Controlling resonant spin photocurrent using magnetic field: Application to a magnetoelectric oxide Cr ₂ O ₃

Recently, a new method for generating unidirectional spin photocurrent that utilizes optical response of magnetic excitations has been proposed. However, experimental verification has been challenging due to the lack of ideal material and lack of a setup that delineates the spin photocurrent from other competing phenomena, such as spin pumping and spin Seeback effect.

In this work, we study a well-known magnetoelectric material Cr₂O₃, which supports magnon spin photocurrent within an effective four-sublattice spin model. Using the nonlinear response theory, we show that the magnon spin current can be generated by both linearly and circularly polarized electromagnetic waves via one-magnon processes. For linearly polarized waves, the spin current arises even in the absence of a static magnetic field. In contrast, circularly polarized waves induce a spin current only when a static magnetic field is present, and the current reverses its direction upon inversion of the field. The sensitivity to the external magnetic field and the contrasting behaviors to the linearly and circularly polarized electromagnetic waves delineate the spin photocurrent from other competing contributions, such as the spin pump and inhomogeneous heating, facilitating experimental verification. Our results show that Cr₂O₃ is an interesting candidate for the experimental investigation of the magnon photocurrent.