New understanding in century-old magneto-optical effects

Magneto-optical (MO) effects, reflecting the basic interactions between light and magnetism, have long been recognized as one of the most attractive subjects in condensed matter physics. Since the discovery of the MO Faraday effect in 1846, studies of MO effects have continued for more than a century and a half. In this article, we introduce three new discoveries in this field. First, MO effects are surprisingly found to exist in antiferromagnets even though the net magnetization is zero, and are very sensitive to the vector spin chirality. Second, they can originate from the scalar spin chirality independent of spin-orbit coupling and band exchange splitting, namely, topological MO effects, which can be further quantized. Third, they are strongly related to the crystal chirality induced by rearrangement of the nonmagnetic atoms, which is called crystal chirality MO effects. These new advances overturn the traditional understanding of MO effects in the past, and may have a far-reaching impact on the development of this field.