  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.
