Electronic structure and stability of the ferrimagnetic ordering in double perovskites

Using results of first-principles band structure calculations and a model tight-binding approach, we investigate the local stability of the half-metallic ferrimagnetic (FiM) states in double perovskites Sr2FeMO6 (M=Mo, Re, and W). In ordered compounds, a generalized double-exchange (DE) mechanism operating in the metallic minority-spin channel, in a hybrid Fe-M-derived t(2g) band, always competes with the strong antiferromagnetic superexchange (SE) interactions in the Fe sublattice mediated by virtual electron hoppings into the unoccupied M(d) states. In the local-spin-density approximation (LSDA), the SE mechanism largely prevails and the FiM phase is unstable with respect to a noncollinear spin-spiral alignment. The situation appears to be more generic. So, the onsite Coulomb repulsion between the Fe(3d) electrons (DeltaU) on the top of the LSDA picture suppresses the SE interactions but may also modify some of the DE interactions through the change of the Fe-M hybridization. The total change of the electronic structure, caused by DeltaU alone, does not explain the local stability of the FiM state. Therefore, we conclude that the FiM phase cannot be stabilized by purely electronic mechanisms. According to our scenario, that is exactly the situation realized in Sr2FeWO6. An interpretation of the experimental FiM ordering observed in Sr2FeMoO6 and Sr2FeReO6 should require an additional mechanism, which destroys the half-metallic character of the electronic structure and suppresses the saturation moment. We consider two possibilities: the alternating breathing distortions of the FeO6 and MO6 octahedra, and the antisite (Fe-M interchange) disorder. In the former case, the oxygen displacement towards the Fe-sites leads to the partial depopulation of the majority-spin Fe(e(g)) band and thereby activates an effective channel for the ferromagnetic DE interactions, similar to colossal-magnetoresistive manganites. In the latter case, the FiM ordering can be stabilized by SE interactions of a small (less than 10%) amount of Fe impurities with the host atoms. We discuss possible implications of these scenarios to different compounds with emphasis on their magnetic and optical properties.