  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 $\xi_{\mathrm{FMR}}=0.45\pm 0.25$ 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 $\xi_{\mathrm{DL}}=1.36\pm 0.66$. In samples with a
pristine (unoxidized) interface, the spin torque efficiency has opposite sign
to that observed in oxidized samples ($\xi_{\mathrm{FMR}}=-0.27\pm 0.14$ for a
5 nm Ni$_{0.8}$Fe$_{0.2}$ layer thickness). We also find a lower bound on the
spin Hall conductivity ($424 \pm 110 \frac{\hbar}{e}$ S/cm) which is
surprisingly consistent with theoretical predictions for the single crystal
Weyl semimetal state of TaAs.
