Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

The valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ has been investigated by an analysis of the Co-L₃ X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L₃ XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.     

Structural and electronic properties of zinc blende BxAl1−xNyP1−y quaternary alloys via ?rst-principle calculations

The structural and electronic properties of cubic zinc blende BN, BP, AlN and AlP compounds and their B_xAl_1−xN_yP_1−y quaternary alloys, have been calculated using the non relativistic full-potential linearized-augmented plane wave FP-LAPW method. The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW) as well as the generalized gradient approximation (GGA) of Perdew-Burke and Ernzerhof (GGA-PBE). The calculated structural properties of BN, BP, AlN and AlP compounds are in good agreement with the available experimental and theoretical data. A nonlinear variation of compositions x and y with the lattice constants, bulk modulus, direct and indirect band gaps is found. The calculated bowing of the fundamental band gaps is in good agreement with the available experimental and theoretical value. To our knowledge this is the first quantitative theoretical investigation on B_xAl_1−xN_yP_1−y quaternary alloy and still awaits experimental confirmations.     

Electrical conductivity and structural stability of SrCo1−xFexO3−δ

Perovskite compounds in the system of SrCo_1−xFe_xO_3−δ (x=0.2, 0.4 and 0.6) were synthesized by solid state reaction. SrCo_1−xFe_xO_3−δ shows the p-type small polaron conduction behavior. Electrical conductivity and oxygen vacancy content decrease with increase in Fe content. The incorporation of Fe increases the structural stability of SrCo_1−xFe_xO_3−δ at low temperatures, while decreasing the structural stability at high temperatures. Oxygen partial pressure has a strong influence on electrical conductivity. At low oxygen partial pressure, SrCo_0.8Fe_0.2O_3−δ will transform from cubic to orthorhombic structure. This structure can remain in 5%H₂/Ar only for a short time and then dissociates into Sr₃Fe₂O_6.64 and Co due to the reduction of B-site elements.     

Structure and magnetism in Ho1−xSrxCoO3−δ

We have magnetically and structurally characterized the Ho_1−xSr_xCoO_3−δ family of materials where 0.67≤x≤0.95. The solid solution range and evolution of the structure as a function of x is established and correlated with the broad range of magnetic behavior observed. The structure is shown to be tetragonal I4/mmm although is possibly cubic when x=0.95. For 0.67≤x≤0.9 the material shows antiferromagnetic long range order and ferromagnetic clusters. At x=0.95 the magnetic transition is at 120 K and the imaginary susceptibility becomes non-zero and the temperature of the cusp in the ac susceptibility shows a frequency dependence indicative of glassiness.     

Studies on sintering of solid state synthesized Ba1−xKxBiO3−δ superconducting phase

Superconducting Ba_1−xK_xBiO_3−δ pellets were synthesized by solid state reaction followed by sintering. Thermo-gravimetric and differential thermo-gravimetric analysis (TG-DTA) of the mixture of nitrates was carried out to study the reactions during the phase formation. The effect of different sintering temperatures on the phase formation was studied. The X-ray diffraction data confirms the formation of superconducting Ba_0.6K_0.4BiO_2.23 phase at 700 °C. The surface morphological studies as a function of sintering temperature were studied by SEM. It is observed that the pellets prepared by solid state reaction followed its sintering at 700°C shows the superconducting transition at 26.8 K.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.     

High temperature transport and thermoelectric properties of Ca3−xErxCo4O9+δ

Ca_3−xEr_xCo₄O_9+δ (x=0, 0.05, 0.15, 0.3 and 0.5) samples were prepared using a sol-gel method followed by hot-pressing sintering technique. Powder X-ray diffraction analysis showed the single-phases of Ca_3−xEr_xCo₄O_9+δ were obtained up to x=0.3 and the crystallinity of the samples decreased with increasing Er-doping amount. The high-temperature (323-1073 K) thermoelectric properties of the samples were investigated. The substitution of Er³⁺ for Ca²⁺ resulted in the decrease of electrical conductivity, thermal conductivity and the increase of Seebeck coefficient for all the samples except the x=0.05 one. The dimensionless figure of merit ZT reached 0.28 at 1073 K for Ca_2.7Er_0.3Co₄O_9+δ, indicating the thermoelectric properties of CaCo₄O_9+δ can be enhanced by doping Er in the system.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.     

Role of growth temperature and oxygen partial pressure on the structural and electrical properties of pulsed laser deposited La1−xSrxnO3−δ thin films

The paper presents the fabrication and characterization of La_0.65Sr_0.35MnO_3−δ (LSMO) polycrystalline thin films deposited directly on Si (1 0 0) substrates using pulsed laser deposition technique. Various deposition parameters like substrate temperature and oxygen partial pressure have been varied systematically to obtain stoichiometric, crack-free films with smooth surface morphology having nearly monodisperse grain size distribution. The substrate temperature variation from 600 to 800 °C had profound effects on the microstructure and topography of the deposited film, with optimum result being obtained at 700 °C. The variation of partial pressure of oxygen controls the deposition kinetics as well as the stoichiometry of the film in terms of oxygen vacancy, which influences the magnetic and electrical transport properties of the manganate films. The microstructure and crystallinity of the deposited films have been studied using X-ray diffraction, scanning electron microscopy and atomic force microscopy. A correlation between the oxygen stoichiometry and micro-structural and transport properties of the deposited films has been obtained.     

Structural and magnetic properties in the self-doped perovskite manganites with nominal composition La0.7Sr0.3−xMnO3−δ

Perovskite manganites with nominal composition La_0.7Sr_0.3−xMnO_3−δ (0.00≤x≤0.20) have been prepared by the sol-gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3?c perovskite structure, while for x>0.10, the samples have two phases with the R3?c perovskite being the dominant phase and Mn₃O₄ being the second phase. A quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that on the basis of the thermal equilibrium theory of crystal defects there are Mn²⁺ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn²⁺ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites.     

Multi-band nature of superconducting anisotropy in PrFeAsO1−δ single crystal

We investigated the magnetic torque of superconducting PrFeAsO_1−δ single crystal using the torque magnetometer at temperatures from 2 K to 200 K in magnetic field of 30 kG. The torque curve tends to have a singular shape compared to other superconductors. The analyses of the PrFeAsO_1−δ torque curves by the single-band and the multi-band Kogan model yield 3 < γ_λ < 5 in 30 kG. With the aid of the multi-band Kogan model, however, we obtain γ_ξ = 0.79 ± 0.01 and γ_λ = 19 ± 3 in 30 kG at 20.5 K. The smallness of γ_ξ(<1) thus obtained is due to the multi-band nature of the iron pnictide.     

Effect of Ar+ irradiation on the electrical conductivity of BaCe0.9Y0.1O3−δ

The effects of 10 keV Ar⁺ ion irradiation on the electrical characteristics of BaCe_0.9Y_0.1O_2.95 subject to fluences of 0, 1.0 × 10¹⁷, 5.0 × 10¹⁷ and 1.0 × 10¹⁸ ions/cm² at room temperature, has been investigated using elastic recoil detection analysis (ERDA), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and alternating current (AC) impedance measurements. It was confirmed from the ERDA results that the hydrogen concentration near the surface increased with increase of Ar⁺ ion fluence. This increase may be associated with the increasing quantities of hydrogen generated by interaction between oxygen vacancies, formed by irradiation, and H₂O from exposure to air. SEM images showed clearly that the number of surface defects due to modification increased with increasing fluence. In addition, the size of the defects showed a tendency to increase with increasing fluence. From the results of XPS analyses, providing information on the electronic states on the surface, it was evident that with increase in the Ar⁺ ion fluence, the quantity of excess oxygen, such as hydroxide, increased in the oxygen 1s XPS spectrum. In addition, it was indirectly found, from decomposition of the Ce 3d, spectrum that the concentration of oxygen vacancies increased with fluence, since the percentage of Ce³⁺ also increased. Accordingly, the surface modification led to the formation of more oxygen vacancies and a greater hydrogen concentration on the surface, since the H₂O interacted with some of them. From the results of the DC conductivity and AC impedance measurements, the proton conductivity was shown to predominate over the temperature range from 473 K to 823 K. It was concluded that the increase in these protons and vacancies generated from surface modification contributed to the increase of proton conductivity.     

Synthesis and magnetic properties of Co1−xZnxFe2O4+γ nanoparticles as materials for magnetic fluid hyperthermia

Nanoparticles of the single spinel phase Co_1−xZn_xFe₂O_4+γ of mean size 3-23 nm, as determined by X-ray diffraction analysis, were synthesized by the co-precipitation method followed by a temperature treatment. Magnetic studies carried out in the range of 4.5-550 K revealed gradual transition from ferrimagnetic to superparamagnetic to paramagnetic behaviour depending on the composition and particle size. The observed behaviour indicates a broad distribution of volume sizes of the nanoparticles. Particular importance can be ascribed to the composition of x=0.6 where the observed transition temperature to the paramagnetic state at 310-334 K suggests applicability of this material for magnetic fluid hyperthermia in a self-controlled regime.     

Room temperature magnetocaloric effect of La-deficient bulk perovskite manganite La0.7MnO3−δ

Room temperature magnetocaloric effect in La-deficient bulk perovskite manganite La_0.7MnO_3−δ prepared by conventional solid-state reaction has been reported. The maximum value of the magnetic entropy change (about−1.32 J/kg K) and the refrigerant capacity (approximately close to 37 J/kg) had been obtained at 290 K corresponding to a magnetic field variation of 1 T for La_0.7MnO_3−δ. It is the strong Jahn-Teller coupling that changes Mn-O bond length and Mn-O-Mn bond angles and then the canted spin arrangement and induces the strong double-exchange coupling to a comparatively high magnetic transition temperature. This Curie temperature near room temperature with easy fabrication and higher chemical stability makes La_0.7MnO_3−δ a potential candidate as a working substance in magnetic refrigeration technology.     

Non-universal dielectric relaxation in SrFeO3 − δ

SrFeO_3 − δ compound is prepared by the thermal decomposition method followed by ball milling. Analysis of Mössbauer spectrum and X-ray diffraction study proves the presence of multi-phase nature, i.e., Sr₈Fe₈O₂₃ and Sr₄Fe₄O₁₁ phases at room temperature. Furthermore, the Mössbauer spectrum at room temperature evidenced the presence of major Fe^3.5+ which is the resultant of equal contributions of Fe⁴⁺ and Fe³⁺. The Nyquist plot at all measured temperatures (80–230 K) suggests that the dielectric response is well associated with single relaxation time (exponential parameter, n∼1$n\sim 1$) i.e., the Debye-type. Modulus analysis exhibits the non-universal dielectric behaviour (stretched exponential parameter, β>1$\beta >1$) below 230 K and the Debye-type responds (β∼1$\beta \sim 1$) at and above 230 K. The Debye-type behaviour exhibited by SrFeO_2.81 at around room temperature in its defect state offers a new opening for this material for multifunctional applications.     

Fluorination of perovskite-related phases of composition La1−xSrxFe1−yCoyO3−δ

We report here on the fluorination of the perovskite-related phases La_1−xSr_xFe_1−yCo_yO_3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe³⁺ ions, which are not possible in oxides containing Fe⁴⁺.     

Influence of the substrate material on the properties of pulsed laser deposited thin Li1+xMn2O4−δ films

Thin Li_1+xMn₂O_4−δ films were deposited on several substrate materials (stainless steel, p-doped silicon and glassy carbon) by pulsed laser deposition. To obtain the correct thin film stoichiometries, targets with a different amount of excess lithium were required (Li_1.03Mn₂O₄ + xLi₂O; x = 2.5 and 7.5 mol%). The resulting polycrystalline thin films were characterized with respect to their morphology and electrochemical activity. It was found that only thin Li_1+xMn₂O_4−δ films deposited on stainless steel and glassy carbon showed the typical insertion and deinsertion peaks of Li⁺ during cycling.     

High-temperature magnetic properties of noninteracting randomly oriented single-domain Fe3 − δO4 nanoparticles

Magnetic measurements have been performed on 40-nm sphere-like Fe_3 − δO₄ (δ=0.043) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe_3 − δO₄ nanoparticles with SiO₂ effectively eliminates magnetic interparticle interactions so that the coercive field HC in the high-temperature range between 300 K and the Curie temperature (855 K) can be well fitted by an expression for noninteracting randomly oriented single-domain particles. From the fitting parameters, the effective anisotropy constant K is found to be (1.38±0.11)×¹⁰⁵ erg/cm³, which is very close to the bulk magnetocrystalline anisotropy constant of 1.35×¹⁰⁵ erg/cm³. Moreover, the inferred mean particle diameter from the fitting parameters is in quantitative agreement with that determined from transmission electron microscope. Such a quantitative agreement between data and theory suggests that the ensemble of our SiO₂-coated sphere-like Fe_3 − δO₄ nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles.