Green's-Function Theory of a Disordered Heisenberg Ferromagnet

Magnetic properties are calculated for a spin one-half Heisenberg ferromagnet in which the exchange interactions deviate randomly in strength from the mean interaction. Disorder produces a low-energy peak and a high-energy tail in the density of spin-wave states for a simple-cubic geometry. The ferromagnetic Curie temperature is shown to decrease linearly with disorder; the temperature-dependent magnetization is not otherwise markedly altered.