Superexchange induced canted ferromagnetism in dilute magnets

Abstract

We argue, in contrast to recent studies, that the antiferromagnetic superexchange coupling between nearest neighbour spins does not fully destroy the ferromagnetism in dilute magnets with long-ranged ferromagnetic couplings. Above a critical coupling, we find a \textit{canted} ferromagnetic phase with unsaturated moment. We have calculated the transition temperature using a simplified local Random Phase Approximation procedure which accounts for the canting. For the dilute magnetic semiconductors, such as GaMnAs, using \textit{ab-initio} couplings allows us to predict the existence of a canted phase and provide an explanation to the apparent contradictions observed in experimental measurements. Finally, we have compared with previous studies that used RKKY couplings and reported non-ferromagnetic state when the superexchange is too strong. Even in this case the ferromagnetism should remain essentially stable in the form of a canted phase.