Superexchange induced canted ferromagnetism in dilute magnets

TL;DR

This paper demonstrates that superexchange interactions can induce a canted ferromagnetic phase in dilute magnets, which remains stable even when strong, explaining experimental observations and contrasting with previous studies.

Contribution

It introduces the concept of a canted ferromagnetic phase caused by superexchange in dilute magnets and predicts its existence using ab-initio couplings.

Findings

Canted ferromagnetic phase exists above a critical superexchange coupling.

Transition temperature can be calculated with a local RPA approach accounting for canting.

Ferromagnetism persists in a canted form even with strong superexchange.

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.