Unexpected behavior of the antiferromagnetic mode of NiO

Although NiO is often considered the classic example of an antiferromagneti c insulator, recent investigations have revealed unexplained features of th e magnon spectrum. The present study of the temperature and polarization be havior of first-order magnetic Raman scattering reveals that the polarizati on selection rules are not described by the generally accepted antisymmetri c scattering tensor. The inclusion of quadratic magnetooptic coupling terms can explain the symmetry of the scattering tensor, but does not lead to re sults consistent with the accepted [112] spin alignment direction. [S0163-1 829(98)06241-9].