Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

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(1-3) and for magneto-optical technologies(4). The MOKE can be additionally useful for the investigation of the antiferromagnetic (AF) state, although thus far limited to insulators(5-9). 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 octupoles(11) 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(12-16)