THEORY OF NONRECIPROCAL OPTICAL EFFECTS IN ANTIFERROMAGNETS - THE CASE OF CR2O3

A microscopic model of nonreciprocal optical effects in antiferromagne ts is developed by considering the case of Cr2O3 where such effects ha ve been observed. These effects are due to a direct coupling between l ight and the antiferromagnetic order parameter. This coupling is media ted by the spin-orbit interaction and involves an interplay between th e breaking of inversion symmetry due to the antiferromagnetic order pa rameter and the trigonal field contribution to the ligand field at the magnetic ion. We evaluate the matrix elements relevant for the nonrec iprocal second harmonic generation and gyrotropic birefringence.