In this work we compare numerically exact Quantum Monte Carlo (QMC) calculations and Green function theory (GFT) calculations of thin ferromagnetic films including second order anisotropies. Thereby we concentrate on easy plane systems, i.e. systems
for which the anisotropy favors a magnetization parallel to the film plane. We discuss these systems in perpendicular external field, i.e. B parallel to the film normal. GFT results are in good agreement with QMC for high enough fields and temperatures. Below a critical field or a critical temperature no collinear stable magnetization exists in GFT. On the other hand QMC gives finite magnetization even below those critical values. This indicates that there occurs a transition from non-collinear to collinear configurations with increasing field or temperature. For slightly tilted external fields a rotation of magnetization from out-of-plane to in-plane orientation is found with decreasing temperature.
