Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Magnetization of La_0.66Ba_0.34MnO₃ and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0<H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.     

Impedance spectroscopy and conduction mechanism of multiferroic (Bi0.6K0.4)(Fe0.6Nb0.4)O3

Polycrystalline (Bi_0.6K_0.4) (Fe_0.6Nb_0.4)O₃ material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of P–E hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2P_r) of 1.027 µC/cm² and coercive field (2E_c) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2M_r) of 0.035 emu/gm and coercive field (2H_c) of 0.211 kOe.     

Magnetic properties and formation of Sr ferrite nanoparticle and Zn,Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σ_s and coercive field strength H_c are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σ_s=74 emu/g and coercive field strength H_c=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe₂O₃ and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn²⁺, Ti⁴⁺ and Ir⁴⁺ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)_x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)_x substituted phase.     

Magnetization anisotropy of the Tm- and Fe-subsystems in Tm2Fe17

Magnetic and neutron diffraction measurements were carried out in order to study the spontaneous and induced spin-reorientation (SR) transition of the “easy axis–easy plane” type in the poly and single-crystalline samples of the hexagonal Tm₂Fe₁₇. We have determined the temperature dependence of the lattice parameters and the angle between the c-axis and the magnetic moment of the Tm-subsystem. We also find that the SR transition is accompanied by a large (about 20%) magnetization change of the Tm subsystem. In order to induce such a SR transition with the external magnetic field, μ₀H_cr=5 T is necessary to be applied along the hard-magnetization direction (the a-axis) at 4.2 K. The H_cr value decreases with an increasing temperature. The magnetization measurements demonstrate that at 10 K the saturation magnetization along the easy-magnetization direction (the c-axis) is smaller than that along the hard-magnetization direction. Based on this observation, we believe that Fe-subsystem of Tm₂Fe₁₇ is likely to have magnetization anisotropy.     

Magnetic properties of iron nanoparticles in a polymer film

We systematically synthesized self-aggregated iron nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) by ion-exchange method. Our experimental results show that iron nanoparticles in MF-4SK exhibit superparamagnetic properties above the blocking temperature. Field-cooled and zero-field-cooled magnetization data show the blocking temperature, T_B≅120 K for the iron concentration of 5×10¹⁹ atoms per 1 g of polymer film at 500 Oe applied field. This result is well matched with the calculation based on the temperature dependence of the coercivity, which shows T_B≅110 K, with the zero temperature coercivity (H_C0) ≅ 420 Oe. The radius of the typical iron particle is determined to be ∼2 nm from transmission electron microscopy (TEM), showing good agreement with the value acquired by Langevin function fit. These experimental evidences suggest that iron nanoparticles in the polymer film obey a single-domain theory.     

FMR investigation of magnetic anisotropies in Fe/Au superlattices

Fe(xML)/Au(xML) superlattices (1⩽x⩽4, ML: monatomic layer thickness) have been investigated by the ferromagnetic resonance method at room temperature. It has been confirmed that out-of-plane anisotropy field H_u shows oscillatory behavior as a function of Fe layer thickness with a period of 1 ML as reported previously from magnetization measurements. In addition, we have found that the in-plane fourfold anisotropy field H₂ oscillates in a similar manner. The easy magnetization axis for x⩾2.25 is Fe [1 1 0] in contrast with the case of bulk Fe, and the values of H₂ show maxima for x=2.5 and 3.5, suggesting that the atomic steps at interfaces are formed along the Fe [1 1 0] direction. Furthermore, the interface roughness for x=non-integer causes wide distributions of H_u and H₂ compared to those for x=integer with flat interfaces.     

Particle size effects on magnetic properties of yttrium iron garnets prepared by a sol–gel method

We present detailed measurements of field—and temperature—dependence of magnetization in nanocrystalline YIG (Y₃Fe₅O₁₂) particles. The fine powders were prepared using sol–gel method. Samples with particle sizes ranging from 45 to 450 nm were obtained. We observe that the saturation magnetization decreases as the particle size is reduced due to enhancement of the surface spin effects. Below a critical diameter D_s≅190 nm, the particles become single domains and the coercive forces reaches a maximum at diameters close to the critical value. As the particle size decreases the coercivity diminishes and at D_p≃35 nm diameters the upper limit of superparamagnetic behavior is reached. A quantitative comparison of temperature and particle size dependence of coercivity shows a satisfactory agreement that is expected for an assembly of randomly oriented particles.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

On the rate constants of OH + HO2 and HO2 + HO2: A comprehensive study of H2O2 thermal decomposition using multi-species laser absorption

Hydrogen peroxide (H₂O₂) and hydroperoxy (HO₂) reactions present in the H₂O₂ thermal decomposition system are important in combustion kinetics. H₂O₂ thermal decomposition has been studied behind reflected shock waves using H₂O and OH diagnostics in previous studies (Hong et al. (2009) [9] and Hong et al. (2010) [6,8]) to determine the rate constants of two major reactions: H₂O₂ + M → 2OH + M (k₁) and OH + H₂O₂ → H₂O + HO₂ (k₂). With the addition of a third diagnostic for HO₂ at 227 nm, the H₂O₂ thermal decomposition system can be comprehensively characterized for the first time. Specifically, the rate constants of two remaining major reactions in the system, OH + HO₂ → H₂O + O₂ (k₃) and HO₂ + HO₂ → H₂O₂ + O₂ (k₄) can be determined with high-fidelity.No strong temperature dependency was found between 1072 and 1283 K for the rate constant of OH + HO₂ → H₂O + O₂, which can be expressed by the combination of two Arrhenius forms: k₃ = 7.0 × 10¹² exp(550/T) + 4.5 × 10¹⁴ exp(?5500/T) [cm³ mol^?1 s^?1]. The rate constants of reaction HO₂ + HO₂ → H₂O₂ + O₂ determined agree very well with those reported by Kappel et al. (2002) [5]; the recommendation therefore remains unchanged: k₄ = 1.0 × 10¹⁴ exp(?5556/T) + 1.9 × 10¹¹⁺exp(709/T) [cm³ mol^?1 s^?1]. All the tests were performed near 1.7 atm.     

Chemical diffusion of water in the double perovskites Ba4Ca2Nb2O11 and Sr6Ta2O11

The mechanism and kinetics of water incorporation in the double perovskites Ва₄Ca₂Nb₂O₁₁ and Sr₆Ta₂O₁₁ has been investigated (T = 300÷500 °C and aH₂O = 1 · 10^− 3÷2.2 · 10^− 2). The formation of hydration products Ba₄Ca₂Nb₂O₁₁·xH₂O and Sr₆Ta₂O₁₁·xH₂O (0.2 < x < 0.50) was limited by the diffusion of H₂O. It has been found that the concentration dependences of D˜_H2O are the same for both samples: small increasing of D˜_H2O with increasing x. The temperature dependences of the chemical diffusion coefficients of water for compositions of Ba₄Ca₂Nb₂O₁₁·0.35H₂O and Sr₆Ta₂O₁₁·0.35H₂O could be described with close activation energies of E_a = 0.38 ± 0.03 eV and E_a = 0.49 ± 0.03 eV, respectively. The chemical diffusion coefficients of water are nearly one order of magnitude smaller for tantalate Sr₆Ta₂O₁₁. This result correlates with lower oxygen and proton conductivities in Sr₆Ta₂O₁₁ as the consequence of lower mobilities.     

A quantitative explanation for the apparent anomalous temperature dependence of OH + HO2 = H2O + O2 through multi-scale modeling

Kinetic models for complex chemical mechanisms are comprised of tens to thousands of reactions with rate constants informed by data from a wide variety of sources – rate constant measurements, global combustion experiments, and theoretical kinetics calculations. In order to integrate information from distinct data types in a self-consistent manner, a framework for combustion model development is presented that encapsulates behavior across a wide range of chemically relevant scales from fundamental molecular interactions to global combustion phenomena. The resulting kinetic model consists of a set of theoretical kinetics parameters (with constrained uncertainties), which are related through kinetics calculations to temperature/pressure/bath-gas-dependent rate constants (with propagated uncertainties), which in turn are related through physical models to combustion behavior (with propagated uncertainties). Direct incorporation of theory in combustion model development is expected to yield more reliable extrapolation of limited data to conditions outside the validation set, which is particularly useful for extrapolating to engine-relevant conditions where relatively limited data are available. Several key features of the approach are demonstrated for the H₂O₂ decomposition mechanism, where a number of its constituent reactions continue to have large uncertainties in their temperature and pressure dependence despite their relevance to high-pressure, low-temperature combustion of a variety of fuels. Here, we use the approach to provide a quantitative explanation for the apparent anomalous temperature dependence of OH + HO₂ = H₂O + O₂ – in a manner consistent with experimental data from the entire temperature range and ab initio transition-state theory within their associated uncertainties. Interestingly, we do find a rate minimum near 1200 K, although the temperature dependence is substantially less pronounced than previously suggested.     

High temperature protonic conduction in Sr-doped LaP3O9

Electrical conduction of Sr-doped LaP₃O₉ ([Sr]/{[La] + [Sr]} = 2–10 mol%) was investigated under 0.4–5 kPa of p(H₂O) and 0.01–100 kPa of p(O₂) or 0.3–3 kPa of p(H₂) at 573–973 K. Sr-doped LaP₃O₉ showed apparent H/D isotope effect on conductivity regardless of the Sr-doping level under both H₂O/O₂ oxidizing and H₂/H₂O reducing conditions at investigated temperatures. Conductivities of the material were almost independent of p(O₂) and p(H₂O). These results demonstrated that the Sr-doped LaP₃O₉ exhibited protonic conduction under wide ranges of p(O₂), p(H₂O) and temperature. The conductivity of the Sr-doped LaP₃O₉ increased with increasing Sr concentration up to its solubility limit, ca. 3 mol%, while the further Sr-doping slightly degraded the conductivity. These indicate that Sr²⁺ substitution for La³⁺ leads to proton dissolution into the material and induced protonic conduction. Conductivities of the 3 mol% Sr-doped sample were 2 × 10^- 6–5 × 10^− 4 S cm^− 1 at 573–973 K.     

Microwave properties of DyBCO monodomain in the mixed state and comparison with other RE-BCO systems

We report on microwave measurements on DyBa₂Cu₃O_7?δ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance R_s(H) in the a–b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K–T_c with a static magnetic field μ₀H < 0.8 T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behavior of the sample under study. The magnetic field dependence of R_s(H) is ascribed to the dissipation caused by vortex motion. The analysis of X_s(H) points to a free-flow regime, thus allowing to obtain the vortex viscosity as a function of temperature. We compare the results with those obtained on RE-BCO systems. In particular, we consider strongly pinned films of YBa₂Cu₃O_7?δ with nanometric BaZrO₃ inclusions.     

Magnetic properties of ball-milled Mn nanoparticles

Nanoparticles of Mn of sizes  < 500 Å were prepared by the ball-milling technique. The temperature dependence of the magnetic susceptibility χ showed systematic variation with particle size. Peaks observed in χ were attributed to the magnetic ordering of the oxides Mn₃O₄and MnO. Peaks found in ∂(χT) / ∂T were associated with the Neel temperature ofα -Mn. We estimated that our samples contain about 0.4% of Mn₃O₄. This low concentration of Mn₃O₄was not detected by X-ray diffraction experiments but contributed significantly to the magnetization measurements.     

Defect structure analysis of B-site doped perovskite-type proton conducting oxide BaCeO3 Part 1: The defect concentration of BaCe0.9M0.1O3−δ (M = Y and Yb)

By means of a high-temperature gravimetry, the defect chemical relationships between oxygen nonstoichiometry and water content in BaCe_0.9M_0.1O_3?δ (M = Y and Yb) were investigated as functions of partial pressure of oxygen, P(O₂), partial pressure of water vapor, P(H₂O), and temperature. Concentrations of protonic defect and that of oxygen vacancy strongly depend on P(H₂O) and temperature, while the dependences on P(O₂) were weak. The equilibrium constants of the water vapor incorporation reaction H₂O + V_O^??? + O_O^× = 2OH_O^? were determined. Concentrations of hole, [h^?], in the dry-atmospheres were determined by the weight gain by the incorporation of oxygen from the gas atmospheres. The [h^?] values increased with decreasing temperature. The [h^?] values were estimated to be about 2 to 3 orders of magnitude less than [OH_O^?] values measured in the wet-atmospheres.     

Yttrium doped La1−xYxO0.9F0.1FeAs superconductors: Hall and thermopower studies

The effect of yttrium substitution at the lanthanum site on the superconducting properties of La_1?xY_xO_0.9F_0.1FeAs (‘x’ = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y₂O₃ is observed. The temperature dependence of resistivity measurements confirm the superconducting transition temperature (T_c) of 34.8 (±0.05) K and corresponding Meissner transition at 34.3 K in the ‘x’ = 0.3 composition which is higher than that reported for the parent phase (LaO_0.9F_0.1FeAs (T_c = 28 K)). Further increase in the concentration of yttrium leads to broadening and suppression of the superconducting transition. The value of H_c2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient (S) shows a negative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient (R_H) also remains negative throughout the temperature range supporting the thermopower results. The lattice parameters (a and c) decreases and the charge-carrier density increases with yttrium doping.     

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In1−xFex)2O3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In₂O₃ samples (In_1−xFe_x)₂O₃ (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In₂O₃ lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In₂O₃ sample. The highest remanence magnetization moment (6.624 × 10⁻⁴ emu/g) is reached in the sample with x = 0.03.     

Effects of doping with nanoscale Co3O4 particles on the superconducting properties of powder-in-tube processed MgB2 tapes

Nanoscale Co₃O₄ particles were doped into MgB₂ tapes with the aim of developing superconducting wires with high-current-carrying capacity. Fe-sheathed MgB₂ tapes with a mono-core were prepared using the in situ powder-in-tube (PIT) process with the addition of 0.2–1.0 mol% Co₃O₄. The critical temperature decreased monotonically with an increasing amount of doped Co₃O₄ particles for all heat-treatment temperatures from 600 to 900 °C. However, the transport critical current density (J_c) at 4.2 K varied with the heat-treatment temperatures. The J_c values in magnetic fields ranging from 7 to 12 T decreased monotonically with increasing Co₃O₄ doping level for a heat-treatment temperature of 600 °C. In contrast, some improvements on the J_c values of the Co₃O₄ doped tapes were observed in the magnetic fields below 10 T for 700 and 800 °C. Furthermore, J_c values in all the fields measured increased as the Co₃O₄ doping level increase from 0 to 1 mol% for 900 °C. This heat-treatment temperature dependence of the J_c values could be explained in terms of the heat-treatment temperature dependence of the irreversibility field with Co₃O₄ doping.     

Optical,electrical and magnetic properties of nanostructured Mn3O4 synthesized through a facile chemical route

Nanostructured Mn₃O₄ sample with an average crystallite size of ∼15 nm is synthesized via the reduction of potassium permanganate using hydrazine. The average particle size obtained from the Transmission Electron Microscopy analysis is in good agreement with the average crystallite size estimated from X-ray diffraction analysis. The presence of Mn⁴⁺ ions at the octahedral sites is inferred from the results of Raman, UV–visible absorption and X-ray photoelectron spectroscopy analyzes. DC electrical conductivity of the sample in the temperature range 313–423 K, is about five orders of magnitude larger than that reported for single crystalline Mn₃O₄ sample. The dominant conduction mechanism is identified to be of the polaronic hopping of holes between cations in the octahedral sites. The zero field cooled and field cooled magnetization of the sample is studied in the range 20–300 K. The Curie temperature for the sample is about 45 K, below which the sample is ferrimagnetic. A blocking temperature of 35 K is observed in the field cooled curve. It is observed that the sample shows hysteresis at temperatures below the Curie temperature with no saturation, even at an applied field (20 kOe). The presence of an ordered core and disordered surface of spin arrangements is observed from the magnetization studies. Above the Curie temperature, the sample shows linear dependence of magnetization on applied field with no hysteresis characteristic of paramagnetic phase.     

Superconductivity and thermal properties of sulphur doped FeTe with effect of oxygen post annealing

Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe_1?xS_x; (x = 0–30 %) system and study the impact of low temperature oxygen (O₂) annealing as well. Rietveld analysis of room temperature X-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity C_p(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe_0.75S_0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe_0.75S_0.25 sample. The upper critical fields H_c2(10%), H_c2(50%) and H_c2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 °C) O₂ annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [C_p(T)] measurements for FeTe_1?xS:O₂ system are presented and discussed.