Theory of half-metallic ferrimagnetism in double perovskites

Double perovskites such as Sr ₂FeMoO ₆ 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.