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Electronic properties of double perovskite compounds

The double perovskites family A2MM′O6 (A being an alkaline‐earth and M, M′ two different transition‐metal elements) is considered as a serious candidate for magnetoelectronic applications. It appears to be fundamental to understand the role of electronic parameters controlling the half‐metallic ground state and high Curie temperature Tc. In this respect it is very interesting that different members of the family present a variety of electronic and magnetic properties. Among them, Sr2FeMoO6 and Sr2FeReO6 are half‐metallic ferromagnets with fairly large Tc(≈450 K) while Sr2FeWO6 is known as an antiferromagnetic insulator with TN ≈ 37 K. As expected, a metal–insulator and magnetic transition has been reported in the substituted compounds Sr2FeMoxW1–xO6. To elucidate the origin of such different behavior in these Sr2FeMO6 double perovskites we consider a correlated electron picture with localized Fe‐spins (d5 configuration) and conduction electrons originating from M (M = Mo, Re or W) together with the double‐exchange type interaction taking place in the hybridized Fe–M t2g band and competing with the antiferromagnetic superexchange interactions between the Fe‐spins. We consider here only the ordered stucture in which Fe and M constitute two interpenetrating sublattices. Using a tight‐binding model and the renormalized perturbation expansion technique, we calculate the density of states and determine the behaviour of the critical temperature as a function of the band filling and the Fe–M charge transfer energy. This allows us to discuss the stability of the half‐metallic ferromagnetic state with increasing charge transfer energy. We will also discuss the electronic properties and the ferro‐antiferromagnetic transition in the substituted compounds like Sr2FeMoxW1–xO6, when disorder among Mo/W takes place in the M‐sublattice. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)