Valence state of iron and molybdenum cations under conditions of anionic deficiency in Sr2FeMoO6–δ

The activation energy of oxygen diffusion in strontium ferromolybdate Sr2FeMoO6–δ was determined by the Merzhanov technique based on the temperature dependences of the oxygen desorption dynamics. It was found that the activation energy has a minimal value of 76.7 kJ/mol at δ = 0.005 and maximum value of 156.3 kJ/mol at δ = 0.06. It was suggested that, with an increase in the oxygen vacancies concentration, an interaction occurs between them and the nearest cations with the subsequent formation of associates of various types that are less mobile than the single anion vacancies. According to the Mössbauer spectroscopy data, it was established that the appearance of oxygen vacancies and their ordering contribute to the isomer shift, and some of the iron ions occupy the tetrahedral (or close to it) positions in the lattice. This indicates the formation of associates of oxygen vacancies. The results of XPS studies have shown that the increase in the concentration of oxygen vacancies results in a decrease of the Mo6+ and Fe2+ concentration. At the same time, the number of Mo5+ and Fe3+ cations increases due to the redistribution of the electron density, and molybdenum cations in a different valence state (Mo4+) appear.publishe

The role of the Fe/Mo cations ordering degree and oxygen non-stoichiometry on the formation of the crystalline and magnetic structure of Sr2FeMoO6‒δ

Single-phase Sr2FeMoO6-δ powders with various oxygen indices (δ) and degrees of the superstructural ordering (P) of the Fe/Mo cations were obtained from SrFeO2.52 and SrMoO4 reagents via solid-state synthesis. It has been established by means of the x-ray and neutron diffraction that, upon reducing the oxygen content and enhancing the superstructural ordering, the lengths of the Fe–O1 and Mo–O2 bonds in the crystal lattice increase, whereas the Fe–O2 and Mo–O1 bond lengths decrease. At the same time, the volume of the unit cell is reduced, which indicates an enhancement of the covalency degree of the bonds and stimulates a redistribution of the electron density, as well as an increase of the concentration of the spin-down charge carriers located in the conduction band on the Mo(t2g)↓ orbitals. This circumstance leads to an increase of the density of states at the Fermi level, accompanied by an amplification of the exchange interaction and elevation of the Curie temperature, which points to the leading role of the spin-polarized charge carriers at the Fermi level in the exchange interaction.publishe

Small-angle neutron scattering and magnetically heterogeneous state in Sr2FeMoO6–δ

Single-phase strontium ferromolybdate (Sr2FeMoO6–δ) samples with different degrees of the superstructural ordering of the Fe/Mo cations were obtained from partially reduced SrFeO3–х, SrMoO4 precursors by the solid-state technology. The study of the temperature dependences of the magnetization measured in the field-cooling and zero-field-cooling regimes indicated an inhomogeneous magnetic state of the samples. The presence of magnetic regions of different nature has also been revealed by the small-angle neutron scattering. For the Sr2FeMoO6–δ samples with different superstructural ordering of the Fe/Mo cations and for all values of the magnetic field induction in the range up to 1.5 T and of the scattering vector in the interval 0.1 >q >0.005 Å–1, the analytical dependence I ~ q–α obeys the Porod law (α ≈ 4), which corresponds to an object with a smooth and well-marked surface and polydisperse grain size. Deviations from the Porod law in the q > 0.1 Å–1 region and a weakening of the neutron scattering in applied magnetic fields may be ascribed to magnetic inhomogeneities with diameters D <6 nm, which are partially destroyed /oriented by magnetic fields В ≥1.5 T.publishe

Magnetization of Magnetically Inhomogeneous Sr₂FeMoO_6-<i>δ</i> Nanoparticles

In this work, we describe the magnetization of nanosized SFMO particles with a narrow size distribution around ca. 70 nm fabricated by the citrate-gel technique. The single-phase composition and superstructure ordering degree were proved by X-ray diffraction, the superparamagnetic behavior by magnetization measurements using zero-field cooled and field-cooled protocols, as well as by electron magnetic resonance. Different contributions to the magnetic anisotropy constant and the temperature dependence of the magnetocrystalline anisotropy are discussed

Tunneling conduction mechanisms in strontium ferromolybdate ceramics with strontium molybdate dielectric intergrain barriers

This work is a contribution to the understanding of the electrical resistivity in strontium ferromolybdate (Sr2FeMoO6-δ, SFMO) ceramics. It demonstrates that an appropriate thermal treatment leads to the formation of dielectric SrMoO4 shells at the surface of SFMO nanograins. In samples without SrMoO4 shells, the sign of the temperature coefficient of resistance changes with increasing temperature from negative at very low temperatures to positive at higher temperatures. Samples exhibiting a negative temperature coefficient contain SrMoO4 shells and demonstrate a behavior of the resistivity that can be described in terms of the fluctuation-induced tunneling model, and near room temperature the conductivity mechanism converts to a variable-range hopping one. The results of this work serve as a starting point for the understanding of the low-field magnetoresistance which is very promising for spintronic device application.publishe

Influence of synthesis conditions on microstructure and phase transformations of annealed Sr 2

The sequence of phase transformations during Sr2FeMoO6−x crystallization by the citrate–gel method was studied for powders synthesized with initial reagent solutions with pH values of 4, 6 and 9. Scanning electron microscopy revealed that the as-produced and annealed powders had the largest Sr2FeMoO6−x agglomerates with diameters in the range of 0.7–1.2 µm. The average grain size of the powders in the dispersion grows from 250 to 550 nm with increasing pH value. The X-ray diffraction analysis of the powders annealed at different temperatures between 770 and 1270 K showed that the composition of the initially formed Sr2FeMoO6−x changes and the molybdenum content increases with further heating. This leads to a change in the Sr2FeMoO6−x crystal lattice parameters and a contraction of the cell volume. An optimized synthesis procedure based on an initial solution of pH 4 allowed a single-phase Sr2FeMoO6−x compound to be obtained with a grain size in the range of 50–120 nm and a superstructural ordering of iron and molybdenum cations of 88%

Magnetoresistive effect in nanosized strontium ferromolybdate with dielectric interlayers

Monophasic iron ferromolybdate nanopowders with a double perovskite structure have been synthesized using the citrate-gel technique at pH=4. A superstructural ordering degree of the iron and molybdenum cations of 88% has been obtained. X-ray diffraction of pressed Sr2FeMoO6−δ pellets subjected to annealing at T=700 K and p(O2)=10 Pa has revealed the formation of the SrMoO4 phase at grain boundaries. The temperature dependence of the electrical resistivity in the range from 4.2 to 300 K switches from a metal type one in the monophasic Sr2FeMoO6-δ to a semiconductor type one in the Sr2FeMoO6−δ–SrMoO4–Sr2FeMoO6−δ structure containing dielectric interlayers, indicating variable range hopping in the latter. In the applied magnetic fields the temperature dependence does not change qualitatively; however, the resistivity decreases with increasing field, i.e., a negative magnetoresistance of up to 41% at T=10 K and B=8 T is observed. The external field forms a collinear spin structure, thus increasing the spin-polarized current through the granular Sr2FeMoO6−δ–SrMoO4–Sr2FeMoO6−δ heterostructure

Influence of synthesis conditions on microstructure and phase transformations of annealed Sr 2 FeMoO 6−x nanopowders formed by the citrate–gel method

The sequence of phase transformations during Sr 2 FeMoO 6−x crystallization by the citrate–gel method was studied for powders syn-thesized with initial reagent solutions with pH values of 4, 6 and 9. Scanning electron microscopy revealed that the as-produced and annealed powders had the largest Sr 2 FeMoO 6−x agglomerates with diameters in the range of 0.7–1.2 µm. The average grain size of the powders in the dispersion grows from 250 to 550 nm with increasing pH value. The X-ray diffraction analysis of the powders annealed at different temperatures between 770 and 1270 K showed that the composition of the initially formed Sr 2 FeMoO 6−x changes and the molybdenum content increases with further heating. This leads to a change in the Sr 2 FeMoO 6−x crystal lattice pa-rameters and a contraction of the cell volume. An optimized synthesis procedure based on an initial solution of pH 4 allowed a single-phase Sr 2 FeMoO 6−x compound to be obtained with a grain size in the range of 50–120 nm and a superstructural ordering of iron and molybdenum cations of 88%.info:eu-repo/semantics/publishe

Strontium ferromolybdate-based magnetic tunnel junctions

Thin-film strontium ferromolybdate is a promising material for applications in room-temperature magnetic tunnel junction devices. These are spin-based, low-power-consuming alternatives to CMOS in non-volatile memories, comparators, analog-to-digital converters, and magnetic sensors. In this work, we consider the main tasks to be solved when creating such devices based on strontium ferromolybdate: (i) selecting an appropriate tunnel barrier material, (ii) determining the role of the interface roughness and its quantification, (iii) determining the influence of the interface dead layer, (iv) establishing appropriate models of the tunnel magnetoresistance, and (v) promoting the low-field magnetoresistance in (111)-oriented thin films. We demonstrate that (i) barrier materials with a lower effective electronegativity than strontium ferromolybdate are beneficial, (ii) diminution of the magnetic offset field (the latter caused by magnetic coupling) requires a wavy surface rather than solely a surface with small roughness, (iii) the interface dead-layer thickness is of the order of 10 nm, (iv) the tunnel magnetoresistance deteriorates due to spin-independent tunneling and magnetically disordered interface layers, and (v) antiphase boundaries along the growth direction promote the negative low-field magnetoresistance by reducing charge carrier scattering in the absence of the field.publishe

Double Perovskite Sr2FeMoO6 Films Prepared by Electrophoretic Deposition

The present work reports on the new approach to create metal-supported Sr2FeMoO6 (SFMO)-based electrodes that have high potential to be applied in solid oxide fuel cells. The SEMO films were formed on stainless steel substrates by electrophoretic deposition (EPD) method. Ethyl alcohol with phosphate ester as a dispersant and isopropyl alcohol with I-2-acetone mixture as a charge additive were considered as an effective medium for EPD of SFMO particles. The synthesis of SFMO powder as well as suspension preparation and deposition kinetics were systematically studied. The effect of applied voltage on the thickness and morphology of SFMO films was established. The microstructure of the deposits was examined by electron microscopy. The thickness, morphology and porosity of the SEMO layers can be fine-tuned by varying solvent, charging additives, deposition time, and applied voltage. According to X-ray photoelectron spectroscopy analysis, it was found that Fe3+-Mo5+ and Fe2+-Mo6+ pairs coexist, whereas the valent balance shifts toward an Fe2+-Mo6+ configuration