Spontaneous non-reciprocal reflection of light from antiferromagnetic Cr₂O₃

The reflection of light from a medium with ordered spin structure characterized by the breakdown of time-reversal and parity symmetry is expected to be non-reciprocal even if the net magnetic moment of the medium equals zero. We report on the first experimental observation of spontaneous non-reciprocal rotation and circular dichroism in spin magnetoelectric Cr₂O₃. Non-reciprocal effects were observed below the antiferromagnetic transition temperature T_N=307 K and their temperature behaviour roughly corresponds to that of the order parameter. Observed values of (1-4)*10^-4 for the magnetoelectric susceptibility in the optical range are several orders of magnitude higher than predicted earlier. This increase of the susceptibility is presumably attributable to electronic dipole transitions in the optical range.