Unidirectional spin Hall magnetoresistance and spin-orbit torques in HM₁/Co/HM₂ trilayer systems

We present a detailed analysis of harmonic longitudinal and Hall voltage measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs) and unidirectional spin Hall magnetoresistance (USMR) is achieved by introducing the second heavy metal (HM) with the opposite sign of the spin Hall angle. The extracted SOT efficiencies are larger for the trilayers as compared to the bilayers, confirming the enhanced values reported for the trilayers with perpendicularly magnetized Co. The maximum effective spin Hall angle found for the Pt/Co/Ta trilayer reaches $\theta_{SH}$ = 20%. The USMR of the trilayer yields up to 27% higher effect as for the respective bilayers with the largest effective USMR amplitude of -0.32 $\times$ 10$^{-5}$ for the Pt/Co/Ta trilayer at a charge current density of 10$^{7}$ A/cm$^2$.