  A mean field approximation of a model for double perovskites that takes into
account the coupling between itinerant electron spins and localized spins is
developed. As in previously reported theoretical results, and contrary to
experimental observation, the critical temperature is suppressed for large
electron density. An effective Heisenberg model reveals the cause of this
discrepancy: the competition between degenerate antiferromagnetic and
ferromagnetic channels. This degeneracy can be broken by the inclusion of a
Hubbard-type U term. It is therefore suggested that electron correlation
effects need to be incorporated in the minimal model of double perovskites in
order to explain the experimental observation of increasing ferromagnetic
critical temperature with increasing electron doping.
