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, $\mathrm{Ta}\mathrm{As}$, are grown by molecular beam epitaxy on (001) $\mathrm{Ga}\mathrm{As}$ and interfaced with a metallic ferromagnet (${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$). Spin-torque ferromagnetic resonance (ST FMR) measurements yield a spin-torque ratio as large as ${\xi }_{\mathrm{FMR}}=0.45\pm 0.25$ in samples with an oxidized interface. By studying ST FMR in these samples with varying ${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$ layer thickness, we find that the dampinglike-torque efficiency is ${\xi }_{\mathrm{DL}}=1.36\pm 0.66$. In samples with a pristine (unoxidized) interface, the spin-torque ratio is ${\xi }_{\mathrm{FMR}}=-0.27\pm 0.14$ and has opposite sign to that observed in oxidized samples. We also find a lower bound on the spin Hall conductivity ($424\pm 110\hslash /e$ S/cm), which is surprisingly consistent with theoretical predictions for the single-crystal Weyl semimetal state of $\mathrm{Ta}\mathrm{As}$.

• Received 21 February 2022
• Revised 22 September 2022
• Accepted 29 September 2022

DOI:https://doi.org/10.1103/PhysRevApplied.18.054004