Physical properties and electronic evolution of Sr2FeMo1-xNbxO6 (0 <= x <= 1)

We investigated magnetic and electronic properties of double perovskites Sr2FeMo1-xNbxO6 (0 <= x <= 1). As Nb substitution x increases, the system gradually loses all ferromagnetic metallic characters such as the ordered magnetic moment, the magnetic transition temperature T-C, the degree of cation-site ordering, and the metallic conductivity. For x>0.7, the ferromagnetic characters disappear. The electronic structure was also investigated to explore its correlation with the physical properties by using the soft x-ray absorption spectroscopy at Fe L-2,L-3, Mo M-2,M-3, and O K edges. The Fe state, which is an Fe2+-Fe3+ mixed valent state close to Fe2.5+ in average at Sr2FeMoO6 (x=0), becomes gradually transformed to be an Fe3+ monovalent state at Sr2FeNbO6 (x=1) with the Nb substitution. The Mo valence state, approximately Mo5.5+ (4d(0.5)), does not show any considerable change for 0 <= x < 1. The replacement of Mo with Nb5+ (4d(0)) reduces the density of states at Fermi level, which corresponds to the Mo 4d-Fe 3d strongly mixed states, and cause the increase in resistivity. These results show that the loss of the ferromagnetic order upon the Nb substitution is directly attributed to the reduction in the Fe-O-Mo-O-Fe hybridized metallic chain population.