  The electronic and magnetic properties of concentrated and diluted
ferromagnetic semiconductors are investigated by using the Kondo lattice model,
which describes an interband exchange coupling between itinerant conduction
electrons and localized magnetic moments. In our calculations, the electronic
problem and the local magnetic problem are solved separately. For the
electronic part an interpolating self-energy approach together with a coherent
potential approximation (CPA) treatment of a dynamical alloy analogy is used to
calculate temperature-dependent quasiparticle densities of states and the
electronic self-energy of the diluted local-moment system. For constructing the
magnetic phase diagram we use a modified RKKY theory by mapping the interband
exchange to an effective Heisenberg model. The exchange integrals appear as
functionals of the diluted electronic self-energy being therefore temperature-
and carrier-concentration-dependent and covering RKKY as well as double
exchange behavior. The disorder of the localized moments in the effective
Heisenberg model is solved by a generalized locator CPA approach. The main
results are: 1) extremely low carrier concentrations are sufficient to induce
ferromagnetism; 2) the Curie temperature exhibits a strikingly non-monotonic
behavior as a function of carrier concentration with a distinct maximum; 3)
$T_C$ curves break down at critical $n/x$ due to antiferromagnetic correlations
and 4) the dilution always lowers $T_C$ but broadens the ferromagnetic region
with respect to carrier concentration.
