  Ferromagnetic systems described by the double exchange model are
investigated. At temperatures close to the Curie temperature, and for a wide
range of doping levels, the system is unstable toward phase separation. The
chemical potential decreases upon increasing doping, due to the significant
dependence of the bandwidth on the number of carriers. The reduction of the
electronic bandwidth by spin disorder leads to an enormously enhanced
thermopower which peaks near T_c, with a sign opposite that predicted by a
rigid band model.
