Microscopic theory of spin pumping in antiferromagnets

Spin pumping refers to the phenomenon that when an AC magnetic field is applied to a bilayer junction of magnet and a metal, an DC spin current is injected from the magnet to the metal through the magnetic resonance in the magnet. In the last decades, the spintronics research has been extended from ferro- or ferri-magnets to antiferromagnets. Therefore, the development and understanding of spin current generation in antiferromagnets [1] are crucial for advancing ultrafast spintronics [2]. Despite various experimental studies of antiferromagnetic spintronics, the theoretical study has not progressed well. For instance, the microscopic theory for spin pumping in ferromagnets has already been established, while such a theory for antiferromagnets has been less explored. Given this situation, we have developed a microscopic theory of spin pumping for easy-axis type antiferromagnets with Néel and canted phases and weak ferromagnets with a Dyzaloshinskii-Moriya (DM) interaction accompanying another canted phase [3]. We quantitatively estimate the dependence of the generated spin current on the external magnetic field, the polarization direction of the AC magnetic field, and its frequency dependence. Furthermore, we compare the results of our theory with recent experimental findings on spin pumping in antiferromagnets. As a result, we demonstrate that our theoretical framework can account for several experimental observations.