Effects of Doping and Coulomb Correlations on Tc and Competing Phases in Half-metallic Double Perovskites

Double perovskites such as $Sr_{2}FeMoO_{6}$ (SFMO) are rare examples of materials with half-metallic ground states and a ferrimagnetic $T_{c}$ much greater than room temperature. We have shown that the electronic and magnetic properties of SFMO are well described by a generalized double exchange model [1] for itinerant electrons from Mo coupled to localized Fe spins. However, the simplest model proves inadequate when SFMO is electron-doped by La-substitution on the Sr sites. Ignoring Coulomb correlations for the itinerant electrons, the ferromagnetism of Fe spins becomes progressively weaker with electron doping, and eventually the model is unstable to a metallic antiferromagnetic ground state. This is in contradiction with experiments [2], which find a ferromagnetic $T_{c}$ increasing with carrier concentration and no evidence for an antiferromagnetic state up to $SrLaFeMoO_{6}$. In this talk we will show that the Hubbard U on the Mo-site is responsible for the observed doping trends. We will show that correlations stabilize the ferromagnetism, with the observed $T_{c}(n)$ behavior, and that the antiferromagnetic metal is not a competitive state for reasonable values of n.\newline[1] O. Erten et al, PRL 107, 257201 (2011)\newline[2] J. Navarro et al, PRB 64, 092411 (2001).