Impurity spin wave models in a simple cubic Heisenberg ferromagnet

A Green function method is developed to describe the excitations of a simple cubic Heisenberg ferromagnet which contains a small number of randomly distributed impurity spins. The method used demonstrates some novel features in the form taken by the averaged Green function describing the system and eliminates the need to work with spurious parameters depending on the square root of the impurity spin. A brief discussion is given of the eigenvalues and densities of states associated with the impurity spin wave modes and of the modification to the host spin wave band.