A mean field theory of magnets with competing double exchange and superexchange interactions

We study the competition between ferromagnetic double exchange (DE) and direct nearest-neighbor antiferromagnetic exchange J_AF in two-dimensional colossal magnetoresistance materials. Towards this end, a single site mean field theory is proposed which incorporates the hopping-mediated nature of the DE contribution. The canted state appears only under highly restricted conditions. Instead, in the regime where the antiferromagnetic and DE ferromagnetic interactions are of comparable scale (and in the classical limit), the system exhibits ferro- or antiferromagnetic order with incomplete saturation of the lattice or sublattice magnetization. We interpret these results as an indication that spin fluctuations remain strong even at low temperatures and discuss the associated experimental implications.