Perovskite Sr2FeMoO6 Research Articles

Half-metallic antiferromagnetic behavior of double perovskite Sr2OsMoO6: First principle calculations

Electronic structure calculations based on density functional theory within the generalized gradient approximation for double perovskite Sr2FeMoO6 and Sr2OsMoO6 have been performed using the accurate full potential augmented spherical wave method. By substituting Fe atoms by Os in the double perovskite structure oxides we have shown that it is possible to realize half-metallic antiferromagnets with 100% spin polarization of the conduction electrons crossing the Fermi level, without showing a net magnetization. To support our results, GGA+U electronic structure calculations have been performed showing that the half-metallic antiferromagnetic state still persists. We conclude that the origin of the antiferromagnetism in Sr2OsMoO6 may be attributed to both superexchange and generalized double exchange mechanisms via the B(3d,5d)–O(2p)–B′(4d) coupling.

The disorder is induced by S doping at O-sites in double perovskite Sr2FeMoO6 compound

We have investigated the crystal, structural and physical properties of polycrystalline Sr2FeMoO5.5S0.5. The structure of this compound is assigned to the tetragonal system with space group I4/mmm and the Rietveld refinement results show that there exist about 23% of anti-sites. Thermal magnetization demonstrates the Curie temperature to be higher than 300K. Thermal magnetization and isothermal magnetic measurements show a magnetic or structural phase transition at around 190K. The saturation moment obtained from Rietveld refinement results and isothermal magnetic curve are 2.21 and 2.04μB (78K), respectively. The evident magnetoresistance is observed under 0.5 and 1.0T magnetic fields in cooling and warming modes, respectively. Variation of the coefficients in T1/2, T2 and T3/2 terms follows the residual resistivity in both modes. The residual resistivity decreases with magnetic field. Under 0T magnetic fields the coefficient in T2 term is inversely proportional to spin-wave stiffness constant, D, in both modes.

Preparation by sol–gel and solid state reaction methods and properties investigation of double perovskite Sr2FeMoO6

Abstract Double perovskite Sr 2 FeMoO 6 was prepared by two ways consisting in sol–gel technique and solid-state reaction method. The resulting powders from gel and mixed oxides precursors showed microstructures consisting of very fine grains (0.5–0.8 μm) and a crystalline perovskite structure. The structural and microstructural properties of the double perovskite Sr 2 FeMoO 6 powders as-prepared and ceramics were compared. Tetragonal Sr 2 FeMoO 6 pellets were prepared from the two powders by spark plasma sintering at: 1000, 1100 and 1200 °C and then annealing at 1200 °C, 2 h in 5%H 2 /Ar. The pellets presented different magnetic characteristics. The saturation magnetization of the samples prepared by sol–gel is close to those prepared by conventional synthesis method.

Quantitative control of Fe/Mo anti-site defect and its effects on the properties of Sr2FeMoO6

Using a facile method, Fe/Mo anti-site defect (ASD) concentration in double perovskite Sr2FeMoO6 is quantitatively controlled within a wide range of 2%–21%. Our systematic investigation demonstrates the significant effects of ASD on the physical properties, such as magnetization, magnetoresistance and the Curie temperature.

Effect of high-pressure on the electronic and magnetic properties in double perovskite oxide Sr2FeMoO6

The high-pressure effect on the physical properties in double perovskite Sr2FeMoO6 is explored theoretically. The structure of Sr2FeMoO6 exhibits I4/mmm symmetry in the whole external hydrostatic pressure applied. A spin crossover of Fe ion from high-spin to low-spin state is found at the pressure of about 33–36 GPa, accompanied by a transition from ferrimagnetic half-metallic to nonmagnetic semiconductor state. When the pressure is above 45 GPa, the nonmagnetic compound behaves as full metal. By the mean-field theory, the estimated Curie temperature of the ferrimagnetic Sr2FeMoO6 increases from 472 to 639 K when the pressure increases from 0 to 30 GPa.

Absence of traditional magnetoresistivity mechanisms in Sr2FeMoO6 thin films grown on SrTiO3, MgO and NdGaO3 substrates

Magnetoresistive double perovskite Sr2FeMoO6 thin films were grown with two different deposition pressures on SrTiO3, MgO and NdGaO3 substrates by pulsed laser deposition and thorough structural, magnetic and magneto-transport characterization was made. According to x-ray diffraction, all the films were phase pure and fully textured. Indication of substrate dependent strain and low angle grain boundaries was found, especially in films on MgO. Both the deposition pressure and the choice of the substrate have a strong influence on the saturation magnetization, Ms, and Curie temperature, TC. The structural and magnetic data indicate the presence of anti-site disorder (ASD) in the films. The temperature dependence of resistivity showed semiconductive behaviour at temperatures below 100 K and metallic behaviour at higher temperatures. The semiconductive behaviour was found to increase with increasing ASD. In good quality films, up to 12% negative magnetoresistance (MR) was observed and films grown on MgO and NGO substrates also showed low field MR. However, the most significant observation of this study was that the magnetoresistivity of these Sr2FeMoO6 thin films could not be explained with any traditional MR mechanism, but carried the clear signature of superposition of different mechanisms, in particular low angle grain boundary tunnelling and suppression of antiferromagnetically ordered domains under a magnetic field.

The structural, electronic and magnetic properties of a symmetrical FeMoO terminated (001)-oriented slab of double perovskite Sr2FeMoO6

The structural, electronic and magnetic properties of a 9-layer symmetrical FeMoO terminated (001)-oriented slab of double perovskite Sr2FeMoO6 have been studied by using the first-principle projector augmented wave potential within the generalized gradient approximation taking into account the on-site Coulomb repulsive (U=2.0eV for Fe and 1.0eV for Mo). An outward relaxation is observed for each layer of the first four layers and the accompanying layer's rumpling has a decreasing trend from layer 1 to layer 4. Along Fe–O–Mo–O–Fe or Mo–O–Fe–O–Mo chain, the oxygen atom is closer to the adjacent Mo atom than to the adjacent Fe atom. The half-metallic nature ensures the symmetrical FeMoO terminated (001)-oriented slab of double perovskite Sr2FeMoO6 to be a potential application in spintronic devices. The Jahn–Teller structural distortions and the outward relaxations lead to the differences between pz state and similar px and py states, between dxy state and similar dyz and dxz states as well as between dz2 and dx2−y2. The reduction in the restrictions from the reduction in the number of the near neighbors, especially the vanishing of the nearest neighbor oxygen atom just above the transition-metal (TM) atoms on the first layer makes not only the remaining axial TM–O distances slightly shorter than the four equatorial TM–O distances but also shifts toward higher energy region the occupied up-spin and down-spin states with respect to the cases of the TM atoms on the third and fifth layers.

Surface spin-glass and exchange bias in Sr2FeMoO6 nanoparticle

Tunneling magnetoresistance in polycrystalline double perovskite Sr2FeMoO6 exhibits many unusual features, which can be efficiently probed by manipulating the tunnel barriers/grain surfaces. Accordingly, many experimental reports appeared on nanosized particles of Sr2FeMoO6 with largely enhanced grain boundary contributions. However, for the first time we report the existence of a spin-glasslike component, along with conventional ferromagnetism, in well-characterized Sr2FeMoO6 nanoparticles, which has been critically confirmed by the perceptible exchange bias effect, observed in these nanoparticles. Our results suggest that the spin-glass component is likely to reside on the surface of each particle, which probably provides useful clues about the unusual tunneling magnetoresistance responses, always exhibited by nanocrystalline Sr2FeMoO6.

High-pressure synthesis of the double perovskiteSr2FeMoO6: increment of the cationic ordering and enhanced magnetic properties

The double perovskite Sr2FeMoO6 has been prepared in polycrystalline form by high-pressure methods, starting from aprecursor developed via a citrate technique, containing an elevated degree ofanti-site disordering. The application of high external pressure (2 GPa) toSr2FeMoO6 promotes the long distance Fe/Mo cationic order, due to the smaller lattice volume of theordered sample. Both the disordered perovskite obtained at ambient pressure and thesample synthesized under high-pressure methods have been characterized by means of x-raydiffraction, neutron powder diffraction and magnetic measurements. The magneticproperties of the two oxides have been compared; the specimen prepared under highpressure not only presents an improved cationic ordering, but also displays a superiorsaturation magnetization and a sharpener ferromagnetic transition at a significantly hightemperature of 430 K.

Effect of La doping on the properties of Sr2−xLaxFeMoO6 double perovskite

We report on the carrier doping effect (achieved through La doping) on the electronic structure, crystalline structure, grain morphology, and electric and magnetic properties of the ferromagnetic perovskite Sr2FeMoO6. X-ray diffraction on Sr2−xLaxFeMoO6 (x=0, 0.25, 0.5, and 1) ceramic powders shows that all compounds have the tetragonal crystal structure with I4∕mmm symmetry. The cell parameter a and the volume of the tetragonal structure increase with the La doping and the grain size is halved with respect to that of undoped Sr2FeMoO6. Mössbauer spectroscopy shows that the presence of La reduces the degree of ordering on the Fe and Mo sites which causes a reduction of the saturation magnetization MS. However, the addition of electrons in the system results in an increase of the Curie temperature of the Sr2FeMoO6. All these results are supported by ab initio calculations which indicate that the half-metallic character is preserved upon doping and that the magnetization decreases for perfect samples by 1μB∕La atom in a formula unit.

Carrier doping dependence of the Tc in double perovskite Sr2FeMoO6

AbstractCarrier doping effects on the Curie temperature Tc of double perovskite Sr2FeMoO6 have been studied along the series polycrystalline Sr2–xAxFeMoO6 (A = Ba and K) samples. The partial substitution of K+ for Sr2+ considerably reduces the Tc from 399 K for x = 0 to 352 K for x = 0.2 with cell volume increasing. This decrease of Tc with K+ doping originates from carrier doping effects in addition to ionic size ones. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanocrystalline Sr2FeMoO6 prepared by citrate-gel method

Double perovskite Sr2FeMoO6 powders with small crystallite size have been synthesised with citrate-gel method. The starting solution pH was varied between 1.5 and 9.0 resulting in large differences in the phase composition and ordering of B′/B″ sites. The samples prepared at 975 °C had crystallite sizes under 40 nm whereas crystallite sizes of the samples prepared at 1050 °C were between 78 and 239 nm. The XRD patterns were refined with spacegroup I 4/m, which gave good results for both batches, although clearly better results were obtained with monoclinic P 21/n spacegroup for the 975 °C batch. The ordering and the saturation magnetization agreed well with each other after treatment at 1050 °C, but the samples prepared at 975 °C had a strongly reduced saturation magnetization from that given by the ordering.

Influence of doping Al at Fe site on the magnetic structure and magnetotransport properties of Sr2FeMoO6

Double perovskite Sr2FeMoO6 with Al doping at Fe site up to 30% were prepared by traditional solid-state reaction. The degree of the cation ordering of Sr2FeMoO6 increases with Al doping obviously as shown by the refinement of room-temperature X-ray diffraction. The saturated magnetic moment per formula unit of these compounds decreases gradually with increasing Al concentration, but the magnetic moment per Fe ion increases markedly, which is consistent with the improvement of cation ordering. Basing on the analysis of magnetic structure with Al doping, doping of Al will lead to the formation of non-magnetic coupled Mo—O—Al—O—Mo chains, which can divide the large Mo—O—Fe ferrimagnetic region into smaller patches and enhance the low field magnetoresistance. The formation of Fe—O—Fe antiphase boundaries are suppressed by the doping Al due to the improvement of the cation ordering, and the high field magnetoresistance that roots in spin-dependent scattering at the antiphase boundaries is reduced. Therefore, the total magnetoresistance reaches a maximum value at x=0.15.

Structural and magnetotransport features in new electron-doped Sr2−xCexFeMoO6double perovskites

A new series of double perovskites of stoichiometry Sr2−xCexFeMoO6 (0 ≤ x ≤ 1) has been prepared by a wet chemistry procedure yielding very reactive precursors, which were subsequently annealed under reducing conditions. The compounds have been studied by X-ray (XRD), neutron powder diffraction (NPD), Mossbauer spectroscopy, magnetic and magnetotransport measurements in polycrystalline samples, aiming to investigate the effect of electron doping in the parent perovskite Sr2FeMoO6. The evolution of the crystal structure has been analyzed from NPD data; a transition from tetragonal (I4/m) to monoclinic (P21/n) has been observed at 0.2 ≤ x ≤ 0.4 as a result of the decrease of the tolerance factor due to the electron injection on B sites and the reduction in ionic size at A positions of the ABO3 perovskite. A bond valence study shows that an important fraction of the injected electrons is localized at Mo sites, which promotes the occurrence of antisite disordering between Fe and Mo cations. The magnetic properties are characteristic of ferromagnetic materials, with an ordering temperature of around 400 K, showing a non-monotonic behavior along the series, which is understood as a compromise between the steric and the electron injection effects. The introduction of a magnetic rare-earth in the Sr sublattice improves the magnetoresistive response at low temperatures, as a result of the polarization of the Ce3+ moments: a magnetoresistance (MR) factor of 45% is observed at 5 K for the x = 0.6 sample. For the first time we show that the introduction of a magnetic element in the A sublattice of a double perovskite can be used as an effective way to enhance the magnetoresistive response in this attractive family of oxides.

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Thin films of ordered double perovskite Sr2FeMoO6 were sputter-deposited on Ba0.4Sr0.6TiO3-buffered SrTiO3 substrates, and their surface properties were examined by atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS). The Mo 3d spectra of Sr2FeMoO6 films showed a multiple peak structure, which originated from the unusual electronic state of metallic Sr2FeMoO6. Systematic XPS studies of the air-exposed surface of Sr2FeMoO6 film indicated that the surface native barrier was an insulating SrMoO4. In addition, a TMR junction with the native barrier was fabricated using a convention photolithographic technique, and an MR ratio of 10 % was observed at 4.2 K.

B-site ordering and magnetic behaviours in Ni-doped double perovskite Sr2FeMoO6

The crystal structure and magnetic properties of polycrystalline Sr2(Fe1−xNix)MoO6 (0.0 ⩽ x ⩽ 0.2) synthesized via a mechanical activation route have been investigated. Rietveld refinement of the x-ray diffraction data indicates that the long-range order parameter (S) among octahedral B sites in Sr2(Fe1−xNix)MoO6 is significantly enhanced by an increasing level of Ni doping. The B-site ordering results in a decrease in the lattice dimensions and an increase in lattice tetragonal distortion. Consequently, magnetization of the Ni-doped Sr2FeMoO6 is progressively improved by an increasing level of Ni doping. The Curie temperature (Tc) in Sr2(Fe1−xNix)MoO6 is also raised systematically by Ni doping, from Tc = 411 K for x = 0 to Tc = 432 K for x = 0.20. Magnetization and Curie temperature in Sr2(Fe1−xNix)MoO6 depend almost linearly on both the level of Ni doping and B-site long-range order parameter S.

Phase Formation and Magnetoresistance of Double Perovskite Sr2FeMoO6

Phase formation behaviors of double perovskite Sr2FeMoO6 (SFMO) derived from mechanical activation were investigated. Polycrystalline double perovskite SFMO of grain sizes in nanometer range were synthesized by heat treatment of the precursors derived from mechanical activation of SrO, Fe2O3, and MoO3 at temperatures in the range of 600°–900°C in flowing atmosphere of H2/Ar. Mechanical activation at room temperature led to formation of SFMO at 700°C, which is about 200°C lower than what is required in the conventional solid state reaction. The magnetization of thus‐derived SFMO increases with increasing heat‐treatment temperature in the range of 700°–900°C. Similarly, its magnetoresistance ratio also increases with increasing heat‐treatment temperature, which is accounted for by the elimination of insulating SrMoO4 impurity phase and enhancement in crystallinity of the double perovskite SFMO phase.

Synthesis and Characterization of Sr3FeMoO6.88: An Oxygen‐Deficient 2D Analogue of the Double Perovskite Sr2FeMoO6.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and Characterization of Sr3FeMoO6.88: An Oxygen-Deficient 2D Analogue of the Double Perovskite Sr2FeMoO6

We have prepared Sr3FeMoO6.88(1), an oxygen-deficient two-dimensional analogue of the double perovskite Sr2FeMoO6, and have investigated its structure and physical properties. According to powder X-ray and neutron diffraction data, Sr3FeMoO6.88 adopts the tetragonal n = 2 Ruddlesden−Popper structure (space group I4/mmm) with a completely disordered B-cation distribution and oxygen vacancies occurring preferentially at the axial O1 sites between the perovskite layers. X-ray absorption near-edge spectroscopy and Mossbauer data are consistent with trivalent Fe and mixed (4+/5+) Mo oxidation state. The compound is semiconducting, showing an intrinsic magnetic transition at ∼85 K and an additional upturn in the magnetic susceptibility at ∼270 K, the latter is associated with intergrowth defects. An appreciable magnetoresistance, reaching the value of ∼−15% at 5 K and 5 T is also observed

Novel method of synthesis for double-perovskite Sr2FEMoO6

Polycrystalline double perovskite Sr2FeMoO6 nanosized powders have been prepared using a wet chemical coprecipitation method of synthesis. The products were ignited in nitrogen at different temperatures in order to examine thermal evolution of precursors. Powders that were fired at an above 850°C for two hours have almost a single-phase structure. Some intermediate phases have been found at low temperatures. Powder X-ray diffraction linewith measurements show an average particle size in the order of 40 nm for samples annealed in 600–1000°C. It is suggested that the samples annealed at these temperatures were both simultaneously paramagnetic and ferromagnetic in nature. The advantages of this method are the low temperature involve in the sample preparation and the use of a non-reducing atmosphere.