Magnetization reorientation and anisotropy in ultrathin magnetic films

The temperature dependence of spontaneous magnetization for every monolayer , as well as of various anisotropies in an ultrathin ferromagnetic film, is investigated by means of the Green-function technique. The Hamiltonian is based on the Heisenberg model with the surface anisotropy and demagnetizati on included. A microscopic theory valid in the whole temperature range is t hen developed to calculate the energy spectrum, spontaneous magnetization, and various anisotropies of the him. The transition temperature for the reo rientation transition between perpendicular and in-plane magnetization, as well as the Curie temperature, is calculated as a function of L, the number of monolayers in the film. Numerical results for L=1-8 are presented. It i s shown that our results are able to account for experimental data that are available for ultrathin Fe films.