Separation of Artifacts from Spin–Torque Ferromagnetic Resonance Measurements of Spin–Orbit Torque for the Low–Symmetry Van der Waals Semi–Metal ZrTe 3
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 ξ$^{FL}_{∥}$ = 0.014 ± 0.004, and an unconventional in-plane field-like torque efficiency |ξ$^{FL}_{∥}$| = 0.003 ± 0.001. As a result, 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.