Uncompensated Linear Dichroism of Magneto‐Optical Kerr Effect in a 2D Altermagnet

Abstract Altermagnetism features compensated spins with spin‐split bands, enabling novel spintronic and magneto‐optical phenomena. The magneto‐optical Kerr effect (MOKE) has been widely used to investigate the magneto‐optical properties of ferromagnets but is suppressed in conventional antiferromagnets. Interestingly, recent studies suggest that MOKE can still be observed in altermagnetic materials. Here, the MOKE effect in a prototype 2D altermagnet is analyzed using first‐principles calculations. Altermagnetism in Cr 2 O 2 provides giant spin splitting with spin‐momentum locking along the k x and k y directions in the reciprocal space, leading to nonzero optical conductivity around the valley bandgaps of 0.58 eV. Polar MOKE is observed without a net magnetization, exhibiting linear dichroism with Kerr rotation angles reversing between the x and y directions due to the spin‐dependent selection rule. Furthermore, a −6% uniaxial compressive strain applied along the y ‐axis decreases the bandgap at the Y point and induces band inversion at the X point, imposing distinct MOKE signals. These findings illustrate the extraordinary relationship between spin splitting and optical properties in the 2D altermagnet Cr 2 O 2 , providing new insights for probing 2D altermagnetism through optics.