Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T30: Focus Session: Exchange Spring Magnets and Multilayers |
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Sponsoring Units: GMAG DMP Chair: Luke Marshall, Northeastern University Room: 206B |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T30.00001: Depth dependent magnetization profiles of hybrid exchange springs R. Knut, T.N. Anh Nguyen, S. Chung, Q. Tuan Le, S.M. Mohseni, V. Fallahi, S. Perdekov, O. Karis, R.K. Dumas, C.W. Miller, J. Akerman We report on the magnetization depth profile of a hybrid exchange spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. Such materials allow for additional control of the magnetization dynamics in magnetic nanostructures, and suggest the possibility of improved spin transfer torque-MRAM switching behavior and thermal stability. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant, and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element-specific x-ray magnetic circular dichroism of the Fe and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T30.00002: Quadratic magneto-optical Kerr effect and chirality of magnetization reversal in Py/SmFe/Py exchange spring magnet thin films Jiyeong Gu, Hanming Yuan Magnetic switching behavior of the exchange spring magnet, Py(Permalloy)/SmFe/Py thin films, was investigated by magneto-optical Kerr effect (MOKE). Exchange spring magnet shows a unique magnetic hysteresis loop due to the non-collinear magnetization developed by magnetic coupling of the soft and hard magnetic layers. Py/SmFe/Py thin films were deposited on silicon substrate by DC magnetron sputtering. Experimental Kerr rotation measurement showed asymmetric hysteresis loops as a result of superimposed quadratic MOKE (QMOKE) contribution to linear MOKE (LMOKE). Overall MOKE signal was separated into LMOKE and QMOKE signals using loop symmetrization and antisymmetrization operation. The longitudinal (M$_{L})$ and transverse (M$_{T})$ magnetization components with respect to the plane of light incidence were extracted and QMOKE signal was simulated from the two mixed terms, M$_{L}$M$_{T\, }$and M$_{L}^{2}$- M$_{T}^{2}$. QMOKE signal not only showed the nature of magnetization reversal in exchange spring magnet system from the information of M$_{T}$ at switching field but also determined the direction of magnetization rotation from the positions of the peaks. Our work demonstrated that the chirality of magnetization reversal can be determined from analysis of QMOKE signal. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T30.00003: Nanoscale Engineering Toward Bulk Exchange-Spring Permanent Magnetic Fe/MnAl Nanocomposites Luke G. Marshall, Ian J. McDonald, Laura H. Lewis The ability to amplify the performance of exchange-spring nanocomposites through correlation and tailoring of chemical order, microstructural scale and magnetic response is necessary for the development of novel high-performance permanent magnets. To this end, rapid solidification of near-equiatomic MnAl for incorporation into the model exchange-spring Fe/MnAl system has produced alloys containing 20-30\% ferromagnetic $\tau$-MnAl with the high-temperature parent hexagonal $\epsilon$-MnAl phase composing the balance. While typically formed by briefly annealing the quenched $\epsilon$-phase, this work has confirmed direct attainment of nanoscaled $\tau$-MnAl via cryogenic milling. Magnetic and structural data indicate a significantly decreased chemical order accompanied by increased lattice strain with increased milling time. Control of the chemical order and nanostructure of near-equiatomic MnAl will favor metastable configurations to foster robust exchange coupling between $\tau$-MnAl and $\alpha$-Fe to create high performance magnetic nanocomposites. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T30.00004: Micromagnetic Modeling of Reversal Nucleation in Core/Shell Exchange-Spring Structures J.S. Jiang, Sam Bader Nanocomposite exchange-spring permanent magnet materials promise superior performance and are a potential solution to the supply criticality in rare earth elements [1]. The nucleation of magnetization reversal in cylindrical and spherical soft core/hard shell exchange-spring structures has been investigated by solving the linearized Brown's equation perturbatively, and has been verified with numerical simulations [2]. Accounting for the magnetostatic self-interaction field leads to a modification to the proposed quasi-coherent ``bulging'' mode[3] of nucleation for small core sizes. The modified curling mode, where the magnetization configuration is vortex-like and flux-closed, becomes favored at large core sizes. The mode crossover occurs at a core diameter of approximately twice the exchange length for the cylindrical geometry. Since flux-closure allows magnetic elements to be densely packed without affecting the nucleation field, a potential direction for improving permanent magnet materials is to induce the modified curling mode by creating a soft-cylinder-in-hard-matrix exchange-spring microstructure.\\[4pt] [1] J. M. D. Coey, \textit{IEEE Trans. Magn.} \textbf{47}, 4671 (2011).\\[0pt] [2] J. S. Jiang, S. D. Bader, \textit{J. Phys.: Condens. Matter}, \textbf{26}, 064214 (2014).\\[0pt] [3] R. Skomski, J. P. Liu, D. J. Sellmyer, \textit{Phys. Rev.} \textbf{B60}, 7359 (1999). [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T30.00005: Fabrication and Characterization of MnBi/Co and MnBi/FeCo Nanocomposite Bulk Magnets Narayan Poudyal, Kinjal Gandha, Wei Wang, Xiaotong Liu, Zhaoguo Qiu, Kevin Elkins, J Ping Liu, Jun Cui We report the fabrication of MnBi/Co and MnBi/FeCo\textbf{ }nanocomposite bulk magnets by consolidating the hard and the soft phase powder particles under a magnetic field followed by subsequent sintering process. The anisotropic micro and submicron hard magnetic MnBi particles were first prepared by low energy cryo ball milling at liquid nitrogen temperature. MnBi/Co and MnBi/FeCo nanocomposite powders were then prepared by using different fraction of chemically synthesized Co nanowires and FeCo nanoparticles as the soft magnetic phase. The saturation magnetization ($M_{s})$ of the composite magnets increases with addition of the soft phase while the coercivity first increases and then decreases. The MnBi/Co and MnBi/FeCo nanocomposite bulk magnets have reached an enhanced magnetization value ($M_{s} \quad =$ 78 and 80.6 emu/g) with 30 wt. {\%} of Co nanowires and FeCo nanoparticles, respectively compared to the single phase MnBi bulk magnet ($M_{s} \quad =$ 52 emu/g). [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T30.00006: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T30.00007: Direct measurement of interlayer interaction in Permalloy/Gd nanodots Pavel Lapa, Junjia Ding, Valentine Novosad, Axel Hoffmann Antiferromagnetic interaction at the interfaces of ferromagnetic transition metals(TM) and Gd is well known phenomenon. However, quantitative description of this interaction still lacks understanding. The main reason is that most experimental data were obtained by inspecting the hysteresis loops of TM/Gd multilayers. First, in plane domain structures of TM and Gd films complicates the description of the magnetization reversal process. Second, experimentally measured parameters are averaged over the domains obfuscating the microscopical picture. We make an effort to overcome these limitations by studying the magnetization reversal process of Permalloy/Gd nanodots. The dots were prepared by combination of optical lithography and magnetron sputtering. Experimentally it is observed that in these dots the antiferromagnetic interlayer interaction tunes magnetic vortex nucleation/annihilation fields. Rise of Gd magnetization at low temperature provides unusual temperature behavior of hysteresis loop. Micromagnetic models in which interlayer interaction energy acts as a fitting parameter is applied to simulate experimental hysteresis curves. The effect of nonmagnetic spacer between Permalloy and Gd is also considered and will be presented. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T30.00008: Detection of Magneto-Crystalline Anisotropy in YIG Films Formed by Aerosol Deposition Scooter Johnson, E.R. Glaser, Konrad Bussmann, Frederic Rachford, Fritz Kub, Charles Eddy, Jr. We have employed aerosol deposition (AD) to form dense polycrystalline films of yttrium iron garnet (YIG) at room temperature in thicknesses of 0.3--11 $\mu$m onto $a$-plane sapphire substrates. AD is a room-temperature process that accelerates a precursor of dry sub-micron-sized crystallites to impact and form a thick dense nano-crystalline film that is well-bonded to the substrate. We present results of ferromagnetic resonance (FMR) taken on the as-deposited films. In addition to the main resonance at 2815 G (in-plane) and 4650 G (out-of-plane) we find a distinct resonance mode $H_r'$ that depends on film thickness. It appears in the 0.3-$\mu$m-thick film, becomes most intense in the 1-$\mu$m-thick film, and saturates for thicker films. $H_r'$ moves with film orientation in the applied field from 4085 G (in-plane) to 3010 G (out-of-plane). FMR performed by rotating the applied field in the film plane shows that $H_r'$ exhibits an anisotropy that reflects the crystallographic orientation of the substrate. These results suggest that during the early stages of growth a magneto-crystalline anisotropy is created in the interface region of the film that may be oriented to the crystallographic axis of the sapphire possibly caused by local heating that may facilitate recrystallization. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T30.00009: The magnetism of TiN Barbara Jones, Chiung-Yuan Lin, Szu-Wen Yang, Pushpa Raghani, Shruba Gangopadhyay Titanium Nitride is a well-known technological material, used typically in thin 3 micron coatings, in which form it is an extremely hard ceramic. It is the most common PVD coating used today. It shows no evidence of magnetism. We have studied isolated atoms of Ti on a CuN/Cu surface, layers of TiN on a Cu surface, and isolated monolayers of TiN, using first-principles density functional theory (DFT). In all of these geometries, TiN is magnetic, its interactions mainly governed by superexchange, resulting in striped magnetic structures. The question of how this dramatic magnetism disappears as more layers are added and TiN approaches bulk is one we have studied using DFT. We discuss the process by which the magnetism of TiN makes the transition between nanoscale and bulk. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T30.00010: Role of Cu in exchange bias in FeMn revealed with neutron scattering Igor V. Roshchin, Pavel N. Lapa, Artur Glavic, Haile Ambaye, Valeria Lauter, Sungkyun Park, Tatiana Eggers, Casey W. Miller We observed an unusual behavior: a non-magnetic material, copper, modifies magnetic properties of antiferromagnetic FeMn in close proximity. Copper is responsible for the ``\textit{intrinsic}'' exchange bias (EB) observed in a multilayered system without a distinct, separate ferromagnetic (FM) layer: Ta(5 nm)/[FeMn(5-45 nm)/Cu(5 nm)]$_{10}$/Ta(5 nm) [1]. This EB occurs between pinned and unpinned uncompensated magnetization (UM) in the FeMn layers. The analysis of the remanent magnetization (M$_{\mathrm{R}})$ shows that the unpinned (ferromagnet-like) UM is distributed uniformly throughout FeMn [1]. Since the magnitude of the EB loop shift (H$_{\mathrm{E}})$ scales with the inverse thickness of the FeMn layer, this EB is clearly an interfacial phenomena. This behavior is similar to that described by Malozemoff's model for the bilayer (antiferromagnet-ferromagnet) EB systems [2]. Thus, the pinned UM should be located near the FeMn interface. Results of polarized neutron reflectometry that explain the role of Cu in the unusual magnetism in these FeMn/Cu multilayers will be presented. \\[4pt] [1] D. Kaya \textit{et al.}, J. Appl. Phys., \textbf{113}, 17D717 (2013).\\[0pt] [2] A.P. Malozemoff, Phys. Rev. B \textbf{35}, 3679 (1987), \textit{ibid}., \textbf{37}, 7673 (1988). [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T30.00011: Multilayers of Co and Pt: the ultrathin limit Michalis Charilaou, Catherine Bordel, Pierre-Emmanuel Berche, Brian Maranville, Frances Hellman Magnetic properties of ultrathin (Co/Pt) multilayers, with overall composition 1Co:3Pt and a sub-monolayer Co coverage have been investigated to provide deeper understanding of magnetism in the ultrathin limit and to better understand the alloy. A comparison between a sub-monolayer multilayer and an fcc CoPt$_{3}$ alloy film with perpendicular magnetic anisotropy (PMA) attributed to growth-induced Co clustering reveals significant differences in magnetization, despite the presence of thin Co platelets in both cases. Two regimes of the ferromagnetic phase transition, Brillouin and non-Brillouin-like, are seen in the experimental $M(T)$ data and have been reproduced in Monte Carlo simulations, associated with different distributions of Co-Co nearest neighbors. Both the average number of Co-Co pairs and the width of the distribution are higher for the alloy than the multilayer, leading to surprisingly higher Curie temperature and a nearly linear temperature-dependence of the magnetization of the alloy film, in contrast to the Brillouin-like behavior of the multilayer. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T30.00012: Direct surface charging and alkali-metal doping for tuning the interlayer exchange coupling and magnetic anisotropy Tamene Dasa, Valeri Stepanyuk Manipulation of surface or interface charges allows one to control the magnetic orders in nanostructures. In this contributuion we show two main surface charge alteration techniques for tuning the interlayer exchange coupling and magnetic anisotropy of ferromagnetic layers interspaced with non-magnetic ones. Our ab-initio study reveals that a modest amount of extra charge could switch the mutual alignment of the magnetic layers from anti-ferromagnetic to ferromagnetic or vice verse, taking Fe/NxCu(Pt)/Fe trilayer as model system. We also propose adsorption of alkali metals as a natural way of surface charging mechanism. Clear evidence is found that the interlayer magnetic order can be tuned only by adsorbing alkali metals on the magnetic layer. A combined effect of non-magnetic overlayers and interlayer magnetic order leads to a high perpendicular magnetic anisotropy in FePt thin films. These findings suggest that the interplay of interlayer magnetic coupling with magnetic anisotropy could play a crucial role for magnetic hardening and controlling the spin states. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T30.00013: Maximization of surface-enhanced transversal magneto-optic Kerr effect in Au/Co/Au thin films Edgar Pati\~no, Cesar Herre\~no In order to maximize the transversal magneto optic Kerr effect (T-MOKE) of a Au/Co/Au structure we propose a method to obtain the optimum thickness values. A criteria based on preserving good plasmonic properties has been included as part of this method. Using the theoretical prediction we grew Au/Co/Au trilayers and perform optical and MO characterization using the Kretschmann configuration. The results admit very easy interpretation in terms of the in-teraction between the magneto-optical and plasmonic proper-ties dictating the optimal thicknesses of the structure. Moreo-ver we have grown and characterized the optimized structure finding good agreement with theory reaching, for a 532 nm green laser, a maximal surface magneto-optic (MO) signal enhancement of close to nine folds with respect to the signal without plasmonic excitation. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T30.00014: Investigation of Boundary Magnetization and Exchange Bias of B-doped Chromia Michael Street, Will Echtenkamp, Christian Binek This work is devoted to utilize the magnetoelectric material Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ (chromia) for spintronics. We exploit the electric switchable boundary magnetization (BM) of chromia to manipulate an adjacent exchange-coupled ferromagnetic material. Using a ferromagnetic Co/Pd multilayer deposited on chromia, reversible, room-temperature isothermal switching of the exchange bias (EB) field has been achieved by reversing the electric field in the presence of a constant magnetic field. To use voltage-controlled BM in chromia as a key component in a spintronic device, the N\'{e}el temperature must be increased above the bulk value of T$_{\mathrm{N}} \quad =$307 K. First principle calculations show that boron doping of chromia can increase T$_{\mathrm{N}}$ by roughly 10{\%} per 1{\%} O site substitution by B. We have grown B-doped chromia samples in a gaseous decaborane background atmosphere. We diagram structural and magnetic characterizations of pure and B-doped chromia. SQUID measurements of the BM of B-doped chromia samples indicate an enhancement of T$_{\mathrm{N\thinspace }}$from the bulk value. This finding has been corroborated by spin dependent inverse photoemission spectroscopy [Ref]. Further, we investigate EB systems using a Co/Pd multilayer deposited on B-doped chromia for independent proof of T$_{\mathrm{N}}$ increase in the experimental context most relevant for voltage-controlled spintronics. [Ref] M. Street, et al., Appl. Phys. Lett. 104, 222402 (2014). This work was supported in part by C-SPIN, a center of STARnet, a SRC program, sponsored by MARCO and DARPA. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T30.00015: Theoretical analysis of magnetic structures in the fcc-Fe/Cu(001) system Yasutomi Tatetsu, Shinji Tsuneyuki, Yoshihiro Gohda Magnetic thin films have been studied due to its peculiar magnetic behaviors compared to their bulk systems. Fe/Cu magnetic thin films have been studied for the past decades, because of their uncertain magnetic structures. As is well known, the ground state of Fe has the bcc structure, but the fcc-Fe, which is stable above 1184 K, can epitaxially grow on a Cu(001) substrate even below room temperature, since the lattice parameters of Cu (3.62{\AA}) and fcc Fe (3.58{\AA}) are quite close to each other. Many kinds of studies for the systems have been reported, but its ground state is controversial. We applied first-principles calculations to the fcc-Fe/Cu(001) system using a computational code OpenMX for understanding of its electronic and magnetic structures. Our structural model is a slab model consisting of seven Cu layers and several Fe layers capped by a 10-{\AA} vacuum layer. According to our collinear-spin calculations, an antiferromagnetic structure is stable in the 4 to 7-ML systems and a ferromagnetic coupling at the top two layers can be obtained, which is in good agreement with experimental results. We also calculated the non-collinear spin configuration, which considers the spin-orbit interaction. [Preview Abstract] |
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