Theory of electromagnon-mediated spin pumping

Spin pumping (SP) is a well-known way generating DC spin current by the irradiation of electromagnetic (EM) waves. Its usual setup is a junction of a ferromagnetic insulator and a paramagnetic heavy metal. Magnons are resonantly created by applying a GHz wave to the ferromagnet, and then spin current is injected to the metal via the angular momentum transfer from magnons to conducting electrons at the interface. On the other hand, in multiferroics driven by EM waves, electron spins can be coupled to not only the magnetic field but also the electric one via the magnetoelectric coupling. Electro-active magnons are called electromagnons, and their resonant frequency is usually in the range of 0.1-1.0 THz. In the present work, we theoretically study the SP in a multiferroicmagnet, focusing on its spiral ordered phase such as perovskite manganites RMnO₃. Based on the non-equilibrium Green’s function method, we derive the formula of DC spin currents pumped by AC electric or magnetic fields, and show that the spin current is created via THz waves in addition to GHz pumping. Comparing our results to the usual pumping in ferromagnets, we show that currently available THz laser is enough to realize the SP in multiferroics.