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

TL;DR

This paper uses many-body Green's function theory to accurately analyze the temperature-dependent magnetic reorientation and anisotropy in a ferromagnetic monolayer, improving upon mean-field approximations.

Contribution

It introduces a comprehensive Green's function approach for magnetic reorientation, capturing spin wave interactions across all temperatures, and compares results with mean-field theory.

Findings

Green's function theory yields different results than mean-field theory.

Magnetization orientation and anisotropy are accurately calculated over temperature.

Significant differences highlight the importance of spin wave interactions.

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 $\la S^\alpha\ra$ ($\alpha=x,y,z$), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle $\Theta_0$ 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.