Spin-orbit torque is a current-induced transfer of angular momentum from an atomic lattice to magnetic order. It is a promising mechanism to write magnetic memories and drive spin torque oscillators. Since its inception, the list of spin-orbit torque mechanisms has grown beyond the conventional spin Hall and Rashba-Edelstein mechanisms to include “unconventional” mechanisms, arising from spin and orbital current generation in ferromagnetic layers, nonmagnetic layers, and their interfaces. In this talk, we use micromagnetic, semiclassical, and first principles calculations to show that unconventional spin-orbit torques are potentially important for devices, from causing nonlocal spin torques in ferromagnetic trilayers to enabling large amplitude, easy-plane spin-orbit torque oscillators.
We will discuss first-principles calculations of spin transport and spin-orbit torques in disordered films and multilayers within the nonequilibrium Green's function technique. For a nonmagnetic Pt film, the behavior of the spin accumulation and the transverse spin current deviate significantly from the conventional spin-diffusion model. The effective transverse spin-diffusion length is much shorter than the longitudinal spin-diffusion length. For ferromagnetic trilayers, we find dampinglike and fieldlike torques with unconventional spin polarizations. The large rotated fieldlike component in Co/Cu/Co trilayers is inconsistent with diffusive transport in the spacer layer but can be explained by nonlocal interactions between the ferromagnetic layers when the mean-free path is not small compared to the spacer thickness.
THz emission spectroscopy reveals to be a very powerful experimental method to investigate the properties of Rashba or topological insulator surface states. The THz emission can be also used in heavy metallic or in more general Rashba systems. We prove here the ability of the present method. In 3d/5d transient metal bilayers and beyond heavy metal structures, Rashba states and Topological insulators are expected candidates for spintronic-terahertz domains due to their high spin to charge conversion properties. In this scheme, we are interested in the samples based on 2D electron gas, topological insulators and Heusler alloys with strong spin-orbit coupling.
Terahertz (THz) emission spectroscopy in spin systems has become a very powerful method to generate THz radiation and to investigate the properties of Rashba or Topological Insulator surface states. The THz emission can be generated in heavy metallic or in more general Rashba systems. In 3d/5d transient metal bilayers THz emission in via the Inverse Spin Hall effect. Beyond heavy metal structures, Rashba states are strong candidates for THz-spintronics owing to their high spin to charge conversion properties. Here we present 2D electron gas with strong Rashba spin-orbit coupling and demonstrate THz emission via the Inverse Edelstein Effect.
Spin-orbit torque in metallic heterostructures arises due to multiple microscopic mechanisms, which presents a challenge for theoretical understanding and interpretation of the experimental data. First-principles calculations provide valuable insight through controlled studies of the dependence of spin-orbit torques on the relevant parameters in realistic disordered heterostructures. Recent results from such calculations and progress in understanding the mechanisms of spin-orbit torque will be discussed. It was found that the damping-like torque in ferromagnet/heavy-metal bilayers tends to have a large interfacial contribution that is comparable to the conventional spin-Hall contribution. Calculations with varying degrees of interfacial intermixing show that it does not strongly affect the damping-like torque but can strongly enhance the field-like torque. Recent results for ferromagnet/normal-metal/ferromagnet trilayers and antiferromagnet/normal-metal bilayers will also be discussed.
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. However, the relation between the parameter $\delta$ and the scattering properties of the interface has been missing. We establish this relation using the properly generalized magnetoelectronic circuit theory, for both normal and ferromagnetic interfaces. It is found that the parameter $\delta$ extracted from the measurements on multilayers scales with the square root of the probability of spin-flip scattering. The spin-flip scattering probabilities are calculated for several specific interfaces using the Landauer-Büttiker method based on the first-principles electronic structure, and the results are compared with experimental data.
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