A molecular-field theory of anisotropic ferromagnetica

Abstract

After a brief introduction discussing the basis of molecular-field theories, we introduce a generalization of Anderson’s treatment of antiferromagnetism which enables us to take higher-order exchange interactions into account as well as anisotropy terms. We restrict ourselves to terms quadratic in the spins. This theory is developed in detail for face-centred cubic, body-centred cubic, simple cubic, and body-centred tetragonal magnetic lattices as well as for rutile structure antiferromagnetics. The general theory is developed in such a way that we can predict the possible preferred directions along which the spins in the antiferromagnetics will be alined. Until more information is available about the values of the various exchange parameters, it is not possible to predict theoretically which of the possible spin arrangements will, in fact, be the most stable one. On comparing experimental evidence on the spin arrangements in face-centred cubic and rutile structure antiferromagnetics we find, however, that almost all arrangements found experimentally are included in those predicted by the theory.