Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic   Films
  The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within Heisenberg spin models. Analytic expressions for the temperature dependent anisotropy are derived from which it is seen that the reduced magnetization in the film surface at finite temperatures plays a crucial role for this transition. Detailed phase diagrams in the temperature-thickness plane are calculated. 
A. Hucht and K. D. Usadel