Eur. Phys. J. B 13, 477-489 (2000)
https://doi.org/10.1007/s100510050059

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

¹ Hahn-Meitner-Institut Berlin, Glienicker Straße 100, 14109 Berlin, Germany
² also Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

Received: 6 April 1999
Revised: 9 July 1999
Published online: 19 October 2012

Abstract

The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the spin components (), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches.