Influence of W/Ta substitution at the Mo site on the Fe valence and magnetoresistive properties of Sr₂FeMoO₆

High-purity Sr₂Fe(Mo_1-xT_x)O₆ samples with T = W, Ta and 0≤x≤1 were synthesized using a solid-state reaction scheme. During synthesis a strongly reducing atmosphere was maintained by confining the precursors and an Fe/FeO oxygen getter to fused-silica tubes. Sample quality and homogeneity were confirmed by X-ray diffraction and energy-dispersive X-ray analysis. For the non-substituted samples, ⁵⁷Fe Mössbauer spectra indicate that the Fe ions occupy a mixed-valence state of +2.5. W⁶⁺ forces increasing amounts of Fe to enter the +2 state, whereas Ta⁵⁺ substitution yields increasing amounts of Fe³⁺. Non-substituted samples exhibit a characteristic tunneling-type magnetoresistance below T_C. Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the Néel temperature of the pure Sr₂FeWO₆ and Sr₂FeTaO₆ phases, respectively. The enhancement is related to the colossal magnetoresistance effect at the paramagnetic to antiferromagnetic transitions of areas rich in W or Ta.