Abstract
High-purity Sr2Fe(Mo1−xTx)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 TC. 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.
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