Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S42: Spin-Orbit TorquesFocus
|
Hide Abstracts |
Sponsoring Units: GMAG Chair: Chunhui Du, University of California, San Diego Room: 709/711 |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S42.00001: Partitioning the spin-orbit torque in heavy metal/ferromagnet heterostructures Fei Xue, Paul Haney In heterostructures composed of heavy metals and ferromagnets, the spin-orbit torque is the result of reduced symmetry and the presence of spin-orbit coupling. In order to optimize the spin-orbit torque efficiency, we provide a general scheme to identify the contributions to the intrinsic spin-orbit torque due to the wave-function change induced by an external electric field in the steady state. Using a toy model of a bilayer system with atomic p-orbitals, we find both spin current from spin Hall effect and Berry curvature-like pseudo-magnetic field contribute to the torque. We next apply our analysis to first-principles calculation of spin-orbit torque in multilayer Pt/Co and 1T’-WTe2/Co systems. The additional in-plane symmetry breaking in 1T’-WTe2 allows for an unconventional out-of-plane torque. We find that the atomic orbital current can be transferred to spin current at the interface heavy metal layer, which is the dominant spin current contribution to the torque. The contributions identified by our partitioning scheme in the ferromagnet/heavy metal heterostructure could shed light on how to improve the spin-orbit torque switch efficiency. |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S42.00002: Bulk versus Interface Contributions to the Spin-Orbit Torque in Ferromagnetic Heterostructures Farzad Mahfouzi, Nicholas Kioussis We present an ab initio-based theoretical framework which elucidates the origin of the bulk versus interface contributions to the spin-orbit torque (SOT) in Heavy-Metal(HM)/Ferromagnet(FM) heterostructures. We find that |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S42.00003: Spin-orbit torque generated by amorphous FexSi1-x Cheng-Hsiang Hsu, Julie E Karel, Niklas Roschewsky, Dinah Simone Bouma, Scott Bennett, Alexander Nguyen, Suraj S Cheema, Shehrin Sayed, Frances Hellman, Sayeef Salahuddin Despite the tremendous amount of work that has gone into spin-orbit torque and spin current generation in heavy metals, questions on the origin of the underlying physics still remain, often stemming from the challenge in differentiating between spin currents that are generated from the bandstructure of the heavy metal, and other sources, such as inversion symmetry breaking at the interface and or scattering. Here we report observation of spin-orbit torque under room temperature in fully amorphous non-magnetic Fe(x)Si(1-x)/cobalt bilayer via spin-torque ferromagnetic resonance and harmonic Hall measurements. Both techniques provide a consistent spin Hall angle of about 3%. According to the conventional theory of the spin Hall effect, a spin current in an amorphous material is not expected to have any contribution from the bandstructure. Despite this fact, our observation of a reasonably strong spin-orbit torque paves a new avenue for understanding the underlying physics of spin-orbit interaction. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S42.00004: Transverse and parallel spin-torque ferromagnetic resonance for improved measurements of spin-orbit torques Saba Karimeddiny, Joseph Mittelstaedt, Robert Buhrman, Daniel Ralph We present a method to quantify the contribution of the spin-pumping/inverse-spin-Hall-effect (SP-ISHE) voltage to spin-torque ferromagnetic resonance (ST-FMR) measurements of spin-orbit torques. In this approach the resonant homodyne voltages transverse to and parallel to the applied microwave current are measured simultaneously. We leverage the fact that spin-pumping only contributes as a symmetric Lorentzian to derive consistency conditions which allow the SP-ISHE to be separated from the rectification signals associated with the spin-orbit torque. The method does not require knowledge of parameters such as the effective spin mixing conductance and spin diffusion lengths of heavy metals. Our findings show that for some material combinations SP-ISHE can alter the ST-FMR signal significantly, and the relative sign of rectification and SP-ISHE is unambiguously determined. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S42.00005: Interfacial contributions to spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers Kirill Belashchenko, Alexey Kovalev, Mark van Schilfgaarde The thickness dependence of spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers is studied [1] using the first-principles non-equilibrium Green's function formalism combined with the Anderson disorder model. A systematic expansion in orthogonal vector spherical harmonics is used for the angular dependence of the torque. The damping-like torque in Co/Pt and Co/Au bilayers can be described as a sum of the spin-Hall contribution, which increases with thickness in agreement with the spin-diffusion model, and a comparable interfacial contribution. The magnetoconductance in the plane perpendicular to the current in Co/Pt bilayers is of the order of a conductance quantum per interfacial atom, exceeding the prediction of the spin-Hall model by more than an order of magnitude. This suggests that the "spin-Hall magnetoresistance," similarly to the damping-like torque, has a large interfacial contribution unrelated to the spin-Hall effect. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S42.00006: Field-free spin-orbit torque switching of Cr-induced perpendicular magnetization Tsao-Chi Chuang, Chi-Feng Pai, Ssu Yen Huang Current-induced spin-orbit torque (SOT) switching in a heterostructure with perpendicular magnetic anisotropy (PMA) has attracted great attention, as a new writing method for spintronic devices. However, this highly attractive switching scheme is often accompanied by an unfavorable external magnetic field. In this work, we show that the 3d Cr, without any heavy metals and the MgO layer, can induce strong interfacial PMA and deliver deterministic SOT switching. Most importantly, we demonstrate field-free SOT switching in 3d Cr without any complex schemes, including asymmetrical layers, pattern structure, and additional ferromagnetic or antiferromagnetic layers. We show that the underlying cause for the deterministic field-free switching lies in the slanted columnar microstructure for the otherwise uniform thin films. The direction of oblique columnar structure dictates the up and down orientations of the PMA layer, resulting in polarity-controlled field-free SOT switching [1]. Our results uncover the significant role of 3d materials and shed light on field-free SOT magnetization switching. |
Thursday, March 5, 2020 12:27PM - 1:03PM |
S42.00007: Anomalous spin-orbit torques in a ferromagnetic metal Invited Speaker: Xin Fan Current-induced spin-orbit torques have attracted a considerable amount of attention in recent years. The typical structure for studying the spin-orbit torque is a ferromagnetic metal/ nonmagnetic material bilayer. Most research has focused on the spin-orbit torque generated externally by the nonmagnetic material, overlooking the possibility that the ferromagnetic metal can generate spin-orbit torque on itself. We recently discovered that a ferromagnetic metal can in fact generate equal and opposite spin-orbit torques at its surfaces, which is termed as the anomalous spin-orbit torque, due to its analogy to the anomalous spin Hall effect. The strong anomalous spin-orbit torque may challenge our current understanding of spin-orbit torque in bilayer/multilayer systems. It also provides new opportunities that ferromagnet itself can be a source of spin-orbit torques. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S42.00008: Spin-orbit driven spin depolarization in the ferromagnetic Weyl semimetal Co3Sn2S2 Sandeep Howlader, Surabhi Saha, Ritesh Kumar, Vipin Nagpal, Satyabrata Patnaik, Tanmoy Das, Goutam Sheet Co3Sn2S2 has recently emerged as a ferromagnetic Weyl semimetal. Previous experiments and theoretical investigations of the band structure have provided an indication of the possibility of half-metallicity in this compound. However, our spectroscopic investigations have shown that a spin-polarized super-current flows through a Nb/Co3Sn2S2 point contact with a large Andreev reflection indicating a huge deviation from half-metallic character. In fact, we have observed a 50% spin polarization at the Fermi level. Theoretical calculation of the electronic density of states with inclusion of spin-orbit coupling showed that this deviation stems from a spin depolarizing effect near the Fermi energy. Additionally, the calculations revealed a particle-hole asymmetry that explains our experimental observation of asymmetric Andreev reflection spectra. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S42.00009: Spin Orbit Torque in Graphene/Co Heterosystem Kenan Song, Aurelien Manchon, Udo Schwingenschlogl The spin-orbit torque (SOT) technique has opened new horizons for the development of innovative magnetic devices beyond memories and data storage. When graphene is attached to the surface of Co (001), the inversion symmetry is broken, which results in the onset of a built-in perpendicular electric field. As a result, in the presence of the large spin-orbit coupling of Co, Rashba effect emerges at the interface. Therefore, a spin density driven by a net current can be achieved at the interface, bringing SOT into such a heterostructure. With density functional theory, we show that Co atoms at the interface exhibit spin-momentum locking, which is in line with the exprtiments. Around the Fermi level, the graphene Dirac cones couple with the Co 3d states around the K and K' points, resulting in a spin texture odd in momentum k. This special spin texture promotes current-driven SOT. We then project the band structure obtained by first principles onto Wannier orbitals to get the tight-binding Hamiltonian. Non-equilibrium properties then are calculated using the Kubo formula. Our results show that the SOT can be used to electrically control the magnetization of the Co layer and to realize fast electronic device. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S42.00010: Time-evolution of spin-orbit torque and magnetization in Rashba ferromagnet from multiscale time-dependent-quantum-transport/classical-micromagnetics formalism UTKARSH BAJPAI, Branislav Nikolic Spin-orbit torque originates in a ferromagnetic material when the nonequilibrium electronic spin density emerging due to spin-orbit coupling is noncollinear to the local magnetization. In this work, we investigate the time-evolution of spin-orbit torque in a Rashba ferromagnet using a recently developed quantum-classical hybrid framework where quantum electrons described by time-dependent nonequilibrium Green functions (TDNEGF) are self consistently coupled to local magnetization dynamics described by classical-micromagnetics using the Landau-Lifshitz-Gilbert (LLG) equation. It has been previously shown that such a self-consistent TDNEGF+LLG framework microscopically generates a spatially inhomogeneous and time-dependent dynamical Gilbert damping [1] through a memory kernel, contrary to conventional micromagnetics where a static Gilbert damping parameter is introduced phenomenologically. Here, we explore the influence of Rashba spin-orbit coupling on the resulting time-dependent spin-orbit torque, magnetization dynamics and dynamical Gilbert damping parameter in our TDNEGF+LLG framework which is further compared to the Gilbert damping parameter and magnetization dynamics evaluated using the scattering matrix theory. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S42.00011: Theoretical study of spin torque and domain wall motion on ferromagnetic Kagome lattice Sehoon Kim, Kentaro Nomura We theoretically study spin torque and domain wall motion induced electrically in a ferromagnetic Kagome lattice, which has spin-orbit coupling. For details, we suppose the exchange interaction with an inhomogeneous magnetic texture of uniaxial magnetic anisotropy and the spin-dependent second-nearest neighbor hopping acting as the spin-orbit coupling. Because of the chiral edge conduction, electric charges are accumulated around the domain wall. We calculate the spin polarization of localized charges as a response of the electric field by the Kubo formula. As a result, the spins of localized charges are polarized and exert spin torque on the domain wall. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S42.00012: Current-induced unidirectional magnetoresistance in FeRh/Pt bilayers Julie Shim, Hilal Saglam, Kisung Kang, Junseok Oh, Yi Li, Wei Zhang, Matthew Gilbert, Andre Schleife, Axel Hoffmann, Joseph Sklenar, Nadya Mason Antiferromagnetic (AFM) spin-orbit torque oscillators are important due to potential applications ranging from magnetic random-access memory to THz emitters. However, AFM spin-torque oscillators operate in the THz-range, and direct observation of THz magnetization dynamics is difficult. Previously, in ferromagnetic spin-torque oscillators, current-induced unidirectional magnetoresistance (UMR) signals accompanied the emission of microwave radiation at the ferromagnetic resonance frequency [1,2], which implies that observations of UMR may provide indirect evidence of AFM-resonance. Here we present transport results on magnetron sputtered AFM FeRh/Pt films, fabricated into nanowire geometries. A current-induced UMR is observed, in addition to anisotropic magnetoresistance effects. These findings suggest a route toward designing AFM spin-orbit torque oscillators. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700