Theory of Half-Metallic Ferrimagnetism in Double Perovskites

Double perovskites such as Sr2FeMoO6 are rare examples of materials with half-metallic ground states and a ferrimagnetic T-c above room temperature. We present a comprehensive theory of the temperature and disorder dependence of their magnetic properties by deriving and validating a new effective spin Hamiltonian for these materials, amenable to large-scale three-dimensional simulations. We show how disorder, ubiquitous in these materials, affects T-c, the magnetization, and the conduction electron polarization. We conclude with a novel proposal to enhance T-c without sacrificing polarization.