderock Division, Naval Surface Warfare Center, W. Bethesda, MD
Resonant ultrasound spectroscopy was used to determine the temperature (T) dependence of the elastic constants of magnetoelastic Fe-Si, from 7 to 300 K. Fe-Si1 is important in the context of highly magnetostrictive Fe-based alloy studies using non-magnetic solute atoms that are similar as well as different: full (Ga, Ge) and empty (Al, Si) d-shell elements or Group III (Ga, Al) and Group IV (Ge, Si) elements. Compositions of Fe100-xSix were chosen in the A2 (x=5), D03 (x=19.8), and mixed (x=11.6) phases. Maximum variations with temperature of the shear constants c’ and c44 (10% and 4%) are found for the pure ordered phase, D03. The longitudinal constant, c11, has a weaker T-dependence (3.6%). Overall, the elastic constants of Fe-Si vs. x, at all temperatures, are significantly higher than those of Fe-Ge of Fe-Al. The resonance data further reveals, through ultrasonic attenuation, the strength of magnetoelastic coupling. When magnetic-domain boundaries are forced to cross in and out of domains (at low fields) due to the propagating stress waves, energy is lost through magnetoelastic coupling, reflected in a lower quality factor (Q) for the resonance. The relative change in Q, κ=[QH=0-Qsat]/Qsat, is used as an indicator of coupling. Using pure modes, the attenuation associated with each shear deformation (tetragonal-κ1 and rhombohedral-κ2) was determined. At 300 K, we find that for 5 and 19.8 at.% Si, 2<κ1<4, while for 11.6 %, κ1~0. The results concur with the existing b1 and λγ,2 300K data. In addition, κ2<1 for all three Fe-Si samples, suggesting extremely low b2 and λε,2 values at 300 K. At 7 K, we find κ1~9 for 19.8 %, 2× higher than at 300 K. The 11.6% sample also gains, κ1~1, and the 5 % sample remains at κ1~2. For all samples at 7 K, κ2<1. Considering the weak T-dependence measured for c’ and c44, the behavior of κ, which is correlated to b, can be transferred to that of λ. As a check, the technique was applied to a highly magnetostrictive sample, Fe80Al20 (λγ,2=190 ppm), yielding κ1~10 and κ2~0, both λ and κ being ~ invariable with T. Through these tests, we present ultrasonic attenuation as a tool to predict the strength of magnetoelastic coupling and magnetostrictive strains in ferromagnetic materials.
Referen ces
[1] Q. Xing, D. Wu and T. A. Lograsso, J. Appl. Phys. 107, 09A911 (2010).
2:18 PM
DH-05. Behavior of Magnetic Field Annealed Galfenol Steel
Michael D. Brooks1, Eric Summers1, Marilyn Wun-Fogle2 and James Restorff2
1Materials Science, ETREMA Products, Inc., Ames, IA; 2Naval Surface Warfare Center, Bethesda, MD
An attractive feature of the Galfenol alloy system is the ability to impart significant anisotropy such that no pre-load mechanism is required to attain maximum magnetostriction. Stress annealing is typically employed to generate this anisotropy. Recent published results1,2 indicate that magnetic field annealing could generate enough stored energy to satisfy most applications. A magnetic field annealing apparatus was designed and built that applies a constant 1 Tesla magnetic field at temperatures between 300 - 700°C in a 7.6 cm square x 15.2 cm long cavity. Data to be presented compares magnetic properties such as; strain, d33, hysteresis, and magnetic permeability of field annealed and stress annealed Galfenol. Figure 1 shows the magnetostriction and magnetic flux density vs. magnetic field for a Galfenol rod in the initialized (no anisotropy), stress annealed, and magnetic field annealed states. The plots show that the Galfenol rod behaves differently after field annealing compared to the stress annealed state. While the saturation strain values are equivalent the magnetic field required to saturate the Galfenol is 70% less in the field annealed state. This behavior is also exhibited in the flux density versus field curve. Both curves have a significant impact on the device design. The type (mechanical or magnetic) and magnitude of stored energy largely determines the magnetic bias point impacting the amount and grade of permanent magnet needed.
References
1J.H. Yoo, J.B. Restorff, M. Wun-Fogle, and A.B. Flatau, “The effect of magnetic field annealing on single crystal iron gallium alloy,” J. App. Phys., 103, 07B325 (2008). 2J.H. Yoo, S.M. Na, J.B. Restorff, M. Wun-Fogle, and A.B. Flatau, “The Effect of Field Annealing on Highly Textured Polycrystalline Galfenol Strips,” IEEE Trans. on Magnetics, 45, Iss. 10, (2009).
2:30 PM
DH-06. Structure and magnetostriction of quenched Fe1-xGax (x = 0.15-0.30) alloys
Xiaoxi Zhu, Jinghua Liu and Chengbao Jiang
Materials science and engineering School, Beihang University, Beijing, China
Non-magnetic Ga addition in Fe enhances the magnetostriction over ten folds. The previous studies show that the complicated structures depend on the Ga content and thermal history. Several different models were presented on the relationship between the suggested crystal structures and the large magnetostriction in Fe-Ga alloys. The structure determination of Fe-Ga alloys by x-ray diffraction is troublesome due to the similarity in the atomic scattering factors of Fe and Ga atoms, leading to the extremely weak peak intensity of the super lattice diffusion scattering. In this paper, based on the Fe atomic moment testing, combined with the lattice parameter determining and the Curie temperature measuring, we hope to explore another approach to identify the structural transition with the increase of Ga content in the quenched Fe1-xGax alloys (0.15≤x ≤0.30). The quenched Fe1-xGax alloys show the structure transition from A2 to D03 and then to B2 in the experimental Ga con tent range with the regular change of the Fe average atomic magnetic moment from 2.25 μB to 1.78 μB and then to1.58 μB , respectively. The relationship between the structure and the magnetostriction is clarified, and the maximum magnetostriction appears in A2 phase. The variation tendency of magnetostriction is well characterized, reflecting the structure transition.
2:42 PM
DH-07. Temperature dependence of magnetization and magnetostriction in Fe81Ga19 alloy
Jinghua Liu and Chengbao Jiang
School of Materials Science and Engineering, Beihang University, Beijing, China
The magnetostriction and magnetization of Fe81Ga19 alloy were investigated for wide temperature range (150 K ~ 402 K). The magnetostriction is nearly constant and varies with -0.3 ppm/K below 350 K. When the temperature raises beyond 350 K the magnetostriction decreases obviously with around -1.3 ppm/K. The saturation magnetization and magneto-crytalline anisotropy of Fe81Ga19 alloy were calculated for different temperature. The saturation magnetization decreases gradually with the increasing temperature and correspondingly satisfies with T3/2 law. The magnetocrystalline anisotropic constant K1 decreases even faster than the magnetization with the increasing temperature. The relation between K1 and normalized magnetization was found to be K1(T)/K1(0) ∝ m4. By analysis the variations of magnetocrystallline anisotropy and elastic module, we obtain the relation between the magnetostriction and temperature. The magnetostriction exhibits abnormal decreasing trend when temperature is beyond 350 K, which probably relates to closing Debye temperature of the material.
References
[1] Y. Millev, Phys. Rev. B 51, 2937 (1995). [2] U. Atzmony, M. Dariel, E. Bauminger, D. Lebenbaum, Phys. Rev. B 7, 4220 (1973). [3] K. Sato, Y. Ishikawa, K. Mori, A. E. Clark, and E. Callen, J. Magn. Magn Mater. 54-57, 875 (1986). [4] J. R. Cullen, A. E. Clark, Phys. Rev. B 15, 4510 (1977). [5] A. E. Clark, K. B. Hathaway, M. Wun-Fogle, J. B. Restorff, T. A. Lograsso, V. M. Keppens, G. Petculescu, R. A. Taylor, J. Appl. Phys. 93, 8621 (2003). [6] A. E. Clark, M. Wun-Fogle, J. B. Restorff, K. W. Dennis, T. A. Lograsso, R. W. McCallum, J. Appl. Phys. 97, 10M316 (2005). [7] R. Kellogg, A. B. Flatau, A. E. Clark, M. Wun-Fogle, T. A. Lograsso, J. Appl. Phys. 91, 7821, (2002). [8] O. Hjortstam, K. Baberschke, J. M. Wills, B. Johansson, O. Eriksson, Phys. Rev. B 55, 15026 (1997).
2:54 PM
DH-08. Structural and Magnetic Characterization of Electrodeposited Magnetostrictive Fe1-xGax/Cu Multilayered Nanowires
K. S. Madhukar Reddy1, Jung Jin Park3, Suok-Min Na3, Mazin Maqableh2, Alison Flatau3 and Bethanie Stadler1, 2
1Chemical Engineering and Materials Science, University of Minnesota - Twin Cities, Minneapolis, MN; 2Electrical and Computer Engineering, University of Minnesota - Twin Cities, Minneapolis, MN; 3Aerospace Engineering, University of Maryland, College Park, MD
In this work, we present optimization studies and the structural/magnetic properties of Giant Magnetostrictive1 Galfenol (Fe1-xGax) continuous and Galfenol/Cu multilayered nanowire arrays. The arrays were electrochemi cally deposited in anodic aluminum oxide (AAO) templates. The growth parameters were optimized by controlling critical factors like hydrodynamic conditions using a novel in-house modified rotating disk electrode-template, nucleation using Cu seed layers and mass transfer using pulse deposition techniques. Extensive structural characterization by XRD, EBSD and TEM revealed a strongly textured growth in both Fe1-xGax and Fe1-xGax/Cu multilayered nanowires. The magnetization reversal mechanisms in various 35 nm diameter nanowire arrays were studied by VSM. It was found that both continuous and multilayered nanowires undergo reversal by nucleation and propagation of a transverse domain wall when reversal fields are applied close to the nanowire axis (Figure 1). However, for perpendicularly applied fields, the reversal mode switched to that of coherent rotation mode. Thus, this study enables use of Galfenol in applications such as artificial cilia, nanoactuators, nanomagnetic sensors, and spintronics.
References
A. E. Clark, J. B. Restorff, M. Wun-Fogle, T. A. Lograsso, D. L. Schlagel, IEEE Trans. Magn. 2000, 36(51), 3238-3240.
3:06 PM
DH-09. The compositional dependence of surface magnetization processes in Fe-Ga magnetostrictive nanowires
Nicoleta Lupu1, Alexandru Pintea1, Mihaela Lostun1, 2 and Horia Chiriac1
1Magnetic Materials and Devices, National Institute of Research and Development for Technical Physics, Iasi, Romania; 2Faculty of Physics, "Alexandru Ioan Cuza" University, Iasi, Romania
The magnetoelastic properties of Fe-Ga alloys are very sensitive to the microstructure, the solubility of Ga in bcc α-Fe and the fabrication process [1], these effects being more prominent for Fe-Ga electrodeposited alloys [2]. In this paper, we are presenting our latest results on the influence of Ga content on surface magnetization processes in Fe-Ga magnetostrictive nanowires prepared by pulse electrodeposition into alumina templates with nanopores of 300 nm in diameter and 55 μm in length. The deposition potential, concentration of the surfactants and pH of the electrodeposition bath all concur in the composition of the nanowires. The EDX analysis is showing a variation in the composition along the nanowires length, from Fe55Ga45 at the bottom of the template (where the electrodeposition process starts) to Fe80Ga20 at the top (where the concentration of Ga3+ ions is reduced). The MFM images taken along the nanowires length are showing magnetic inhomogeneities in the top part of the nanowires (10-15% from the total nanowires length of 50 μm), and more homogeneous magnetic structures at the bottom part. The different magnetic behavior along the nanowires is also proved by the local magnetic loops measured by longitudinal magneto-optical Kerr (MOKE) effect (the rotation of the plane of polarization is proportional with the magnetization component parallel with the plane of incidence), using a polarized light generated by a He-Ne laser with a diameter of 10 μm. An applied field of maximum 3 kOe was applied along the nanowires length using an electromagnet. The hysteresis loops taken at each 10 μm along the nanowires length, indicated a progressive decrease of the coercive field from 400 to 250 Oe with the Ga content decrease from 45 to 20 at.% (the last one is the eutectic composition). Support from PN II-Partnerships Programme (Contract No. 12-114/2008) is highly acknowledged.
References
[1] T.A. Lograsso, A.R . Ross, D.L. Schlagel, A.E. Clark, M. Wun-Fogle, J. Alloys Compd. 350 (2003) 95. [2] P.D. McGary, K.S. Madhukar Reddy, G.D. Haugstad, B.J. H. Stadler, J. Electrochem. Soc. 157 (2010) D656.
3:18 PM
DH-10. Magnetic domain manipulation in magnetostrictive Fe70Ga30 thin films via direct application of strain fields observed with Lorentz Microscopy
Paris Alexander and John Cumings
Materials Science & Engineering, University of Maryland, College Park, MD
The controlled and reversible switching of magnetic domains using static electric fields has been previously demonstrated via magneto-electric (ME) coupling in a multiferroic system. However, the nature of the interfacial transfer of strain in such devices is not well understood. We use Lorentz-force transmission electron microscopy to observe the magnetic domain structure dynamics resulting from direct application of strain on magnetostrictive iron gallium (Fe70Ga30) thin films. Iron-gallium films are deposited on flexible free standing membranes, and using a mechanically manipulated tip, a strain is applied directly to the film. The varied hysteretic behaviors under applied magnetic and strain fields will be presented, and the relationship between the induced magneto-crystalline anisotropy and local magnetization will be discussed.
References
This work was supported by the NSF-MRSEC at the University of Maryland, DMR 0520471.
3:30 PM
DH-11. The compressive stress effect on the magnetostriction and magnetization for Sm-Dy-Fe composites
Bowen Wang1, Zhihua Wang1, Yanming Hao2, Ling Weng1, Wenmei Huang1 and Weili Yan1
1Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Hebei University of Technology, Tianjin, China; 2Department of Physics,College of Science, Tianjin University of Science & Technology, Tianjin, China
The Sm-Dy-Fe magnetostrictive materials possess a large but negative magnetostriction and has shown great potential in applications at low frequency. To create a complement to Sm-Dy-Fe materials for higher-frequency applications, Sm-Dy-Fe composites have been developed and they broaden the useful range of the giant magnetostrictive materials into the ultrasonic regime [1]. However, the variation of magnetostriction and magnetization for Sm-Dy-Fe composites with compressive stress has not been studied up to now. And the compressive stress will make an effect on the magnetostriction of Sm-Dy-Fe materials according to magnetostrictive theory. Therefore, it is interesting to investigate the stress effect on the magnetostriction and magneization for Sm-Dy-Fe composites. Polycrystalline Sm0.88Dy0.12Fe2 ingots were prepared by arc-melting the appropriate constituent metals in a high purity argon atmosphere. The homogenized ingots were ground into particles and then the particles and polymer were put into molds to form rod samples under a compaction pressure of 800 MPa along the rod direction. After samples were removed from the mold, they were placed in the magnetic field of 200 kA/m, and in the vacuum at 373 K for 2h to ensure full cure of the polymer. The measurements of magnetostriction and magnetization of samples were made under the different compressive stress. It is found that the magnetostriction parallel to the rod direction is almost unchanged with increasing the compressive stress up to 20 MPa and shows a large unsaturated magnetostriction of −620×10-6 at 600 kA/m, which is larger than that of Ref. [2]. The magnetostriction perpendicular to rod axis direction increases a little with increasing the compressive stress when the magnetic field is larger than 300 kA/m. The variation of magnetostriction and magnetization with compressive stress and has been discussed. This result is very important to application of Sm-Dy-Fe composites.
References
1.Siu Wing Or, Wai Yin Wong, Dynamic magnetomechanical properties of Sm0.88Dy0.12Fe1.93/epoxy composites, J. Magn. Magn. Mater. 304(2006)e439. 2.Fang Yang, Chung Ming Leung, Siu Wing Or, Wei Liu, Xiangke Lv, Zhidong Zhang, J. Alloys Compd. 509 (2011) 4954.
3:42 PM
DH-12. Anomalous lattice softening of Ni-Mn-Ga austenite due to magneto-elastic coupling
Oleg Heczko1, Hanus Seiner2, Jaromir Kopecek1, Petr Sedlak2 and Michal Landa2
1Institute of Physics ASCR, Prague, Czech Republic; 2Institute of Thermomechanics ASCR, Prague, Czech Republic
Ni-Mn-Ga is the most important example of ferro-magnetic shape memory alloy exhibiting up to 10% of magnetic field induced deformation in the modulated martensite phase.1 The lattice softening of parent cubic phase is a precursor of (pre)martensitic transformation. In ferromagnetic material the lattice softening must be affected by magneto-elastic interaction. In Ni-Mn-Ga cubic austenite we experimentally observed very strong softening effect. Using ultrasound methods we measured large increase of shear elastic constant in magnetic field from 3.6 GPa in demagnetized state to 5.9 GPa at magnetic saturation while the other elastic constants were field-independent. Measured shear coefficient was approximately proportional to the square of magnetization and this behaviour did not change up to Curie temperature. Above Curie point the coefficient was independent of the field. Additionally, huge magneto-mechanical damping in demagnetized state decreased nearly six-fold in saturation. The observed softening can be ascribed to unique combination of very low magnetic anisotropy of order 100 J/m3 and relatively high magnetostriction of order 100 ppm. This low magnetic anisotropy allows the magnetostriction to contribute considerably to the shear strains in ultrasound measurements. This magneto-elastic phenomenon is fully analogous to the ΔE effect observed in other ferromagnetic materials with low magnetic anisotropy and high magnetostriction. Anomalous elastic softening and resulting very high elastic anisotropy in ferromagnetic cubic phase of Ni-Mn-Ga is quite exceptional among the known cubic materials. It may have profound consequences for the transformation to modulated martensite and thus for the magnetic shape memory effect. Therefore further exploration of this softening phenomenon in other ferromagnetic shape memory alloys is very important and the first results for ferromagnetic shape memory alloy Co-Ni-Al will be presented for comparison.
References
1 A. Sozinov, A. A. Likhachev, N. Lanska, and K. Ullakko, Appl. Phys. Lett. 80, 1746 (2002)
3:54 PM
DH-13. Co- and In- doped NiMnGa multifunctional alloys: giant effects under high pressure and high magnetic field
Simone Fabbrici1, Jiri Kamarad2, Zdenek Arnold2, Francesca Casoli1, Antonio Paoluzi1, Lara Righi3, David Serrate 4, Pedro Algarabel4, Mathias Doerr5, Erik van Elferen6 and Franca Albertini1
1IMEM-CNR, Parma, Italy; 2Institute of Physics, AS CR, Prague, Czech Republic; 3University of Parma, Chemistry dep., Parma, Italy; 4University of Zaragoza, Inst. de Ciencia de Materiales de Aragon, Zaragoza, Spain; 5Tech University of Dresden, Inst. Festkorperphys, Dresden, Germany; 6Radboud University of Nijmegen, High Field Magnet Lab, Nijmegen, Netherlands
The vast family of Ni-Mn based Heusler alloys provide an extended playground of physical properties. The interplay between a reversible martensitic transformation (MT) and magnetically ordered states gives rise to a series of functional properties, such as giant magneto- and baro-caloric effects, giant magnetoresistance, magnetic shape memory and magnetic superelasticity, that can be exploited for developing innovative devices. The path towards the technological application of these materials relies on the enhancement of the sensitivity of the MT to external forces, such as magnetic field (dTM/dH) or pressure (dTM/dp), which lead to the actuation of the reversible transformation itself. We have shown that by proper Co-doping Mn-rich Ni2MnGa alloys it is possible to revert the order of the structural and magnetic transitions, giving rise to a reverse MT between a paramagnetic low temperature phase and a ferromagnetic high temperature one [1]. The reverse transformation increases the magnetization jump between the two phases and consequently the dTM/dH well beyond the maximum values shown by the ternary alloy NiMnGa. The corresponding magnetocaloric effects are greatly enhanced [2]. Here we present the effects of hydrostatic pressure on magnetism and martensitic transformation; Co-doping increases the sensitivity of the MT to the applied pressure, resulting in remarkable values of the dTM/dp parameter. A possible explanation to such behavior could be related to the structural modifications induced by the stoichiometric changes; temperature dependent X-ray diffraction shows that the volume difference between the parent and product phases is greatly enhanced by Co; ΔV/V values higher than any other NiMnY Heusler alloy are found (Y being a IIIa-Va element). Recently performed magnetostriction measurements in extremely high magnetic fields (up to 30T) confirmed the remarkable structural (ΔV/V) and magnetic (dTM/dH) changes related to the MT and account for the behavior of these alloys in extreme conditions. Finally, we show how the additional doping of In further improves the aforementioned functional properties, pushing these materials among the most promising candidates for future applications.
References
[1] S. Fabbrici et al., Appl. Phys. Lett. 95, 022508 (2009) [2] S. Fabbrici et al., Acta Mater 59, 412-419 (2011)
4:06 PM
DH-14. In situ tailoring of magnetization configuration in ferromagnetic films under external stress
Wiem Karboul Trojet, Damien Faurie, Yves Roussigné and Salim Mourad Cherif
Laboratoire des sciences et procédées des materiaux LSPM, Villetaneuse, France
When a stress is applied to a ferromagnetic film, its magnetic structure and magnetization behaviour can be changed through magnetoelastic coupling. We have deposited NiFe films using thermal evaporation on a dogbone polyimide (Kapton®) compliant substrate. The growth angle and thickness have been found to strongly affect the domain structure as well as the magnetic properties. In fact, the shado wing effects induce anisotropy and thus strongly influence the macroscopic magnetic properties. The effect of mechanical stress on the domains configuration was studied by means of a micro-tensile testing combined in situ with magnetic force microscopy imaging. In the absence of applied stress and magnetic field, the sample shows stripe domains with a tilted direction regarding to the tensile axis. This behavior is linked to the deposition conditions and was confirmed by the ferromagnetic resonance measurements. After saturating the films, the application of an increasing stress regenerates progressively a domains structure. This behavior is analyzed within the frame of a competition between the applied magnetic field, the initial anisotropy induced by film elaboration process and the stress-induced magnetoelastic anisotropy.
4:18 PM
DH-15. Model and experimental research on magneto-thermo-mechanical characterization of Terfenol-D
Li Wang1, Bowen Wang1, 2, Zhihua Wang1, Ling Weng1, Wenmei Huang1 and Weili Yan1
1Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Hebei University of Technology, Tianjin 300130, China; 2International Center for Materials Physics, the Academy of Sciences, Shenyang 110015, China
Giant magnetostrictive materials can be used in magneto-mechanical coupling devices, such as transducers and an actuators, on the basis of their superior material properties. A significant feature of magnetostrictive materials is the magnetomechanical effect, that is, a larger deformation appears when both a magnetic field and a compressive stress are applied [1]. In fact, the magnetostrictive materials are also sensitive to their working temperature [2]. Therefore, it is necessary to establish a model to describe the relation of magneto-thermo-mechanical characteristics for giant magnetostrictive materials. In this paper, a nonlinear model is founded to determine the relation of magneto-thermo-mechanical characteristics for giant magnetostrictive materials, based on the Jiles-Atherton model, magnetomechanical theory and thermo dynamic relations. The variations of saturation magnetization and magnetic susceptibility with temperature are simultaneously analyzed in the model. The proposed model can be used to calculate the magnetostriction and magnetization characteristic of Terfenol-D rod in different temperatures and under different stress. The coupling effects of axial stress, magnetic field and temperature on the strain and magnetization as well as Young’s modulus of a Terfenol-D rod are both experimentally tested and numerically simulated according to the proposed model. Comparison between the calculating and experimental results shows that the proposed model can better describe the relation of magneto-thermo-mechanical characteristics of Terfenol-D rod under a varying temperature, compressive stress and magnetic field. Therefore, the proposed model possesses an important significance for the design of magnetostrictive devices.
References
1. X.J. Zheng and L. Sun, A nonlinear constitutive model of magneto-thermo-mechanical coupling for giant magnetostrictive materials. J. Appl. Phys. 100(2006)063906 2. Y.R. Liang and X.J. Zheng, Experimental researches on magneto-thermo-mechanical characterization of Terfenol-D. J. Acta Mechanica Solida Sinica. 20(2007)283.
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Damien Le Roy, Univ of Nebraska
DP-01. Microstructures and magnetic properties of L10 FePt films deposited onto NaCl-type films
Shen-Chi Chen1, Ta-Huang Sun2 and Po-Cheng Kuo3
1Department of Materials Engineering and Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei, Taiwan; 2Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan; 3Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan
The single-layered Fe54Pt46 films of 30 nm thick with high perpendicular magnetic anisotropy are achieved by depositing the film onto heated Si (100) substrate of 620°C. Its perpendicular coercivity, saturation magnetization and perpendicular squareness are as high as 14 kOe, 473 emu/cm3 and 0.96, respectively. The perpendicular magnetic anisotropy of the FePt films degrades when a 5-nm NiO film is introduced under FePt films. Further increasing the thickness of NiO film to 10 nm, the perpendicular coercivity of the film decreases greatly to around 4 kOe. Compared with NiO underlayer, when a 10-nm MgO underlayer is introduced, the perpendicular coercivity of the FePt film is sill as high as above 12 kOe. Furthermore, when the thickness of the MgO underlayer decreases to 5 nm, the (111)FePt peak of the FePt films almost disappears and both (001)FePt and (002)FePt peaks are enhanced greatly, indicating that the perpendicular magnetic anisotropy of FePt film can be improved by introduction of a thinner MgO underlayer. Its perpendicular coercivity keeps at around 14 kOe, but perpendicular squareness increases to 1. Auger electron spectroscopy (AES) analysis shows that Ni atoms diffuse into the FePt magnetic layer from the NiO underlayer is more serious than that of Mg atoms in FePt/MgO bilayer films. Ni atoms are mainly reacted with Fe atoms and NiFe compounds are formed, which results in large decrease in perpendicular magnetic anisotropy and perpendicular coercivity of the FePt/NiO bilayer films with thicker NiO underlayer.
DP-02. Effects of phase distribution and grain size on the effective anisotropy and coercivity of Nanocomposite PtCo Permanent Alloy
Tao Liu, Wei Li, Wei Sun and Zhaohui Guo
Division of Functional Material, Central Iron & Steel Research Institute, Beijing, China
Phase distribution and grain size are the key factors controlling the coercivity of nanocomposite permanent magnets. Feng et al. have investigated the effective anisotropy and coercivity of nanocomposite Nd2Fe14B/α-Fe magnets. [1] The latest research shows that the PtCo alloy is composed of nano-structured soft and hard magnetic phases. [2] There exist three types of grain interfaces with different exchange-coupling strengths in nanocomposite PtCo permanent alloy. It is necessary to describe the proportions of these interfaces. We assume that soft and hard phases distribute randomly in it and that all grains have cubic shapes. Fig.1(a)-1(c) show the dependence of the coercivity Hc on the hard-grain size Dh. For a given volume fraction vh and soft-grain size Ds, Hc shows a maximum value as a function of the hard-grain size Dh. That is because with increasing hard-grain size, the anisotropy of individual grains increases, while the partial proportions of the interface area decrease, which would lead to a maximum value of the effective anisotropy. Fig.1(d) shows the comparisons between t he calculated coercivity Hc and the experimental results. In the calculation, the values of the parameters vh, Dh and Ds are identical with those of Refs.2, the microstructural fitting parameters pc is equal to 0.685.
References
[1] W.C. Feng, R.W. Gao, S.S. Yan, W. Li and M.G. Zhu, J. Appl. Phys. 98, 044305(2005) [2] Q.F. Xiao, E. Brück, Z.D. Zhang, F.R. de Boer and K.H.J. Buschow, J. Alloys Compd., 364,64(2004)
DP-03. Formation mechanism and magnetic properties of cubic and cuboctahedron FePt Nanoparticles
Bian Baoru1, 2, Liu JingJing1, 2, Han Xianghua1, 2, Liu J. Ping1, 3 and Du Juan1, 2
1Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 2Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, China; 3Department of Physics, University of Texas at Arlington, Arlington, TX
Synthesis of magnetic nanoparticles has been studied extensively due to their great potentials for magnetic resonance imaging, data storage and high performance permanent magnets. It has been found that the propertices of nanoparticles are strongly influenced by their shape. In general, shape-controlling of nanoparticles is believed to be a kinetically controlled process, in which low-energy facets persist while high-energy facets vanish, leading to a particular resultant shape enclosed by the low-energy surfaces 1. Previous studies on nanoparticle shape control by using different surfactants has been reported . Here, we present a novel and simple method to control the shape of FePt nanoparticles by controlling the refluxing temperature. Monodisperse fcc structured FePt nanocubes(fig.1(a)) rounded by {111} planes with the average size of 7 nm were synthesized by temperature control. By adjusting the refluxing temperature, the main terminating planes of FePt NPs became {100} and {111} planes, which leads to a cuboctahedron shape(fig.1(b)). The mechanism of the different shape formation of the FePt NPs has been systematically discussed. The magnetic measurements showed that the order of saturated magnetization (Ms), Ms (nanocube)> Ms (cuboctahedron). This method may give a good reference to synthesize other type of magnetic particles.
References
1. Yin, Y. & Alivisatos, A. P. Colloidal nanocrystal synthesis and the organic-inorganic interface. Nature 437, 664-670 (2005)
DP-04. Magnetic domain observation of FePt/MgO/FePt trilayer circular dots
Hiroki Iwama, Shinji Matsumoto, Syohei Wakamatsu, Katsuya Sugawara, Kotaro Sato, Masaaki Doi and Toshiyuki Shima
Tohoku Gakuin University, Tagajo, Japan
Recently, L10 FePt ordered alloy with high magnetocrystalline anisotropy (Ku=7.0×107 erg/cc) has attracted much attention, since they are believed to be good candidates for future magnetic devises such as next-generation ultrahigh-density magnetic storage media and biasing nanomagnets [1]. It is also thought to overcome the problem of thermal fluctuation in nano-meters scale. A lot of studies such as thin films and self-assembled nano particles have been investigated [2], [3]. However, the magnetic properties of the multilayer structure of FePt alloy have not been completely clarified. In order to understand the magnetic coupling between the adjacent FePt layers with different chemical ordering, FePt/MgO/FePt trilayer films and circular dots have been fabricated and their structure, magnetic properties and magnetic domain structure have been investigated. Thin films were prepared by using an UHV-compatible magnetron sputtering system. A Cr seed layer of 1 nm and Au buffer layer 40 nm were deposited on a MgO (100) single crystalline substrate at room temperature (RT). FePt bottom layer of 5~20 nm was co-deposited with Fe and Pt at 300 0C. Then the intermediate layer of MgO layer of 2~10 nm were deposited at RT. FePt top layer of 5~10 nm was deposited at 300 0C. Finally, the samples were annealed at Ta of 500 ~ 600 0C. The magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer. The structure analysis was performed by X-ray diffraction (XRD) with Cu Kα radiation. The circular dot arrays were fabricated using an electron beam lithography and Ar ion etching system. The magnetic domain structure was observed by magnetic force microscope (MFM) in applied fields up to ±6 kOe. From magnetization curves, remarkable steps of the magnetization in the second quadrant can be seen for all the samples. This is thought to arise from the difference in the chemical ordering of top and bottom FePt layers. The multiple domain structure was observed for FePt circular dots with diameter of 300 nm. Three different magnetization states were clearly observed in the magnetic domain structure at different applied magnetic fields for FePt(10 nm)/MgO(10 nm)/FePt(10 nm) film.
References
[1] T. Shima et al., Appl. Phys. Lett. 99. 033516 (2006). [2] T. Shima et al., Appl. Phys. Lett. 81. 1050 (2002). [3] S. Matsumoto et al., J. Phys.: Conf. Ser. 266. 012038 (2011).
DP-05. Magnetization processes in micron-scale (CoFe/Pt)n multilayers with perpendicular anisotropy: First-order reversal curves measured by extraordinary Hall effect
Z. Diao1, 2, N. Decorde1, 2, P. Stamenov1, 2, K. Rode1, 2, G. Feng1, 2 and John Michael David Coey1, 2
1Physics, Trnity College Dublin, Dublin, Ireland; 2CRANN, Trinity College Dublin, Dublin, Ireland
The thin film multilayer stacks (Co/Pt)n and (Co/Pd)n can be regarded as model systems for investigating perpendicular magnetic anisotropy. They are considered as potential media for perpendicular magnetic recording, and more recently, they have been applied as perpendicular ferromagnetic electrodes in spin-valves and magnetic tunnel junctions. Their strong perpendicular anisotropy originates from interface symmetry breaking and a magneto-elastic anisotropy term due to interface strain. However, the magnetization processes in perpendicular multilayers are complex. Previously, first-order reversal curves (FORCs) measured by an alternating gradient magnetometer have been used to characterize switching in macroscale (Co/Pt)n thin film stacks [1, 2]. In contrast to previous work, here we report the reversal curves of micrometer-scale (CoFe/Pt)n multilayers, measured by the extraordinary Hall effect. Our stacks with n = 10, 20, and 50 were deposited by DC magnetron sputtering. The stacks were patterned into Hall bars, with an active area of 3 μm × 3 μm. Their perpendicular anisotropy is characterized by a 'quality factor' Q of 1.06, 0.94 and 0.69, respectively. FORC diagrams were calculated from a group of magnetization curves following the approach described by Pike et al. [3]. We identify two major peaks in all three FORC diagrams, which are associated with the nucleation of reversed magnetic domains and domain annihilation before saturation, r espectively. The peaks separate from each other when n increases from 10 to 50. Key features of the FORC diagrams can be reproduced by a Preisach-like model with two sets of hysterons. FORCs protruding out of major hysteresis loops, as previously reported [2], are also observed, but in a more identifiable manner. We attribute the effect to the lateral confinement in our micron-sized samples, which introduces increased frustration during domain growth. This is supported by the observed maze domain pattern under magnetic force microscopy.
References
[1] J. E. Davies, O. Hellwig, E. E. Fullerton, G. Denbeaux, J. B. Kortright, and K. Liu, Phys. Rev. B 70 (2004) 224434. [2] J. E. Davies, O. Hellwig, E. E. Fullerton, M. Winklhofer, R. D. Shull, and K. Liu, Appl. Phys. Lett. 95 (2009) 022505. [3] C. R. Pike, A. P. Roberts, and K. L. Verosub, J. Appl. Phys. 85 (1999) 6660 - 6667.
DP-06. Ultrathin M-type Strontium ferrite hexagonal platelets synthesized via CTAB-assisted chemical co-precipitation technique
D. Y. Chen, Y. Y. Meng, D. C. Zeng, Z. W. Liu, X. C. Zhong, H. Y. Yu and W. Q. Qiu
South China University of Technology, Guangzhou, China
M-type strontium ferrite particles (50-150nm) were synthesized via co-precipitation method with cetyltrimethyl ammonium bromide (CTAB) as a surfactant. Effect of CTAB content (x=0~9 wt.%) on the formation, structure, morphology and magnetic properties of strontium ferrite particles were studied. Single phase M-type ferrites with ultrathin hexagonal platelet-like (thickness:10-20nm) morphology were obtained in the existence of CTAB, whereas secondary phase α-Fe2O3 was detected in the samples prepared without CTAB, exhibiting rod-like shape. Both the saturation magnetization and coercivity increased up to x = 6 wt.% and the optimum values were 68.7 emu/g and 6620 Oe, respectively. CTAB may act as a crystallization master, controlling the nucleation and growth of the particles. Two distinct formation mechanisms (FIG.1) have been proposed to explain these results. According to the lowest energy principle, spherical SrFeO3-x and Fe2O3 precursors tend to form without CTAB addition hence rod-like products were produced due to the rough reaction interface. In contrast, owing to selective absorption of CTAB to a certain crystal plane, flaky precursors can be obtained to build the smooth reaction interface which drives the hexaferrite crystals grow in a preferred direction along c-axis. The growth rate is low along the {00l} direction but high in {hk0}. Consequently, ultrathin hexagonal platelets were obtained. These nanoparticles are promising for the building blocks of hard magnets and recordings.
DP-07. Particle Size Dependent Structural and Magnetic Properties of CoCrFeO4 Nanoparticles
Sagar E. Shirsath1, Vishnu S. Shinde2, Ram H. Kadam3, Ali Ghasemi4 and Akimitsu Morisako5
1Department of Physics, Vivekanand College, Aurangabad, India; 2Department of Chemistry, Shree Shivaji College, Omerga, Osmanabad, India; 3Materials Research Laboratory, Srikrishna Mahavidyalaya Gunjoti, Gunjoti, Omerga, Osmanabad, India; 4Materials Engineering Department, Malek Ashtar University of Technology, Shahin Shahr, Islamic Republic of Iran; 5Spin Device Technology Centre, Faculty of Engineering, Shinshu University, Nagano, Japan
Nanoparticles of CoCrFeO4 ferrite in the particle size range of 9 - 38 nm have been prepared by a sol-ge l auto combustion method. Synthesized powders were annealed at four different temperatures viz. 400 0C, 600 0C, 800 0C and 1000 0C. X-ray diffraction pattern was indexed by a Rietveld program to calculate accurate unit cell dimension. Particle sizes are determined by X-ray analysis and transmission electron microscope (TEM). The study confirmed the formation of CoCrFeO4 ferrite nanoparticles in the range of 9-38 nm depending on the annealing temperature. The size of the particles increases with annealing temperature and sintering time. The size of the nanoparticles increase linearly with sintering temperature and time, most probably due to coalescence that increases as sintering temperature increases. The saturation magnetization increases from 62 to 81 emu/g and coercivity initially increases up to 300 Oe and then decreases with increase in particle size, sintering temperature. Coercivity (1030 Oe) is observed on cooling down to 4 K while typical blocking effects are observed below about 243 K. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles. The larger particles seem to be blocked at higher temperatures as compared to the smaller particles at the same field. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles. The dielectric constant increases with increase in sintering temperature and particle size.
DP-08. The comparison between magnetic and reflection loss characteristics of substituted strontium ferrite and nanocomposites of ferrite/carbon nanotubes
Ali Ghasemi, Xiaoxi Liu and Akimitsu Morisako
Shinshu University, Nagano, Japan
Recently we have focused on the microwave absorption characteristics of ferrites [1-5].Site preference and magnetic properties of SrFe12−x(Ni0.5Co0.5Sn)x/2O19 nanoparticles with x=0-2.5 were studied using both 57Fe and 119Sn Mössbauer spectroscopies.The results show that the magnetization and magnetocrystalline anisotropy are closely related to the distributions of Ni-Co-Sn ions on the five inequivalent crystal sites.In the second stage the nanocomposite of substituted ferrite/carbon nanotube (CNTs)were synthesized. The structural and magnetic properties of nanocomposite were studied by XRD,TEM and VSM. The complex permittivity, permeability, and reflection loss properties of the ferrite and nanocomposites were studied separately. Figure 1 displays the variation of reflection loss versus frequency of nanocomposite at different CNTs volume percentage. The enhanced reflection loss of substituted ferrite/CNTs nanocomposites in compared to substituted ferrite is attributed to the better magnetic loss and matched characteristic impedance, rather than improvement in dielectric loss. It was found that nanomposites display a great potential application as thinner and lighter wide-band electromagnetic wave absorbers.
References
[1] A. Ghasemi, V. Šepelák, X. X. Liu, A. Morisako, J. Appl. Phys. 107, (2010) 09A743-09A745. [2] A. Ghasemi, V. Šepelák, X. X. Liu, A. Morisako, J. Appl. Phys. 107 (2010) 09A734-09A737. [3]A. Ghasemi, V. Šepelák, X. Liu, A. Morisako, IEEE Trans. Magn. 45, 2456-2460, (2009). [4] A. Ghasemi, X. Liu, A. Morisako, IEEE Trans. Magn. 45, 4420-4424, (2009). [5] A. Ghasemi, V. Šepelák, X. X. Liu, A. Morisako, accepted for publication in IEEE Trans. Magn.
DP-09. Tailoring the exchange spring behaviour for the hard soft ferrite nanocomposite SrFe12O19/CoFe2O4
Debangsu Roy, K. V. Sreenivasulu and P S Anil Kumar
Department of Physic s, Indian Institute of Science, Bangalore, India
Exchange spring magnets are a special class of permanent magnets consisting of two suitably dispersed ferromagnetic/ ferrimagnetic phases which are exchange coupled [1]. In the past, we were successful to achieve the exchange spring behaviour for the first time in the oxide ferrite nanocomposite of BaFe12O19 and Ni0.8Zn0.2Fe2O4 [2]. We have also succeeded to enhance the (BH)max of the nanocomposite of Fe3O4 and Barium Calcium Ferrite compared to the parent Barium Calcium Ferrite using the same concept [3]. The effect of the soft phase(Ni0.8Zn0.2Fe2O4) on the magnetization reversal and coercivity mechanism of the nanocomposite Ni0.8Zn0.2Fe2O4/BaFe12O19 [4] has also been studied. In the present work we are showing the exchange spring behaviour in the nanocomposite of hard Strontium Ferrite (SrFe12O19) and soft cobalt ferrite (CoFe2O4). We also have studied the effect of the particle size tailoring in this nanocomposite on the exchange spring behaviour. It is found that depending on the microstructural engineering of the soft phase i.e cobalt ferrite (CoFe2O4), we could switch between exchange spring to NON-exchange spring nature of the nanocomposite. SEM micrograph on both kinds of samples has been acquired to analyze the particle size. The Henkel plot measurement has been done for both the samples to estimate the interacting field. The nucleation field for the exchange spring nanocomposite has been obtained from the magnetic field dependence of the DC demagnetization remanence. The estimated nucleation field for the exchange spring sample is further correlated to the grain size of the soft phase (CoFe2O4). Magnetic viscosity measurement has also been done on both the samples to correlate the effect of the different grain size of the soft phase in the hard-soft nanocomposite. All these studies are revealing the importance of the microstructural engineering to obtain the exchange spring behaviour in the oxide nanocomposite.
References
[1] E. F. Kneller and R.Hawig, IEEE Trans. Magn., 27, 3588 (1991). [2] D.Roy, C. Shivkumara and P.S. Anil Kumar, J. Magn. Magn. Mater., 321 L11 (2009). [3] D.Roy, and P.S. Anil Kumar, J. Appl. Phys., 106 073902 (2009). [4] D.Roy, C. Shivkumara and P.S. Anil Kumar, J. Appl. Phys., 109, 07A761 (2011).
DP-10. Size effects on magnetic properties of nanocrystalline Sr2CuCo2Fe24O41 prepared by Co-precipitation method
Praveena Kuruva, K. Sadhana and S. Srinath
School of Physics, University of Hyderabad, Hyderabad, India
The nanocrystalline z-type hexaferrite powders were synthesized by Co-precipitation method using aqueous solutions of strontium, copper, cobalt iron and nitrates. The as synthesized powders were characterized using XRD, TEM and TG/DTA. The average particle size of the powders was found to be 48nm.The effect of calcinations temperature on crystallinity, morphology and magnetic properties have been investigated. The sintered samples were characterized using XRD, FE-SEM and EDS. The hexagonal ferrite formation was observed around 900°C which is confirmed with the TG/DTA curves. The plate shape grains were observed in FE-SEM at 950°C. The grain sizes of sintered hexaferrites are in the range of 220nm to 850nm. The value of Ms= 70 emu/g and Hc=540 Oe was obtained at 900°C/5h.
DP-11. Screen-printing of ferrite ma gnetic nanoparticles produced by carbon combustion synthesis of oxides
Chamath Dannangoda1, Eduard Galstyan2, Dmitri Litvinov3 and Karen Martirosyan1
1Physics, University of Texas at Brownsville, Brownsville, TX; 2Texas Center for Superconductivity, University of Houston, Houston, TX; 3Department of Electrical and Computer Engineering, University of Houston, Houston, TX
We investigate the feasibility of screen-printing process of hard ferrite magnetic nanoparticles produced by carbon combustion synthesis of oxides (CCSO). The use of nanoparticles, instead of micrograin powders, allows fabrication of thin films structures uniquely suitable in print technology. In CCSO the exothermic oxidation of carbon generates a smolder thermal reaction wave that propagates through the solid reactant mixture converting it to the desired oxides [1-4]. The ethylene glycol and oleic acid were added to the powder. The modified ferrite nanoparticles were dispersed in alcohol solvent with ultrasonic oscillation and centrifuging treatment to remove excess of oleic acid and ethylene glycol. Then the nanoparticles were dried in a vacuum oven at 50°C for 4h. The nanocomposites solutions were prepared by mixing surface modified ferrites with 2 wt.% poly(methyl methacrylate) dissolved in N,N-dimethylformamide. The AFM image of thin film layer shows the distribution of the nanoparticles. The saturation magnetization (Ms) and coercivity (Hc) of Ba-ferrite composites were varied 10-34 emu/g, and 1000-2000 Oe respectively.
References
1. K.S. Martirosyan and D. Luss, US Patent # 7897135, 2011. 2. K.S. Martirosyan, D. Luss: Ind. Eng. Chem. Res., 46, 1492, (2007). 3. K.S. Martirosyan, L. Chang, J. Rantschler, S. Khizroev, D. Luss, D. Litvinov: IEEE Transactions on Magnetics, 43,6, 3118,(2007). 4. K.S. Martirosyan, E. Galstyan, S. M. Hossain, Yi-Ju Wang, D. Litvinov, Mat. Sci. and Eng. B, 176, 8, 2011.
DP-12. Synthesis, Characterization and Functionalization of CoFe2O4 Nanoparticles with Piper hispidinervium C. DC oil
Tatiane Quetly M. Silva1, Jorge L. Lopez Aguilar1, Roberto Paniago2 and Hans D. Pfannes2
1Centro de Ciências Biológicas e da Natureza - CCBN, Universidade Federal do Acre - UFAC, Rio Branco, Brazil; 2Departamento de Fisica, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil
We investigated by Mössbauer spectroscopy in the range of 4.2-300 K, cobalt ferrite nanoparticles synthesized by chemical coprecipitation of FeCl3.6H2O and Co(NO3)2.6H2O in an alkaline medium and functionalized with Piper hispidinervium C. DC oil. The oil is rich in safrole. Safrole is an important compound with application in several industries, such as pharmaceutics, cosmetics. After its chemical conversion, for example, it is possible to obtain heliotropin, a fixative agent for fragrances. The oil was submitted to saponification reaction with NaOH 30% and annealing at 70oC by 5h. The product was acidified with HCl for to obtain fatty acids. The average crystallite size was 8nm, as determined from X-ray line broadening measurements of the powder sample. The saturation magnetization was 28emu/g at 4.2K. The Mössbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamegnetic relaxation. The spectrum obtained at 4.2 K is dominated by the six-line components due to CoFe2O4 particles. The blocking temperature (TB ≈200 K) was calculated from the temperature dependence of the Mössbauer spectra. The line form in spectra Mössbauer vary with the temperatures it was simulated using a model of superparamagnetic relaxation of two levels (spin ½) and theory stochastic. It was taken into account that a distribution of the size of the particles that obeys a log-normal distribution. At 4.2 K the spectra were very resolved and adjusted with two sextets that correspond to the sites tetrahedral (A) and octahedral [B]. The Mössbauer spectrum was fitted with two magnetic subspectra with internal fields Hint of 491 kOe and 515kOe, respectively.
DP-13. Structure and Magnetoelectronic Properties of Nickel and Cobalt Oxide Nanopolymers
Liudmila A. Pozhar
Department of Physics, University of Idaho, Moscow, ID
Ab initio, quantum many body theory-based computational methods1,2 have been used to "synthesize" and study magneto-electronic properties of small, non-stoichiometric Ni-O and Co-O quantum dots (QDs) and wires (QWs) from about 1 nm to 6 nm in dimensions at conditions designed to model nanostructure synthesis in quantum confinement or on surfaces, and in "vacuum" (when no foreign atoms are present). QDs and QWs so obtained exhibit prominent magnetic properties defined by uncompensated electron spins localized on Ni or Co atoms, and the direct optical transition energies from about 0.1 eV to over 4 eV. Depending on synthesis conditions, small molecules with one or two Ni or Co atoms can form both flat "antiferromagnetic" singlets with antiparallel uncompensated electron spins, and "ferromagnetic" spin multiplets (triplets and pentets) with parallel uncompensated electron spins. Stretchable Ni-O and Co-O bonds enable the development of spatial isomers to accommodate constraints of nucleation in quantum confinement or on surfaces. Somewhat larger QDs containing from 3 to 8 Ni or Co atoms have three-dimensional geometry and are higher spin multiplets. Nanoclusters possessing from 10 to about 30 Ni or Co atoms are flat nanoribbons (NRBs) with in-plane strings of Ni or Co atoms "coated" by oxygen atoms. Midsize Ni-O NRBs with 10 to 20 Ni atoms are "antiferromagnetic" singlets, while larger Ni-O NRBs with over 20 Ni atoms are "ferromagnetic" spin multiplets. In contrast, all Co-O NRBs with over 8 Co atoms are "ferromagnetic" spin multiplets. Larger NRBs of over 10 Ni or Co atoms are polymers: they are "self-assembled" in the process of proportionate addition of Ni/Co and O atoms to the smallest NRBs containing 10 Ni or Co atoms. The obtained results may help understanding physical mechanisms governing exchange bias in larger magnetic structures.
References
1. L.A. Pozhar, EuroPhys. J. D 57, 343 (2010). 2. L.A. Pozhar, A.T. Yeates, F. Szmulowicz and W.C. Mitchel, Phys. Rev. B 74, 085306 (11) (2006).
DP-14. Controlled Synthesis of Superparamagnetic Iron-Oxide Nanoparticles by Phase Transformation
Mark A. Laurenzi III1 and Everett E. Carpenter2
1Physics, Catholic University, Washington DC, DC; 2Chemistry, Virginia Commonwealth university, Richmond, VA
This work presents the results of a Fe(II) concentration dependent synthesis process. Iron-oxide nanoparticles with uniform distribution in dimension and single domain structure in the range 0<x<10 (nm) transformed throughout the volume of a Na-Ca-Fe-B (NCFB) amorphous matrix under heat treatment. X-ray diffraction (XRD), transmission electron microscopy (TEM), Mossbauer spectroscopy (MS) and superconducting quantum interference device (SQUID) support a hypothesis that the initial Fe(II) concentrati on controls nanoparticle nucleation and growth during synthesis. The Fe(II) concentration is established by reduction of the NCFB system at high temperature. The reduction proceeds according to the oxygen partial pressure resulting from exposing the matrix to a fixed ratio of CO/CO2 at high temperature. The oxidation state Fe(II) is estimated by MS. Three systems with well separated initial Fe(II) oxidation states under XRD and TEM show increasing dimension with increasing Fe(II) concentration. The diffraction features of all three systems are identical with those in literature of iron-oxide nanoparticles. The average dimension by XRD and TEM are L(vol) and D(area), respectively. L(vol) is estimated by the Stokes-Williamson equation whereas; D(area) is estimated in the usual way. The estimated dimension increases with increasing Fe(II) concentration. The single domain region for iron-oxide nanoparticles is known in literature to subsume the range in dimension reported. This, in light of the small difference in volume and area weighted dimension estimates, strongly indicates uniform distribution in dimension and single domain structure. Furthermore, all three systems are superparamagnetic at 300K and display hysteresis at 10K. This indicates that 10K is well below the blocking temperature. The remnant magnetization Mr and coercivity Hc of all three systems increase with increasing dimension. This confirms the previous results since it is well known in literature that increasing Hc with increasing dimension is characteristic of single domain structures.
DP-15. High temperature magnetic properties of Co1-xMgxFe2O4 nanoparticles prepared by forced hydrolysis method
Adolfo Franco Jr1 and F. C. e Silva1, 2
1Instituto de Física, Universidade Federal de Goias, Goiânia, Brazil; 2Instituto de Química, Universidade Federal de Goias, Goiânia, Brazil
Ferrite materials are taken as important materials from the point of view of their technological applications. Some of them can be found in telecommunications, magnetic recording, as permanent magnets, and bio-applications as magnetic fluids. Most of these technological applications require temperatures higher than room temperature and depend strongly on both size and shape of the nanoparticles, which are closely related to the method of preparation. In this work, nanoparticles of cobalt ferrites doped with magnesium ions, were synthesized by forced hydrolysis method. X-ray diffraction patterns of all systems showed broad peaks consistent with the cubic inverse spinel structure of cobalt ferrite. The average crystallite sizes, determined from the prominent peaks of the diffraction using Scherrer’s equation and TEM micrographs, was found to be 5.0 ± 0.9 nm in irregular and roughly spherical morphology. The magnetic properties measurements were carried out by using a Vibrating Sample Magnetometer (VSM) with high temperature furnace. The experiments were performed over a broad range of temperature varying from room temperature to 870 K in magnetic fields up to 2 T. Magnetization measurements show that all specimens were superparamagnetic at room temperature and the saturation magnetization (Ms), depends on the molar magnesium concentration and the temperature as well. However Ms is nearly the same at 100 K above room temperature which is suitable for bio-applications as magnetic fluids. Also the Curie temperature, Tc was measured by applying a magnetic field of 100 Oe and heating the samples from room temperature through the Curie temperature at a rate of 5 oC/min. Tc decreases with increasing the molar magnesium concentration; being 820 K and 610 K for x = 0.8 and 1.6, respectively. The effective anisotropy constants (Keff) determined at room temperature decreased with the substitution of Co2+ ion by the nonmagnetic Mg2+ ions in the spinel structure. These results were discussed in terms of interparticle interactions induced by the thermal fluctuations, cation distribution and other imperfections that could increase with temperature.
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: S. N. (Prem) Piramanayagam, DSI
DQ-01. Fabrication and magnetic properties of CoPt (101) films for bit patterned media with inclined anisotropy toward recording density of 5 Tbit/in2.
Kazuhiko Shintaku, Saori Chiba, Takanori Kiya and Kiyoshi Yamakawa
Akita Industrial Technology Center, Akita, Japan
A recording layer with inclined anisotropy was proposed to be effective in reducing magnetostatic interaction between adjacent magnetic dots in a bit patterned medium for high recording densities of more than 2 Tbit/in2 [1][2]. From the simulation results, strongly inclined anisotropy of 30 degrees from the film plane was needed for the recording density of 5 Tbit/in2. A c-axis of an hcp CoPt film with preferred crystallographic orientation of (101) plane is inclined at about 30 degrees from the film plane. In this report, method of obtaining CoPt films with preferred (101) crystallographic orientation was studied. Samples were prepared by dc and rf magnetron sputtering in an Ar gas atmosphere. The substrates were φ 2.5-inch glass disks. The designed film structure was CoPt (15 nm)/CrPt (10 nm)/ Pt (10 nm)/Ta (10 nm)/glass substrate. The CoPt and CrPt are recording and intermediate layer, respectively. The Pt underlayer was effective in increasing the crystal orientation of the CrPt. The Ta underlayer was greatly effective in enhancing the crystal orientation of the whole film. The sputtering conditions were determined so that the growth of the CoPt (001) plane could be restrained and that of the CoPt (101) plane could be preferred. From an X-ray diffraction pattern, a Pt fcc (111) and (222), a CrPt bcc (110), and a CoPt hcp (101) peak were clearly observed. The full width at half maximum value of the rocking curve (Δθ 50) of the CrPt (110) peak was 5.0°, while that of the CoPt (101) peak was 5.7°. All layers seem to be heteroepitaxially grown in this system. From the high-resolution cross-sectional transmission electron microscope (TEM) image of the CoPt layer, c-axes inclined at about 30 degrees from the film plane were clearly observed. The film structure of the whole film and magnetic properties of the CoPt layer will be discussed in detail later. Moreover, an oblique deposition method will be reported by using a sputtering system to avoid the randomly distributed anisotropy in the film plane prepared by using above process. This work was partially supported by NEDO Green IT project.
References
[1] N. Honda, K. Yamakawa, and K. Ouchi, “Simulation Study of High-Density Bit-Patterned Media with Inclined Anisotropy,” IEEE Trans. Magn., 44, 3438-3441(2008). [2] N. Honda, K. Yamakawa, and K. Ouchi, “Simulation Study of Bit Patterned Media with Weakly Inclined Anisotropy,” IEEE Trans. Magn., 46, 1806-1808 (2010).
DQ-02. Fabrication of [001] L10-FePtRh ferro-antiferromagnetic pattern by flat-patterning method
Takashi Hasegawa1, Tatsuya Tomioka1, Yuji Kondo2, Haruki Yamane1 and Shunji Ishio2
1Department of Materials Science and Engineering, Akita University, Akita City, Japan; 2Akita Industrial Technology Center (AIT), Akita, Japan
For the fabrication of BPM, (i) lithographic patterning, (ii) chemical methods and (iii) flat-patterning that modifies the physical properties of films keeping surface flatness have been proposed so far. In methods (i) and (ii), removing the material between the dots or assembling of the particles leads to non-uniformity in the height of the disk surface, which causes the head floating problem in the near-contact recording scheme. In method (iii), a ferromagnetic (FM) film changes to a paramagnetic (PM) or antiferromagnetic (AF) phase locally by ion irradiation or atomic diffusion [1]. The L10 FePt1-xRhx film has a high Ku and shows a FM-AF transition [2]. The films with 0≤x≤0.32 are FM, and with 0.34≤x≤0.40 are AF. In this study, a FM-AF pattern using the [001]-oriented L10-FePtRh film fabricated by method (iii) was investigated. Figure 1(a) shows a MFM image of the FM-AF pattern. FM dots with diameter (D) of 50 nm are observed, and no magnetic forces are detected on the area between dots. Dot size and position distributions (σ) are 3.0 nm and 3.7 nm, respectively. Average MFM contrast and contrast distribution (σ) are 3.9 deg and 0.99 deg, respectively. Figure 1(b) shows a polar kerr hysterisis curve perpendicular to the film plane. Coersivity and its distribution (δHc) are 2.5 kOe and 4.2 kOe, respectively.
References
[1] T. Hasegawa et al., J. Appl. Phys., 109, 07B705-1 (2011). [2] T. Hasegawa et al., J. Appl. Phys., 106, 103928-1 (2009).
DQ-03. Control of Magnetic Properties of MnBiCu Thin Films by Kr+ Ion Irradiation
Qianqian Xu1, Ryutarou Kanbara1, Takeshi Kato1, Satoshi Iwata1 and Shigeru Tsunashima2
1Department of Quantum Engineering, Nagoya University, Nagoya, Japan; 2Department of Research, Nagoya Industrial Science Research Institute, Nagoya, Japan
We have previously reported that the ferrimagnetism of CrPt3 was destroyed by a low Kr+ ion dose of 2×1014 ions/cm2, and that the planar bit patterned media were fabricated using this ion irradiation[1]. In this paper, ion irradiation on MnBi and MnBiCu films has been studied, since these films are known to exhibit large perpendicular anisotropy and are fabricated by low process temperature around 300°C which is much lower than that of CrPt3(850°C). MnBi and MnBiCu thin films with a thickness of 15 nm were prepared by magnetron sputtering of Mn-Cu/Bi multilayers followed by vacuum annealing at 350°C. The Mn52Bi48 film exhibited surface roughness of 17.5 nm, and the roughness was improved by adding Cu. For Mn52Bi28Cu20 film, surface roughness of 2.5 nm was obtained. The Mn52Bi28Cu20 film exhibited a saturation magnetization Ms of 210 emu/cc, the coercivity Hc of 3.4 kOe, and perpendicular anisotropy of 1.7×106 erg/cc. Figure 1 shows the dependence of the Ms and Hc of the Mn52Bi28Cu20 film on the 30 keV Kr+ ion dose intensity. The Ms and Hc decreased by the irradiation more than 1012 ions/cm2 and disappeared at a low Kr+ ion dose of 5×1013 ions/cm2. Thus, MnBiCu is considered to be a candidate for planer bit patterned media fabricated by the ion irradiation.
< div height="0">References
[1] T. Kato et al., J. Appl. Phys., 105, 07C117 (2009)
DQ-04. Fabrication of exchange-coupled Fe/FePt dots by anodized aluminum oxide template
Ju-Cheng Hsiao, Yen-Chun Huang, Liang-Wei Wang, Jung-Wei Liao and Chih-Huang Lai
Materials Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan
We fabricated exchange-coupled composite (ECC) Fe/FePt dots by using porous alumina template. The layer structure was Al(100 nm)/Ti(5 nm)/Fe(2 nm)/L10 FePt(5 nm)//SiO2/Si substrate. L10 FePt was fabricated by atomic-scale multi-layer sputtering1 and post annealing with RTA at 400 °C. Fe/FePt composite films have a smaller coercivity than FePt only films and still exhibit a large perpendicular anisotropy. The Ti layer was used for the adhesion layer between Al and Fe/FePt films, and also prevented the Fe layer from oxidization during the anodization process. The anodization was performed in 0.3 M sulfuric acid under DC voltage of 25V. The sample was immersed in 6 wt% phosphoric acid to enlarge the pore diameter. After forming anodized aluminum oxide (AAO) template, a Ta layer was directly deposited on the top of the AAO. Part of Ta went into the bottom of pores and was used as the hard mask for the subsequent etching after removing AAO. Etching was performed by using reactive ion etching (RIE) with CO/NH3 gas mixture. The SEM image and the size distribution of Fe/FePt dots are shown in Fig. 1(a). The average size of Fe/FePt dots is 33.4 nm. We also fabricated FePt dot array with a similar size by the same process. Both FePt and Fe/FePt dots showed strong perpendicular anisotropy. Due to the exchange coupling between Fe and FePt, the Fe/FePt dots showed a smaller coercivity than FePt dots, shown in Fig. 1(b). Furthermore, the Fe/FePt dots exhibited narrower switching field distribution (SFD) than FePt dots. Therefore, the ECC patterned Fe/FePt dots may possess improved writability and SNR than single-layered FePt dots.
References
1. Y. C. Wu, L. W. Wang and C. H. Lai, Applied Physics Letters 91 (2007).
DQ-05. Time Dependent Estimation of the Switching Field Distribution of Bit Patterned Media with 17- and 35-nm Pitch CoPt Dots
Akira Kikitsu, Yosuke Isowaki, Tomoyuki Maeda and Yoshiyuki Kamata
Storage Materials & Devices Laboratory, Toshiba Corp., Corporate R&D Center, Kawasaki, Japan
One of the key challenges of the bit patterned media (BPM)is a reduction of the switching field distribution (SFD) of magnetic dots. In the previous study, it was shown that the intrinsic SFD became small at the short time region by the influence of the thermal fluctuation[1]. In this study, dipole SFD was investigated by the time dependent estimation. Difference between intrinsic and dipole SFDs as well as the dot size dependence are discussed. Co80Pt20 alloy film was etched through a self-assembled polymer mask with 17- and 35-nm pitch[1]. Magnetic properties were estimated by a polar Kerr measurement under a static and pulse (150 μs) field. Intrinsic SFD was estimated by the delta-Hc method and dipole SFD was estimated by the differentiation of majour loop. Figure shows the time dependence of intrinsic and dipole SFD. Ratio of the SFDs to the coercivity are plotted in a lower panel. It is found that dipole SFD is almost constant against the measurement time. This result is due to the little influene of the thermal fluctuation on the magne tization (exponential to time) compare to the coercivity (logarithmic to time) for 1 to nano second time domain. Both BPMs have similar dipole SFD owing to their pitch and average magnetization.
References
[1] A. Kikitsu et al., abstract of 55th MMM conference, CF-01 (2010). A part of this work was funded by NEDO Green IT project.
DQ-06. Switching properties of exchange coupled magnetic dot arrays for next-generation bit patterned media
Yuji Kondo1, Takanori Kiya1, Takashi Hasegawa2, Shunji Ishio2, Jun Ariake1 and Naoki Honda3
1Akita Industrial Technology Center, Akita, Japan; 2Faculty of Engineering and Resource Science, Akita University, Akita, Japan; 3Faculty of Engineering, Tohoku Institute of Technology, Sendai, Japan
As the dot density of bit patterned media (BPM) becomes high, the switching field width of the dot array increases because of increased magnetostatic interaction between the dots. We have proposed the exchange coupled BPM between the dots as a technique to reduce the switching field width [1]. The switching properties of Co80Pt20 dot arrays with the controlled exchange coupling were studied. Dot arrays with the height of 7 nm, the diameter of 17 nm and the period of 30 nm were patterned using an electron beam lithography. The exchange coupling between the dots was controlled with residual film thickness (tr=0 ~ 1.5 nm) during ion milling. Magnetization curves were measured using anomalous Hall effects and magnetic domains were observed by magnetic force microscopy (MFM). Fig. 1 shows the field angle dependence of the coercivity and MFM images for the dot arrays with various residual film thicknesses after DC demagnetization, respectively. Fig. 1 (a) indicates deviation of the switching property with the residual thickness from the Stoner-Wohlfarth type. SFD was also reduced with the thickness, but the magnetic cluster size became large with the thickness (b to d), which would cause write errors on adjacent tracks. However, the exchange coupling could be easily optimized with the residual film thickness.
References
[1] Y. Kondo et al., Digests of Intermag Conference 2009, CP-03, 2009.
DQ-07. Reduction of Switching Field Distribution in Bit-Patterned Media
Mojtaba Ranjbar1, 2, S. N. Piramanayagam1, Seng kai Wong1, Rachid Sbiaa1 and Tow Chong Chong1, 2
1Data Storage Institute, (A*STAR) Agency for Science, Technology and Research, Singapore, Singapore; 2Electrical and computer engineering Department, National University of Singapore, Singapore, Singapore
Switching field distribution (SFD) is one of the major challenges in bit-patterned media (BPM). In addition to the intrinsic contributions to the SFD, the magnetostatic interactions from the neighboring bits also widen the SFD, especially at high densities. Antiferromagnetically coupled BPM is one approach to study and understand this problem [1]. In this paper, we demonstrate experimentally that capped BPM - where a thin layer of magnetic material provides an optimum exchange coupling - is another suitable technique to reduce SFD. Thin films of the type [Co(0.3nm)/Pd(0.8nm)]xN were deposited on Ta/Pd seedlayers using dc magnetron sputtering system at 1.5 mTorr Ar pressure, where N represents the number of bilayers. The films were pat terned using HSQ resist and electron beam lithography. Ion-milling was carried out in such a way that the magnetic layers in films with N=5 and 10 were fully etched but those with N=15 and 25 were etched partially, resulting in an exchange coupling through the unetched magnetic layers. Hall probes were further fabricated on the patterned dots. Anomalous Hall Voltage (AHV) measurements were made on the dots at various applied fields and angles. Figure 1 shows the hysteresis loops of the thin film samples, studied by AGM and the corresponding patterned dots studied by AHV. The coercivity of all the thin film samples are lower and almost identical and can be understood to be arising from the domain wall motion initiated from the nucleation sites with lower reversal fields. The fully patterned sample shows a significant increase of coercivity to about 10 kOe. The capped BPM, on the other hand shows a lower coercivity and narrower SFD. These results indicate that Capped BPM is a potential candidate to reduce SFD.
References
1. S. N. Piramanayagam et al., J. Appl. Phys. 105, 07C118 (2009).
DQ-08. Switching Field Distribution Analysis on L10-FePt ECC Bit Patterned Media
Hao Wang1, Weimin Li2, 3, Tofizur Rahman1, Haibao Zhao1, Jun Ding2, Yunjie Chen3 and Jianping Wang1
1Electrical and Computer Engineering, Large Lakes Oberservatory, Minneapolis, MN; 2Materials Science and Engineering, National University of Singapore, Singapore, Singapore; 3Data Storage Institute, A*STAR, Singapore, Singapore
L10-FePt bit patterned media with exchange coupled composite (ECC) structure has been considered as one of candidates for future magnetic recording media. In this paper, ECC L10-FePt patterned structure with 32nm bit size in large area was fabricated using di-block-copolymer lithography1. Switching field distribution (SFD) of the fabricated ECC patterned media was analyzed using different techniques. Typically, the dipole interaction has to be removed to extract the intrinsic SFD for perpendicular granular media. However, for bit patterned media, the magnetization reversal process of a certain recording bit could be influenced by the dipole field from neighboring bits. SFD broadening is expected based on traditional testing methods. In this work, we not only measured the SFD by subtracting the dipole interaction, but also took MFM images to track the remanent states of the media to observe the SFD broadening phenomenon. MFM images were captured with different magnetic fields2. Typical results were shown in Fig.1. The ECC structure with high saturation magnetization was found to enhance the SFD broadening. On the contrary, the ECC structure with lower Ms narrowed the SFD. Micromagnetic simulation was carried out to confirm this conclusion. Acknowledgement: This work has been supported by INSIC and we thank Drs. Yousuke Isowaki, Yoshiyuki Kamata and Akira Kikitsu from Toshiba Central Research Lab and Drs. Jianzhong Shi and Tianli Huang from Data Storage Institute.
References
[1] H. Wang, T. Rahman, H. Zhao, Y. Isowaki, Y. Kamata, A. Kikitsu, and J.P Wang, J. Appl. Phys. 109, 07B754 (2011). [2] W. M. Li, Y. J. Chen, T. L. Huang, J. M. Xue, and J. Ding, J. Appl. Phys. 109, 07B758 (2011).
DQ-09. Angular dependence and temperature effect on switching field distribution of Co/Pd based bit patterned media
Weimin Li1, 2, Xuelian Huang1, Jianzhong Shi 2 and Jun Ding1
1material science and engineering, National Univ Singapore, Singapore, Singapore; 2Data Storage Institute, Agency for Science, Technology and Research (A*STAR), Singapore, Singapore
A key issue in patterned-media is the switching field distribution (SFD) needs to be sufficiently narrow to secure exact addressability. However, its mechanism is not fully understood. In this work, angle and temperature dependences of Hcr (coercivity) and SFD are studied based on patterned bits (size=80nm, pitch=100nm) fabricated on Co/Pd films[1]. From our observation, the single isolated bit switched coherently as Stoner-Wohlfarth particles, as upper image shown in fig.1(a). However, dipolar interaction will affect magnetization reversal and then broaden SFD. The relative SFD (SFD/Hcr) as a function of the angles by different method (experimental, LLG simulation, formula calculation) are obtained and compared, as lower image shown in fig.1(a). As angle increase, SFD/Hcr slightly increase, however, the intrinsic SFD/Hcr is not related to angles. In the formula calculation, we assumed that every bit switched uniformly, so it shows relatively lower SFD/Hcr values. While LLG simulation shows higher SFD/Hcr because each bit is divided to many cells, the involving of spins will increase the SFD. Experimental results which show highest SFD/Hcr indicating that there are other factors which will broaden SFD (such as defects, thermal effects, etc.) besides dipolar interactions. SFD at both 77K and 300K are obtained from standard deviation of normalized M-H curves, as dashed lines shown in fig.1(b). Insert images are remanent magnetization states for several applied field at 77K. It shows that coercivity increased as the temperature, which can be explained that thermal energy decreases the switching energy barrier. However, the relative SFD is constant with temperature, indicating that SFD are not caused by random thermal fluctuations.
References
1. W. M. Li, Y. J. Chen, T. L. Huang, J. M. Xue and J. Ding, Journal of Applied Physics 109 (7), 07B758-753 (2011).
DQ-10. Scanning magnetoresistance microscope investigation of site specific reversal of graded bit patterned media
Long V. Chang1, Paul Ruchhoeft1, Sakhrat Khizroev2 and Dmitri Litvinov1
1Electrical and Computer Engineering, University of Houston, Houston, TX; 2Electrical and Computer Engineering, Florida International University, Miami, FL
Bit patterned media development has been hindered by fabrication challenges concerted towards improving the switching field distribution. The origin of the switching field distribution is under intense scrutiny and research efforts suggest the source is a magnetic anisotropy distribution [1]. We further explore this theory by fabricating bit patterned media with a binary anisotropy distribution and performed write displacement experiments with a scanning magnetoresistance microscope (SMRM). A binary anisotropy distribution in graded bit patterned media is achieved by angled helium ion irradiation. Light ion irradiation is known to reduce the magnetic anisotropy of Co/Pd multilayers. To confine the damage to a local region on the bits, bit patterned media was fabricated with 100 nm of residual resist. The residual resist serves as an ion stopping layer capable of blocking helium ions up to 2500 eV. An off-axis broad beam irradiation creates localized edge damage producing a binary anisotropy distribution in every bit as confirmed by Fig. 1. The switching behavi or of these bits has been explored numerically and it was found that reversal begins at the lower anisotropy region that was damaged by ions. The simulation result is confirmed experimentally with a SMRM which raster scans a recording head in contact with the bit patterned media. By applying the minimum switching field to switch a bit at various write pole displacements, a binary reversal map is produced revealing the position of the nucleation sites. The fidelity of the SMRM has been established to reliably investigate our hypothesis on 100 nm wide bit patterned media.
References
[1] J.M. Shaw, SE Russek, et al., Phys. Rev. B, 78, 024414 (2008)
DQ-11. A Two-Dimensional Coding for Patterned Media Recording
Gukhui Kim and Jaejin Lee
Soongsil Univ., Seoul, Republic of Korea
In bit patterned media, amplitude of a bit signal can be affected by signals of adjacent bits [1][2]. The amplitude is the most destructive when the bit has the same value with surrounded 8 bits [3][4]. The 7/9 modulation code [3] and the 5/6 modulation code [4] are 2-D modulation codes which prevent this worst case. The two codes place redundant bits in predefined positions in every 3-by-3 array. In [3][4], those codes show better performance compared with no coding scheme. We propose a 7/8 modulation code whose code rate is better than 5/6. The codeword consists of 4-by-2 bit array (c={ci}, i=0,…,7). In this way, the neighboring 2-by-2 bit array of the codeword is always included in every 3-by-3 array. The proposed code avoids the case of all ‘1’s or ‘0’s in the neighboring 2-by-2 array. As all 3-by-3 bits include these 4 bits, proposed code can prevent the worst case. Encoder inputs source data (d={di}, i=0,…,6). If (d0,d1,d2,d3) have the same value, encoder outputs a codeword by using the logic system in Fig.1(a). If those bits are not identical, encoder checks the other three bits. When the (d4,d5,d6) is ‘0’, encoder outputs a codeword (ci=di, i=0,…,6, c7=1). Otherwise, the encoder outptuts a codeword (ci=di, i=0,…,6, c7=0). The decoder checks (c6,c7) bits. If both two bits are ‘1’, decoder restores outputs (d′={d′i}, i=0,…,6) by using system in Fig.1(b). Otherwise, it decides to (d′i=ci, i=0,…,6). By using this coding scheme, magnetic patterned media can prevent the case of all identical bits in every 3-by-3 array and it guarantees better BER performance than no coding scheme.
References
1. A. Moser, "Magnetic recording: Advancing into the future," J. Phys.D, Appl. Phys., vol.35, no.19, pp. R157-R167, 2002. 2. S. K. Nair and R. M. H. New, “Patterned media recording: Noise and channel equalization,” IEEE Trans. Magnetics, vol. 34, no. 4, pp 1916-1918, 1998. 3. J. P. J. Groenland and L. Abelmann, "Two-dimensional coding for probe recording on magnetic patterned media," IEEE Trans. on Magnetics, vol. 43, No. 6, June 2007. 4. X. Shao, L. Alink, H. Groenland, L. Abelmann and C. H. Slump, "A simple two-dimensional coding scheme for bit patterned media," Proc. IEEE Intermag 2011, DE-06, April 2011.
DQ-12. Unequal Error Correction Strategy for Multi-track Processing in Bit Patterned Media Systems
Pornchai Supnithi1 and Lin M. Myint2
1Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand; 2School of Management Technology, Shinawatra University, Bangkok, Thailand
For bit patterned media (BPM) recording systems, multi-track processing offers the performance gains in the 2-D interference channels [1]-[3]. In multi-track processing, when each non-overlapping N tracks is processed, the detection of the main track has all the necessary data from the neighboring tracks to process, but the outer tracks may not, hence, the unequal bit error rates of these tracks will be observed as shown in Fig. 1. In this work, we investigate the use of the unequal error protection (UEP) among tracks to tackle this issue, and design the suitable combination of structured LDPC codes with unequal code rates to improve the overall performance in the system. We consider the system in [2], the readback signals from 3 adjacent tracks are detected at a time and each track is handled by the turbo equalization with one soft-output Viterbi algorithm (SOVA) detector and a low-density parity-check (LDPC) decoder. One drawback is that the SOVA detectors of the upper and lower tracks receive partial ITI estimations only, unlike the processing of the middle track. Therefore, we use the UEP technique for the multi-track processing. In the UEP system, we employ the structured LDPC codes with different code rates for the three tracks. We assign the higher code rates to the two outer tracks and the lower code rate to the middle tracks. We compare the performance of UEP system with that of the equal error protection (EEP) system. Similar to the UEP system, an EEP system is set up using the three LDPC codes with the same code rates for all three tracks. For a fair comparison, the average code rates in the EEP and UEP cases are about 0.89. In Figure 2, the UEP system does improve over the EEP scheme at the low SNRs, but it offers the improved performance over the EEP at SNR above 5.2 dB.
References
[1] B. Kurkoski, IEICE Trans. Fundamentals, vol. E91, no. 10, pp. 2696-2703, Oct. 2008. [2] L.M.M. Myint, P. Supnithi, and P. Tantaswadi, IEEE Trans. Magn., vol. 45, no. 10, Oct. 2009. [3] W. Chang and J.R. Cruz, IEEE Trans. Magn., vol. 46, no. 11, Nov. 2010.
DQ-13. Computational analysis of microwave assisted magnetization reversal for exchange coupled composite grain
Yoshitoki Furomoto1, Ayumu Kato1, Terumitsu Tanaka1, Anis Faridah Md Nor2 and Kimihide Matsuyama1
1Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan; 2Department of Physics, Malaya University, Kuala Lumpur, Malaysia
Microwave assisted magnetization reversal (MAMR)[1] is a promising candidate for future magnetic recording. Li et al. proposed ECC medium for solving the potential problems in microwave power and frequency for MAMR[2]. Bertotti et al. theoretically analyzed MAMR process taking a single spin for example[3], which showed the LLG equation can be written in the other forms[4]. The equations allow three solutions “stable”, “unstable” and “saddle”. However, the theory cannot be applied to single spin with incident angle, θ, between anisotropy axis and DC reverse field, HDC, or ECC grain. In this study, distributions of the regions obtained by three solutions were estimated for ECC grain. Figure 1(a) shows the critical fields in a single spin as a parameter of θ. Microwave frequency is set to 80 GHz which is optimal frequency for MAMR of the single spin. Critical DC field jumps at Hac=3.8 kOe at θ =0 indicating bifurcation between the stable and unstable regions. The bifurcation shi fts toward the lower Hac with increase in θ. Figure 1(b) shows critical curves for an ECC grain with θ =0 and 45 degree. The optimum frequency for MAMR is 14 GHz which is significantly lower than that of the single spin. The jump at Hac=1.8 kOe for θ =0 is possibly bifurcation between the stable and unstable regions. However, Hac and Hdc of the bifurcation are as small as 1.8 kOe and 2.5 kOe, respectively. These values are much smaller than those for the single spin. The bifurcation shifts toward lower Hac with increase of θ in the same manner as that for the single spin.
References
[1] J. -G. Zhu, X. Zhu and Y. Tang, IEEE Trans. Magn. 44, 125(2008). [2] S. Li, B. Livshitz, H. N. Bertam, M. Schabes, T. Schrefl, E. E. Fullerton and V. Lomakin, Appl. Phys. Lett. 94, 202509(2009). [3]Giorgio Bertotti , Claudio Serpico , and Isaak D. Mayergoyz, Phys. Rev. Lett. 86, 724 (2001) [4] Werner Scholz and Sharat Batra, J. Appl. Phys. 103, 07F539(2008)
DQ-14. Reduction in switching fields by thermal activation in microwave assisted magnetization reversal
Terumitsu Tanaka1, Yoshitoki Furomoto1, Ayumu Kato1, Anis Faridah Md Nor2 and Kimihide Matsuyama1
1Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan; 2Department of Physics, Malaya University, Kuala Lumpur, Malaysia
Microwave assisted magnetization reversal (MAMR)[1] has potential application in future magnetic recording. The critical curves for MAMR in a single-domain particle is theoretically estimated by Bertotti et al.[2, 3], which well explains steady state magnetization switching at 0 K. In this study, MAMR properties in a single-domain particle were simulated solving the Landau-Lifshitz-Gilbert equation taking account of temperature elevation due to ferro-magnetic resonance. Anisotropy field and damping constant at 0 K were set to be 50 kOe and 0.01, respectively. Fig. 1(a) shows the critical curves of magnetization switching at 0 K and 400 K. DC field, HDC, and 50 GHz of circular polarized field, HMW, were included in the calculation. The critical curve for 0K well agrees with theoretical calculation[2]. When the grain is heated up to 400 K, magnetization switching probability widely distributes in HMW at around 20 kOe of HDC. The critical values of HMW are greater than that at 0 K nevertheless thermal activation decreases energy barrier height. This implies stochastic thermal fields reduce efficiency in MAMR in the region. On the other hand, thermal activation gives 25 % of reduction in critical value of HMW. Fig. 1(b) shows that the relationship between critical HDC and incident angle of DC field. The figure translates thermal activation assists particle crossing the energy barrier for switching in the case of suitable incident angles. The minimal value of critical HDC smaller than 15 kOe was obtained at 135 degree at 400 K, which is achievable for recording head of recent hard disk drive.
References
[1] J. O. Artman, S. H. Charap, and D. J. Seagle, IEEE Trans. Magn. MAG-19, 1814 (1983). [2] G. Bertotti, C. Serpico, and I. Mayergoyz, Phys. Rev. Lett. 86, 724 (2001). [3] S. Batra and W. Scholz, IEEE Trans. Magn. 44, 3392 (2008).
DQ-15. Ferromagnetic resonance properties of granular Co-Cr-Pt films measured using micro-fabricated cop lanar waveguides
Takafumi Kobayashi1, Naoko Ishida1 and Yukio Nozaki1, 2
1Dept. of Physics, Keio University, Yokohama, Japan; 2JST, CREST, Tokyo, Japan
The dynamical magnetization process caused by an application of alternating magnetic field with a frequency of a few tens gigahertz attracts much attention for the application to microwave-assisted magnetic recording. Recently, the microwave-assisted magnetization reversal (MAMR) in perpendicularly magnetized films was experimentally demonstrated[1,2]. However, the MAMR on a granular film generally utilized for the medium of hard-disk drives (HDDs) has not been reported yet. To realize the MAMR in practical HDDs, it is significant to quantitatively understand the influence of both the exchange and the dipole interactions among the grains on a nonlinear damping process of magnetization. In this study, the ferromagnetic resonance (FMR) properties of Co-Cr-Pt based granular film have been studied by means of vector network analyzer FMR spectroscopy. Electrically shorted coplanar waveguide (CPW) was fabricated on 13 nm thick Co-Cr-Pt granular film with a lateral size of 10×100 μm2 using liftoff technique. The width and length of the center conductor of CPW were 2 and 100 μm, respectively. The coercive field, nucleation field, saturation field and anisotropy field of the granular film are 0.18, -0.18, 0.72 and 1.46 T, respectively. The saturation magnetization is 0.47 T. Figure 1 shows the microwave reflection coefficient of the CPW as a function of the external magnetic field applied perpendicular to the film plane. The frequency of the microwave was fixed at 30, 35 or 40 GHz. The asymmetric FMR spectra observed at 30 and 35 GHz are attributed to the magnetic hysteresis of the granular film. The broad linewidth of the FMR spectrum suggests the large distribution of resonance frequency caused by the dynamic dipole interaction.
References
[1] Y. Nozaki, N. Narita, T. Tanaka, and K. Matsuyama, Appl. Phys. Lett. 95, 082505 (2009). [2] T. Yoshioka, T. Nozaki, T. Seki, M. Shiraishi, T. Shinjo, Y. Suzuki, and Y. Uehara, Appl. Phys. Express 3, 013002 (2010).
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Xi Chen, Seagate Technology
DR-01. Multipole expansion technique for the magnetostatic field computation in patterned magnetic films
Alessandra Manzin and Oriano Bottauscio
Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy
In the last years, there has been a growing interest in magnetic antidot films to investigate their potential application in recording media and sensor devices [1-3]. The micromagnetic simulation of the switching mechanism in antidot arrays requires an accurate evaluation of the local magnetostatic fields induced by the hole presence. However, this task can be very time-consuming when modeling films with size of the order of some micrometers. Possible approaches for reducing the CPU time include the use of fast Fourier transform (FFT) [4], fast multipole method (FMM) [5-7] and FFT on multipoles [8]. This work is focused on the calculation of the magnetostatic field in magnetic antidot films with square lattice and square holes, varying film and hole sizes. To speed-up the computation and limit the memory requirements, the magnetostatic field due to "far" dipoles is evaluated by a multipole expansion technique. The patterned film i s decomposed into identical macrocells representing the spatial period, in turn discretized into a microscale grid. Locally, the field is the sum of two terms. The first one is computed by taking into account the contributions of a limited number of surrounding macrocells that identify a “near” zone. The second term derives from a multipole expansion in terms of spherical harmonics and considers the contributions of the remaining macrocells [8, 9].
References
[1] N. G. Deshpande, M. S. Seo, X. R. Jin, S. J. Lee, Y. P. Lee, J. Y. Rhee, and K. W. Kim, Appl. Phys. Lett. 96, 122503 (2010). [2] R. Cheng, B. L. Justus, A. Rosenberg, D. N. Mcllroy, Z. Holman, D. Zhang, and Y. Kranov, J. Appl. Phys. 108, 086110 (2010). [3] D. Tripathy, P. Vavassori, J. M. Porro, A. O. Adeyeye, and N. Singh, Appl. Phys. Lett. 97, 042512 (2010). [4] R. D. McMichael, M. J. Donahue, D. G. Porter, and Jason Eicke, J. Appl. Phys. 85, 5816 (1999). [5] Y. Takahashi, S. Wakao, T. Iwashita, and M. Kanazawa, J. Appl. Phys. 105, 07D514 (2009). [6] B. Van de Wiele, F. Olyslager, and L. Dupré, J. Comp. Phys. 227, 9913 (2008). [7] P. B. Visscher and D. M. Apalkov, Physica B 343, 184 (2004). [8] H. H. Long, E. T. Ong, Z. J. Liu, and E. P. Li, IEEE Trans. Magn. 42, 295 (2006). [9] A. Manzin and O. Bottauscio, IEEE Trans. Magn. 45, 5208 (2009).
DR-02. Model study for spinlogic devices combining micromagnetic simulations with spin transport
Andrew Tuggle1, 2 and Claudia Mewes1, 2
1Center for Materials for Information Technology (MINT), University of Alabama, Tuscaloosa, AL; 2Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL
Based on the success and intensive research in the field of spintronics, we investigate a form of spin-based logic (spinlogic). The research in the field of spinlogic is just emerging, and it is still unclear which approach will lead to a new extremely energy-efficient way to perform logic operations. Some of the pursued approaches are based on domain wall motion, magnetic quantum dot automata, spin wave propagation or different three-terminal architectures. One promising approach is a three-terminal device taking advantage of Spin Transfer Torques (STT) to perform logic operations. These devices intimately couple micromagnetic and spin transport effects, so that traditional micromagnetics simulation codes do not provide an adequate description of device dynamics, required for successful demonstration of logic device function and for developing an effective dynamic switching model. A new finite element analysis (FEA) code, SpinFlow 3D, combines micromagnetic and spin transport simulations in a self-consistent way, yielding the appropriate level of description for STT-switched logic devices, thus providing theoretical guidance for device design and characterization. We present first results for spin transfer torque switched logic devices, including critical current densities.
DR-03. Effects of notch shape on the magnetic domain wall motion in nanowires with in-plane or perpendicular magnetic anisotropy
Su Jung Noh1, Yasuyoshi Miyamoto2, Mitsunobu Okuda2, Naoto Hayashi2 and Young K. Kim1
1Department of Materials Science and Engineering, Korea University, Seoul, Republic of Korea; 2Imaging & Storage Devices Research Division, NHK Science and Technology Research Laboratories, Tokyo, Japan
Magnetic domain wall (DW) motion in nanowires (NWs) is extensively investigat ed for potential applications in spintronic devices [1]. For the precise storage of magnetic data, the control of DW pinning and depinning is critical. Here, we report micromagnetic modeling results of the DW motions in notched or anti-notched NWs with in-plane or perpendicular magnetic anisotropy (IMA or PMA). The notch of NW acts as a potential well on DW motions, while the anti-notch works as an energy barrier [2]. Fig.1(a) shows the modeling NW structures with triangular-shaped notches and anti-notches. In the NWs with IMA, the DW was depinned from anti-notches under the current density of 5x108 A/cm2. However, the DW vibrated during pinning at the notch because it was not able to overcome pinning potential of notches at the same current density (Fig.1(b)). It seems easier for the DWs to overcome the pinning energy barrier of the anti-notches than the pinning potential well of the notches. Unlike IMA case, DW moves faster and easier through the notches than through the anti-notches in the NW with PMA at the same current density. The total energy (sum of exchange, anisotropy and demagnetizing energy) of the DW is lower in notched NW (Fig.1(c)), which leads to easier DW depinning.
References
[1] S. S. P. Parkin, M. Hayashi and L. Thomas, Science 320, 190 (2008). [2] C. W. Sandweg et al., J. Appl. Phys. 103, 093906 (2008).
DR-04. Effect of Enhanced Damping Caused by Spin-Motive Force on Vortex Dynamics
Jung-Hwan Moon and Kyung Jin Lee
Dept. of Mat. Sci. & Eng., Korea University, Seoul, Republic of Korea
A vortex state is a stable magnetic configuration for a patterned disk. Field or current-driven vortex dynamics has been extensively investigated. One of the most important parameters to determine the vortex dynamics is the Gilbert damping that has been generally assumed to be constant. However, Zhang proposed that the spatiotemporal change in the magnetization generates an additional damping due to the spin-motive force, which has the form of 3x3 tensor [1]. Thus, the effect of the spin-motive-force-induced damping tensor should be carefully examined to correctly understand the vortex dynamics. In this work we investigated the effect of the damping tensor on the resonance frequency and the core velocity by means of micromagnetic simulation. We used a Permalloy disk with diameter of 270 nm and thickness of 20 nm, where the vortex state is the equilibrium state. Fig. 1(a) shows that the resonance frequency of vortex gyration does not change even with considering the damping tensor. It is because the resonance frequency is solely determined by potential well determined by the disk geometry. Fig. 1(b) shows the core velocity as a function of the time, where the vortex core is excited by a pulsed field. The core velocity decays more rapidly when the damping tensor is considered. By fitting the result with the exponential decay function, the increase in energy dissipation rate is found to be equivalent to the additional Gilbert damping constant of 0.0026, which could be significant in a magnetic material having a small intrinsic damping constant. The value of time-dependent damping tensor and its effect on vortex dynamics such as vortex core reversal will be presented in detail.
References
[1] S. Zhang and S. S.-L. Zhang, Phys. Rev. Lett. 102, 086601 (2009).
DR-05. Magnetization dynamics of a ferromagnetic quantum dot under spin bias
Zhuo Bin Siu1, 3, Mansoor Jalil2, 1 and Seng Ghee Tan3
1NUS Graduate S chool for Integrative Sciences and Engineering, National Univ Singapore, Singapore, Singapore; 2Electronic and Computer Engineering, National University of Singapore, Singapore, Singapore; 3Data Storage Institute, Singapore, Singapore
In the phenomenon of Spin Transfer Torque[1,2], the flow of a spin polarized current through a magnetized material imposes a torque on the magnetization. The magnetization dynamics are usually described by the Landau-Lifshitz (LL) equation, while in nanoscale electronics the spin current can be calculated using the non-equilibrium Green's function (NEGF) formalism. Here we combine the two formalisms to model the magnetization dynamics of a ferromagnetic quantum dot (QD) sandwiched between two semi-infinite leads. Fig. 1a shows a quantum dot initially magnetized in the +z direction connected to a spin bias[3] in the y direction. The spin polarization of the QD electron states under the initial magnetization are first calculated using the NEGF formalism. The resulting spin polarization then acts as an effective magnetic field in the LL equation for calculating the magnetization. This new magnetization is used for the NEGF calculation of the new QD spin polarization. The cycle of NEGF and LL calculations are carried out repeatedly to obtain the time evolution of the QD magnetization. The resulting trajectory of the magnetization is shown in Fig. 1b. This scheme captures the interplay between magnetization, spin current and spin torque and allows the magnetization and spin polarization to be determined self consistently.
References
[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) [2] L. Berger, Phys. Rev. B 54, 9353 (1996) [3] D. K. Wang, Q. F. Sun, and H. Guo, Phys. Rev. B 69, 205312 (2004)
DR-06. Revisit of Magnetization Reversal With Spin Transfer Torque in the Finite Temperature
Xingzhi Cheng
Electrical & Computer Engineering, National University of Singapore, Singapore, Singapore
Revisit of Magnetization Reversal With Spin Transfer Torque in the Finite Temperature Magnetization reversal in the presence of spin transfer torque has been widely studied in the past decade for its potential applications in manipulating the magnetization dynamics. Theoretical studies have been conducted in predicting the switching properties, such as the thermal stability, with respect to the external coupling in a finite temperature condition. We reexamine some previous theoretical studies which simplify the spin transfer torque effects into reduction of the effective energy barrier Eb_{eff} [1]. We found that a missing torque ( \alpha/Ms * m x p ) besides the well adopted spin torque ( a_J m x (mxp) ) has been ignored during the transformation from Gilbert damping picture to the Landau-Lifshitz-Gilbert (LLG) picture. Both Fokker-Planck equation corresponding to the stochastic LLG equation with complete spin torque items and its solution is re-derived. We found that the spin transfer torque effects can be categorized into two type applied fields: a derivative of linear potential (explained) and an unexplained field. While a general closed form solution to the steady state is not available, we are able extract asymptotical results in both the weak and strong coupling limit. We also revisit one of the recent discussion on this topic [2], in which we notice a missing item was found in the Fokker-Planck coefficients (kCot(\theta)) and therefore leads to a defect in the discussion. However we are supporting their main conclusion that the effected energy barrier is affected by a factor of (1- I /Ic)^2 in some aspects.
References
[1] Z . Li and S. Zhang, Phys. Rev. B. 69, 134416 (2004) [2] T. Taniguchi and H. Imamura, Phys. Rev. B, 83, 054432 (2011)
DR-07. Modeling of pulsed laser heating in magnetic nanowires
Iordana Astefanoaei, Ioan Dumitru and Alexandru Stancu
Department of Physics, Alexandru Ioan Cuza University, Iasi, Romania
In the last years, the interaction between magnetic nanowires and laser light becomes an important topic[1]. Study of the thermal stability in the magnetic nanowires during laser-induced heating presents a special interest for technological applications as heat assisted magnetic recording(HAMR)[2]. The main goal of this paper is to describe the thermal field produced by a laser pulse in a magnetic nanowire system. It was proposed a two-dimensional analytical model that predicts the spatio-temporal temperature distribution in the magnetic nanowire during laser-induced heating. Considering the laser as a heating source - applied at the end of a nanowire included into a membrane, the temperature was obtained by solving the heat conduction equation with proper thermal boundary conditions. The laser pulse was considered 50 ns. The thermal stability of the nanowire depends on its size parameters (radius and length) and laser intensity. It is observed an increase of temperature in time do to the laser heating and a decrease along wire axes. The magnetic behavior of the nanowire was analyzed using the Micromag application. The thermal dependence of the anisotropy constant was considered. The switching mechanism starts from the end with the higher temperature and propagates along wire axis to the coolest end. The magnetic stability of wire after pulse switching-off was analyzed. The results show that by changing the values of the size parameters of the nanowire and laser intensity,the thermal and magnetic stability of the system can be controlled. Acknowledgement: The authors acknowledge the financial help from the CNMP 12093 HIFI project.
References
[1] A. Moser et al., J. Phys. D: Appl. Phys. 35, R157 (2002). [2] Shengbin Hu, Baoxi Xu, Hongxing Yuan, Yunjie Chen, Jun Zhang and Rong Ji, Journal of Magnetism and Magnetic Materials, vol 303,2(62-65)(2006).
DR-08. Temperature effects in perpendicular spin transfer torque magnetic random access memory
Dorin Cimpoesu and Alexandru Stancu
Physics, Alexandru Ioan Cuza University of Iasi, Iasi, Romania
Magnetic random access memory (MRAM) based on spin transfer torque (STT) [1] is of great interest as a promising candidate for both embedded and standalone memory applications, because it combines advantages such as high speed, nonvolatility, low power consumption, high density, and high endurance in a single CMOS-compatible memory technology. With memory area density increasing and the memory cell size further shrinking the study of thermo-fluctuations in these magnetic structures becomes of extreme importance for their recording thermal stability. On the other hand there is currently a considerable interest in thermally assisted writing of MRAM, in order to increase memory density. In order to have a robust STT-MRAM technological method the most important factors influencing this type of switching have to be systematically analyzed. This paper is the result of such an analysis based on stochastic Landau-Lifshitz-Gilbert [3] simulations. Our work is based on a macro-spin model, the free layer being assumed to be a single domain. We to study and compare two general architectures: perpendicular STT-MRAM, with fully perpendicular magnetization in both free and fixed lay ers; and combined STT-RAM, where an in-plane free layer is switched by the combined effect of spin transfer torque from two in-plane and out-of-plane polarizers. The use of a perpendicular polarizing layer in combination with an in-plane free layer has been proposed in Ref. [2] as a means of reducing the switching time and write energy of STT-MRAM cells. The STT-MRAM switching is examined in terms of applied current pulse amplitude and shape, waiting time, initial conditions, and also as a function of the Gilbert damping constant. Thermally activated switching is also analyzed and we show that the fast dynamic reversal is a combination of precessional and thermally activated switching. The switching speed is increased as the time the system spends in the intermediate state is decreased due to thermal fluctuations. In the paper we present how the thermal fluctuations affect the switching behavior, the reliability and the writing speed of perpendicular STT-MRAM devices.
References
[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996). [2] A. D. Kent et al., Appl. Phys. Lett. 84, 3897 (2004). [3] W.F. Brown Jr., Phys. Rev. 130 (1963) 1677.
DR-09. Tailoring vortex core in confined magnetic nanostructures
Ana L. Dantas1, Thiago S. Moura2, Felipe F. Olivieira2, Gustavo G. Rebouças3 and Artur S. Carriço2
1Departamento de Física, Universidade do Estado do Rio Grande do Norte, Mossoró, Brazil; 2Departamento de Física, Universidade Federal do Rio Grande do Norte, Natal, Brazil; 3Departamento de Física, Universidade Federal Rural do Semi-Árido, Angicos, Brazil
We present a study of vortex formation in nano-sized disk and square dots of Fe and Py. For small lateral dimensions the vortex core diameter depends on the nanoparticles shape, size, and height. We focus on nanoelements with a top surface area ranging from 2600nm2 to 5600nm2. We have found that for nanoparticles with the same surface area, the vortex core radius of a nanodisk is smaller than that of a square nanoelement. The difference turns out negligible for dimensions much larger than exchange length. Square Fe nanoparticles with a top surface area of 2600nm2 and height of 30nm have a vortex core diameter of 36nm, while circular nanoparticles with same area and height display a vortex with a diameter of 30nm. The corresponding parameters for Py nanoparticles are 42nm and 36nm. Interface bias leads to a shift of the vortex core from the nanoelement center and a reduction of the perpendicular component of the magnetization in the vortex core center. As a result, in the off-center position, the core diameter increases to minimize the increase in the exchange energy. Furthermore, the interface exchange coupling pins only the spins from the interface layer. Thus, the vortex evolves gradually from the magnetic pattern at the interface layer, to the free surface layer pattern. The vortex core is located in slightly different positions from layer to layer, leading to an increase in the vortex core diameter. For a square Fe 57nm x 57nm x 24nm, an interface field of 8kOe leads to a vortex core shift of 7.5nm and a large increase of 78% in the core diameter, which increases from the unbiased nanoelement value of 33nm to 42nm. For a Fe 75nm x 75nm x 24nm the off-center shift is 12nm and an increase in the core diameter of 60%.
DR-10. Magnetic configuration of nanodots with perpendicular anisotropy
Erico R. Novais1, Pedro Landeros2, Andreia G. Barbosa3, Maximiliano D. Martins3 , Flavio Garcia4 and Alberto P. Guimaraes1
1CBPF, Rio de Janeiro, Brazil; 2Physics Department, UTFSM, Valparaiso, Chile; 3CDTN, Belo Horizonte, Brazil; 4LNLS, Campinas, Brazil
Nanodots with magnetic vortices have many potential applications, such as vortex magnetic memories (VRAMs) and spin transfer nano-oscillators (STNOs). Adding a perpendicular anisotropy term to the magnetic energy of the nanodot, one may modify the vortex core properties. This can be obtained, e.g., in Co nanodots by varying the thickness of the Co layer in a Co/Pt stack. Here we discuss the spin configurations of circular and elliptical nanodots for different values of the perpendicular anisotropy. We show for nanodisks that micromagnetic simulations and analytical results give the same magnetic configurations. Increasing the perpendicular anisotropy, the vortex core radii increase, and this leads to modifications in the phase diagrams, with the appearance of new configurations. The knowledge of these phase diagrams is relevant for the choice of optimum nanodot dimensions designed for applications. We also show that MFM measurements on Co/Pt multilayers confirm the trend of the vortex core diameters with varying Co layer thicknesses.
References
Garcia et al. Appl. Phys. Lett., 97, 022501 (2010).
DR-11. Spin Transfer torques in Antiferromagnets
Hamed B. Saidaoui, Xuhui Wang and Aurelien Manchon
Physical Sciences, King Abdullah University Of Sciences and Technology, Thuwal, Saudi Arabia
Two phenomena are at the centre of spintronics in magnetic multilayers. The Giant magnetoresistance (GMR) shows the resistance change depending on the relative orientation between local moments. On the other hand, the polarised itinerant electrons exert torques on the local magnetic moments, i.e. a mechanism known as spin transfer torque, which leads to phenomena such as magnetisation reversal [1,2,3]. Interestingly, Nunez et al [4]. showed theoretically that spin-valves based on antiferromagnetic (AFM) electrodes, as a rising candidate in spintronic applications, also display outstanding spintronics properties such AFM-GMR and Spin Torque. The spin torque in AFM presents a bulk contribution unlike the counterpart in ferromagnets that occur in the interface [4]. Since antiferromagnets do not possess net magnetization, the effect of the torque can be detected through a change in the exchange bias in a regular metallic spin-valve. However, besides promising experimental indications[5], no accurate studies of the nature of spin torque on antiferromagnets in a regular metallic spin-valve have been provided. Alternatively, only a few theoretical analyses are available either based on non-equilibrium Green’s functions [6], or Density Functional theory [7]. In the present study, we apply the well known non-equilibrium Green's function formalism to investigate the spin transport in layered structure as Ferromagnet/Normal metal/Antiferromagnet and Ferromagnet/Insulator/Antiferromagnet Numerical calculations are performed to extract the spin torques exerted on the antiferromagnetic layer. We will t show the behavior of the torque and its bias dependence in both structures as well as the variation of the torque with relative angle between magnetic moments. The case of Ferromagnet/Insulator/Antiferromagnet is of particular interest, displaying non-linear bias voltage dependences. We will compare results we get with previous theoretical results. Experimental configurations will be suggested and discussed.
References
[1] J. C . Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) [2] L. Berger, J. Appl. Phys. 81, 4880 (1997) [3]E. B. Myers, D. C. Ralph, J. A. Katine, R. N. Louie, and R. A. Buhrman, Science 285, 867 (1999) [4] A. S. Nunez, R. A. Duine, P. Haney, and A. H. MacDonald, Phys. Rev. B 73, 214426 (2006) [5] Z. Wei, A. Sharma, A.S. Nunez, P.M. Haney, R.A. Duine, J. Bass, A.H. MacDonald, and M. Tsoi, Phys. Rev. Lett. 98, 116603 (2007) [6] R. A. Duine et al., Phys. Rev. B 75, 014433 (2007) [7] Y. Xu, S. Wang, and K. Xia, Phys. Rev. Lett. 100, 226602 (2008)
DR-12. Simulation of a Spin Field Effect Transistor based on magnetic impurity doped ZnO
Ratheesh Ramachandran Thankalekshmi and Alok C. Rastogi
Electrical and Computer Engineering, State University of New York, Binghamton University, Binghamton, NY
Spin Field Effect Transistor (Spin-FET) is the building block of futuristic integrated spintronic devices. The spin-FET device structure proposed by Datta-Das is based on conventional MOSFET modeled as a resistive switch. This paper theoretically analyzes and simulates the conductance modulation of the electron channel in terms of variation in polarized spin density in response to the gate voltage. Simulation of spin-FET is done in Matlab assuming a source injects 100% spin polarized electrons into the channel comprising of magnetic impurity doped ZnO semiconductor. The channel length is assumed within spin diffusion length. The drain (sensor) detects the spin orientation of the channel electrons. Spin switched change in the channel charge has been modeled theoretically and is correlated with the channel conductance. Analysis establishes significance of the electric field for spin injection at the source node. At low field injection relative to the gate field the channel conductance is largely unaffected by the field at the drain (sensor). Simulation shows that in this field region, spin dependent channel conductance varies linearly in response to the gate voltage interpreted in terms of Rashba field. Reduction of channel conductance at high field spin injection scales with the source bias relative to the sensor node. Simulation of the theoretical model in this field region shows a significant exponential dependence of the channel conductance on the gate voltage. Theoretical analysis shows by inclusion of the spin degree of freedom in conventional MOSFET, functionalities of a spin-FET can be realized. Simulation studies establish the range of channel parameters; spin injection field relative to the sensor and the gate field for spin switched changes in the electron channel conductance. By affecting significant changes in the channel resistance through least change in the gate voltage, these parameters may form a basis for spin-FET design for future low power spintronic devices
References
[1] Control of Spin Precession in a Spin-Injected Field Effect Transistor ,Hyun Cheol Koo, Jae Hyun Kwon, Jonghwa Eom, Joonyeon Chang, Suk Hee Han, and Mark Johnson ,Science 325 (5947), 1515 [2] Weak Localization and Electron−Electron Interactions in Indium-Doped ZnO Nanowires , Richard S. Thompson, Dongdong Li, Christopher M. Witte, Jia G. Lu Nano Letters 2009 9 (12), 3991-3995 [3] Carrier concentration dependence of band gap shift in n-type ZnO:Al films J. G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, Z. Ye, Y. J. Zeng, Y. Z. Zhang, L. P. Zhu, H. P. He, and B. H. Zhao, J. Appl. Phys. 101, 083705 (2007) [4] Basic materials physics of transparent conducting oxides P. P. Edwards, A. Porch, M. O. Jones, V. Morgan and R. Perks, Dalton Transactions, 19, 2995 (2004) [5]Large Rashba Splitting in InAs Quantum Wells due to Electron Wave Function Penetration into the Barrier Layers, D. Grundler, Phys. Rev. Lett. 84, 6074 (2000). [6] St rong circular photogalvanic effect in ZnO epitaxial films,Q. Zhang, X. Q. Wang, C. M. Yin, F. J. Xu, N. Tang, B. Shen, Y. H. Chen, K. Chang, W. K. Ge, Y. Ishitani, and A. Yoshikawa, Appl. Phys. Lett. 97, 041907 (2010) [7] Spin-Polarized Transport in Inhomogeneous Magnetic Semiconductors:Theory of Magnetic/Nonmagnetic p-n Junctions, I. Zutic, J. Fabian, and S. Das Sarma, Phys. Rev. Lett. 88, 066603 (2002) [8] Theory of spin-polarized bipolar transport in magnetic p-n junctions, J. Fabian, I. Zutic, and S. Das Sarma, Phys. Rev. B 66, 165301 (2002) [9] Zinc Oxide Nanostructures: Synthesis and Properties, Z. Y. Fan and J. G. Lu, J. Nanosci. Nanotechnol. 5, 1561 (2005) [10] Electronic analog of the electro-optic modulator Datta and Das, Appl. Phys. Lett., 56, (1990) 665
DR-13. Ultrafast magnetization switching driven by current pulses in a spin-valve with in-plane and out-of-plane dual polarizers
Zhiwei Hou, Zeyi Song, Jianwei Zhang and Yaowen Liu
Department of Physics, Tongji University, Shanghai, China
Current-driven magnetization switching holds great promise in the applications of the nonvolatile magnetic random-access memory (MRAM). An unavoidable intrinsic feature is that the spin-torques is almost zero in the initial stage when the layer magnetizations are collinearly aligned, which results in the relative long switching time. A particularly elegant approach to reduce the switching time is by designing a nanopillar that the spin-polarizing layer is perpendicular to the free layer in a ~90° configuration[1], so that a large spin-torque can be generated in the initial stage to tilt the free layer magnetization out of plane. Recently, the ultrafast magnetization switching on the scale of sub-nanosecond current pulses has been reported in some promising spin-valve or magnetic tunnel junction structures that incorporate an in-plane free layer and two polarizers with in-plane (IP) and out-of-plane (OP) magnetization configurations [2-4]. In this work, a combined analytic and simulation study will be presented to predict the magnetization dynamics in a spin valve with the dual polarizers. The phase diagram governed by the spin-torque strengths has been established and shows three different parameter regions: no switching, switching and periodic oscillations. The spin-torques which are able to switch the free layer have been analytically predicted and supported by simulations, showing that the torque strength originated from the OP polarizer should be much smaller than that from the IP polarizer. The free layer in a spin-valve with dual polarizers could switches tens of times faster than the spin-valve with a single in-plane polarizer by tuning the current pulses.
References
[1] A. D. Kent, et al., Appl. Phys. Lett. 84, 3897 (2004). [2] O.J. Lee, et al., Appl. Phys. Lett. 95, 012506 (2009). [3] C. Papusoi, et al.,Appl. Phys. Lett. 95, 072506 (2009). [4] G. E. Rowlands, et al., Appl. Phys. Lett. 98, 102509 (2011).
DR-14. Kinetic Equation Description of Spin Relaxation in Wurtzite Structure
Fatih Dogan and Aurelien Manchon
Material Science, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
In spin-driven electronic devices, feasibility of industrial application highly depends on the existence and long spin polarization lifetime in the device [1]. Excess spin in the device goes through scattering mechanisms that randomize the spin, causing loss of information. To maximize the spin diffusion length in non-magnetic semiconductors, many experimental and theoretical tools have been pro posed. Due to the variety of physical systems to be analyzed (Si, GaAs, GaN etc…), generalized theories are introduced that are commonly used to interpret experimental data. These spin-flip scattering processes, EY, DP and BAP mechanisms [2], assume an overall picture that is not band structure specific and reduces the material dependences to a few established parameters. These analytical expressions present intrinsic limitations [compare fits to experimental data by ref. 3 and ref. 4 for instance]. Moreover, whereas most of the theoretical studies focused on cubic structures (GaAs, Si, cubic GaN), only few studies have been proposed in Wurtzite structures. In these structures, the presence of a bulk Rashba-type spin-orbit interaction introduces an additional coherent spin precession mechanism that may strongly affect the overall spin relaxation [5, 6]. We present a theoretical description based on Green function analysis of kinetic spin Bloch equation [3]. Defining each scattering process microscopically (impurity scattering, coulomb scattering, exchange interaction) the analysis treats each scattering mechanism independently in the momentum space and importance of the specific mechanism can be viewed independently. In our analysis, dispersion of the spin carrier and/or scatterer can be specified, and the contributions for individual cases can be specifically weighted. We introduce a spin imbalance in a 3D bulk system, and let it evolve in time to observe how each scatterer affects the spin distribution. We use the analysis to compare Wurtzite and cubic structures. Specifically, we look at bulk ZnO systems, and determine the importance of the contribution of bulk Rashba spin-orbit coupling to the spin relaxation in Wurtzite structures. Coherent spin precession and anisotropic spin diffusion are analyzed.
References
[1] Igor \^{Z}uti\'{c}, Jaroslav Fabian, S. Das Sarma Review of Modern Physics 76, 323 (2004) [2] M.W. Wu, J. H. Jiang, M. Q. Weng Physics Reports 493, 61 (2010) [3] J. H. Jiang and M. W. Wu, Physical Review B 79, 125206 (2009) [4] Srinivasan Krishnamurthy, Mark van Schilfgaarde and Nathan Newman, App. Phys. Lett. 83, 1761 (2003) [5] J. H. Buss, J. Rudolph, F. Natali, F. Semond, D. Hägele, App. Phys. Lett. 95, 192107 (2009) [6] Wan-Tsang Wang, et al, App. Phys. Lett. 91, 082110 (2007)
DR-15. Magnetostatic interactions between nanowires and nanotubes
Juan Escrig, Alejandro Pereira and Dora Altbir
Departamento de Física, Universidad de Santiago de Chile, Santiago, Chile
Although an array composed of a few ferromagnetic nanowires or nanotubes could in principle seem a quite simple problem to study and model, it is striking to notice how complex this problem can turn out to be. The effects of interparticle interactions are in general complicated by the fact that the dipolar fields depend on the magnetization state of each element, which in turn depends on the fields due to adjacent elements. Therefore, the modeling of interacting arrays of particles is often subject to strong simplifications as, for example, modeling the particle using a one dimensional modified classical Ising model [1]. Zhan et al. [2] used the dipole approximation including additionally a length correction. Velazquez and Vazquez [3] considered each microwire as a dipole, in a way that the axial field generated by a microwire is proportional to its magnetization. They also calculated the magnetostatic field and expanded it in multipolar terms, [4] showing that the non-dipolar contributions of the field are non-negligible for distances considered in experiments. Besides, the magnetostatic interaction energy between two magnetic elements of arbitrary shape was derived by Beleggiaet al. [5] using a Fourier space approach. A detailed study of the magnetostatic interaction between two parallel wires placed side by side has been shown [6]. Recently, we have investigated the effect of magnetostatic interaction on the hysteresis loops, due to the vertical displacement of the wires [7] and the full long-range magnetostatic interaction between two nanotubes [8]. Clearly, for the development of magnetic devices based on these particles, a clear understanding of the magnetostatic interaction between particles is of main importance. Thus, in this work we present an analytical model for the full long-range magnetostatic interaction between two cylindrical particles exploring the possibility of varying the magnetic coupling as a function of the particle’s position. JE acknowledges the Millennium Science Nucleus Basic and Applied Magnetism (project P10-061F), Financiamiento Basal para Centros Cientificos y Tecnologicos de Excelencia, and Fondecyt 1110784.
References
[1] L C Sampaio, E H C P Sinnecker, G R C Cernicchiaro, M Knobel, M Vazquez, and J Velazquez 2000 Phys. Rev. B 61, 8976. [2] Q -F Zhan, J -H Gao, Y -Q Liang, N -L Di, and Z -H Cheng 2005 Phys. Rev. B 72, 024428. [3] J Velazquez, and M Vazquez 2002 J. Magn. Magn. Mater. 249, 89. [4] J Velaszquez, K R Pirota, and M Vazquez 2003 IEEE Trans. Magn. 39, 3049. [5] M Beleggia, S Tandon, Y Zhu, and M De Graef 2004 J. Magn. Magn. Mater. 278, 270. [6] D Laroze, J Escrig, P Landeros, D Altbir, M Vazquez, and P Vargas 2007 Nanotechnology 18, 415708. [7] A Pereira, J C Denardin, and J Escrig 2009 J. Appl. Phys. 105, 083903. [8] J Escrig, S Allende, D Altbir, and M Bahiana 2008 Appl. Phys. Lett. 93, 023101.
1:00 PM - 5:00 PM, Saguaro Ballroom
Chair: Markus Muenzenberg, Göttingen University
DS-01. Ferromagnetic Resonance and Gilbert Damping Behaviors of Co80Fe10B10 Thin Films in the Microwave Range
Dung Shing Hung1, 2, Yu-Che Chiu2, Faris B. Abdul Ahad2, 3 and Shang Fan Lee2
1Department of Information and Telecommunications Engineering, Ming Chuan University, Taipei, Taiwan; 2Institute of Physics and Nano Science and Technology Program, TIGP, Academia Sinica, Taipei, Taiwan; 3Department of Engineering and System Science, NTHU, Hsinchu, Taiwan
From the LLG equation, dM/dt = -|γ| μ0 M×Heff+(α/Ms)M×dM/dt, α is the Gilbert damping coefficient that is correlated to magnetic relaxation and also a crucial factor in many aspects. In this paper, we will demonstrate the ferromagnetic resonance (FMR) behaviors of the Co80Fe10B10 (CoFeB) thin films through microwave transmission-line techniques. There were two different methods used. First, a frequency-domain measurement with a fixed magnetic field was applied, we could get the FMR frequency fFMR. According to the Kittle equation (fFMR )2=[(γμ0)/2π]2 (Meff+Hk+Happl )(Happl+Hk) [1], the γ values could be obtained by fitting the fFMR versus Happl. The data and fitting results agreed well and were shown in Fig.1. In the second method, a magnetic field-domain measurement with a fixed microwave frequency f was given. The linewidth (ΔHappl) around the resonance field (Hres) could be obtained from the spectra. From ΔHappl=ΔH0+4πα/(γμ0< /sub>) f, we fit ΔHappl to f and the slope gave us the information of calculating their α and γ values. In this study, the results exhibited the thickness-dependence damping behaviors of the CoFeB thin films. The analyses showed that α increased with increasing 1/d values, just as shown in fig.2. This is closed to the Fuchs-Sondheimer predication [2]. In this paper, we will demonstrate the details of the magnetic behaviors of CoFeB thin films studied by the microwave measurement and analyses.
References
[1] C. Bilzer, J. Appl. Phys. 100, 053903 (2006) [2] S. Ingvarsson, Phys. Rev. B 66 214416 (2002)
DS-02. Micromagnetic analysis of the magnetization dynamics in Cobalt nanorings driven by the Oersted field
Eduardo Martinez
Fisica Aplicada, Universidad de Salamanca, Salamanca, Spain
Recent experiments have shown that the Oersted field, generated by passing a current through an atomic force microscope tip at the center of a ferromagnetic ring, can manipulate its magnetization, and promote its chirality reversal. These observations could improve the exploration of magnetic states for designs of data storage in such rings. Here, a systematic study of these processes is carried out by means of micromagnetic modeling. As an example, Fig. 1(a) depicts the equilibrium onion-state in a Co ring with a width of w=60nm, thickness of t=3nm, and external radius of Rout=151nm (after application and relaxation of an in-plane saturation field). It consists on two 180 domain walls (DWs): a head-to-head and a tail-to-tail. A conducting wire (RT=20nm) placed in the center of the ring was used to inject a current, and the magnetization dynamics induced by the induced Oersted field was evaluated by solving the Landau-Lifshitz-Gilbert eq. Under injection of a stationary current of 47mA, the DWs approach each other, and a equilibrium state with a 360 wall is adopted (Fig. 1(b)). For a higher current (48mA), the DWs annihilate and the magnetization adopt an azimuthal vortex pattern (Fig. 1(c)) with the chirality of the Oersted field. A detailed description of these processes, which are in good agreement with experiments [1], will be discussed in the presentation.
References
[1] T. Yang et al. Appl. Phys. Lett. 98, 242505 (2011).
DS-03. Microscopic Dipole-Exchange Theory for Magnonic Crystals: Application to Ferromagnetic Films with Patterned Surfaces
Hoa T. Nguyen and Michael G. Cottam
Department of Physics and Astronomy, University of Western Ontario, London, ON, Canada
The emerging field of magnonics has recently attracted much attention, since it allows for the manipulation of the collective spin-wave properties in periodic magnetic nanostructures (see, e.g., [1,2]). Here calculations are reported for the spin-wave bands and gaps in magnonic crystals formed by the patterning of one (or both) of the surfaces of ferromagnetic films with thickness in the range 10 - 50 nm. Specifically, a patterning in terms of an array (with period typically 100 - 400 nm) of rectangular grooves is considered, e.g., as illustrated for patterning on one surface. The theory uses a microscopic (or Hamiltonian-based) method that incorporates exchange and magnetic dipole-dipole interactions, as well as effects of single-ion anisotropy and an external in-plane magnetic field parallel to the grooves. This approach has been generalized from its application to individual magnetic nanoelements, such as stripes [3], to magnonic arrays of stripes coup led by the dipolar fields across nonmagnetic spacers [4] to this present work, where an important difference is that the coupling between different periodic elements of the magnonic crystal involves both exchange and dipolar terms. It is found that this type of structure allows for greater control of the spin-wave bands and gaps as the depth and the width of the grooves are varied. The microscopic method is convenient for taking into account the inhomogeneous field associated with the edges and corners of the grooves. Numerical examples are given for Permalloy structures, considering cases of the stripe patterning on both one and two surfaces, and the applicability to Brillouin light scattering is discussed.
References
[1] V. V. Kruglyak, S. O. Demokritov, and D. Grundler, J. Phys. D 43, 264001 (2010). [2] S-K Kim, J. Phys. D 43, 264004 (2010). [3] H. T. Nguyen, T. M. Nguyen, and M. G. Cottam, Phys. Rev. B 76, 134413 (2007). [4] H. T. Nguyen and M. G. Cottam, J. Phys. D 44 (2011), in press.
DS-04. Fractal basin boundaries in magnetization relaxations of nanomagnets subject to weak AC excitations.
Claudio Serpico1, Domenico P. Ansalone2, Massimiliano d'Aquino3, Giorgio Bertotti2 and Isaak D. Mayergoyz4
1Dip. di Ingegneria Elettrica, Univ. of Naples Federico II, Napoli, Italy; 2INRIM, Turin, Italy; 3Dip. per le Tecnologie, Univ. of Naples Parthenope, Naples, Italy; 4Electrical and Computer Engineering, Univ. of Maryland College Park, College Park, MD
Magnetization relaxation dynamics appear in many situations of fundamental and technological interest, such as the operation of spinvalve elements, precessional switching of the magnetization or switching induced by spin polarized currents [1]. In this work, we analyze the influence of weak time-harmonic (AC) external excitations on magnetization relaxation dynamics in a uniformly magnetized particle (nanomagnet). We show that the transient relaxation of magnetization toward states of lower energy may become chaotic. In the case of constant external fields, the magnetization energy landscape of the magnet has two wells corresponding to two stable magnetization equilibria. In this situation, magnetization relaxation originating from any magnetization state will tend, after a transient, toward one of the two stable equilibria. This leads to a natural partitioning of the unit sphere, representing magnetization orientations, into two regions (basins of attraction) by associating each initial state to the corresponding final stable equilibrium. The boundaries separating the two basins are smooth curves. This picture is considerably modified when a weak time-periodic external excitation (field or injected current) is applied to the system. Magnetization relaxations tend toward two stable periodic solutions located in the vicinity of two energy minima. For extremely small AC excitations, the unit sphere is still partitioned in two basins of attractions associated to these two periodic stable solutions and separated by smooth boundaries. However, if the amplitude of the AC excitations is above a certain threshold the smooth boundaries of the two basins are destroyed and the basins of attraction become separated by fractal boundaries [2]. The final periodic solution approached in long-time limit depends sensitively and discontinuously on the initial conditions and the long-time predictability of magnetization dynamics is practically lost. In the work we illustrate this complex phenomena through numerical simulations and we provide analytical formulas, based on a perturbation technique, to compute the threshold which corresponds to the destruction of the smooth basin boundaries.
References
[1]G. Bertotti, I.D. Mayergoyz, C. Serpico, “Nonlinear magnetization dynamics in nanosystems”, Elsevier, 2009. [2]Y. Ueda, S. Yoshida, H. B. Stewart and J. M. T. Thompson Phil. Trans. Royal Soc.: Physical Sciences and Engineering Vol. 332, No. 1624, 1990
DS-05. Temperature dependence of the dynamics of a synthetic antiferromagnet. Frequency and time domain investigation.
Dorin Cimpoesu and Alexandru Stancu
Physics, Alexandru Ioan Cuza University of Iasi, Iasi, Romania
Synthetic Antiferromagnet (SAF) structures are very important in designing modern spintronic devices such as magnetic random access memory (MRAM). The performance of the devices using SAF structures relies on their switching characteristics. Recently has been proposed to use microwave assisted switching (MAS) technique in order to reduce the write field in MRAM [1,2]. The microwave induced magnetization reversal is fundamentally different from that of a static field, because a static field is not an energy source while a microwave can be. As the devices using SAF structures require a short access time and the magnetization is forced by pulsed or ac magnetic fields to switch at nano and subnanosecond time scales, the understanding of the mechanisms involved in the dynamic processes is crucial for the development and design of future nanoscale devices. Also with devices size further shrinking the study of thermo-fluctuations in these magnetic structures becomes of extreme importance for their thermal stability. In this paper, based on the stochastic Landau-Lifshitz-Gilbert (LLG) equation [3], we perform a study of the dynamics of nanosized SAF elements, both in time and frequency domain over large static and pulse field ranges, like in the pulsed inductive microwave magnetometry (PIMM) method. Also we are studying the case of a continuous and pulsed microwave excitation. When the SAF's layers are dynamically excited by relative small magnetic fields, the two coupled layers resonate in a collective way with the two eigenmodes (acoustical and optical) [2]. As the amplitude of the excitation increases we identify new oscillation modes. Large amplitude oscillations can present also a chaotic behavior. Numerical integration of LLG equation in this case can be extremely sensitive to the integration time step, and even to the numerical algorithms used, as a consequence of repeated amplification of truncation and rounding errors, giving rise to a "computed chaos." In the full paper we show how we handled this problem, and also paper we discuss also how the thermal fluctuations affect the dynamics behavior of SAF elements.
References
[1] S. S. Cherepov, V. Korenivski, and D. C. Worledge, IEEE Trans. Magn. 46, 2112 (2010). [2] A. Konovalenko, E. Lindgren, S. S. Cherepov, V. Korenivski, and D. C. Worledge Phys. Rev. B 80, 144425 (2009). [3] W.F. Brown Jr., Phys. Rev. 130 (1963) 1677.
DS-06. Broadband Ferromagnetic Resonance Spectroscopy of Permalloy Triangular Nanorings
Junjia Ding1, Mikhail P. Kostylev3 and Adekunle O. Adeyeye1, 2
1Department of Electrical and Computer Engineering, National Univ Singapore, Singapore, Singapore; 2Advanced Materials for Micro- and Nano- Systems, Singapore-MIT Alliance, Singapore, Singapore; 3School of Physics, University of Western Austral