The magnetic reorientation transition in thin ferromagnetic films   treated by many-body Green's function theory
  This contribution describes the reorientation of the magnetization of thin ferromagnetic Heisenberg films as function of the temperature and/or an external field. Working in a rotating frame allows an exact treatment of the single-ion anisotropy when going to higher-order Green's functions. Terms due to the exchange interaction are treated by a generalized Tyablikov (RPA) decoupling. 
P. Fr\"obrich, and P.J. Kuntz