Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

The reorientation of the magnetization of a ferromagnetic monolayer is calc ulated with the help of many-body Green's function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magneti c properties over the entire temperature range of interest since interactio ns between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic fie ld is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy ter rn. The orientation of the magnetization is determined by t he spin components (S-alpha) (alpha = x, y, z), which are calculated with t he help of the spectral theorem. The knowledge of the orientation angle The ta(0) allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Gree n's function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches.