Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V22: Spin-Orbit Coupling and Antisymmetric Exchange at Metal InterfacesFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Graham Rowlands, BBN Technology - Massachusetts Room: LACC 402A |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V22.00001: Strong Enhancement Of The Spin Hall Effect By Spin Fluctuations Near The Curie Point Of FexPt1-x Alloys Yongxi Ou, Daniel Ralph, Robert Buhrman Robust spin Hall effects (SHE) in non-magnetic heavy metal systems are either attributed to an intrinsic band-structure effect or to extrinsic spin-dependent scattering from impurities, namely side-jump or skew scattering. Here we will report on extraordinarily strong skew scattering, attributable to spin fluctuations, in ferromagnetic FexPt1-x alloys near their Curie point, tunable with x. This results in a damping-like spin-orbit torque being exerted on an adjacent ferromagnetic layer that is strongly temperature dependent in this transition region, with a peak value that indicates a lower bound 0.34 ± 0.02 for the peak spin Hall ratio within the FePt. We also observe a pronounced peak in the effective spin-mixing conductance of the FM/FePt interface, and determine the spin diffusion length in these FexPt1-x alloys. These results establish new opportunities for fundamental studies of spin dynamics and transport in ferromagnetic systems with strong spin fluctuations, and a new pathway for efficiently generating strong spin currents for applications. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V22.00002: Effects of non-local emergent electromagnetic fields driven by Rashba spin-orbit coupling Junji Fujimoto, Gen Tatara We theoretically study effects of emergent electromagnetic fields induced by the Rashba spin-orbit coupling (RSOC) at a distance from the exchange coupling (ExC), considering, for example, the tri-layer metallic structure of ferromagnetic (FM), normal (NM), and heavy (HM) metals, where ExC is only in FM, and RSOC is near the interface between NM and HM. In such a structure, the magnetization dynamics in FM may give rise to the spin-motive force at the interface between NM and HM due to RSOC. We show that the motive force can be understood as that driven by a non-local emergent electric field, in which RSOC connected to ExC through the diffusion of electrons. We also find that a magnetic field as the counterpart of the electric field does not propagate in equilibrium but propagates when the electric current parallel to the layers flows. The Rashba-Edelstein magnetoresistance can be understood as a normal magnetoresistance due to the non-local emergent magnetic field. The calculations are done using the Green function and the Feynman diagram without setting specific geometries like the above tri-layer structure. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V22.00003: Spin transport through ferromagnetic|nonmagnetic interfaces at finite temperatures: An ab-initio study of spin-memory loss Kriti Gupta, Rien J.H. Wesselink, Zhe Yuan, Paul Kelly The discontinuity of spin currents at an interface is a direct consequence of enhanced spin-orbit splitting. The spin-memory loss (SML) which characterizes this discontinuity is a key to quantitatively understand the effect of interface spin orbit coupling (ISOC) on spin transport. While ISOC can make a significant contribution in a variety of experiments dealing with multilayer ferromagnetic(F)|nonmagnetic(N) geometries, it is usually neglected in their interpretation to avoid introducing too many free parameters. To study devices operating at ambient temperature it is desirable to understand how thermal disorder affects interface behavior. Unfortunately experimental studies of SML have been limited to low temperatures. In this work, we determine the SML, interface resistance and spin-asymmetry parameters as a function of temperature for a Py(Ni80Fe20)|Pt interface. Using first-principles scattering theory, we calculate the conductance as well as local charge and spin currents, modeling temperature-induced disorder within the adiabatic approximation with frozen thermal lattice and (for F materials) spin disorder. Our results for the spin current are then interpreted in terms of a generalized Valet-Fert model in order to extract the above parameters. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V22.00004: Spin Hall Effect Induced in Ferromagnetic Metal-Heavy Metal Interface : a First Principles study* Duc Cuong Do, Soon Cheol Hong, Sung-Hyon Rhim Spin Hall effect, a generation of transverse spin current from the longitudinal current, has received a lot of attention in both theory and experiment, which have many potentials in spintronic devices. Here, spin Hall effect is expected in FM/HM interface. In this study, we investigate various types of FM/HM interfaces (FM=Fe and Co) and (HM=Pt, Pd, Ta, W) using density functional theory with Wannier function approach to explicitly calculate the transverse spin Hall conductivity in the framework of Kubo formula. The spin Hall conductivity found at some interfaces may invite serious exploration. To understand the origin of spin Hall effect at interfaces, the analysis of k-resolved Berry curvature based on band structure are exhaustively presented. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V22.00005: Optimizing the Spin Hall Angle in Ultrathin Metallic Films Christian Herschbach, Dmitry Fedorov, Martin Gradhand, Ingrid Mertig The spin Hall effect (SHE) is one of the key effects in modern spintronics creating pure spin currents directly in nonmagnetic materials. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V22.00006: Spin Injection and Detection via the Anomalous Spin Hall Effect of a Ferromagnetic Metal Kumar Sourav Das, Wim Y. Schoemaker, Bart Van Wees, Ivan Vera Marun We report a novel spin injection and detection mechanism via the anomalous Hall effect in a ferromagnetic metal. The anomalous spin Hall effect refers to the transverse spin accumulation generated within the ferromagnet due to the anomalous Hall effect. We utilize the anomalous spin Hall effect and its reciprocal effect to electrically inject and detect magnons in a magnetic insulator (YIG) in a non-local geometry, using a ferromagnetic metal (permalloy). Our experiments reveal that permalloy can have a higher spin injection and detection efficiency to that of platinum, owing to the anomalous spin Hall effect. The discovery of this new effect in ferromagnetic metals opens up the possibility to utilize a variety of ferromagnets as more efficient injectors and detectors of spin currents as compared to expensive heavy metals with high spin-orbit coupling, like platinum. Crucially, we demonstrate that the spin injection and detection via the anomalous spin Hall effect are tunable through the magnetization of the ferromagnet, thus creating new possibilities for spintronic applications. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V22.00007: Abstract Withdrawn
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Thursday, March 8, 2018 3:54PM - 4:06PM |
V22.00008: Spin-memory loss due to spin-orbit coupling at ferromagnet/heavy-metal interfaces: Ab initio spin-density matrix approach Kapildeb Dolui, Branislav Nikolic Spin-memory loss (SML) of electrons traversing ferromagnetic-metal/heavy-metal (FM/HM), FM/normal-metal (FM/NM) and HM/NM interfaces is a fundamental phenomenon that must be invoked to explain consistently large number of spintronic experiments. However, its strength extracted by fitting experimental data to phenomenological semiclassical theory, which replaces each interface by a fictitious bulk layer, is poorly understood from a microscopic quantum framework and/or materials properties. Here we describe ensemble of flowing spin quantum states using spin-density matrix, so that SML is measured like any decoherence process by the decay of its off-diagonal elements or, equivalently, by the reduction of the magnitude of polarization vector. By combining this framework with density functional theory (DFT) calculations, we examine how all three components of the polarization vector change at Co/Ta, Co/Pt, Co/Cu, Pt/Cu and Pt/Au interfaces embedded within Cu/FM/HM/Cu vertical heterostructures. In addition, we use ab initio Green's functions to compute spectral function and spin texture over FM, HM and NM monolayers around these interfaces which quantify interfacial spin-orbit coupling, thereby explaining the microscopic origin of SML [Reference: arXiv:1708.07105]. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V22.00009: Giant, Temperature-Dependent Spin Hall Torque from Rare-Earth Mixed-Valence YbAl3 Neal Reynolds, Shouvik Chatterjee, Ariel Seidner, Darrell Schlom, Kyle Shen, Daniel Ralph The spin Hall effect occurs in non-magnetic systems and gives rise to a spin current that flows transverse to an applied electric field. Theoretical predictions and recent experimental work reveal that the strength of the spin Hall effect can be enhanced by the presence of 4f-derived states at the Fermi level. YbAl3 is a rare-earth mixed-valence metal in which the Yb 4f-derived states become increasingly itinerant and shift towards the Fermi level as temperature is lowered below room temperature. Using temperature-dependent spin torque ferromagnetic resonance measurements of the spin Hall effect in YbAl3/Fe bilayers, we find an effective spin Hall conductivity that increases in concert with the increase in the 4f density of states near the Fermi level. At the peak, the effective spin Hall conductivity is enormous, at least an order of magnitude larger than in any other material system reported previously. We discuss the implications of this result for optimizing the spin Hall effect in light of the evolution of the electronic structure of YbAl3 with temperature. |
Thursday, March 8, 2018 4:18PM - 4:54PM |
V22.00010: Interfacial Dzyaloshinskii-Moriya Interaction in Pt/CoFeB Films: Effect of the Heavy-Metal Thickness Invited Speaker: Silvia Tacchi Recently, the Dzyaloshinskii-Moriya interaction (DMI) [1], i.e., the antisymmetric exchange interaction, has attracted a great deal of attention due to its crucial role in the stabilization of chiral spin textures such as spin spirals and magnetic skyrmion lattices. A considerable interfacial DMI arises from the inversion symmetry breaking at interface between a ferromagnetic layer and a nonmagnetic one having a large spin-orbit coupling. Due to the enormous potential of chiral magnetic domain walls and skyrmions for the development of a new generation of spintronic devices, it’s now crucial to obtain a detailed knowledge of the interfacial DMI in ultrathin films. In this context Brillouin light scattering (BLS) revealed to be a powerful tool to investigate interfacial DMI. In a BLS experiment, the presence of an interfacial DMI manifests itself in an asymmetric dispersion relations of the counterpropagating Damon-Eshbach spin wave modes. In this work, BLS has been exploited to study the interfacial DMI in ultrathin CoFeB films in contact with a Pt layer with variable thickness.[2] We found that the strength of the interfacial DMI increases with Pt thickness (dPt), up to reach a saturation value for dPt larger than a few nanometers. The experimental results are quantitatively explained by a theory based on an extension of the 3-site indirect exchange mechanism [3], where the asymmetric exchange interaction between two neighboring ferromagnetic atoms is mediated by a third nonmagnetic atom having a large spin-orbit coupling. We found that the increase of the interface DMI as a function of the Pt thickness is due to the cumulative electron hopping between the atomic spins at the interface and the non-magnetic atoms in the heavy metal. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V22.00011: Link between Intrinsic Spin Hall Effect and Geometrical Curvature of the Fermi Surface Elena Derunova, Yan Sun, Stuart S Parkin, Mazhar Ali Spin and Anamalous Hall effects (SHE and AHE) are when orthogonal spin and charge currents are generated from an applied E-field in the absence of external B-fields. Both are of great interest from both a fundamental physics perspective as well as for technological applications. Currently, few good SHE materials are known and a method for searching for new SHE materials is lacking. Here we present a new way of finding SHE materials based on using the geometrical interpretation of the Berry curvature, in analogy to previous work in the field of the AHE. First, we compute the electronic structure of a material with and without spin orbit coupling (SOC) and then combine that with analysis of the non-SOC Fermi surface. We show that areas of high fermi surface curvature w/o SOC and corresponding spin Berry curvature w/SOC came from areas in the electronic structure where anti-crossing points exist w/o SOC and are gapped by the inclusion of SOC. We explain, mathematically, why this must be the case and propose a route to a quantitative SHC calculation based on the Fermi surface curvature. Also, we show that even this qualitative understanding can be used to rapidly screen through prospective SHE materials and quickly find strong candidates for calculation using the Kubo formalism. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V22.00012: Reorientable Spin Direction for Spin Current Produced by the Anomalous Hall Effect Jonathan Gibbons, David MacNeill, Robert Buhrman, Daniel Ralph Ferromagnets with strong spin-orbit interactions are expected to generate spin currents via the anomalous Hall effect. These spin currents are predicted to differ from those generated by the spin Hall effect from a nonmagnetic material in that they should have a spin polarization always parallel to the magnetization of the source layer. Hence, by manipulation this magnetic state, one may control the resulting spin-orbit torques and generate torques that are forbidden by symmetry for the conventional spin Hall effect. We report quantitative measurements of spin torques in pinned ferromagnet/spacer/free ferromagnet samples, as a function of changing the relative angle between the magnetizations of the two magnetic layers. We show experimentally that the spin direction of the spin current generated by a magnetic layer can be reoriented by turning its magnetization direction, and that this allows the spin-orbit torque arising from the anomalous Hall effect to be non-zero in a geometry for which the spin Hall torque generated by non-magnetic materials is identically zero. We will report initial results on the relative strength of this anomalous Hall torque from different ferromagnetic materials. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V22.00013: A highly efficient spin Hall effect generator of spin currents Lijun Zhu, Daniel Ralph, Robert Buhrman Current-induced spin-orbit torques (SOTs) in heavy metal/ferromagnet (HM/FM) multilayers, where the spin Hall effect (SHE) in the HM layer is the dominant spin current source, can play an efficient role in manipulating magnetization at the nanoscale. Despite extensive efforts and recent demonstrations of fast SOT switching of in-plane magnetized MTJs, the energy efficiency of SOT devices based on the conventional spin Hall materials (e.g. W, Pt, and Ta) has been limited by a relatively small spin Hall angle, or a high resistivity of the HM, and/or damping enhancement due to strong spin memory loss at the HM/FM interface. On the other hand, for perpendicularly magnetized systems the DMI at the HM/FM interface requires an external magnetic field, or its equivalent, that is larger than the DMI field applied along the bias current direction in order to switch the magnetization, which leads to device complexity. Here we will report a highly efficient SHE spin current generator which simultaneously shows a giant ant-damping spin-torque efficiency of ~0.35, a relatively low resistivity of ~ 83 μΩ cm, reduced spin memory loss at the FM (e.g. FeCoB) interface, and as well a DMI that is tunable, to zero if desired. |
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