First order green-function theory of isotropic Heisenberg ferromagnet

The Heisenberg ferromagnet with general spin S is considered within Green-function theory and spectral density method. New difference equations of the first order determining one- and two-particle correlation functions are derived and solved. The spectral density method is used to close Oguchi's variational theory without additional decoupling assumptions. The temperature renormalized spectrum is found to be a series expansion in that the first term coincide with RPA result and the first two terms correspond essentially to Callen's result. Low temperature expansions for the renormalization factor and the magnetization are given and shown to coincide with Callen's result.