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 μ0H up to 10 T and temperature T down to 2 K. The fieldlike torque hFL changes sign with varying Co thickness tCo, 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 hFL. The sign of hFL coincides with the sign of UMR induced by spin-dependent scattering. However, the dampinglike torque hDL is proportional to the inverse Co thickness, with no sign reversal for all measured temperatures and tCo, indicating that hDL originates solely from the SHE. The generation of hDL 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.