In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by Fe57 Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.

Electronic, magnetic, and structural phase transitions in nearly stoichiometric TbBaFe 2 O 5ϩw (0.00Ͻw Ͻ0.05) have been investigated. At high temperatures this compound is a paramagnetic, mixed-valence (Fe 2.5ϩ ) conductor with identical square-pyramidal coordinations at all iron atoms. Upon cooling below T N ϭ450 K, an antiferromagnetic ͑AFM͒ spin order appears, accompanied by a magnetostrictive orthorhombic distortion. At lower temperatures the increasing distortion sets the frame for a first attempt to order charges. Mössbauer spectroscopy shows that one squeezed and one expanded square pyramid appear with different orientations of their magnetic and electric field tensors, each centered by its own mixed-valence iron state, one Fe 2.5ϩ⑀ , the other Fe 2.5Ϫ⑀ . The lattice retains its distortion, but a small, structurally homogeneous, and continuous increase in volume is experienced. At somewhat lower temperature (T V ) a discontinuous increase of the orthorhombic distortion occurs, marking the second attempt to order charges, now with the classical symptoms of the Verwey transition: a large change in volume, entropy, and electrical conductivity. Below T V , a normal Fe 3ϩ high-spin state in a symmetrical square-pyramidal coordination appears, whereas Fe 2ϩ is distorted. The long-range order of this arrangement is solved from high-resolution powder neutron diffraction data. Rietveld refinements show that the charge-ordered spins have AFM interactions in all three directions (G type͒ whereas in the mixed-valence state a ferromagnetic ͑FM͒ interaction appears between the iron atoms facing each other across the Tb layer. This FM interaction is suggested to be essential for the appearance of the mixed-valence state via the double-exchange sharing of the Fe 2ϩ -originated electron. This also allows for the total ordered spin moment being unchanged at the Verwey transition, following one single Brillouin curve. Analogous cases are pointed out where the Verwey transition proceeds in a similar manner, also at the molecular level.

A mixed-valence state, formally denoted as Fe 2.5ϩ , is observed in the 300 K Mössbauer spectra of the most reduced samples of SmBaFe 2 O 5ϩw . Upon cooling below the Verwey-type transition temperature (T V Ϸ200 K), the component assigned to Fe 2.5ϩ separates into a high-spin Fe 3ϩ state and an Fe 2ϩ state with an unusually low internal field. The separation of the mixed-valence state at T V is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.

No abstract

Polycrystalline Sr 2 FeMoO 6 samples are synthesized by an encapsulation technique under very low oxygen partial pressures utilizing the Fe/FeO redox couple as an oxygen getter. A route based on thermal decomposition of metal complexes, using ethylenediaminetetraacetic acid as the complexant, is developed to prepare a precursor powder. Single-phase samples are readily obtained even at temperatures as low as 900 °C. The degree of Fe/Mo ordering and the grain size are well-controlled by sintering temperature and time. Homogeneous morphology of the samples is confirmed from scanning electron microscopy images. Rietveld refinements of X-ray diffraction patterns indicate that the degree of order (S) at the Fe/Mo sites is higher than 0.95 for samples sintered at 1150 °C for 100 h. For the same sample a record-high saturation magnetization of 3.96 µ B , which is very close to the theoretical value of 4 µ B , is obtained. It is considered that the high degree of order stems from the uniform mixing of starting reactants in an atomic scale and also from the stability of overall cation stoichiometry during sample sintering in an encapsulated ampule. It is found that the Curie temperature exhibits a nonmonotonic dependence on S, with the maximum at S ≈ 0.84. Additionally, evidence for super-paramagnetic-type behavior in the present solution-derived samples is obtained from the Mo ¨ssbauer data.

No abstract

High-purity Sr 2 Fe(Mo 1Ϫx T x )O 6 samples with TϭW, Ta and 0рxр1 were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier 57 Fe Mössbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of ϩ2.5. ͓J. Lindén et al. Appl. Phys. Lett. 76 ͑2000͒ 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 2.5ϩ . 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 Sr 2 FeWO 6 and Sr 2 FeTaO 6 phases, respectively. The enhancement appears to be related to the colossal magnetoresistance ͑CMR͒ 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 Sr 2 FeMoO 6 more efficiently than W VI .

No abstract