Coupled ferro-antiferromagnetic Heisenberg bilayers investigated by many-body Green's function theory

TL;DR

This paper develops a many-body Green's function theory for coupled ferro- and antiferromagnetic Heisenberg bilayers, revealing significant differences from mean field theory and lattice-dependent critical temperature behaviors.

Contribution

It introduces a non-collinear Green's function approach to analyze coupled magnetic layers, highlighting lattice-dependent effects on critical temperatures.

Findings

Critical temperature decreases with interlayer coupling for simple cubic lattice.

Critical temperature increases with interlayer coupling for fcc lattice.

Significant differences between Green's function theory and mean field theory results.

Abstract

A theory of coupled ferro- and antiferromagnetic Heisenberg layers is developed within the framework of many-body Green's function theory (GFT) that allows non-collinear magnetic arrangements by introducing sublattice structures. As an example, the coupled ferro- antiferromagnetic (FM-AFM) bilayer is investigated. We compare the results with those of bilayers with purely ferromagnetic or antiferromagnetic couplings. In each case we also show the corresponding results of mean field theory (MFT), in which magnon excitations are completely neglected. There are significant differences between GFT and MFT. A remarkable finding is that for the coupled FM-AFM bilayer the critical temperature decreases with increasing interlayer coupling strength for a simple cubic lattice, whereas the opposite is true for an fcc lattice as well as for MFT for both lattice types.