Crystal fields and magnetic anisotropy in the molecular field approximation. I. General considerations

Abstract

The consequence of incorporating crystal field interactions into the molecular field theory of magnetism are investigated, with special reference to noncubic symmetry. Spin-orbit coupling is assumed to dominate both the exchange and the crystal field interactions, but no approximations are made concerning the relative magnitudes of exchange and crystal field. Topics discussed include differential susceptibilities in the paramagnetic and ferromagnetic phases, the effect of the crystal field on the Curie temperature and on the saturation magnetization, the temperature dependence of the direction of spontaneous magnetization, and crystal field quenching of the cooperative phase. The theory is illustrated by numerical calculations for ions with an incomplete f shell in a crystal field of hexagonal symmetry.