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

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 Cr₂O₂ 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 Cr₂O₂, providing new insights for probing 2D altermagnetism through optics.