Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R16: Focus Session: Magnetic Thin Films |
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Sponsoring Units: GMAG DMP Chair: Roland Kawakami, University of California, Riverside Room: 318 |
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R16.00001: Ferroelectric control of magnetocrystalline anisotropy at Co/poly(vinylidene fluoride) interfaces J.P. Velev, P.V. Lukashev, T.R. Paudel, J.M. Lopez-Encarnacion, S. Adenwalla, E.Y. Tsymbal Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and strain coupling. In this work we use density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy which controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50{\%}, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R16.00002: The study of perpendicular magnetic anisotropy and Boron diffusion in Ta--CoFeB--MgO--CoFeB--Ta magnetic tunnel junction using polarized neutron reflectometry Valeria Lauter, H. Ambaye, T. Zhu, Y. Yang, R.C. Yu, J.Q. Xiao The current-induced spin transfer torque (STT) plays an important role in spintronic devices. However, the level of current density needed to reorient the magnetization is presently too high for most commercial applications, and reducing the current density is the challenging basis for recent research in spintronics. The magnetic tunnel junction (MTJ) with a perpendicular magnetic anisotropy (PMA) enables a small critical current density for current-induced magnetization switching and provides a pathway for such STT devices. We investigated the origin of PMA in CoFeB sandwiched by MgO and Ti layers using the anomalous Hall effect (AHE) and polarized neutron reflectometry (PNR). It is found that the PMA properties of CoFeB layers deposited above and under MgO layer are different and PNR measurements confirmed that a large PMA in the CoFeB above MgO layer is related to its low magnetization. From PNR experiments, we obtained the details of the magnetic and structural depth profiles inside the film. Using the sensitivity of neutrons to the absorption cross-section of boron, we unambiguously determined the depth profile of the boron distribution and showed that after annealing, most of the boron diffused to form a 2-nm-thick interface layer between the CoFeB and tantalum layers. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R16.00003: Spin-polarized ion scattering spectroscopy study on Si/Fe(100) surfaces Taku Suzuki, Shunichi Hishita We investigated surface magnetic structure in the initial stage of Si deposition on an Fe(100) surface by spin-polarized ion scattering spectroscopy (SP-ISS). [1] We found silicidation at the Si/Fe interface after Si deposition followed by annealing at 823 K. The silicidation occurs by the incorporation of silicon into the Fe substrate via the substitutional site of bcc Fe. After annealing, the incorporated Si atoms are distributed in surface layers several nanometers thick. The SP-ISS analysis revealed that the average magnetic moment of Fe in the silicide surface layer is about 70\% of that of Fe in the Fe(100) surface layer, whereas that of Si is almost zero. These surface magnetic moments are discussed in terms of the local magnetic environment. It is likely that the outermost surface of the silicide layer has an atomic arrangement similar to that of Fe$_3$Si(100) with surface termination by the Fe-Si plane.\\[4pt] [1] T.T.Suzuki and S.Hishita, Appl.Surf.Sci.259(2012)166. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R16.00004: Microscopic Structure of Mn Atom Chains on the Si(001) Surface Investigated by Scanning Tunneling Microscopy A. Fuhrer, F. J. Rue{\ss}, N. Moll, A. Curioni, D. Widmer The Si(001) 2x1 reconstructed surface has the interesting property that many metal atom species form nearly perfect 1D atomic wires oriented perpendicular to the Si dimer rows during deposition at room temperature. These wires are thought to consist of metal dimers located between the dimer rows linking up to form atomic chains. More recent experiments indicated that similar wire formation occurs for Mn which, with its half filled d-shell, has interesting magnetic properties e.g. when used as a dopant in dilute magnetic semiconductors. In our experiments we use scanning tunneling microscopy to study the atomic structure of these Mn-wires in detail and find that it is different from that of the other known metal wires. We show that two distinct types of Mn wires occur, with an asymmetric appearance relative to the underlying Si lattice. While one type of asymmetry can be linked to the buckling of the Si dimers near the Mn-wires the other is found to be intrinsic to the microscopic structure of the Mn-wires. We further compare high-resolution bias-dependent constant current images with images simulated for a Mn trimer wire structure using density functional calculations employing the CPMD code. \\[4pt] A. Fuhrer, F. Rue{\ss}, N. Moll, A. Curioni, D. Widmer, PRL {\bf109}, 146102 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R16.00005: The failure of DFT computations for a stepped-substrate-supported monatomic highly-correlated wire system Nader Zaki, Richard M. Osgood, Andrew J. Millis, Chris A. Marianetti The ab-initio method, density functional theory (DFT), has been immensely successful in its ability to predict physical properties of condensed matter systems. In particular, DFT calculations have proven to be quantitatively accurate in predicting structural properties in a wide range of materials and qualitative failures are rare. Here, however, we show that DFT can fail qualitatively to correctly predict the dimerized structural phase for a recently reported experimentally realized monatomic Co wire system that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate [1]. We attribute this failure to DFT's over-prediction of hybridization of the Co wire with the underlying Cu substrate. We demonstrate that this over-hybridization leads to weakening of the magnetic coupling along the wire, which is responsible for dimerization, while increasing the stiffness of the wire due to strengthening of the non-magnetic elastic term. Additionally, we show that accounting for local interactions via DFT$+U$ also fails at predicting the correct structural phase. [1] N. Zaki et al, arXiv:1208.0612 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R16.00006: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R16.00007: Temperature-dependent proximity magnetism in Pt Weng L. Lim, John C. Owens, Neema Ebrahim-Zadeh, Hilary G. E. Hentschel, Sergei Urazhdin We report the observation of a significant magnetic coupling between two ferromagnets (FM) separated by a thin Pt layer. The coupling remains ferromagnetic regardless of the Pt thickness, and exhibits a strong dependence on temperature $T$. These features of the coupling cannot be explained by the well-known RKKY mechanism of coupling between FM separated by a nonmagnetic spacer. We use a phenomenological model to demonstrate that the observed effects are consistent with the existence of temperature-dependent magnetic ordering induced in Pt in proximity to the interfaces with FM, consistent with a recent report on the ferromagnetic characteristics in Pt films grown on ferromagnetic insulators [1]. The magnetization in Pt decays away from the interfaces with a characteristic length scale that increases with decreasing temperature, and reaches 0.8 nm at $T<25$ K. Our results suggest that, in Pt/FM heterostructures, the magnetic and spin-dependent transport properties of Pt and FM are mutually affected, opening possibilities for engineering of new magnetoelectronic metamaterials. [1] S. Y. Huang et al, Phys. Rev. Lett. 109, 107204 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R16.00008: Tailoring magnetic properties of thin films with quantum well states and external electric field Tamene R. Dasa, Valeriy S. Stepanyuk Dependence of magnetic anisotropy energy (MAE) and spin-polarization of magnetic multilayers on the layer thickness is studied with {\it ab initio} techniques. For thin Fe films adsorbed on a Pt surface a reversal of the MAE (rotation of the easy axis) is observed with changing film thickness. Moreover, our calculation show that capping of magnetic films with Pt in most cases leads to a strong increase of MAE. Both of the later phenomena are traced back to spin-dependent quantum-well states (QWS) in the magnetic thin films. Combining the newly gained understanding with the well-known fact, that quantum well states can be tuned by external electric fields acting on the system, we show that, similar to the case of the quasi 1-D systems [1], the MAE in thin magnetic metallic films can be tailored with external electric field. For example, applying an electric field of $-1~V$/{\AA} to a multilayer of Pt/Co/Pt(001), its MAE can be changed by more than 50\%. To finalize the paper, changes in spin-polarization and the Stark-like shift accompanying exposure of the system to external electric fields are outlined and discussed. \\[4pt] [1] T.~R.~Dasa, P.~A.~Ignatiev, and V.~S.~Stepanyuk, Phys. Rev. B \textbf{85}, 205447 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R16.00009: Giant proximity effect in ferromagnetic bilayers Silvia Ramos, Tim Charlton, Jorge Quintanilla, Andreas Suter, Jagadeesh Moodera, Thomas Prokscha, Zaher Salman, Ted Forgan The proximity effect is a phenomenon where an ordered state leaks from a material into an adjacent one over some finite distance, $\xi$. For superconductors, this distance is $\sim$ the coherence length. Nevertheless much longer-range, ``giant'' proximity effects have been observed in cuprate junctions. This surprising effect can be understood as a consequence of critical opalescence. Since this occurs near all second order phase transitions, giant proximity effects should be very general and, in particular, they should be present in magnetic systems. The ferromagnetic proximity effect has the advantage that its order parameter (magnetization) can be observed directly. We investigate the above phenomenon in Co/EuS bilayer films, where both materials undergo ferromagnetic transitions but at rather different temperatures (bulk $T_C$ of 1400K for Co and 16.6K for EuS). A dramatic increase in the range of the proximity effect is expected near the $T_C$ of EuS. We present the results of our measurements of the magnetization profiles as a function of temperature, carried out using the complementary techniques of low energy muon rotation and polarized neutron reflectivity. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R16.00010: Asymmetric magnetic switching behavior of Py/SmFe/Py exchange spring magnet Jiyeong Gu, Hanming Yuan Magnetic switching behavior of the symmetric exchange spring magnet, Py(Permalloy)/SmFe (or SmCo)/Py thin films, was investigated. Exchange spring magnet shows a unique magnetic hysteresis curve due to the non-collinear magnetization developed by magnetic coupling of the soft and hard magnetic layers. Using Magneto Optical Kerr Effect (MOKE) we could separately measure the magnetic hysteresis loops of the top and the bottom Py layers. We found the magnetic hysteresis loops for the bottom and the top Py layers are different indicating the switching behavior is not symmetric. The coercivity of the bottom Py layer is much smaller than that of the top Py layer. From the comparison of MOKE data to the one measured by Alternating Gradient Magnetometer, we observed that the top Py layer and hard layer switch together representing the top Py layer is strongly coupled to the hard layer and shows a single switching instead of spiral magnetization distribution; while the bottom Py layer shows a non-collinear magnetization behavior as we expect from a typical exchange spring magnet. Thickness of the soft and hard layers was systematically varied to further investigate the asymmetric switching behavior of double exchange spring magnet thin films. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R16.00011: Epitaxial Growth of Heusler Co$_{2}$MnSi Heterostructures: Electronic and Structural Properties Thomas Neulinger, Sahil Patel, Alexander Kozhanov, Brian Schultz, Chris Palmstr{\O}m The Heusler alloy Co$_{2}$MnSi is predicted to be a half-metal, a material that is spin-polarized at the Fermi energy. We have demonstrated growth by molecular beam epitaxy of Co$_{2}$MnSi, Cr/Co$_{2}$MnSi, and a complete Co$_{2}$MnSi/MgO/Co$_{2}$MnSi(001) magnetic tunnel junction on epitaxial GaAs(001) surfaces without air exposure. Epitaxial Cr layers have been used to exchange bias Co$_{2}$MnSi. In-situ electron diffraction and scanning tunneling microscopy, and ex-situ X-ray diffraction techniques are used to characterize the crystal quality. The magnetic properties are investigated using vibrating sample and superconducting quantum interference device magnetometry. We present these results and will compare them with temperature dependent magnetotransport and tunneling spectroscopy measurements, with emphasis on the influence of Co$_{2}$MnSi surface termination. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R16.00012: Magnetostatics and magnetodynamics in Co$_{2}$MnSi on GaAs (001) Michael Pechan, Daniel Stanley, Michael Sinko, Sahil Patel, Alexander Kozhanov, Brian Schultz, Chris Palmstrom We present an investigation of the magnetic properties of Co$_{2}$MnSi films grown by molecular beam epitaxy on lattice matched Sc$_{0.3}$Er$_{0.7}$As films grown on GaAs (001) substrates with various capping layers (Cr, Al, Au). Co$_{2}$MnSi thickness varied from 3 to 21.4 nm. X-ray diffraction analysis confirmed the single crystal nature and crystallographic orientations of the films. Magnetization measurements reveal square loops with low in-plane saturation fields and very narrow (few Oe) coercive fields. An interesting feature of the loops in several of the samples is the presence of a small ($<$10 Oe) exchange-bias field observed at room temperature. Room temperature ferromagnetic resonance (FMR) measurements were carried out at 35 GHz as a function of in-plane angle to quantify the anisotropy in these structures. Resonances follow the typical derivative lineshape with relatively narrow line widths ranging from 30 to 140 Oe, consistent with high quality Heusler alloy film formation. Four-fold anisotropy is clearly observed in all samples confirming the high quality single-crystal nature of the films. A small unidirectional anisotropy associated with the exchange bias mentioned above is also observed. We will also present results on preliminary MJT structures. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R16.00013: topological chiral spin-wave modes in dipolar ferromagnetic thin films Ryuichi Shindou, Ryo Matsumoto, Jun-ichiro Ohe, Shuichi Murakami Magnetic dipole-dipole interaction in ferromagnet plays role of locking a relative angle between the spin space and the orbital space, just in the same way as the relativistic spin-orbit interaction does in ferromagnetic metals, leading to their quantum anomalous Hall effect. Focusing on this similarity, we theoretically design a couple of periodically-structured ferromagnetic thin film models which support unidirectional (chiral) propagations of spin-waves along its sample boundaries in their dipolar regime. Contrary to the Daemon-Eshbach surface mode, the chiral direction and the number of such spin-wave edge modes are determined by so-called topological Chern integer associated with `Bloch wavefunctions' for the volume-type spin wave modes. Namely, even if the direction of the magnetic field is fixed, the chiral direction can be still either left-handed or right-handed, depending on the periodic structuring and the frequency range, which is not the case with the Daemon-Eshbach mode. By introducing proper `atomic orbitals' for the proposed thin film models, we present a simple tight-binding description for the proposed topological chiral edge modes. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R16.00014: Equilibrium properties of Ising metamagnetic films James Mayberry, Michel Pleimling Artificial antiferromagnets have attracted attention lately due to the potential for technological applications. We model these systems as thin Ising metamagnetic films and study their equilibrium properties using Monte Carlo simulations. In variance with previous work but in agreement with the experimental systems, we consider films comprised of ``sets'' of planes, with an antiferromagnetic coupling between sets and a ferromagnetic coupling within sets. This allows us to consider different situations by varying the number of planes in each set. Studying the magnetization density and response functions as a function of temperature and magnetic field, we determine the corresponding phase diagrams. We discuss how a change of the number of planes in each set changes the equilibrium phase diagram. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R16.00015: Characteriziation of $Ni_2MnGa$ Ferromagnetic Shape Memory Alloy nanowires P. Gyawali, Keshab R. Sapkota, B. Dahal, R. Dulal, I. L. Pegg, J. Philip Heusler type $Ni_2MnGa$ ferromagnetic shape memory alloy has been extensively studied in thin films and in bulk. The structural transition to martinsitic phase occurs thermodynamically reversibly within the ferromagnetic region.. For the technological application, magnetic field is used to induce the motion of twin boundaries in martinsite phase. $Ni_2MnGa$ nanowires were grown for the first time using electrospinning method. Structrual characterization were done using XRD and EDX. Nanowires exhibit tetragonal structure with a = b = 5.85 \AA and c/a = 0.96. Magnetic measurements show the pre- martinsite transformation. Curie temperature of nanowires is about 360 K. [Preview Abstract] |
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