Investigation on Electrical and Magnetic Properties of Gd-doped BiFeO<Subscript>3</Subscript>

Multiferroic ceramic samples of Bi_1−x Gd _x FeO₃ (x=0, 0.05, 0.1 and 0.15) have been prepared by rapid liquid-phase sintering technique. The effect of Gd substitution on ferroelectric and magnetic properties of Bi_1−x Gd _x FeO₃ ceramics has been investigated. The results of X-ray diffraction (XRD) patterns show that the single-phase BiFeO₃ sample has a rhombohedral structure and Gd³⁺ substitution for Bi³⁺ has not affected its structure. Experimental results suggest that for Bi_1−x Gd _x FeO₃ system, the ferroelectric and magnetic properties of BiFeO₃ are improved by Gd doping and the loop area increases with the Gd content. When x=0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr=1.62 μC/cm², which is about 578.6% higher than that of BiFeO₃.     

Effect of Nb substitution on the structural, dielectric and magnetic properties of multiferroic BiFe1−xNbxO3 ceramics

Polycrystalline BiFe_1−xNb_xO₃ ceramics have been synthesized by standard solid-state reaction method. The effect of Nb substitution on the dielectric, magnetic and magnetoelectric properties of the BiFeO₃ multiferroic perovskite was studied. X-ray diffraction pattern revealed that all the samples with x = 0.00-0.10 showed rhombohedral perovskite structure. We obtained single phase upto doping concentration of x = 0.05 and with further increase in Nb concentration, some impurity peaks appeared. An anomaly in the dielectric constant (?) and dielectric loss (tan (δ)) in the vicinity of the antiferromagnetic Néel temperature (T_N) was observed. Nb substitution reduced the antiferromagnetic Néel temperature (T_N) in BiFe_1−xNb_xO₃. Proper amount of Nb could decrease the dielectric loss. Magnetic hysteresis loops measured at 5 K/300 K and temperature dependent magnetization curves indicated ferromagnetism in Nb substituted BiFeO₃ ceramics. The room temperature magnetic moment was found to increase with increase in Nb concentration. The dependence of dielectric constant on the magnetic field is an evidence of magnetoelectric coupling in BiFe_1−xNb_xO₃ ceramics.     

Effects of Gd doping on the sintering and microwave dielectric properties of BiNbO<Subscript>4</Subscript> ceramics

Gd³⁺ was chosen as a substitute for Bi³⁺ in BiNbO₄ ceramics, and the substitution effects on the sintering performance and microwave dielectric properties were studied in this paper. The high temperature triclinic phase was observed only in the Bi_0.98Gd_0.02NbO₄ ceramics when sintered at 920 °C. Both bulk densities and dielectric constant (ε_r) increased with the sintering temperature, while decreased with the Gd content. The quality factor (Q) exhibited a correlation to the Gd content and the microstructures of Bi_1−x Gd _x NbO₄ ceramics. At the sintering temperature of 900 °C, Bi_0.992Gd_0.008NbO₄ ceramics exhibited microwave dielectric properties of ε_r ∼ 43.87, Q × f ∼ 16,852 GHz (at 4.3 GHz), and its temperature coefficient of resonant frequency (τ_f) was found to be near-to-zero.     

Fabrication and characterization of compositionally graded Bi1−x Gd x FeO3 thin films

An undoped BiFeO₃ thin film, Gd doped Bi_0.95Gd_0.05FeO₃ thin film with a constant composition, Gd up-graded doped Bi_1?x Gd _x FeO₃ and Gd down-graded doped Bi_1?x Gd _x FeO₃ thin films were successfully grown on Pt (111)/Ti/SiO₂/Si (100) substrates using a sol-gel and spin coating technique. The crystal structure, ferroelectric and dielectric characteristics as well as the leakage currents of these samples were thoroughly investigated. The XRD (X-Ray Diffraction) patterns indicate that all these thin films consist of solely perovskite phase with polycrystalline structure. No other secondary phases have been detected. Clear polarization-electric field (P-E) hysteresis loops of all these thin films demonstrate that the incorporation of Gd³⁺ into the Bi site of BFO thin film have enhanced the ferroelectric performance of pure BiFeO₃ thin film, and the Gd down-graded doped Bi_1?x Gd _x FeO₃ thin film has the best ferroelectric properties. Compared to other thin films, the optimal ferroelectric behavior of the Gd down-graded doped Bi_1?x Gd _x FeO₃ thin film results from its large dielectric constant, low dissipation factor and low leakage current.     

Relaxor behavior and ferroelectric properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-K<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-KNbO<Subscript>3</Subscript> lead-free ceramics

Lead-free ferroelectric ceramics of (1−x) [0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃]-x KNbO₃(x = 0, 0.02, 0.04, and 0.06) were prepared by the conventional ceramic fabrication technique. The crystal structure, dielectric properties and P-E hysteresis loops were investigated. XRD data showed that all compositions could form pure perovskite structure. Temperature dependence of dielectric constant ε _r and dissipation factor tanδ measurement between room temperature and 500^∘C revealed that the compounds experience phase transitions that from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric in the range of x = 0–0.04. The frequency dependent dielectric constant showed these compounds were relaxor ferroelectric. At low frequency and high temperature, dielectric constant and dissipation factor increased sharply attributed to the superparaelectric clusters after the KNbO₃ doped.     

Microstructures and thermoelectric properties of p-type pseudo-binary AgxBi0.5Sb1.5−xTe3 (x = 0.05–0.4) alloys prepared by cold pressing

The p-type pseudo-binary Ag_xBi_0.5Sb_1.5−xTe₃ (x = 0.05–0.4) alloys were prepared by cold pressing. The thermal conductivities (κ) were calculated from the values of heat capacities, densities and thermal diffusivities measured, and range approximately from 0.66 to 0.56 (W K^− 1 m^− 1) for the Ag_xBi_0.5Sb_1.5−xTe₃ alloy with molar fraction x being 0.4. Combining with the electrical properties obtained in the previous study, the maximum dimensionless figure of merit ZT of 1.1 was obtained at the temperature of 558 K.     

The structural and electrical property of CuCr1 − xNixO2 delafossite compounds

A series of CuCr_1 − xNi_xO₂ (0 ≤ x ≤ 0.06) polycrystalline samples was prepared. The electrical conductivity was measured in the temperature range of 160–300 K. It was found that the electrical conductivity (σ) increases rapidly with the doping of Ni²⁺ ions. At room temperature, the σ is 0.047 S cm^− 1 for the sample with x = 0.06, which is two orders of magnitude larger than that of the CuCrO₂ sample (9.49E− 4 S cm^− 1). The Seebeck coefficients are positive for all samples, which indicate p-type conducting of the samples. The experimental results imply that it is possible to get higher electrical conductivity p-type transparent conducting oxides (TCO) from CuMO₂ by doping with divalent ions.     

Microwave dielectric and elastic properties of glass-added Ba6−3xR8+2xTi18O54 (x = 2/3)

Microwave dielectric properties of Ba_6−3xSm_8+2xTi₁₈O₅₄ (x = 2/3) [BST] ceramics with the addition of 0–3 wt.% of various glasses have been studied. It has been found that the addition of 0.5 wt.% of the glasses decreases the sintering temperature by about 150 °C. In general, addition of 0.5 wt.% of Zn, Mg and Pb-based glasses deteriorate the quality factor, whereas aluminum and barium borosilicates do not decrease it considerably. The quality factor and dielectric constant decrease with increasing amount of glass. The temperature coefficient of resonant frequency shifts towards positive or negative depending on the composition of the glass. A glass–ceramic composite with a dielectric constant 64, Q × f nearly 8500 GHz and near to zero τ_f could be obtained at a sintering temperature of 1175 °C when 3–4 wt.% Al₂O₃–B₂O₃–SiO₂ glass was added to BST ceramic. The Young's modulus decreases with increasing amount of glass, irrespective of the composition of glass.     

Magnetism in Si1 − xMnx diluted magnetic semiconductor thin films

We have studied the electrical and magnetic properties of p-type semiconductor thin films of Si_1 − xMn_x/Si (x = 0.036 and 0.05) grown by molecular beam epitaxy. Experimental results reveal that the resistivity of the samples decreases gradually with increasing measurement temperature, which can be described well by Mott's variable-range-hopping model. All the samples exhibit the ferromagnetic ordering above room temperature. Among these samples, Si_0.95Mn_0.05 has a higher hole density and magnetization. This indicates an enhancement of hole-mediated ferromagnetic exchange interactions when the Mn-doping concentration is increased.     

Thermal expansion properties of Ln2−xCrxMo3O12 (Ln = Er and Y)

Structures and thermal expansion properties of Ln_2−xCr_xMo₃O₁₂ (Ln = Er and Y) have been investigated by X-ray powder diffraction. Rietveld analysis results of Ln_2−xCr_xMo₃O₁₂ indicate that compounds Er_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.3) and Y_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.2) crystallize in orthorhombic structure and exhibit negative thermal expansion, while both monoclinic and orthorhombic compounds Er_2−xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) and Y_2−xCr_xMo₃O₁₂ (1.8 ≤ x ≤ 2.0) possess positive coefficient of thermal expansion. The coefficients of linear thermal expansion of orthorhombic Ln_2−xCr_xMo₃O₁₂ change from negative to positive with increasing chromium content. Thermogravimetric and differential scanning calorimetry have been used to study the hygroscopicity and the phase transition temperature.     

Composition dependence of optical constants of Ge1 − xSe2Pbx thin films

Optical constants of vacuum-evaporated thin films in the Ge_1 − xSe₂Pb_x (x = 0, 0.2, 0.4, 0.6) system were calculated from reflectance and transmittance spectra. It is found that the films exhibit a non-direct gap, which decreases with increasing Pb content. The variation in the refractive index and the imaginary part of the dielectric constant with photon energy is reported. The relationship between the optical gap and chemical composition in chalcogenide glasses is discussed in terms of the average heat of atomization.     

Multiferroic properties of ErMnO3

Measurements of magnetic and dielectric properties of single crystalline ErMnO₃ establish the Néel and ferroelectric transition temperatures to be 77 K and 588 K respectively. The dielectric constant of ErMnO₃ shows an anomalous jump at T_N. At higher temperatures, the dielectric constant undergoes a significant decrease on application of magnetic fields. The study clearly exhibits multiferroic and magnetoelectric nature of ErMnO₃.     

High-temperature oxygen non-stoichiometry, conductivity and structure in strontium-rich nickelates La2−xSrxNiO4−δ (x = 1 and 1.4)

Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La_2−xSr_xNiO_4−δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20–1050 °C. These phases retain the K₂NiF₄-type structure of La₂NiO₄ (tetragonal, space group I4/mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La₂NiO_4±δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni³⁺/Ni⁴⁺ to a Ni²⁺/Ni³⁺ equilibrium on heating.     

Synthesis of layered Li1.2+x[Ni0.25Mn0.75]0.8−xO2 materials (0 ≤ x ≤ 4/55) via a new simple microwave heating method and their electrochemical properties

Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) was prepared by a new simple microwave heating method and the effect of extra Li⁺ content on electrochemistry of Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) was firstly revealed. X-ray diffraction identified that they had layered α-NaFeO₂ structure (space group R-3m). Linear variation of lattice constant as a function of x value supported the formation of solid solution, that is, extra Li⁺ is possibly incorporated in structure of layered Li_1.2Ni_0.2Mn_0.6O₂ (x = 0), accompanying oxidization of Ni²⁺ to Ni³⁺ to form Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55). This was confirmed by X-ray photoelectron spectroscopy that Ni³⁺ appeared and increased in content with increasing x value. Charge–discharge tests showed that Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) truly displayed different electrochemical properties (different initial charge–discharge plots, capacities and cycleability). Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) in this work delivered the highest discharge capacity of 219 mAh g⁻¹ between 4.8 and 2.0 V. Increasing Li content (x value in Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂) reduced charge–discharge capacities, but significantly enhancing cycleability.     

Microstructure, hysteresis and microwave absorption analysis of Ba(1 − x)SrxFe12O19 ferrite

M-type hexagonal ferrite series, Ba_(1−x)Sr_xFe₁₂O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0), has been synthesized by conventional ceramic method. Hysteresis parameters have been investigated at an applied field of 10 kOe and absorption has been studied at X-band as a function of thickness, substitution and frequency. Microstructure and X-ray diffraction confirmed hexagonal structure of ferrite. The substitution causes profound increase in absorption, coercivity and magnetization. The magnetic parameters have been characterized by taking into account microstructure and preferential site occupancy. Curie temperature decreases with substitution due to the formation of spin canting structure.     

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi_1.7Pb _0.3-xGd_xSr₂Ca₃Cu₄O_12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd³⁺ doping for Pb²⁺ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_c, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_c decreases with increasing temperature and/or Gd concentration, as expected.     

Compositional dependence of the optical parameters for Bi5GexSe65−x (30 ≤ x ≤ 45) films

Different compositions of Bi₅Ge_xSe_95−x (x = 30, 35, 40 and 45 at %) thin films were deposited onto cleaned glass substrates by thermal evaporation method. The structural characterization revealed that, the as-prepared films of x = 30, 35 and 40 at. % are in amorphous state but there are few tiny crystalline peaks of relatively low intensity for the film with x = 45 at. %. The chemical composition of the as-prepared Bi₅Ge_xSe_65−x films has been checked using energy dispersive X-ray spectroscopy (EDX). The optical properties for the as-deposited Bi₅Ge_xSe_65−x thin films have been studied. The additions of Ge content were found to affect the optical constants (refractive index, n and the extinction coefficient, k). Tauc’s relation for the allowed indirect transition is successfully describing the mechanism of the optical absorption. It was found that, the optical energy gap (E_g) decreases with the increase in Ge content. These obtained results were discussed in terms of the chemical bond approach proposed by Bicermo and Ovshinsky. The composition dependence of the refractive index was discussed in terms of the single oscillator model.     

Microstructures and multiferroic properties of YFe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> ceramics prepared by spark plasma sintering

YFe_1−x Mn _x O₃ (x = 0.1, 0.2, and 0.4) dense ceramics were prepared by SPS (spark plasma sintering). The dielectric characteristics were evaluated over broad temperature and frequency ranges, and an obvious dielectric relaxation was observed. It was a thermally activated process following the Arrhenius law, and the activated energy decreased with increasing Mn-content. Obvious ferroelectric hysteresis loops were observed at 153 and 123 K for different compositions. The weak ferromagnetic characteristics were detected at room temperature for all compositions, and the remnant magnetization was reduced with increasing Mn-content. These results indicated the multiferroic properties in the Mn-substituted YFeO₃ ceramics.     

Effect of novel (Gd,Cu) substitution on the electrical properties and magnetoelectric coupling of bismuth ferrite ceramics

The influence of Gd dopant and (Gd, Cu) dopants on the ferroelectric, dielectric and magnetoelectric properties of single phase BiFeO₃ (BFO) were investigated. Nanoparticles of undoped BiFeO₃, Bi_0.95Gd_0.05FeO₃ and Bi_1?xGd_xFe_0.98Cu_0.02O₃ (x?=?1, 2, 3, 4 and 5%) were prepared by sol–gel method. X-ray diffraction reveals that all the samples crystallize in rhombohedral phase. The simultaneous Gd and Cu doping at BFO lattice has significantly enhanced the ferroelectric properties of BFO compared to that of BFO. Substitution of Gd alone at the Bi site, gave rise to attractively enhanced remnant polarization. Though the (Gd, Cu) doped BFO samples exhibit relatively less enhancement, their values of remnant polarization are appreciable. Doping of (Gd, Cu) in the BFO lattice leads to an appreciable dielectric properties. An effective magnetoelectric coupling has been recorded for doped BFO when compared to BFO.     

Influence of Dy-doping on ferroelectric and dielectric properties in Bi<Subscript>1.05−x</Subscript>Dy<Subscript>x</Subscript>FeO<Subscript>3</Subscript> ceramics

Bi_1.05−xDy_xFeO₃ (BDFO) (x = 0−0.2) ceramics were synthesized by solid-state reaction method. The influence of Dy dopant on crystal structural, dielectric and ferroelectric properties was investigated. The lattice parameter and the Curie temperature of BDFO were degraded continuously with increasing contents of Dy³⁺ cations. Leakage current density, ferroelectric polarization and dielectric loss were improved by appropriate Dy doping. When x = 0.1, BDFO showed the best electric properties. At applied electric field of 53 kV/cm, the remnant polarization (2P _r ) was 12.2 μC/cm².These improvements in electric properties in BDFO ceramics could have resulted from the relatively low oxygen vacancy concentration and structural distortion.