Moessbauer, magnetization and crystal structure studies of the double perovskites Sr2FeMo1-xWxO6, x = 0, 0.1, 0.2, 0.3 and 0.4

The structural, magnetic and electronic properties of the double perovskite series Sr2FeMo1-xWxO6, where x = 0, 0.1, 0.2, 0.3 and 0.4, are investigated using x-ray diffraction, Moessbauer spectroscopy and magnetization measurements. The crystal structure has been studied at 450 K and at room temperature. There is a structural transition from tetragonal to cubic for x≤0.2. The Curie temperature (TC) shows a slight increase for x=0.1 and 0.2 (TC=435 and 432 K, respectively), compared to the value (TC 415 K) of the x=0 end member. The largest magnetoresistance at 300 K was found for the x=0 and 0.2 compounds. The concentration of antisite defects determined by Moessbauer spectroscopy and x-ray analysis indicate that: (i) cation ordering at the B sites improves with increasing W content and (ii) the antisite defects formed in the B sites of the double perovskite structure do not aggregate to form bigger intragrain clusters with different local chemical compositions. The x dependence of the magnetic moment, which exceeds 4μB per formula unit when x>0.3, is explained in a ferrimagnetic model with, respectively, antiparallel and parallel coupling of antisite iron and molybdenum to their iron or molybdenum cation neighbours. There is a small contribution of 0.2 μB/W to the ↓ sublattice moment. The W-containing materials are not half-metallic