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 demagnetization included. A microscopic theory valid in the whole temperature range is then developed to calculate the energy spectrum, spontaneous magnetization, and various anisotropies of the film. The transition temperature for the reorientation 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 is shown that our results are able to account for experimental data that are available for ultrathin Fe films.