Spin wave effects in transport between a ferromagnet and a Weyl semimetal surface

We experimentally investigate spin-polarized transport between a ferromagnetic Ni electrode and a surface of Weyl semimetal, realized in a thick WTe${}_2$ single crystal. For highly-transparent Ni-WTe${}_2$ planar junctions, we observe non-Ohmic $dV\mathrm{/}dI(I)$ behavior with an overall increase of differential resistance $dV\mathrm{/}dI$ with current bias, which is accomplished by current-induced switchings. This behavior is inconsistent with trivial interface scattering, but it is well known for spin-polarized transport with magnon emission. Thus, we interpret the experimental results in terms of spin wave excitation in spin textures in the WTe${}_2$ topological surface states, which is supported by the obtained magnetic field and temperature $dV\mathrm{/}dI(I)$ dependencies.