Transverse and parallel spin-torque ferromagnetic resonance for improved measurements of spin-orbit torques

We present a method to quantify the contribution of the spin-pumping/inverse-spin-Hall-effect (SP-ISHE) voltage to spin-torque ferromagnetic resonance (ST-FMR) measurements of spin-orbit torques. In this approach the resonant homodyne voltages transverse to and parallel to the applied microwave current are measured simultaneously. We leverage the fact that spin-pumping only contributes as a symmetric Lorentzian to derive consistency conditions which allow the SP-ISHE to be separated from the rectification signals associated with the spin-orbit torque. The method does not require knowledge of parameters such as the effective spin mixing conductance and spin diffusion lengths of heavy metals. Our findings show that for some material combinations SP-ISHE can alter the ST-FMR signal significantly, and the relative sign of rectification and SP-ISHE is unambiguously determined.