J. Lindén

Evidence for valence fluctuation of Fe in Sr2FeMoO6−w double perovskite

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 57Fe Mossbauer spectroscopy. From 5 K to room temperature the Mossbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.

Magnetoresistance effect in the fluctuating-valence BaSmFe2O5+w system

The occurrence of negative magnetoresistance (MR) in semiconductive BaSmFe2O5+w double-perovskite samples is demonstrated. A peak in the MR value was observed at the Verwey–type transition temperature. The transition signifies the charge separation of the Fe2.5+ fluctuating mixed valence state into high-spin Fe2+ and Fe3+. The samples were ferrimagnetic with a Curie temperature of ∼710 K. Upon oxidizing/reducing the samples the size of the MR peak and the temperature at which the peak occurred varied. The largest MR value observed was 1.4% at 7 T.

Large low-field magnetoresistance effect in Sr2FeMoO6 homocomposites

Homocomposites consisting of two single-phase Sr2FeMoO6 components with different grain sizes were prepared by a sol-gel method. Large low-field magnetoresistance (LFMR) effect was achieved for the composites. Experimental results show that the LFMR strongly depends on both the relative amounts of the two components and their grain sizes. The magnetoresistance value is found to be proportional to the square of the relative magnetization (M∕Ms). We suggest that the LFMR enhancement in the homocomposites compared with parent Sr2FeMoO6 has its origin in the enhanced intergranular effects.

Coexistence of intrinsic and extrinsic magnetoresistance in the double-perovskite Sr2Fe(Mo1−xWx)O6−w system

In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6−w system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1−xWx)O6−w samples with 0⩽x⩽1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN≈50 K for x⩾0.6. In samples with 0.7⩽x⩽0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Neel temperature. The peak was found to have its origin in the colossal magnetoresistance effect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMoO6−w. The coexistence of the two types of mag...

Iron mixed-valence compounds, BaSm(Cu0.5+xFe0.5−x)2O5+δ: I: Synthesis and chemical characterization

Oxygen-deficient BaSm(Cu 0.5+x Fe 0.5-x ) 2 O 5+δ (x = -0.5, -0.45, -0.4, -0.1 and 0.0) double-perovskite samples have been successfully obtained by means of a novel synthesis technique utilizing an Fe/FeO getter encapsulated together with the starting material mixture in an evacuated silica-glass tube. The reducing conditions during the synthesis correspond to an oxygen partial pressure of 7.6 × 10 -16 atm. From X-ray diffraction (XRD), transmission electron micrographs (TEM) and energy-dispersive X-ray (EDX) analyses, the samples have been confirmed to be of essentially single phase. By post-annealing the as-synthesized samples under O 2 or H 2 /Ar mixed gas, the amount of excess oxygen was controlled. Approaching δ ≃ 0 required prolonged annealing periods under reducing conditions. Samples having -0.4 < x < -0.1 did not form as a single-phase material under the present synthesis conditions.

Effect of LiF addition on the formation of the superconducting YBa2Cu4O8 phase

Several possible flux materials have been investigated for preparing the superconducting YBa2Cu4O8 phase under 1 atm of oxygen. LiF was a promising flux and the YBa2Cu4O8 phase was successfully obtained at 815 °C with 0.5–1.0 wt.% LiF. Large amounts of LiF (>1.0 wt.%) accelerated the formation of side products, i.e. BaCuO2 and Y2BaCuO5, instead of the superconducting phase. LiF was also used as a flux in the syntheses of the superconducting Y0.9Ca0.1Ba2Cu4O8 and EuBa2Cu4O8 phases, and synthesis conditions for these compounds were optimized.

Hole-doping effect on the Verwey-type transition and magnetoresistivity of Ba(Sm,Ca)Fe2O5+δ

Abstract Two series of samples of the oxygen-deficient double-perovskite phase, Ba(Sm,Ca)Fe 2 O 5+ δ were synthesized employing a sample encapsulation technique that utilizes Fe metal as an oxygen getter. The hole-doping level on the FeO 2 plane was controlled by varying the amount of excess oxygen or the Ca concentration at the Sm site. Earlier it had been found that at room temperature five-coordinated Fe 2+ and Fe 3+ species form pairs by sharing a d electron, leading to the formation of a Fe 2.5+ fluctuating valence state [Phys. Rev. B 60 (1999) 15,251]. At T V , a Verwey-type transition occurred, signifying the charge separation of the Fe 2.5+ fluctuating valence state into high-spin Fe 2+ and Fe 3+ . Moreover, related to the Verwey-type transition a negative magnetoresistance peak with a magnitude of a few percent was observed [Appl. Phys. Lett. 77 (2000) 1683]. The position of the peak was found to correspond to the jumps seen in the susceptibility and resistivity vs. temperature curves. For the present samples, it is found that the value of T V is severely decreased upon increasing the Ca concentration, as seen by susceptibility and magnetoresistivity measurements. Introduction of excess oxygen leads to a less severe decrease of the transition temperature. The observed behavior of T V was reproduced by a simple model based on combinatorial-entropy calculations.

Magnetic properties, oxygen content and metal valences in BaRE(Cu0.5Fe0.5) 2O5+δ with RE = Lu, Yb, Y, Eu, Sm, Nd and Pr

Abstract Oxygen-deficient double-perovskite samples of BaRE(Cu 0.5 Fe 0.5 ) 2 O 5+ δ (RE=Lu, Yb, Y, Eu, Sm, Nd and Pr; 0≤ δ ≤0.45) have been characterized for their magnetic properties. For the samples with RE=Lu–Sm, the oxygen content could be raised above δ ≈0 only by means of high-pressure heat treatment. In the case of the larger rare earth elements, i.e. Nd and Pr, excess oxygen was easily introduced under normal pressure as well. The (Cu 0.5 Fe 0.5 )O 2 planes were found to exhibit antiferromagnetism at or below room temperature and weak spontaneous magnetization below ∼80 K. The Neel temperature decreases with increasing amount of excess oxygen. Based on the charge-neutrality principle and the 57 Fe Mossbauer data, the average valences of Fe and Cu were deduced. The ratio of six-coordinated Fe 3+ to Fe 5+ depends on both the choice of RE and the sample preparation process. The excess oxygen introduced into the RE layer upon the high-pressure heat treatment increases not only the valence of Fe but also the valence of Cu. The behavior of an observed susceptibility anomaly around 80 K was studied upon varying δ .

Iron mixed-valence compounds, BaSm(Cu0.5+xFe0.5−x)2O5+δ: II: The Fe2.5+ state and magnetic properties

Recently, a fluctuating mixed-valence state formally denoted as Fe 2.5+ was observed in BaSmFe 2 O 5+δ double-perovskite samples, having δ < 0.5 [J. Linden, P. Karen, A. Kjekshus, J. Miettinen, T. Pietari, M. Karppinen, Phys. Rev. B 60 (1999) 15251.]. Upon cooling such samples to the Verwey-type transition temperature (T V 200 K), a mixing-separation transition of the valence state occurs. In order to study in detail the formation of the mixed-valence state BaSm(Cu 0.5+x Fe 0.5-x ) 2 O 5+δ , samples with a certain amount of Fe (0-10% and 40-50%) replaced by Cu have been characterized by 57 Fe Mossbauer spectroscopy and magnetization measurements. All samples were found to be magnetically ordered at room temperature. The amount of Fe entering the Fe 2.5+ state decreased rapidly as the Cu doping level was increased, as judged by the Mossbauer spectra. Increasing the Cu concentration also led to an increase of T V .

Observation of Mixed-Valence State in the BaSm(Cu1-xFex)2O5+δ Double-Perovskite Phase

A set of Fe-based, double-perovskite solid solutions BaSm(Cu1-x Fe x )2O5+δ (x= 1.0, 0.95, 0.90 and 0.50) was synthesized. The samples were characterized by 57Fe Mossbauer spectroscopy at 77 and 300 K. All samples were magnetically ordered at 300 K. In reduced samples, a single component, formally denoted as Fe2.5+, dominated the spectra. Upon oxidizing or doping the samples with Cu the intensity of this component decreases dramatically. The overall amounts of di-, tri-, and mixed-valent Fe corresponded well to the values of x and δ. Below the Verwey-type transition temperature (T V ≈ 200 K) the mixed-valence state separates into pure high- spin Fe3+ and Fe2+, as verified by Mossbauer spectroscopy. Resistivity measurements showed that the samples are semiconducting at 300 K. Upon decreasing temperature, the resistivity increases exponentially, but at T V there is a change in activation energy.