Control of Fe valence state and magnetoresistance by means of (formula presented) and W substitution in (formula presented)

High-purity Ta and (formula presented) were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier (formula presented) Mössbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of (formula presented) [J. Lindén et al. Appl. Phys. Lett. 76 (2000) 2925.] (formula presented) substitution causes increasing amounts of Fe to enter the II state, whereas (formula presented) substitution yields increasing amounts of (formula presented) Both substitution schemes lead to a decrease in the intensity of the component assigned to (formula presented) Nonsubstituted samples exhibit a characteristic tunneling-type magnetoresistance below (formula presented) Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the Néel temperature of the pure (formula presented) and (formula presented) 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 (formula presented) disrupts the double-exchange interaction responsible for the magnetism in the (formula presented) more efficiently than (formula presented).