Large magneto-optical Kerr effect induced by collinear antiferromagnetic order.

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.