Separation of Artifacts from Spin-Torque Ferromagnetic Resonance Measurements of Spin-Orbit Torque for the Low-Symmetry Van der Waals Semi-Metal ZrTe3

Spin-orbit torques generated by exfoliated layers of the low-symmetry semi-metal ZrTe3 are measured using the spin-torque ferromagnetic resonance (ST-FMR) technique. When the ZrTe3 has a thickness greater than about 10 nm, artifacts due to spin pumping and/or resonant heating can cause the standard ST-FMR analysis to overestimate the true magnitude of the torque efficiency by as much as a factor of 30, and to indicate incorrectly that the spin-orbit torque depends strongly on the ZrTe3 layer thickness. Artifact-free measurements can still be achieved over a substantial thickness range by the method developed recently to detect ST-FMR signals in the Hall geometry as well as the longitudinal geometry. ZrTe3/Permalloy samples generate a conventional in-plane anti-damping spin torque efficiency xi||DL = 0.014 +/- 0.004, and an unconventional in-plane field-like torque efficiency |xi||FL| = 0.003 +/- 0.001. The out-of-plane anti-damping torque is negligible. It is suggested that artifacts similarly interfere with the standard ST-FMR analysis for other van der Waals samples thicker than about 10 nm.