Microscopic model, spin-wave theory, and competing orders in double perovskites

We present a microscopic theory of carrier-induced ferrimagnetism in metallic double perovskite compounds such as Sr₂FeMoO₆ and Sr₂FeReO₆ which have recently attracted intense interest for their possible applications to magnetotransport devices. The theory is based on an effective “Kondo-like” Hamiltonian treated here within the large-S expansion. We find that depending on the value of the carrier density the ground state is either a ferrimagnet or a layered antiferromagnet. The ferrimagnetic state has a robust half-metallic electronic structure. The transition to antiferromagnetic phase is first order, accompanied with the regime of phase separation. We study spin-wave spectrum including quantum corrections and find strongly enhanced quantum effects in the vicinity of zero-temperature phase transition.