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.