Effect of Mo/W disorder on the Curie temperature of Sr₂FeMo _xW_1-xO₆ double perovskite

The double perovskites Sr₂FeMO₆ have received a lot of attention, especially Sr₂FeMoO₆ because of its fairly high Curie temperature (T_C = 450 K), half-metallic character, large magnetoresistance and potential applications. On the other hand, Sr ₂FeWO₆ is insulating and antiferromagnetic with T _N = 37 K. With a double exchange type model it has been shown that a ferromagnetic-antiferromagnetic transition can be driven by super-exchange interactions with increasing Fe-M (M=Mo,W) charge transfer energy. Therefore, the charge transfer energy is expected to be larger in FeW than in FeMo compounds. The effect of the Mo/W diagonal disorder in Sr₂FeMo _xW_1-xO₆ has been studied using the tight-binding model and the renormalization perturbation expansion method. For different values of the disorder we calculate the density of states and determine the behavior of the Curie temperature as a function of the Mo/W concentration. The magnetic and the electronic properties in these compounds are discussed in view of these results.