Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques

Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torque (SOT) in heavy-metal/ferromagnet bilayer structures. In the course of a measurement, other resonant pro-cesses such as spin pumping (SP) and heating can cause spin-current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects. In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOT. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in Pt(6 nm)/Co40Fe40B20 samples the contribution from the artifacts is small compared with the SOT rectification signal for Co40Fe40B20 layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of the Co40Fe40B20 thickness that we suggest may be due to competition between a resonant heating effect and the SP-ISHE contribution.