Magnetic and electronic properties of double perovskites and estimation of their Curie temperatures by ab initio calculations

First-principles electronic structure calculations have been carried out on ordered double perovskites (Sr2BBO6)-B-'-O-' (for B-'=Cr or Fe and B-' 4d and 5d transition-metal elements) with increasing number of valence electrons at the B-' sites and on Ba2MnReO6 as well as Ba2FeMoO6. The Curie temperatures are estimated ab initio from the electronic structures obtained with the local spin-density functional approximation, full-potential generalized gradient approximation and/or the LDA+U method (LDA, local density approximation; U, Hubbard parameter). Frozen spin spirals are used to model the excited states needed to evaluate the spherical approximation for the Curie temperatures. In cases where the induced moments on the oxygen were found to be large, the determination of the Curie temperature is improved by additional exchange functions between the oxygen atoms and between oxygen and B-' and B-' atoms. Pronounced systematics can be found among the experimental and/or calculated Curie temperatures and the total valence electrons of the transition-metal elements.