Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure

摘要

We investigate a spin-electricity conversion effect in a topologicalinsulator/ferromagnet heterostructure. In the spin-momentum-locked surfacestate, an electric current generates nonequilibrium spin accumulation, whichcauses a spin-orbit torque that acts on the ferromagnet. When spins in theferromagnet are completely parallel to the accumulated spin, this spin-orbittorque is zero. In the presence of spin excitations, however, a couplingbetween magnons and electrons enables us to obtain a nonvanishing torque. Inthis paper, we consider a model of the heterostructure in which athree-dimensional magnon gas is coupled with a two-dimensional massless Diracelectron system at the interface. We calculate the torque induced by anelectric field, which can be interpreted as a magnon spin current, up to thelowest order of the electron-magnon interaction. We derive the expressions forhigh and low temperatures and estimate the order of magnitude of the inducedspin current for realistic materials at room temperature.