Electronic properties of double perovskite compounds

The double perovskites family A₂MM'O₆ (A being an alkaline-earth and M, M' two different transitionmetal 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 T_c. In this respect it is very interesting that different members of the family present a variety of electronic and magnetic properties. Among them, Sr ₂FeMoO₆ and Sr₂FeReO₆ are halfmetallic ferromagnets with fairly large T_c(≈450 K) while Sr₂FeWO₆ is known as an antiferromagnetic insulator with T_n*ap;37 K. As expected, a metal-insulator and magnetic transition has been reported in the substituted compounds Sr2FeMo^W,_XO6. To elucidate the origin of such different behavior in these Sr₂FeMO ₆ 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 doubleexchange type interaction taking place in the hybridized Fe-M t_2g 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 hi the substituted compounds like Sr ₂FeMo_xW_1-xO_6, when disorder among Mo/W takes place in the M- sublattice.