The Néel Theory of Ferrimagnetism

The Néel theory of ferrimagnetism is a molecular-field theory of magnetic ordering in systems which contain nonequivalent substructures of magnetic ions. The two principal results of the theory are (a) the curve of reciprocal susceptibility vs temperature above the Curie temperature is an hyperbola, and (b) the curve of spontaneous magnetization vs temperature below the Curie temperature may have any one of a number of unusual forms. The exact form depends on the distribution of magnetic ions between the substructures and on the relative values of the molecular field coefficients. The agreement between the predictions of the Néel theory and some experimental observations on ferrites is discussed briefly.