Many-body Green's function theory of Heisenberg films

The treatment of Heisenberg films with many-body Green's function theory (GFT) is reviewed. The basic equations of GFT are derived in sufficient detail so that the rest of the paper can be understood without having to consult further literature. The main part of the paper is concerned with applications of the formalism to ferromagnetic, antiferromagnetic and coupled ferromagnetic-antiferromagnetic Heisenberg films based on a generalized Tyablikov (RPA) decoupling of the exchange interaction and exchange anisotropy terms and an Anderson–Callen decoupling for a weak single-ion anisotropy. We not only give a consistent description of our own work but also refer extensively to related investigations. We discuss in particular the reorientation of the magnetization as a function of the temperature and film thickness. If the single-ion anisotropy is strong, it can be treated exactly by going to higher-order Green's functions. We also discuss the extension of the theory beyond RPA. Finally the limitations of GFT are pointed out.