Thermal properties of surface and bulk spin waves in metamagnets FeBr2 and FeCl2

Surface and bulk spin wave properties are studied in metamagnets, which consist of ferromagnetically ordered layers with the intralayer ferromagnetic exchange coupling being much stronger than the interlayer antiferromagnetic exchange. Using a Green's function method, dispersion relations are calculated for the S = I metamagnets FeBr2 and FeCl2 over a broad range, of temperatures by considering layer dependent thermal averages for the spins. Results are also deduced for the temperature dependence of the integrated intensities of acoustic and optical spin waves close to the surface.