Control of Fe valence state and magnetoresistance by means of T=Ta and W substitution in Sr₂Fe(Mo_1-xT_x)O₆

High-purity Sr₂Fe(Mo_1-xT_x)O₆ samples with T=W, Ta and 0≤×≤1 were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier ⁵⁷Fe Mössbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of +2.5. [J. Lindén et al. 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 Fe^2.5+. 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 Néel 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.