Control of Fe valence state and magnetoresistance by means of T=Ta and W substitution in Sr2Fe(Mo1-xTx)O6 -: art. no. 184408

High-purity Sr2Fe(Mo1-xTx)O-6 samples with T=W, Ta and 0less than or equal toxless than or equal to1 were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier Fe-57 Mossbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of +2.5. [J. Linden Appl. Phys. Lett. 76 (2000) 2925.] W-VI substitution causes increasing amounts of Fe to enter the II state, whereas Ta-V substitution yields increasing amounts of Fe-III. Both substitution schemes lead to a decrease in the intensity of the component assigned to Fe2.5+. Nonsubstituted 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 Neel temperature of the pure Sr2FeWO6 and Sr2FeTaO6 phases, respectively. The enhancement appears to be related to the colossal magnetoresistance (CMR) effect at the paramagnetic to antiferromagnetic transitions in the areas rich in W or Ta. The transition and consequently the region of non-zero CMR effect are rather broad due to the glass-like behavior of the highly-substituted samples within the low-temperature region. Ta substitution had a stronger influence on the transport properties, magnetization and mixed valency than W substitution had. It is suggested that Ta-V disrupts the double-exchange interaction responsible for the magnetism in the Sr2FeMoO6 more efficiently than W-VI.