Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B15: Focus Session: Spinorbit Effects in Spindependent Transport and Dynamics 
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Sponsoring Units: GMAG DMP Chair: Xin Fan, University of Delaware Room: 317 
Monday, March 18, 2013 11:15AM  11:27AM 
B15.00001: Observation of spin Hall effective field Xin Fan, Jun Wu, Yunpeng Chen, Matthew Jerry, Huaiwu Zhang, John Xiao Recent development in spin Hall driven spin transfer torque has attracted intensive interests$^{1}$. Liu \textit{et. al.} has shown that the spin transfer torque induced by the spin Hall effect in a normal metalferromagnetic metal bilayer can switch the magnetization of the ferromagnetic layer, which may be a potential candidate for magnetic random access memory$^{2}$. The switching of the magnetization was primarily attributed to the Slonczewski torque$^{3}$. We show that besides the Slonczewski torque, the spin Hall effect also produces an effective field that can also facilitate the magnetization reversal. This effective field persists even with a Cu spacer layer, and reduces quickly with the increase of the ferromagnetic layer thickness. The observation of the spin Hall effective field shall have ramification on the understanding of both spin transfer torque and spin Hall effect. 1. K. Ando \textit{et. al.}, Electric manipulation of spin relaxation using the spin Hall effect, Physical Review Letters, 101, 036601 (2008). 2. L. Liu \textit{et. al}., SpinTorque Switching with the Giant Spin Hall Effect of Tantalum. \textit{Science} 336, 555558 (2012). 3. J. Slonczewski, Currentdriven excitation of magnetic multilayers. \textit{Journal of Magnetism and Magnetic Materials}, 159, L1L7 (1996). [Preview Abstract] 
Monday, March 18, 2013 11:27AM  11:39AM 
B15.00002: Analysis of the spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids Matthias Althammer, Sibylle Meyer, Michael Schreier, Mathias Weiler, Stephan Gepr\"ags, Matthias Opel, Hans Huebl, Rudolf Gross, Timo Kuschel, Christoph Klewe, JanMichael Schmalhorst, G\"unter Reiss, Arunava Gupta, YanTing Chen, Gerrit E.W. Bauer, Hiroyasu Nakayama, Eiji Saitoh, Sebastian T.B. Goennenwein Pure spin currents, i.e. the net flow of spin angular momentum without an accompanying charge current, represent a new paradigm for spin transport and spintronics. We have experimentally studied a new type of magnetoresistance effect, which arises from the interaction of charge and spin current flows in ferromagnetic insulator/normal metal hybrid structures. In more detail, we measured the resistance of yttrium iron garnet(YIG)/Pt, YIG/nonferromagnet/Pt, nickel ferrite/Pt, and magnetite/Pt hybrid structures as a function of the magnitude and the orientation of an external magnetic field. The resistance changes observed can be quantitatively traced back to the combined action of spin Hall and inverse spin Hall effect in the Pt metal layer, and are thus termed spin Hall magnetoresistance (SMR). We show that the SMR is qualitatively different from the conventional anisotropic magnetoresistance effect arising in magnetic metals. Furthermore, the SMR enables us to quantify the spin Hall angle in our Pt layers. [Preview Abstract] 
Monday, March 18, 2013 11:39AM  11:51AM 
B15.00003: Transverse magnetoresistance and size effects of thin gold films: Experiment and theory. Raul C. Munoz, S. Oyarzun, R. Henriquez, M.A. Suarez, L. Moraga, G. Kremer We report new experimental data regarding the transverse magnetoresistance measured with the electric field \textbf{E} oriented perpendicular to the magnetic field \textbf{B}, both fields (\textbf{E}, \textbf{B}) contained within the plane of the film (the MacDonald configuration) performed in a family of gold films of different thickness. The signal at 4 K can be univocally attributed to electronsurface scattering. Transport measurements were performed at low temperatures T (4K $\le $ T $\le $ 50K) under magnetic field strengths B (1.5 T $\le $ B $\le $ 9 T). The magnetoresistance signal exhibits a marked thickness dependence, and its curvature as a function of magnetic field B varies with film thickness. We also present a new theoretical description based upon a solution of Boltzmann Transport Equation [MacDonald D. C. K. and Sarginson K., 1950 \textit{Proc. Roy. Soc. (London)} \textbf{A 203} 223], computed using the method of characteristics proposed by Chambers [Chambers R. G., 1950 \textit{Proc. Roy. Soc. (London)} \textbf{A 202} 378]. The theoretical description of the magnetic field dependence of the magnetoresistance requires a Hall field that varies with the thickness of the film; this Hall field is tuned to reproduce the experimental data. [Preview Abstract] 
Monday, March 18, 2013 11:51AM  12:27PM 
B15.00004: Angular dependence of spinorbit spin transfer torque Invited Speaker: KyungJin Lee Magnetocrystalline anisotropy arises from the modification of electron states by spinorbit coupling and is determined by integrating over all occupied electron states. On the other hand, currentinduced spin transfer torques arise from the changes in torques that arise from changes in electron populations in the presence of a current. In this respect, spin transfer torques caused by spinorbit coupling can be interpreted as currentinduced corrections to the magnetic anisotropy. From this perspective, we expect a close relationship between the magnetic anisotropy and spinorbit spin torques. We theoretically study this relationship between magnetic anisotropy and spinorbit spin torque for a ferromagnet subject to Rashba spinorbit coupling. For a twodimensional freeelectron model, we find that Rashba spinorbit coupling results in perpendicular magnetic anisotropy and fieldlike currentinduced spin transfer torques. Both quantities acquire nontrivial angular dependence as the spinorbit coupling becomes comparable to the sd exchange interaction. This nontrivial angular dependence can be understood from Fermi surface distortion. In the limits where either the spinorbit coupling or the sd exchange interaction is much greater than the other, the Fermi surface consists of two concentric circles, but when they are comparable it distorts. These freeelectron calculations are in qualitative agreement with ab initio calculations for Co$$Pt bilayers, suggesting that the spinorbit coupling at the interface is nonnegligible in comparison to the sd exchange interaction there. The nontrivial angular dependence of spinorbit spin torque may be used as an indicator of strong interfacial spinorbit coupling, because the spinorbit spin torque that is induced by the spin Hall effect, has a simple $\sin\theta$ dependence where $\theta$ is the angle between the magnetization and the spin injected into a ferromagnet. This work has been done in collaboration with M. D. Stiles and P. M. Haney. [Preview Abstract] 
Monday, March 18, 2013 12:27PM  12:39PM 
B15.00005: Spin transfer torques in magnetic bilayers with strong spin orbit coupling M.D. Stiles, Paul M. Haney, HyunWoo Lee, KyungJin Lee, Aurelien Manchon Current driven magnetic dynamics in ferromagnetic thin films on top of nonmagnetic films with strong spin orbit coupling show strong currentinduced torques. Several theoretical models have been proposed to explain these torques. In one model, the current flowing through the nonmagnetic layer gives rise to a spin Hall current, leading to a spin current incident on the interface between the two layers. This spin current causes spin transfer torques similar to those that are important in magnetic multilayers with current flowing perpendicular to the plane. Another model proposes a torque due to the spinorbit coupling at the interface where the inversion symmetry found in the bulk materials is broken. We model the spin transport with a semiclassical Boltzmann equation approach. Both torques are present in this model and for reasonable parameter sets are largely independent of each other. We compute the dependence of the torques on the thickness of the layers and find that it is difficult to reproduce the large sensitivity to the thickness of the ferromagnetic layer as found in several experiments. This disagreement indicates that structural or electronic properties are probably changing with the thickness of the films studied in experiments. [Preview Abstract] 
Monday, March 18, 2013 12:39PM  12:51PM 
B15.00006: Spin transfer torque devices utilizing the spin Hall effect of tungsten ChiFeng Pai, Luqiao Liu, Yun Li, HsinWei Tseng, Daniel C. Ralph, Robert A. Buhrman It is recently been shown that the spin Hall effect (SHE) in $\beta $Ta generates a transverse spin current that is sufficient for efficiently reversing the moment of adjacent thin film nanomagnets through the spin torque (ST) mechanism. Here we report the existence of an even larger SHE in $\beta $W thin films. Using spin torque induced ferromagnetic resonance (STFMR) with a $\beta $W/CoFeB bilayer microstrip we have determined the magnitude of the spin Hall angle $\theta $ to be 0.30$\pm $0.02, which is twice as large as the previously reported value for $\beta $Ta ($\sim $0.15). From switching data obtained with 3terminal devices consisting of a $\beta $W channel and an adjacent CoFeB/MgO/CoFeB magnetic tunnel junction, we have independently determined \textbar $\theta $\textbar $=$ 0.33$\pm $0.06. We will also report on the variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable ($\alpha $ and $\beta )$ phase composition. Finally we have studied the SHE exhibited by several other 4d and 5d transition metals using the techniques mentioned above and we will report on those results. [Preview Abstract] 
Monday, March 18, 2013 12:51PM  1:03PM 
B15.00007: SpinHall and spinpumping effect observed in W/FeCoB thin films Yun Li, Chifeng Pai, Hsinwei Tseng, Luis Leao, Dan Ralph, Robert Buhrman The spinHall effect (SHE) and its reciprocal, the inverse spinHall effect (ISHE), are of great importance in spintronics since they enable, respectively, the conversion of a longitudinal charge current to a transverse spin current and the reverse process. Here we will report on a ferromagnetic resonance (FMR) study of FeCoB/W thin film bilayer structures that incorporate different W thicknesses and hence difference phases. A very large negative spin Hall angle has been observed in the $\beta $W samples and confirmed by spintorque switching studies. Alternatively FMR measurements with bilayers containing $\alpha $W suggests a strong positive SHE, but this interpretation of the experiment is not consistent with spin torque switching studies utilizing $\alpha $W. Since the $\alpha $W FMR results also show an enhanced magnetic damping we tentatively attribute these results to a significantly enhanced spin pumping effect in $\alpha $W, relative to $\beta $W. Magnetization measurements indicate that the two different types of FeCoB/W bilayers have substantially different interfacial magnetic anisotropy coefficients. We will discuss these results, together with the differing temperature dependence of the FMR signal in the two cases, which help point the way to understanding the origin of the giant SHE in $\beta $W and the strong ISHE in $\alpha $W. [Preview Abstract] 
Monday, March 18, 2013 1:03PM  1:15PM 
B15.00008: Spinorbitinduced spinpolarized surface states in oneatomiclayer Pb films on Si(111) Hyungjun Lee, Hyoung Joon Choi As a route to spintronics without magnetism, spinorbit coupling (SOC) generates and manipulates the spinpolarized carriers, thereby providing key ingredients for spin fieldeffect transistors. Along this line, we investigated the spinorbit induced effects in Pb monolayers on Si(111) substrates, modeled by $\sqrt{3}\times\sqrt{3}$ phase with Pb coverage of 4/3 ML, based on firstprinciples calculations with the inclusion of SOC. We focus on the electronic structures of surface states with characteristic Rashbatype spin splitting and spin texture as well as the charge flow pattern by calculating the current density distribution for the spinpolarized surface states. We also discuss our results on the difference from the spin splitting in the Shockley surface states on Au(111) surface. This work was supported by the NRF of Korea (Grant No. 20110018306), and computational resources have been provided by KISTI Supercomputing Center (Project No. KSC2012C214). [Preview Abstract] 

B15.00009: ABSTRACT WITHDRAWN 
Monday, March 18, 2013 1:27PM  1:39PM 
B15.00010: Spin Hall Effect induced Anisotropic Magnetoresistance Priscila Gonzalez Barba, SeeHun Yan, Luc Thomas, KwangSu Ryu, Stuart Parkin, Aurelien Manchon Spinorbitinduced anisotropic transport in magnetic materials, studied for more than a century, has recently experienced a renewed interest thanks to the formulation of anisotropic spin scattering in terms of Berry's curvature. Anisotropic magnetoresistance (AMR) is related to the scattering of the transport electrons on the orbitals of localized electrons, depending on the magnetization direction. The contributions of the interfaces on AMR has been scarcely studied. We consider a trilayer composed of one ferromagnetic layer sandwiched between two normal metals. The normal metals display spin Hall effect (SHE), whereas the ferromagnetic layer polarize the flowing current. We propose that SHE present in the top and bottom layers might contribute to the AMR. The charge and spin currents are analyzed by driftdiffusion equations including the role of inverse SHE as well as anomalous Hall effect. Longitudinal and transverse spin accumulations at the interfaces are captured through spin dependent conductance and the mixing conductance. It is shown that the presence of a spin accumulation in the normal metal close to the interface is transformed into a charge current through inverse SHE hence altering the conductivity of the normal metal. The obtained total resistivity calculation indicates its own spin accumulation profile dependance. [Preview Abstract] 
Monday, March 18, 2013 1:39PM  1:51PM 
B15.00011: Influence of spinorbit interactions on the electronic structure and magnetic properties of IrMn alloys Hua Chen, Panteleimon Lapas, Fengcheng Wu, Allan H. MacDonald We will present a theoretical study of the electronic and magnetic properties of noncollinear antiferromagnetic metals with strong spinorbit interactions, focusing on the particular case of the IrMn alloy. IrMn alloys are important antiferromagnetic materials often used as the pinning layer in spinvalve structures. Their electronic structure has so far not been extensively studied; in particular the influence of spinorbit interactions which are strong in this material has not yet been addressed. We start from ab initio calculations for ordered IrMn$_{3}$ crystals, and analyze the relationships between band degeneracy, noncollinearity of the Mn spins, and the large spinorbit coupling of Ir. We will also study the spin wave spectra in the ordered IrMn$_{3}$, and finally comment on the influence of transport currents on magnetization structure and dynamics in antiferromagnets in general, and noncollinear systems in particular. [Preview Abstract] 
Monday, March 18, 2013 1:51PM  2:03PM 
B15.00012: Theory of Tunneling Anisotropic Magnetoresistance Using the TightBinding Green's Function Approach Vivek Amin, Jan Zemen, Jan Masek, Jairo Sinova, Tomas Jungwirth An increasing experimental and theoretical understanding of magnetic tunnel junctions has led to widespread application within magnetic hard drives and furthered our understanding of spin valvelike processes fundamental to Spintronics. Crucial to this understanding is the investigation of tunneling processes between single ferromagnetic layers and tunnel barriers. We present a theoretical study of the Tunneling Anisotropic Magnetoresistance (TAMR) in a Co/Pt junction with a tunnel barrier. We calculate conductance as a function of magnetization direction using the LandauerButtiker formula. The system Hamiltonian is obtained by means of a suitable tightbinding model fitted to abinitio calculations, while the transmission is computed via the Green's function formalism. [Preview Abstract] 
Monday, March 18, 2013 2:03PM  2:15PM 
B15.00013: Inelastic process at finite temperature in 2D mutilband systems Jacob Gayles, Huawei Gao, Jairo Sinova Despite the recent progress in the understanding of the contributions to the anomalous Hall effect, however there is still a lacking in understanding the role of inelastic processes at finite temperatures and the role of strong disorder. We use numerical methods and the Kubo Formalism to explore this regime multiband systems with spinorbit coupling. Some experiments with the use of residual conductivity have been interpreted so that extrinsic mechanisms have a strong dependence on the increase in temperature while the anomalous hall conductivity reaches a steady state value. [Preview Abstract] 
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