SOME ASPECTS OF THE THEORY OF MAGNETS WITH COMPETING DOUBLE EXCHANGE AND SUPEREXCHANGE INTERACTIONS

In colossal magnetoresistance materials, ferromagnetic double exchange presumably coexists with a direct nearest-neighbor antiferromagnetic interaction. We construct a single-site mean-field theory that explici tly takes into account the different nature of carrier-mediated ferrom agnetism vs Heisenberg-like superexchange. We find, in contrast to pre vious results in the literature, that the competition between these tw o exchange interactions leads to ferromagnetic or antiferromagnetic or der with incomplete saturation of the magnetization (or sublattice mag netization), rather than spin canting. The associated experimental imp lications are discussed.