The reorientation of the magnetization of a ferromagnetic monolayer calculated with the help of many-body Green's function theory. allows, in contrast to other spin wave theories, a satisfactory of magnetic properties over the entire temperature range of since interactions between spin waves are taken into account. A Hamiltonian plus a-order uniaxial single-ion anisotropy and an external magnetic field is by the Tyablikov (Random Phase Approximation: RPA) decoupling of the interaction term and the Anderson-Callen decoupling of the term. The orientation of the magnetization determined by the spin components\ S^\α\ (\α=x,y,z), which are calculated with the help the spectral theorem. The knowledge of the orientation angle \Θ₀ a-perturbative determination of the temperature dependence of the second-order anisotropy coefficient. for the Green's function theory are compared with those with mean-field theory (MFT). We find significant differences between approaches.
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A 2012 study studied this question.