Giant spin torque efficiency in naturally oxidized polycrystalline TaAs thin films

Abstract

We report the measurement of efficient charge-to-spin conversion at room temperature in Weyl semimetal/ferromagnet heterostructures with both oxidized and pristine interfaces. Polycrystalline films of the Weyl semimetal, TaAs, are grown by molecular beam epitaxy on (001) GaAs and interfaced with a metallic ferromagnet (Ni${}_{0.8}$Fe${}_{0.2}$). Spin torque ferromagnetic resonance (ST-FMR) measurements in samples with an oxidized interface yield a spin torque efficiency as large as $\hat{I}{¾}_{\mathrm{FMR}}=0.45\pm 0.25$ for a 8 nm Ni${}_{0.8}$Fe${}_{0.2}$ layer thickness. By studying ST-FMR in these samples with varying Ni${}_{0.8}$Fe${}_{0.2}$ layer thickness, we extract a damping-like torque efficiency as high as $\hat{I}{¾}_{\mathrm{DL}}=1.36\pm 0.66$. In samples with a pristine (unoxidized) interface, the spin torque efficiency has opposite sign to that observed in oxidized samples ($\hat{I}{¾}_{\mathrm{FMR}}=-0.27\pm 0.14$ for a 5 nm Ni${}_{0.8}$Fe${}_{0.2}$ layer thickness). We also find a lower bound on the spin Hall conductivity ($424\pm 110\frac{\hslash }{e}$ S/cm) which is surprisingly consistent with theoretical predictions for the single crystal Weyl semimetal state of TaAs.