Interfacial antidamping spin-orbit torques in topological insulator/ferromagnet bilayers induced by skew scattering

Spin-orbit torque (SOT) [1] in topological-insulator/ferromagnet (TI/FM) bilayers offers a promising route toward highly efficient magnetic random access memory, as demonstrated by recent experiments [2] where FM magnetization has been switched along the direction perpendicular to the interface using current densities that are two orders of magnitude smaller than those originally used in heavy-metal/FM bilayers and traditional spin-transfer torque in magnetic tunnel junctions. However, the microscopic mechanisms behind this efficiency -- the interplay between spin-momentum locked surface states, two-dimensional electron gas typically present due to band bending and defects in the bulk, and impurities on the surface -- are still poorly understood. Here we show that a nonperturbative treatment of proximity magnetic effects, spin-orbit coupling and disorder in a minimal model of a TI/FM bilayer leads to new types of both antidamping-like and field-like SOTs of pure interfacial origin, which are overlooked by previous microscopic theories. Most notably, a robust skew scattering mechanism is found to enable a current-induced nonequilibrium spin density in all three spatial directions [3,4], instead of the in-plane longitudinal polarization usually found [5] (in clean systems) in response to an injected transverse charge current. We present analytical expressions for the spin-density–charge-current response function in the weak disorder limit and perform a detailed numerical analysis to obtain the ensuing magnetization dynamics in the FM layer [6]. We find that the standard antidamping-like and field-like SOTs are strongly renormalized by the interfacial skew scattering mechanism, thus demonstrating that the usually assumed to be necessary [1] effects stemming from three-dimensional transport are not essential.