Connections between spin-orbit torques and unidirectional magnetoresistance in ferromagnetic-metal–heavy-metal heterostructures

We investigate the connections between current-induced spin-orbit torques and unidirectional magnetoresistance (UMR) by performing second harmonic longitudinal resistance measurements on Co/Pt bilayer with magnetic-field ${\ensuremath{\mu}}_{0}H$ up to 10 T and temperature $T$ down to 2 K. The fieldlike torque ${h}_{\mathrm{FL}}$ changes sign with varying Co thickness ${t}_{\mathrm{Co}}$, which indicates that competing mechanisms, i.e., the spin Hall effect (SHE) and the inverse spin galvanic effect (iSGE), are responsible for the generation of ${h}_{\mathrm{FL}}$. The sign of ${h}_{\mathrm{FL}}$ coincides with the sign of UMR induced by spin-dependent scattering. However, the dampinglike torque ${h}_{\mathrm{DL}}$ is proportional to the inverse Co thickness, with no sign reversal for all measured temperatures and ${t}_{\mathrm{Co}}$, indicating that ${h}_{\mathrm{DL}}$ originates solely from the SHE. The generation of ${h}_{\mathrm{DL}}$ via iSGE can be further excluded because of the observation of an $H$-linear dependence of UMR, which, in turn, indicates the negligible exchange coupling between spin accumulation and ferromagnetic metal.