High-purity Sr2Fe(Mo1-xTx)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.] WVI substitution causes increasing amounts of Fe to enter the II state, whereas TaV substitution yields increasing amounts of FeIII. 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 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 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 TaV disrupts the double-exchange interaction responsible for the magnetism in the Sr2FeMoO6 more efficiently than WVI.
|Julkaisu||Physical Review B - Condensed Matter and Materials Physics|
|Tila||Julkaistu - 1 marraskuuta 2002|
|OKM-julkaisutyyppi||A1 Julkaistu artikkeli, soviteltu|