Control of Fe valence state and magnetoresistance by means ofT=Taand W substitution inSr2Fe(Mo1−xTx)O6

Abstract

High-purity ${\mathrm{Sr}}_{2}\mathrm{Fe}({\mathrm{Mo}}_{1\ensuremath{-}x}{T}_{x}){\mathrm{O}}_{6}$ samples with $T=\mathrm{W},$ Ta and $0l~xl~1$ were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier ${}^{57}\mathrm{Fe}$ M\ossbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of $+2.5.$ [J. Lind\'en et al. Appl. Phys. Lett. 76 (2000) 2925.] ${\mathrm{W}}^{\mathrm{VI}}$ substitution causes increasing amounts of Fe to enter the II state, whereas ${\mathrm{Ta}}^{\mathrm{V}}$ substitution yields increasing amounts of ${\mathrm{Fe}}^{\mathrm{III}}.$ Both substitution schemes lead to a decrease in the intensity of the component assigned to ${\mathrm{Fe}}^{2.5+}.$ Nonsubstituted samples exhibit a characteristic tunneling-type magnetoresistance below ${T}_{\mathrm{C}}.$ Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the N\'eel temperature of the pure ${\mathrm{Sr}}_{2}{\mathrm{FeWO}}_{6}$ and ${\mathrm{Sr}}_{2}\mathrm{Fe}{\mathrm{TaO}}_{6}$ 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 ${\mathrm{Ta}}^{\mathrm{V}}$ disrupts the double-exchange interaction responsible for the magnetism in the ${\mathrm{Sr}}_{2}\mathrm{Fe}{\mathrm{MoO}}_{6}$ more efficiently than ${\mathrm{W}}^{\mathrm{VI}}.$