Local-band theory of itinerant ferromagnetism. IV. Equivalent Heisenberg model

The main results of the picture of itinerant ferromagnetism previously presented are obtained by the functional-integral method of Stratonovitch and Hubbard. A new form of this technique is introduced which is spin rotationally invariant in all approximations and which respects the Pauli principle. By a generalization of the saddle-point method, the itinerant-electron thermodynamics is transformed to that of a classical Heisenberg model with Ruderman-Kittel exchange.