Photovoltaic effect, e.g., solar cells, converts light into DC electric current. This phenomenon takes place in various setups such as in noncentrosymmetric crystals and semiconductor pn junctions. Recently, we proposed a theory for producing DC spin current in magnets using electromagnetic waves, i.e., the spin-current counterpart of the solar cells. Our calculation shows that the nonlinear conductivity for the spin current is nonzero in a variety of noncentrosymmetric magnets, implying that the phenomenon is ubiquitous in inversion-asymmetric materials with magnetic excitations. Intuitively, this phenomenon is a bulk photovoltaic effect of magnetic excitations, where electrons and holes, visible light, and inversion-asymmetric semiconductors are replaced with magnons or spinons, THz or GHz waves, and asymmetric magnetic insulators, respectively. We also show that the photon-driven spin current is shift current type, and as a result, the current is stable against impurity scattering. This bulk photovoltaic spin current is in sharp contrast to that of well-known spin pumping that takes place at the interface between a magnet and a metal.
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