Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C24: Spin-Orbit Coupling at Interfaces: Blessing or Curse for Future Spintronic Devices?Invited
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Sponsoring Units: GMAG Chair: Mark Stiles, NIST Room: New Orleans Theater C |
Monday, March 13, 2017 2:30PM - 3:06PM |
C24.00001: Origin of spin-orbit torques in thin film heterostructures Invited Speaker: Dan Ralph Spin-orbit torques arising from current flow in heavy-metal thin films have the potential to enable very efficient manipulation of magnetic devices. I will describe recent studies aiming to better understand the mechanisms behind these torques and to enhance their effectiveness through improved control over materials and interfaces. Studies as a function of inserting a spacer layer between the heavy metal and ferromagnet indicate that both the anti-damping and effective-field components of spin-orbit torque originate from the spin Hall effect within the heavy metal, rather than a Rashba-Edelstein effect at an interface of the ferromagnet, but spin scattering at both interfaces of the magnet can still affect the strength of the torque components. We have also studied how the introduction of impurities into heavy metals influences the spin-orbit torque, finding significant improvements in both the strength of the torque per unit current and the energy cost for switching. This is consistent with expectations if the spin Hall effect is dominated by an intrinsic band structure mechanism. In addition to enhancing the strength of spin-orbit torques, we are learning to manipulate their direction, by generating spin currents using materials that break inversion symmetry within the sample plane. I will describe measurements in which a thin layer of WTe$_{\mathrm{2}}$, a low-symmetry transition metal dichalcogenide, produces an out-of-plane anti-damping spin-orbit torque, an orientation that is forbidden by symmetry in more conventional devices. This work is done in close collaboration with the group of Bob Buhrman at Cornell University. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:42PM |
C24.00002: Spin transport at interfaces with spin-orbit coupling Invited Speaker: Vivek Amin Spintronic devices typically contain multiple layers and utilize spin-orbit coupling through a variety of effects, such as anisotropy, damping, or novel transport processes like the spin Hall effect.~These devices are most simply analyzed when spin-orbit coupling plays a dominant role in the bulk layers rather than at the interfaces.~However, recent observations of novel phenomena suggest the importance of strong spin-orbit coupling at interfaces.~These phenomena can be desirable (e.g. perpendicular magnetic anisotropy, spin-to-charge conversion, topologically-protected magnetization textures) or parasitic (e.g. spin memory loss).~While the precise role of interfacial spin-orbit coupling on transport remains unclear, substantial evidence indicates that it can no longer be ignored.~We discuss the theory of spin transport at interfaces with spin-orbit coupling, focusing on phenomenological models and highlighting new effects.~In particular, we show that interfaces with spin-orbit coupling can generate spin currents with spin polarizations in unconventional directions (i.e not orthogonal to both the charge current and spin flow).~These spin currents are a direct consequence of interfacial spin-orbit coupling and could assist in switching magnetic layers with perpendicular magnetic anisotropy.~We present the boundary conditions needed for drift-diffusion models to capture interfacial spin-orbit effects, and discuss the interpretation of experiments in which interfacial spin-orbit coupling might play a significant role. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 4:18PM |
C24.00003: Theory of spin loss at metallic interfaces Invited Speaker: Kirill Belashchenko Interfacial spin-flip scattering plays an important role in magnetoelectronic devices. Spin loss at metallic interfaces has usually been quantified by matching the magnetoresistance data for multilayers to the Valet-Fert model, while treating each interface as a fictitious bulk layer whose thickness is $\delta$ times the spin-diffusion length. In this work, the relation of the parameter $\delta$ to the spin-flip transmission and reflection probabilities at an individual interface is established using the properly generalized magnetoelectronic circuit theory. It is found that the parameter $\delta$ extracted from the measurements on multilayers is proportional to the square root of the probability of spin-flip scattering. The spin-flip scattering probabilities are calculated for several specific interfaces using the Landauer-B\"uttiker method based on the first-principles electronic structure, and the results are compared with experimental data. The implications of these findings for spintronic devices will be discussed. [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:54PM |
C24.00004: Spin-to-charge conversion at interfaces: spin pumping, Rashba coupling, and topological insulators. Invited Speaker: J-Carlos ROJAS-SANCHEZ My talk focuses on the conversion between spin and charge currents by exploitation of the spin-orbit coupling (SOC) in the 2DEG states at Topological Insulator or Rashba Interfaces and the resulting perspective for low power spintronic devices. I will show results of spin to charge conversion in spin pumping experiments on \textbf{Bi/Ag Rashba interfaces }[1] and thin films of the \textbf{newly discovered topological insulator }$\alpha $\textbf{-Sn}, and their analysis in term of \textbf{inverse Edelstein Length}. I will show experimental evidence that direct contact of metallic ferromagnetic layer is detrimental for the surfaces states of topological insulators [2]. I will also discuss additional examples of conversion between spin-to-charge at \textbf{GeTe} [3], \textbf{LAO/STO }[4]\textbf{ and Fe/Ge(111) }[5]\textbf{ Rashba }interfaces. I will use the conversion parameters obtained at room temperature with $\alpha $-Sn to demonstrate the very large \textbf{advantage of the SOC effects in 2D interface states }with respect to the Spin Hall Effect (SHE) of 3D metals. [1]J.-C. Rojas-S\'{a}nchez et al. Nat. Comm 4, 2943 (2013). [2] J.-C. Rojas-S\'{a}nchez et al. Phys. Rev. Lett. 116, 096602 (2016). ArXiv 1509.02973 (2015) [3] C. Rinaldi, J.-C. Rojas-S\'{a}nchez et al. Appl. Phys. Lett. Mat. 4, 032501 (2016) [4] E. Lesne, J.-C. Rojas-S\'{a}nchez et al. Nat. Mat. Doi~: 10.1038/nmat4726 (2016) [5] S. Oyarzun, J.-C. Rojas-S\'{a}nchez et al. Nat. Comm. Accepted (2016). [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:30PM |
C24.00005: Spin-Orbit Torques and Magnetoresistance in 5d and 4d Metal Systems Invited Speaker: Pietro Gambardella Heterostructures composed of ferromagnetic (FM) and heavy metal (HM) layers have been studied for decades due to their application as perpendicular magnetic recording media. Only recently, however, experiments and theory have evidenced a wide range of magnetotransport phenomena that have little or no counterpart in single FM layers. The absorption and reflection of spin currents induced by spin-orbit coupling in FM/HM bilayers are responsible for the generation of spin-orbit torques (SOT) as well as for unusual magnetoresistive phenomena. The origin of such spin currents is still widely debated. In this talk we will compare the SOT in 5d and 4d metal systems, namely in Co/Pt and Co/Pd bilayers, showing how the reduced bulk spin Hall effect of Pd allows for the detection of interface-related field-like and damping-like SOT, highlighting diverse effects contributing to the total spin current. Further, we will present a comparative study of the spin-orbit torques and magnetoresistance in the linear and nonlinear (current-dependent) regimes. The magnetoresistance of Co/HM bilayers (HM $=$ Ta, W, Pt) is phenomenologically similar to the spin Hall magnetoresistance (SMR) of YIG/Pt, but has a much larger anisotropy, of the order of 0.5 {\%}, which increases with the atomic number of the HM. Additionally, we find a novel magnetoresistance term that is directly proportional to the current and to the transverse component of the magnetization. This so-called unidirectional magnetoresistance changes sign upon inversion of either current or magnetization and correlates with the amplitude of the damping-like SOT. [Preview Abstract] |
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