In modern technology, optical readout of magnetic information is conventionally achieved by the magneto-optical Kerr effect, i.e., polarization rotation of reflected light. The Kerr rotation is sensitive to time-reversal symmetry breaking and generally proportional to magnetization, enabling optical readout of the ↑ and ↓ spin states in ferromagnets. By contrast, antiferromagnets with a collinear antiparallel spin arrangement have long been considered inactive to such magneto-optical responses, because of Formula: see text (time-reversal Formula: see text followed by translation t) symmetry and lack of macroscopic magnetization. Here, we report the identification of a large magneto-optical Kerr effect induced by collinear antiferromagnetic order, through detailed measurements of a room-temperature antiferromagnetic insulator α Formula: see text Formula: see text. Our first-principles calculations successfully reproduce both the absolute magnitude and spectral shape of the Kerr rotation and ellipticity with remarkable accuracy, which unambiguously proves that it originates from a Formula: see text-symmetry-broken collinear antiferromagnetic order, rather than magnetization. This compound hosts temperature-dependent transition between easy-plane and easy-axis antiferromagnetic states, and their contrasting behaviors suggest that the selection rule is governed by the detail of magnetic symmetry. The present results demonstrate that even a simple collinear antiferromagnetic order can induce a large magneto-optical Kerr effect, and highlight Formula: see text-symmetry-broken antiferromagnets as a promising material platform for highly sensitive optical detection of Formula: see text and Formula: see text spin states.
Yoshimochi et al. (Thu,) studied this question.