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Giant Dampinglike-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 (Ni0.8Fe0.2). Spin-torque ferromagnetic resonance (ST FMR) measurements yield a spin-torque ratio as large as ?FMR=0.45±0.25 in samples with an oxidized interface. By studying ST FMR in these samples with varying Ni0.8Fe0.2 layer thickness, we find that the dampinglike-torque efficiency is ?DL=1.36±0.66. In samples with a pristine (unoxidized) interface, the spin-torque ratio is ?FMR=-0.27±0.14 and has opposite sign to that observed in oxidized samples. We also find a lower bound on the spin Hall conductivity (424±110?/e S/cm), which is surprisingly consistent with theoretical pbkp_redictions for the single-crystal Weyl semimetal state of TaAs.