We propose a self-consistent approximate solution of the s-f model for
describing the exchange coupling of a local moment system with a part
ially filled energy band. Induced electronic correlations account for
the characteristic quasiparticle band effects which become manifest vi
a striking temperature dependencies, band deformations and splittings.
For weak s-f exchange interactions a 'Stoner-like' spin splitting of
the conduction band proportional to the f magnetization occurs. As soo
n as the coupling exceeds a critical value an additional spin splittin
g of the quasiparticle dispersion sets in, which is due to different e
lementary excitations. One of these appears as a repeated emission and
reabsorption of a magnon by the conduction electron, resulting in an
effective electron-magnon attraction. This gives rise to a polaronlike
quasiparticle (a 'magnetic polaron'). Other elementary processes are
connected to magnon emission or absorption by the conduction electron
('scattering states'). The polarization of the conduction band due to
the s-f exchange interaction J feeds back to the localized spin system
leading to an indirect coupling between the spins. For weak s-f coupl
ing the RKKY mechanism dominates (T-c proportional to J(2)), but with
remarkable deviations for intermediate and strong couplings. The Curie
temperature saturates with increasing J, where the saturation value i
s strongly dependent on the band occupation n. The oscillating behavio
ur of the effective exchange integral connecting the localized spins r
estricts ferromagnetism to special regions for n. The magnetization cu
rve, the spin polarization of the itinerant electrons, and f-f as well
as s-f spin correlation functions are worked out for a simple cubic l
attice and discussed in terms of the band occupation n and the s-f exc
hange coupling J.