The magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials as a powerful probe for electronic and magnetic properties^1,2,3 and for magneto-optical technologies⁴. The MOKE can be additionally useful for the investigation of the antiferromagnetic (AF) state, although thus far limited to insulators^5,6,7,8,9. Here, we report the first observation of the MOKE in an AF metal. In particular, we find that the non-collinear AF metal Mn₃Sn (ref. ¹⁰) exhibits a large zero-field Kerr rotation angle of 20 mdeg at room temperature, comparable to ferromagnetic metals. Our first-principles calculations clarify that ferroic ordering of magnetic octupoles¹¹ produces a large MOKE even in its fully compensated AF state. This large MOKE further allows imaging of the magnetic octupole domains and their reversal. The observation of a large MOKE in an AF metal will open new avenues for the study of domain dynamics as well as spintronics using antiferromagnets^{12,13,14,15,16}.

磁光克尔效应 (MOKE) 已在各种铁磁和亚铁磁材料中进行了深入研究，作为电子和磁特性^1,2,3以及磁光技术⁴的强大探针。MOKE 对于反铁磁 (AF) 态的研究也很有用，尽管迄今为止仅限于绝缘体^5,6,7,8,9。在这里，我们报告了在 AF 金属中首次观察到 MOKE。特别是，我们发现非共线AF金属Mn ₃ Sn（参考文献¹⁰）在室温下表现出20 mdeg的大零场克尔旋转角，与铁磁金属相当。我们的第一原理计算表明，即使在完全补偿的 AF 状态下，磁性八极子¹¹的铁有序也会产生较大的 MOKE。这个大 MOKE 进一步允许对磁八极域及其反转进行成像。对 AF 金属中大型 MOKE 的观察将为域动力学以及使用反铁磁体的自旋电子学的研究开辟新途径^{12,13,14,15,16}。