The effect of pressure on the spin density wave transition in the layered cobaltites [Ca2CoO3]0.62[CoO2] and [Ca2Co4/3Cu2/3O4]0.62[CoO2]

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ⁺SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca₂CoO₃]_0.62[CoO₂] and [Ca₂Co_4/3Cu_2/3O₄]_0.62[CoO₂]. A transverse field μ⁺SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca₂CoO₃] and/or [Ca₂Co_4/3Cu_2/3O₄]) but in the CoO₂ plane.     

Anomalous magnetoresistance in [ Sr2Bi2-xPbxO4] RS[ CoO2] y (x = 0, 0.3, and 0.4; y ≈ 1.85) single crystals

Magnetoresistance (MR) of the [ Sr₂Bi_2-xPb_xO₄] ^RS[ CoO₂] _y (x = 0, 0.3, and 0.4; y ≈ 1.85; RS is referred to rocksalt block) single crystals is investigated systematically. A nonmonotonic variation of the isothermal in-plane and out-of-plane MR with the field is observed. The out-of-plane MR is positive at high temperatures and increases with decreasing T, and exhibits a pronounced hump, then changes the sign from positive to negative at a certain temperature. These results strongly suggest that the observed MR consists of two contributions: one negative and one positive component. The isothermal MR in high magnetic fields follows a H² law. The negative contribution comes from spin scattering of carriers by localized-magnetic-moments based on the Khosla-Fischer model.     

Influence of the Ca:Pb ratio on the superconducting properties of Bi-based ceramics of type 2223

From a study of the influence of the Ca: Pb ratio on the superconducting properties of Bibased ceramics of 2223 type it follows that as a second parameter the employed gas atmosphere must be taken into consideration. In case of air preparation good intragrain and intergrain properties are obtained for Ca rich samples (e.g. Bi_1.8Pb_0.2Sr_1.9Ca_2.1 Cu₃O_z); whereas for Pb rich materials (e.g. Bi_1.7Pb_0.5Sr_1.9Ca_1.9 Cu₃O_z) improved conditions are encountered by employment of an Ar/O₂ (8%) atmosphere during synthesis. Independently, the transition temperature is situated near 108 K.     

Effect of Sm,Co codoping on the dielectric and magnetoelectric properties of BiFeO3 polycrystalline ceramics

Multiferroic Bi_0.95Sm_0.05Fe_1−xCo_xO₃ (x=0−0.1) ceramics were prepared by the rapid liquid phase sintering method. For all the samples studied, the dielectric constant and dielectric loss decrease with increasing frequency in the range from 1 kHz to 1 MHz. It shows that the dielectric constant of Bi_0.95Sm_0.05FeO₃ at 10 kHz is about forty times larger than that of pure BiFeO₃. This dramatic change in the dielectric properties of Bi_0.95Sm_0.05Fe_1−xCo_xO₃ (x=0−0.1) samples can be understood in terms of the space charge limited conduction associated with crystal defects, which was indicated by the increase of magnetoelectric effect with doping Co³⁺ under applied magnetic field from 1 to 8 kOe. It was believed that the ferroelectric polarization enhancement comes from the exchange interaction between the Sm³⁺ and Fe³⁺ or Co³⁺ ions for Bi_0.95Sm_0.05Fe_0.95Co_0.05O₃ at room temperature.     

Spin state transition in Ca-doped Na0.7CoO2 with the nominal Co valence below 3.16

In order to clarify the nature of the transition around 300 K in Ca-doped Na_0.7CoO₂, the magnetism of Na_0.7Ca_yCoO₂ with y=0.035 and 0.07 was investigated in a positive muon spin rotation and relaxation (μ⁺SR) experiment. Transverse field μ⁺SR measurements showed that the spin state of the Co ions in Na_0.7Ca_0.07CoO₂ changes at around 300 K; i.e. the exponential relaxation rate vs. T curve exhibited a large maximum around 300 K with an accompanying small peak in the muonic Knight shift, whereas no changes were found in the asymmetry, similar to [Ca₂CoO₃]_0.62[CoO₂] at around 400 K.Although the spin-density-wave (SDW) state exists for Na_xCoO₂ with x≥0.75 at low temperatures, zero-field μ⁺SR spectra in the Ca-doped samples showed no muon spin precessions down to 1.8 K but only fast relaxations indicating disorder. This is probably because the Ca²⁺ ions in the Na planes alter the charge and/or spin distribution in the CoO₂ planes. As a result, the SDW order is hindered, as the nominal Co valence is decreased below 3.16.     

Anionic substitutions in Bi-2223 superconductor

Samples of nominal stoichiometry Bi_1.6Pb_0.4Sr₂Ca_2.5Cu_3.5O_x−zF_z and Bi_1.6Pb_0.4Sr₂Ca_2.5Cu_3.5O_x−z(SO₄)_z (z=0, 0.3, 0.6 and 0.9) were prepared by standard solid state reaction from appropriate oxides, carbonates, copper (II) fluoride, and copper (II) sulphate. Superconducting properties of the samples were characterized by measuring of temperature dependency of their electrical resistivity and critical current density, their phase composition was determined by X-ray diffraction (XRD) and element composition by X-ray fluorescence (XRF). While greater amount of fluorine widens the superconducting transition and worsens transport properties, the less doped sample shows increase in critical current density. Sulphur-doped samples show no superconducting transition at all.     

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.     

High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution

We have prepared polycrystalline Ca_3−xEu_xCo₄O_9+δ (x=0, 0.15, 0.3 and 0.45) samples using a sol-gel process followed by SPS sintering and investigated the Eu substitution effects on their high-temperature thermoelectric properties. With the Eu substitution, both the electrical resistivity and thermopower increase monotonously. This could be attributed to the decrease of hole concentrations by substitution of trivalent Eu³⁺ for divalent Ca²⁺. The Eu substituted samples (x=0.15, x=0.3) have lower thermal conductivity than Ca₃Co₄O_9+δ due to their lower electronic and lattice thermal conductivity. The dimensionless figure of merit ZT reaches 0.3 at 1000 K for the sample of Ca_2.7Eu_0.3Co₄O_9+δ.     

Effect of doping by low content of yttrium at Ca and Sr sites of Bi(Pb)-2212 superconducting ceramics

In this work, a comparative study of the effect of doping by low content of Y³⁺ between (Ca site) and out of (Sr site) the CuO₂ planes of Bi(Pb)-2212 phase is presented. Ceramics of Bi_1.6Pb_0.4Sr₂Ca_1−xY_xCu₂O_8+d (called CY series) and Bi_1.6Pb_0.4Sr_2−xY_xCaCu₂O_8+d (called SY series) with x=0, 0.025, 0.05, 0.075 and 0.1 are elaborated in air by conventional solid state reaction. They are characterized by X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM), density, Vickers microhardness and resistivity measurements. The refinement of cell parameters is done by considering the structural modulation. In comparison with the undoped sample (x=0), the cell parameters a, b and c are reduced by the doping for both series while the b component of the modulation vector increases. A good correlation between the variations of the bulk density and the Vickers microhardness with x is obtained. For both series, the SEM analysis shows that the doped samples exhibit a reduced grain size than that of the undoped one. The variation of resistivity with temperature shows that all samples exhibit a metallic-like character in the normal state. For all doping levels, the CY series presents higher onset critical transition temperature than that of the undoped sample, which is equal to 85.43 K. The opposite is obtained for SY series. The highest value of this temperature is obtained for x=0.075 in the doped samples and is about 92.15 and 79.96 K for CY and SY series, respectively. These values may correspond to a near optimally doped state since the slope (dρ/dT) of resistivity shows a maximum at the same value of x=0.075. For both series, when Y³⁺ is introduced into the samples, the residual resistivity decreases first for x=0.025 and increases gradually after this value until x=0.1.     

Magnetic and charge ordering properties of Bi0.6−xEuxCa0.4MnO3 (0.0≤x≤0.6)

We have studied structure, magnetic and transport properties of polycrystalline Bi_0.6−xEu_xCa_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) perovskite manganites. Magnetic measurements show that the charge-ordering temperature (T_CO) decreases with increasing x up to x=0.4 and then slightly increases with further increasing x up to x=0.6. Further, the antiferromagnetic (AFM) ordering temperature (T_N) decreases with increasing x. At T<T_N a transition to metamagnetic glass like state is also seen. Eu doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.2 and then an increase in resistivity up to x=0.5. We propose that the local lattice distortion induced by the size mismatch between the A-site cations and 6s² character of Bi³⁺ lone pair electron are responsible for the observed variation in physical properties.     

Increase in the diamagnetic response from low-density Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag composites

Low-density polycrystalline Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors with a foamlike microstructure and composites consisting of this superconductor and silver in an amount of 20, 25, and 30 vol % are synthesized. The microstructure, as well as the temperature and field dependences of the magnetization, M(T) and M(H), are studied. It is found that, in Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors and Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x + Ag composites, the diamagnetic response is enhanced and the screening properties are improved compared with high-temperature polycrystalline superconductors with the same composition that are prepared by the standard technology. The observed effect is explained by the features of magnetic flux penetration into a porous medium.     

Magnetic effects in Co3−x O4 defect films from the data of emission Mössbauer spectroscopy

The emission Mössbauer spectra of Co_3?x O₄ defect films are measured in external magnetic fields at strengths of 0.6–3.5 T. It is shown that the memory effect is observed in the spectra of Co_3?x O₄ defect films after exposure to an external magnetic field. At temperatures above the Néel temperature T _N=26 K, the memory effect manifests itself in an increase in the relative contribution from the spectral line of the tetrahedral A sublattice in the spinel structure of Co_3?x O₄ crystallites ([Co _0.83 ²⁺ ]_tetr[Co _2.20 ³⁺ ]_octO₄ and [Co _0.95 ²⁺ ]_tetr[Co _2.10 ³⁺ ]_octO₄ prior to and after the magnetic field treatment, respectively). The isomer shifts δ and the quadrupole splitting ΔE of the spectral lines for both A and B sublattices also change from δ_A=?0.19 mm/s, δ_B=?0.31 mm/s, and ΔE _B =0.83 mm/s to the values δ_A=?0.24 mm/s, δ_B=?0.33 mm/s, and ΔE _B =0.60 mm/s, which are close to δ and ΔE for stoichiometric Co₃O₄ oxide. In the low-temperature spectra (T<T _N), the memory effect additionally shows itself as a decrease in the hyperfine magnetic field H _hf of the spectral component for the A sublattice as compared to that in the spectra measured prior to the magnetic field treatment. It is assumed that the concentration of cation vacancies decreases (and, correspondingly, the fraction of reduced Co²⁺ cations increases) in the Co_3?x O₄ defect films under the action of an external magnetic field. A possible mechanism of this process is proposed.     

Synthesis, structure and luminescence properties of Y(V,P)O4:Eu,Bi phosphors

YVO₄:Eu³⁺-based red-emitting phosphors with the compositions of Y_0.95−xVO₄:0.05Eu³⁺,xBi³⁺ (x=0.01, 0.03, 0.05, 0.07 and 0.09) and Y_0.90(V_1−zP_z)O₄:0.05Eu³⁺,0.05Bi³⁺ (z=0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) were synthesized by the high temperature solid-state method. The as-prepared phosphors have the similar tetragonal phase structure and their morphologies varied with the relative content ratio of V to P. The photoluminescence spectra for the as-synthesized phosphors show that a dominant red emission line at around 619 nm, which is due to the Eu³⁺ electric dipole transition of ⁵D₀-⁷F₂, is observed under different excitation wavelengths (254 and 365 nm). Further, the emission intensities of ⁵D₀-⁷F₂ transition upon 365 nm excitation increase sharply owing to the Bi³⁺ doping. Energy transfer process, luminescent lifetime and quantum efficiency for the selected Y_0.90(V_1−yP_y)O₄:0.05Eu³⁺,xBi³⁺phosphors were also studied in detail.     

Crystal structure and magnetic properties of Mn substituted ludwigite Co3O2BO3

The needle shape single crystals Co_3−x Mn_xO₂BO₃ with ludwigite structure have been prepared. According to the X-ray diffraction data the preferable character of distinct crystallographic positions occupation by Mn ions is established. Magnetization field and temperature dependencies are measured. Paramagnetic Curie temperature value Θ=−100 K points out the predominance of antiferromagnetic interactions. Spin-glass magnetic ordering takes the onset at T_N=41 K. The crystallographic and magnetic properties of Co₃O₂BO₃:Mn are compared with the same for the isostructural analogs Co₃O₂BO₃ and CoO₂BO₃:Fe.     

A novel UV-emitting phosphor: Li6CaB3O8.5:Pb

Pure Li₆CaB₃O_8.5 and Li₆Ca_1−xPb_xB₃O_8.5 (0.005≤x≤0.04) materials were prepared by a solution combustion synthesis method. The phase of synthesized materials was determined using the powder XRD and FTIR. The synthesized materials were investigated using spectrofluorometer at room temperature. The emission and excitation bands of the synthesized phosphors were observed at 307 and 268 nm, respectively. The dependence of the emission intensity on the Pb²⁺ concentration for the Li₆Ca_1−xPb_xB₃O_8.5 (0.005≤x≤0.04) was studied and observed that the optimum concentration of Pb²⁺ in phosphor is 0.01 mol. The Stokes shift of the synthesized phosphor was calculated to be 4740 cm^-1.     

Effects of double filling of Pb and Ba on thermoelectric properties of PbxBayCo4Sb11.5Te0.5 compounds by HPHT

Skutterudite compounds Pb_xBa_yCo₄Sb_11.5Te_0.5 (x≤0.23,y≤0.27) with bcc crystal structure have been prepared by the high pressure and high temperature (HPHT) method. The study explored a chemical method for filling Pb and Ba atoms into the voids of CoSb₃ to optimize the thermoelectric figure of merit ZT in the system of Pb_yBa_xCo₄Sb_11.5Te_0.5. The structure of Pb_xBa_yCo₄Sb_11.5Te_0.5 skutterudites was evaluated by means of X-ray diffraction. The Seebeck coefficient, electrical resistivity and power factor were performed from room temperature to 710 K. Compared with Co₄Sb_11.5Te_0.5, the thermal conductivity of Pb and Ba double-filled samples was reduced evidently. Among all filled samples, Pb_0.03Ba_0.27Co₄Sb_11.5Te_0.5 showed the highest power factor of 31.64 μW cm⁻¹ K⁻² at 663 K. Pb_0.05Ba_0.25Co₄Sb_11.5Te_0.5 showed the lowest thermal conductivity of 2.73 W m⁻¹ K⁻¹ at 663 K, and its maximum ZT value reached 0.63 at 673 K.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

Effects of In-substitution on the microstructure and magnetic properties of Bi-CVG ferrite with low temperature sintering

[Y_1.05Bi_0.75Ca_1.2](Fe_4.4−xIn_xV_0.6)O₁₂(In_x:Bi-CVG) ferrite material has been prepared successfully by a solid-state reaction method. The effects of In³⁺ substitution and sintering temperatures on the bulk density, microstructure and magnetic properties are performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), materials automatic test system (MATS) and microwave ferrite parameters meter. The results show that In³⁺ can lower the sintering temperatures and enhance the magnetic properties of Bi-CVG ferrite. Besides, all sintered specimens with different In³⁺ contents show a single garnet crystal structure. The specimen of [Y_1.05Bi_0.75Ca_1.2](Fe₄In_0.4V_0.6)O₁₂ sintered at 1075 °C shows homogenous distribution of grain size and densified microstructures. The ferromagnetic resonance linewidth (ΔH) has an increase with In³⁺ contents. Additionally, the sample has the optimum magnetic properties: ρ=5.23 g/cm³, B_r=31.3 mT, H_c=378.8 A/m, 4πM_s=506.2×10⁻⁴ T.     

Thermoelectric properties of the Bi2−xYxRu2O7 (x=0-2) pyrochlores

Electrical conductivity and Seebeck coefficient for the Bi_2−xY_xRu₂O₇ pyrochlores with x=0.0,0.5,1.0,1.5,2.0 were measured in the temperature range of 473-1073 K in air. With increasing Bi content, the temperature dependence of the electrical conductivity changed from semiconducting to metallic. The signs of the Seebeck coefficient were positive in the measured temperature range for all the samples, indicating that the major carriers were holes. The temperature dependence of the Seebeck coefficient for the Y₂Ru₂O₇ indicated the thermal activation-type behavior of the holes, while that for the Bi_2−xY_xRu₂O₇ with x=0.0-1.5 indicated the itinerant behavior of the holes. The change in the conduction behavior from semiconductor to metal with increasing Bi content is consistent with the increase in the overlap between the Ru4d t_2g and O2p orbitals, but the mixing of Bi6s, 6p states at E_F may not be ruled out. The thermoelectric power factors for the Bi_2−xY_xRu₂O₇ with x=1.5 and 2.0 were lower than 10⁻⁵ W m⁻¹ K⁻² and those with x=0.0,0.5,1.0 were around 1-3×10⁻⁵ W m⁻¹ K⁻².     

Nonlinear dielectric properties of amorphous Bi1.8Pb0.3Sr2Ca2Cu3−x KxOz (x = 0.2–0.3)

The dielectric constant of amorphous Bi_1.8Pb_0.3Sr₂Ca₂Cu_3?x K_xO_x (x = 0.2–0.3) is measured as a function of the static electric field strength. It is found that, as the field strength increases, the dielectric constant decreases following a quadratic law. This behavior of the dielectric constant in a static electric field indicates nonlinear dielectric properties, which can be described within the thermodynamic theory of ferroelectricity.