  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.
