Symmetry-Driven Giant Magneto–Optical Kerr Effects in Altermagnetic Insulator

Altermagnets have attracted tremendous interest for revealing intriguing physics and promising spintronics applications. In contrast to conventional antiferromagnets, altermagnets break both PT and Tτ symmetries, and simultaneously exhibit spin-split band structures with a vanishing net magnetization. To quantify altermagnetic insulators without conduction electrons, we propose to use the magneto–optical Kerr effect (MOKE). In particular, we demonstrate not only the giant MOKE responses, but also their connection with the orientations of Néel vectors at room temperature in the altermagnetic insulator hematite (α-Fe₂O₃). Specifically, under the Néel vector along the [ 1 ¯ 100 ] axis, we find a giant polar Kerr rotation angle of 103.7 mdeg in the (11 2 ¯ 0) plane, which is allowed by the magnetic space group C2′/c′. Under the Néel vector along the [ 11 2 ¯ 0 ] axis, we find a longitudinal Kerr angle of 9.6 mdeg in the (0001) plane, which is allowed by the magnetic space group C2/c. Further, we show that such pronounced MOKE effects directly enable optical imaging of altermagnetic domains, together with their reversible domain wall (DW) motion. Our studies not only suggest that MOKE can be used to identify altermagnetic candidates, but also signify the feasibility of exploring altermagnetic optical and DW spintronics, which could largely expand the current research paradigm of altermagnetism.