  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.
