Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X38: Spin Transfer and Spin TorquesFocus Live

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Sponsoring Units: GMAG DMP FIAP Chair: Luqiao Liu, Massachusetts Institute of Technology MIT 
Friday, March 19, 2021 8:00AM  8:12AM Live 
X38.00001: Microwave and spin transfer torque driven coherent control in ferromagnets Marina Brik, Nirel Bernstein, Amir Capua Coherent control is a method used to manipulate the state of matter using oscillatory electromagnetic radiation which relies on the nonadiabatic interaction. It is commonly applied in quantum processing applications. This technique is interesting in the context of ferromagnetic materials because of the ability to combine it with spintronics for the purpose of fundamental spin transport research, lowpower information processing, and potentially future quantum bit (Qubit) applications. In this work we address the theoretical grounds of coherent manipulation in practical ferromagnetic systems. We study electromagnetic radiation driven interaction that is enhanced in the presence of spin polarized currents and map the conditions that allow coherent manipulation for which Rabi oscillations take place. We discuss the Gilbert losses in the context of effective coherence decay rates and show that it is possible to control these rates by application of a static spin current. The case of coherent manipulation using oscillatory spin currents that is free of radiation is discussed as well. Our work paves the way towards spin current amplification as well as radiationfree coherent control schemes that may potentially lead to novel Qubits that are robust and scalable. 
Friday, March 19, 2021 8:12AM  8:24AM Live 
X38.00002: Temperaturedependent spintransport and currentinduced torques in
superconductor/ferromagnet heterostructures Manuel Müller, Lukas Liensberger, Luis Flacke, Hans Huebl, Akashdeep Kamra, Wolfgang Belzig, Rudolf Gross, Mathias Weiler, Matthias Althammer Proximity effects at superconductor(SC)/ferromagnet(FM) interfaces provide novel functionality in the field of superconducting spintronics. We investigate the injection of quasiparticle spin currents in a NbN/Permalloy (Py) heterostructures with a Pt spin sink layer. To this end, we excite ferromagnetic resonance in the Pylayer via the microwave driving field of a coplanar waveguide (CPW). A phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inductive coupling strength between sample and CPW as a function of temperature [1]. Below the superconducting transition temperature Tc, we observe a blocking effect of pure spin current transport in the NbN layer. Moreover, below Tc we find a large fieldlike currentinduced torque. Our findings, reveal symmetry and strength of spintocharge current conversion in SC/FM heterostructures and provide guidance for future superconducting spintronics devices [2]. 
Friday, March 19, 2021 8:24AM  8:36AM Live 
X38.00003: Magnetic Droplet Solitons from Unconventional OutofPlane Spin Orbit Torques Robin Klause, Axel F Hoffmann Mirror symmetry breaking in noncolinear antiferromagnets due to magnetic order can generate unconventional outofplane (OOP) spin orbit torques (SOTs). Unlike conventional inplane (IP) SOTs, these torques can be used to manipulate magnetization in materials with perpendicular magnetic anisotropy (PMA). Towards this end, we investigated with micromagnetic simulations if these unconventional OOP SOTs can generate droplet solitons in rectangular geometries. The IP and OOP angles of the spin polarization play an important role for the droplet nucleation, while the applied magnetic field and current affect the precession frequency. Depending on the duration of the current pulse the nucleation of multiple droplets is possible, which interact with each other. Finally we discuss strategies for experimental implementations for spinorbit torque driven droplets. 
Friday, March 19, 2021 8:36AM  8:48AM Live 
X38.00004: Origin of Nonlinear Damping due to Mode Coupling in AutoOscillatory Modes Strongly Driven by SpinOrbit Torque Inhee Lee, Chi Zhang, Simranjeet Singh, Brendan McCullian, P Chris Hammel We investigate the physical origin of nonlinear damping due to modemode coupling between several autooscillatory modes driven by spinorbit torque in constricted Py/Pt heterostructures by examining the dependence of autooscillation on temperature and applied field angle. We observe a transition in the nonlinear damping of the autooscillation modes extracted from the total oscillation power as a function of drive current, which coincides with the onset of power redistribution amongst several modes and the reversal of the slope of the linewidth change vs. drive current from negative to positive for all modes. This indicates the activation of a new relaxation process by nonlinear magnonmagnon scattering between the modes. We also find that both nonlinear damping and threshold current are temperature independent in high drive current regime after the transition, suggesting that the mode coupling occurs dominantly through a nonthermal magnon scattering process via a dipole or exchange interaction rather than via thermally excited magnonmediated scattering. 
Friday, March 19, 2021 8:48AM  9:00AM Live 
X38.00005: Controlling itinerant spin dynamics for enhanced spin transfer torque Yuriy Semenov, Xinyi Xu, Ki Wook Kim The process of spin transfer torque is based on the conservation of total angular momentum of the carrier spins S and localized spins that constitute the magnetization M. Since the electron transit time through a thin magnetic layer is usually much shorter than the precession period of the magnetization M, electron spin dynamics S=S(t) can be evaluated with a quasistatic exchange field B∥M in the Bloch equations. While the injected spins make rapid rotations around M with frequency γB, they also acquire individually small phase differences due to the dispersion in the electron paths through the magnetic layer. In this work, we reexamine the STT process by explicitly considering the dephasing between the injected electron spins. Our theoretical analysis shows that the ensembleaveraged net torque experienced by the magnet is explicitly dependent on the dephasing time τ^{* }and transit time t_{d}, given in an oscillatory form. For the trivial case of a short dephasing time (t^{* }« t_{d}), this general expression reproduces the wellknown equation for spin torque ~M×(B×M). When τ^{* }≥ t_{d}, the result clearly deviates from the conventional understanding, enhancing or suppressing the torque with a choice of layer thickness. This effect may be particularly prominent in an antiferromagnet. 
Friday, March 19, 2021 9:00AM  9:12AM Live 
X38.00006: Anomalous spin Hall and inverse spin Hall effects in magnetic systems Xiang Rong Wang We predict anomalous spinHall effects (SHEs) and anomalous inverse SHEs in magnetic systems. A spin current with propagation and polarization collinear can be generated by a charge current along the order parameter. Spin currents polarized along the order parameter (the charge current) and propagating along the charge current (the order parameter) are generated by the charge current perpendicular to the order parameter. In anomalous inverse SHEs, the charge current, due to a spin current with propagation and polarization collinear, is proportional and is along the projection of order parameter in the plane perpendicular to the spin current. For an applied spin current with mutually perpendicular propagation and polarization directions, a charge current along the spin current propagation direction is generated if the order parameter is collinear with the polarization of the spin current. Also, a charge current along the polarization of spin current is generated if the order parameter is collinear with the propagation direction of the spin current. 
Friday, March 19, 2021 9:12AM  9:24AM Live 
X38.00007: Interfacial crystal Hall effect reversible by ferroelectric polarization DingFu Shao, Jun Ding, Gautam Gurung, Shuhui Zhang, Evgeny Y Tsymbal Recently, an anomalous Hall effect (AHE) has been predicted to occur in collinear bulk antiferromagnets with a noncentrosymmetric nonmagnetic sublattice, coined the crystal Hall effect (CHE). The CHE may be interesting for application in spintronics due to its reversal with switching the nonmagnetic sublattice, provided that suitable means for realizing this property are found. Here, we predict the appearance of the CHE in heterostructures composed of compensated antiferromagnetic metals and nonmagnetic insulators due to reduced symmetry at the interface. We further show that such an interfacial crystal Hall effect (ICHE) can be made reversible in engineered heterostructures where an antiferromagnetic layer is sandwiched between two identical ferroelectric layers. We explicitly demonstrate these phenomena using density functional theory calculations for three heterostructure systems based on realistic materials. Our predictions open a new perspective for spintronics where the AHE can be reversed by an electric field. 
Friday, March 19, 2021 9:24AM  9:36AM Live 
X38.00008: Gatetunable spin torque and edge current in magnetized graphene nanoribbons caused by spinorbit coupling Matheus Martins de Sousa, Wei Chen Motivated by recent experiments that demonstrate gatetunable spinorbit coupling (SOC) in graphene heterostructures, we investigate the effect of SOC on the magnetic response of graphene. In particular, we demonstrate that in the magnetized zigzag nanoribbons, the geometric confinement combined with the Rashba SOC cause peculiar magnetic 
Friday, March 19, 2021 9:36AM  9:48AM Live 
X38.00009: Magnetic skyrmions probed by SPSTM: topology imprinted on the charge current and spin transfer torque Krisztian Palotas, Levente Rozsa, László Szunyogh The controlled creation/annihilation of individual magnetic skyrmions have been demonstrated by using spinpolarized scanning tunneling microscopy (SPSTM) [Science 341, 636], where the spinpolarized current exerts a torque on the spin moments of the sample. However, the detailed microscopic mechanism of this process is presently unknown. Our work contributes to this understanding by a theoretical investigation of the tunneling electron charge and spin transport probing magnetic skyrmions. The spinpolarized charge current (I) and tunneling spin transport vector quantities, the longitudinal spin current and the spin transfer torque (STT), are consistently calculated within a simple electron transport theory [PRB 94, 064434]. The electron tunneling model is extended to SPSTM in high spatial resolution, and applied to magnetic skyrmions [PRB 97, 174402; PRB 98, 094409]. Besides the vector spin transport characteristics, the relationships among conventional charge current SPSTM images [PRB 96, 024410], the magnitudes of the spin transport quantities [PRB 97, 174402], and the topology of various skyrmionic objects are analyzed [J. Magn. Magn. Mater. 519, 167440]. It is also shown that at specific SPSTM tip positions the STT efficiency (STT/I) can reach very large values ~h/e. 
Friday, March 19, 2021 9:48AM  10:00AM Live 
X38.00010: Magnetization control by angular momentum transfer from surface acoustic wave to ferromagnetic spin moments Ryo Sasaki, Yoichi Nii, Yoshinori Onose An angular momentum interconversion between electron spin and other types of angular momentum is useful for developing new spintronic functionalities. The conversions from photon angular momentum and mechanical rotation to ferromagnetic spin moment have been well studied. Although recent studies theoretically suggested that circular vibration of atoms may work as phonon angular momentum, a direct experimental demonstration of conversion to spin moments remains to be performed. 
Friday, March 19, 2021 10:00AM  10:12AM Live 
X38.00011: Energy and momentum conservation in spin transfer Alexander Mitrofanov, Sergei Urazhdin Spin transfer (ST) effect is a consequence of angular momentum conservation, one of the three fundamental conservation laws in nature. We use fully quantum simulations of scattering of spinpolarized electron wavepacket by a ferromagnet modelled as a 1d spin1/2 chain, to show that energy and momentum conservation laws impose strong restrictions on the characteristics of magnetic excitations generated by spin transfer. We found that two groups of magnons are generated by ST, forwardpropagating magnons with large energy, and backwardpropagating magnons with small energy. Analysis shows that the backwardpropagating magnons are generated by the reflected electron component, while the forwardpropagating magnons  by the transmitted electron component, with the values of energy and momenta of the generated magnons determined by the changes of the corresponding characteristics of the electron wave. We also show a converse effect – dependence of the scattered electron’s characteristics on the dynamical characteristics of the magnetic system. Our results demonstrate that quantum magnetism plays an important role in magnetoelectronic phenomena. 
Friday, March 19, 2021 10:12AM  10:24AM Live 
X38.00012: Ballistictodiffusive transition in spin chains with broken integrability João Ferreira, Michele Filippone We study the ballistictodiffusive transition induced by the weak breaking of integrability in a boundarydriven XXZ spinchain. Studying the evolution of the spin current density J^{s} as a function of the system size L, we show that, accounting for boundary effects, the transition has a nontrivial universal behavior close to the XX limit. It is controlled by the scattering length L^{*}~ V^{2}, where V is the strength of the integrability breaking term. 
Friday, March 19, 2021 10:24AM  10:36AM Live 
X38.00013: Spin current driven Rabi oscillations in a ferromagnetic film Nirel Bernstein, Marina Brik, Benjamin Assouline, SeeHun Yang, Amir Capua Rabi oscillations describe the process whereby electromagnetic radiation interacts coherently with spin states in a nonequilibrium interaction so called the nonadiabatic regime [1]. 
Friday, March 19, 2021 10:36AM  10:48AM Live 
X38.00014: Spin transfer torque driven by interfacial roughness and spinorbital scattering Pengtao Shen, M. Mehraeen, Shulei Zhang Spin transfer torques allow efficient electric control of magnetization dynamics in nanoscale heterostructures. In this work, we theoretically investigate a spintransfer torque effect in a ferromagnetic metal layer emanating from surface roughness and Rashba spinorbit scattering in the presence of an inplane charge current. A full quantum mechanical approach is used to evaluate the chargecurrentspincurrent response function, which allows us to analyze the effect of quantum interference between scattering states near the Rashba interface. In the ballistic regime, we find that while a fieldlike torque can be induced by applying an inplane current regardless of the surface roughness, a dampinglike torque only emerges from a rough Rashba interface whereby the interference between scattering states is partially suppressed. 
Friday, March 19, 2021 10:48AM  11:00AM Live 
X38.00015: Planar Hall Driven Torque in a Ferromagnet/Nonmagnet/Ferromagnet System Christopher Safranski, JunWen XU, Andrew D Kent, Jonathan Z Sun Electrical manipulation of magnetization is an intensely studied topic with goals of producing energy efficient nanodevices. The charge to spin current conversion in bilayers of magnetic and nonmagnetic materials is one area of investigation. Typically studied materials have been mostly limited to the generation of inplane polarized spin currents. We investigate spin currents produced by the planar Hall effect in Co/Ni multilayers, which carry a polarization dictated by the FM magnetization direction. In a sample based on CoNi/Au/CoFeB, spin torque ferromagnetic resonance is used to measure the dampinglike torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torques produced by the planar Hall effect. We find the strength of this effect to be comparable to that of the spin Hall effect. However, unlike the spin Hall effect, it can produce a partially out of plane spin polarization. Our results indicate that the planar Hall effect holds potential as a spin current source with a controllable polarization direction. 
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