Curie temperature of Kondo lattice films with finite itinerant charge carrier density

We present a model study of ferromagnetic films consisting of free Bloch electrons coupled to localized moments (Kondo lattice films). By mapping the local interaction onto an effective Heisenberg Hamiltonian we obtain temperature and carrier density dependent exchange integrals mediating the interaction between local moments via the conduction electrons. The non-perturbative approach recovers analytically the weak-coupling RKKY interaction and yields convincing numerical results in the strong coupling (double exchange) regime. The Curie temperature is calculated for various coupling strengths, band fillings, and numbers of layers. The results are compared with total energy calculations. We discuss the influence of charge transfer between film layers and of anisotropy on the Curie temperature. The model we investigate is considered relevant for the understanding of the basic magnetic properties of manganites, diluted magnetic semiconductors, and rare earth substances as, e.g., Gadolinium.