Microscopic Model of Nonreciprocal Optical Effects inCr2O3

We develop a microscopic model that explains nonreciprocal optical effects in centrosymmetric ${\mathrm{Cr}}_2{\mathrm{O}}_3$. It is shown that light can couple directly to the antiferromagnetic order parameter. This coupling is mediated by the spin-orbit interaction and involves an interplay between the breaking of inversion symmetry due to the antiferromagnetic order parameter and the trigonal field contribution to the ligand field at the ${\mathrm{Cr}}^{3+}$ ion.