Separation of Artifacts from Spin‐Torque Ferromagnetic Resonance Measurements of Spin‐Orbit Torque for the Low‐Symmetry Van der Waals Semi‐Metal ZrTe ₃

Abstract Spin‐orbit torques generated by exfoliated layers of the low‐symmetry semi‐metal ZrTe 3 are measured using the spin‐torque ferromagnetic resonance (ST‐FMR) technique. When the ZrTe 3 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 ZrTe 3 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. ZrTe 3 /Permalloy samples generate a conventional in‐plane anti‐damping spin torque efficiency = 0.014 ± 0.004, and an unconventional in‐plane field‐like torque efficiency = 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.