Nonperturbative approach to interfacial spin-orbit torques induced by the Rashba effect

Current-induced spin-orbit torque (SOT) in normal metal/ferromagnet (NM/FM) bilayers bears great promise for the development of low-power spin-based devices, but the microscopic origin of purely interfacial SOTs in ultrathin systems is not yet fully understood. Here, we show that a linear response theory with a nonperturbative treatment of spin-dependent interactions and impurity scattering potential predicts dampinglike (DL) SOTs that are strictly absent in perturbative approaches. The technique is applied to a two-dimensional Rashba-coupled FM (the paradigmatic model of a NM/FM interface), where higher-order scattering processes encoding skew scattering from nonmagnetic impurities allow for current-induced spin polarization with nonzero components along all spatial directions. This is in stark contrast to previous results of perturbative methods (neglecting skew scattering), which predict a coplanar spin-polarization locked perpendicular to the charge current as a result of the conventional Rashba-Edelstein effect. Furthermore, the angular dependence of ensuing SOTs and their de-pendence upon the scattering potential strength is analyzed numerically. Simple analytic expressions for the spin-density-charge-current response function and related SOT efficiencies are obtained in the weak scattering limit. We find that the extrinsic DL torques driven by impurity scattering reaches efficiencies of up to 7% of the fieldlike (Rashba-Edelstein) torque. Our microscopic theory shows that bulk phenomena, such as the spin Hall effect, are not a necessity in the generation of the DL SOTs of the type observed in experiments on ultrathin systems.

Automated summary

The microscopic origin of purely interfacial spin-orbit torques in ultrathin systems is not yet fully understood. In this study, a linear response theory with a nonperturbative treatment is used to predict dampinglike spin-orbit torques that are strictly absent in perturbative approaches. The results show that current-induced spin polarization with nonzero components along all spatial directions can occur in ultrathin systems, contrary to previous perturbative predictions. The angular dependence and dependence upon the scattering potential strength of the resulting spin-orbit torques are analyzed numerically.