Theory of field induced spin reorientation transition in thin Heisenberg   films
  We consider the spin reorientation transition in a ferromagnetic Heisenberg monolayer with a second order single ion anisotropy as a function of temperature and external field. Up to now analytical methods give satisfying results only for the special case that the external field is aligned parallel to the easy axis of the crystal. We propose a theory based on a generalization of the Callen decoupling, which can be used for arbritrary direction of the external field. Excellent agreement between our results and Quantum Monte Carlo data is found for the field induced reorientation at finite temperatures. Additionally, we discuss the temperature dependence of the transition in detail. 
S. Schwieger, J. Kienert, and W.Nolting