Theory of half-metallic double perovskites. I. Double exchange mechanism

The double perovskite material Sr₂FeMoO₆ has the rare and desirable combination of a half-metallic ground state with 100% spin polarization and ferrimagnetic T_c 420 K, well above room temperature. In this two-part paper, we present a comprehensive theoretical study of the magnetic and electronic properties of half-metallic double perovskites. In this paper we present exact diagonalization calculations of the "fast" Mo electronic degrees coupled to "slow" Fe core spin fluctuations treated by classical Monte Carlo techniques. From the temperature dependence of the spin-resolved density of states, we show that the electronic polarization at the chemical potential is proportional to magnetization as a function of temperature. We also consider the effects of disorder and show that excess Fe leaves the ground state half-metallic while antisite disorder greatly reduces the polarization. In the companion paper we will derive an effective classical spin Hamiltonian that provides a new framework for understanding the magnetic properties of half-metallic double perovskites including the effects of disorder. Our results on the dependence of the spin polarization on temperature and disorder have important implications for spintronics.