Many-body theory of magnetism for ions with complicated level structure

Abstract

The problem of incorporating the effects of many-body static spin correlations in excited orbital crystal-field levels is discussed by introducing the concept of a correlated effective field. The resulting theory has the conceptual simplicity of molecular-field theory and can be used for problems where excited orbital crystal-field energies, exchange energies, and thermal energies are all of the same order of magnitude. Correlations are determined by forcing a consistency with the fluctuation theorem and the resulting statistical theory is shown to have an accuracy equivalent to that of random-phase Green's function techniques in the quenched-orbital (spin-only) Heisenberg limit.