Magnetic and electrical properties of layered magnets Tl(Cr,Mn,Co)Se2

Tl(Cr,Mn,Co)Se₂ crystals were synthesized at T ≈ 1050 K. X-ray diffraction analysis showed that TlCrSe₂, TlMnSe₂, and TlCoSe₂ compounds crystallize in the hexagonal crystal system with the lattice parameters: a = 3.6999 Å, c = 22.6901 Å, c/a ≈ 6.133, z = 3, ρ _x = 6.209 g/cm³; a = 6.53 Å, c = 23.96 Å, c/a ≈ 3.669, z = 8, ρ _x = 6.71 g/cm³; and a = 3.747 Å, c = 22.772 Å, c/a ≈ 6.077, z = 3, ρ _x = 7.577 g/cm³, respectively. Magnetic and electrical studies in the temperature range from 80–400 K showed that TlCrSe₂ is a semiconductor ferromagnet, TlMnSe₂ is a semiconductor antiferromagnet, and TlCoSe₂ is a ferrimagnet with a conductivity characteristic of metals. A rather large deviation in the experimental effective magnetic moment for TlCrSe₂ (3.05 μB) from the theoretical value (3.85 μB) is attributed to two-dimensional magnetic ordering in the paramagnetic region of the noticeably layered ferromagnet TlCrSe₂. In TlCrSe₂, a correlation between magnetic and electrical properties was detected.     

Investigation of Structural and Electrical Properties of B-Site Complex Ion (Mg1/3Nb2/3)4+-Modified High-Curie-Temperature BiFeO3-BaTiO3 Ceramics

B-site complex ion (Mg_1/3Nb_2/3)-modified high-temperature ceramics 0.71BiFeO₃-0.29BaTi_1?x (Mg_1/3Nb_2/3) _x O₃ (BF-BTMNx) have been fabricated by the conventional solid-state reaction method. The compositional dependence of the?phase structure, electrical properties, and depolarization temperature of the ceramics was studied. The main phase structure of BF-BTMNx ceramics is perovskite phase with pseudocubic symmetry. The experimental results show that the dielectric and piezoelectric properties, and temperature stability strongly depend on the (Mg_1/3Nb_2/3)⁴⁺ content. The optimum (Mg_1/3Nb_2/3) content enhances the piezoelectric properties, Curie temperature, and depolarization temperature. The ceramic with x = 1% exhibited enhanced electrical properties of d ₃₃ = 158 pC/N and k _p = 0.322, combined with high-temperature stability with Curie temperature of T _c = 453°C and depolarization temperature of T _d = 400°C. These results show that the ceramic with x = 1% is a promising lead-free high-temperature piezoelectric material.     

High-Temperature Capacitor Based on Ca-Doped Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> Ceramics

High-temperature capacitors were prepared by the conventional oxide method based on Bi_0.5Na_0.5TiO₃-BaTiO₃-CaTiO₃ (BNT-BT-CT) lead-free piezoelectric ceramics. BNT-BT is one of the promising candidates as a high-temperature relaxor, and has a high Curie temperature and broadened dielectric constant. The addition of CT increases the dielectric constant at lower temperatures and decreases the dielectric constant at higher temperatures, so that the variation of capacitance is decreased. The effect of BT on the temperature characteristic of dielectric constant is contrary to that of CT. A single-phase rhombohedral perovskite and square grains were obtained in this study. With the proper amount of BT and CT additions, the high-temperature specification can be met: from −55°C to 200°C, the variation of capacitance is within ±15% of room-temperature capacitance.     

Dielectric Properties and Defect Chemistry of WO3-Doped K0.5Na0.5NbO3 Ceramics

The dielectric properties and conductivity behavior of WO₃-doped K_0.5Na_0.5 NbO₃ ceramics were investigated as a function of temperature (25°C to 600°C) and frequency (40 Hz to 10⁶ Hz). The dielectric loss and direct-current (DC) conductivity of the ceramics depend strongly on the tungsten content. A high-temperature dielectric relaxation near temperature of 500°C was observed and analyzed using the semiempirical complex Cole–Cole equation. The activation energy of the dielectric relaxation was estimated to be ～2 eV and increased with increasing WO₃. The frequency-dependent conductivity can be well described by the universal dielectric response law. The activation energy obtained from the DC conductivity changes from 0.93 eV to 1.49 eV. A possible mechanism for the high-temperature dielectric relaxation and conductivity is proposed based on the activation energy value and defect compensation.     

Dielectric,Impedance and Conduction Behavior of Double Perovskite Pr<Subscript>2</Subscript>CuTiO<Subscript>6</Subscript> Ceramics

Polycrystalline Pr₂CuTiO₆ (PCT) ceramics exhibits dielectric, impedance and modulus characteristics as a possible material for microelectronic devices. PCT was synthesized through the standard solid-state reaction method. The dielectric permittivity, impedance and electric modulus of PCT have been studied in a wide frequency (100 Hz–1 MHz) and temperature (303–593 K) range. Structural analysis of the compound revealed a monoclinic phase at room temperature. Complex impedance Cole–Cole plots are used to interpret the relaxation mechanism, and grain boundary contributions towards conductivity have been estimated. From electrical modulus formalism polarization and conductivity relaxation behavior in PCT have been discussed. Normalization of the imaginary part of impedance (Z″) and the normalized imaginary part of modulus (M″) indicates contributions from both long-range and localized relaxation effects. The grain boundary resistance along with their relaxation frequencies are plotted in the form of an Arrhenius plot with activation energy 0.45 eV and 0.46 eV, respectively. The ac conductivity mechanism has been discussed.     

Effect of Mn and Nb Doped BaTiO3 in the Dielectric Properties in the Ba(Mn1/2Nb1/2)xTi1−xO3

Polycrystalline samples of BaTi_1?x (Mn_0.5Nb_0.5) _x O₃ with x = 0.025, 0.05, 0.075, 0.1, 0.125, 0.15, and 0.175 have been synthesized by the high-temperature solid-state reaction technique. The effects of cationic substitution of manganese and niobium for titanium at B sites of the BaTiO₃ perovskite lattice on symmetry and dielectric properties were investigated. X-ray diffraction at room temperature and dielectric permittivity in the temperature range from 85 K to 500 K and frequency range from 100 Hz to 2 × 10⁵ Hz were studied. The evolution from a normal ferroelectric to a relaxor ferroelectric is emphasized. T _C or T _m decreases when both manganese and niobium are introduced into the lattice of BaTiO₃. High dielectric constant of around 9000 at T _C = 280 K was found for Ba Ti_0.925(Mn_0.5Nb_0.5)_0.075O₃ ceramic. A relaxor ferroelectric with ΔT _m = 60 K and $ \varepsilon_{\rm{r}}^{\prime } $ of about 3500 at 10 kHz with T _m = 150 K was found for the BaTi_0.85(Mn_0.5Nb_0.5)_0.15O₃ sample.     

Study on dielectric properties of lanthanum copper tantalate ceramics by impedance spectroscopy

A new perovskite ceramic material with the composition La_2/3CuTa₄O₁₂ was synthesized by solid state reaction method. SEM observations revealed dense and fine-grained microstructure and not uniform grain size distribution in the ceramics sintered in the temperature range 1180–1220 °C. Impedance spectroscopy studies were carried out in the temperature range from −55 to 700 °C at frequencies 10 Hz–2 MHz. Three types of dielectric responses were recognized, attributed to grains, grain boundaries and the electrode-sample interface. The relaxation times determined from the impedance data follow the Arrhenius law with the activation energies of 0.25 eV and 0.94 eV for grains and grain boundaries, respectively. DC electrical conductivity of the investigated ceramics was investigated in the temperature range 20–700 °C. The determined activation energy of electrical conduction was about 0.5 eV. The dielectric properties of La_2/3CuTa₄O₁₂ ceramics were compared with those of other A_2/3CuTa₄O₁₂ (A=Y, Nd, Sm, Gd, Dy, Bi) materials, being the spontaneously formed internal barrier layer capacitors, and some essential differences were found.     

Ferromagnetic Y2CoMnO6: Spin-Glass-Like Behavior and Dielectric Relaxation

The magnetic and dielectric properties of a Y₂MnCoO₆ ceramic have been investigated. The x-ray diffraction pattern shows that the sample has the monoclinic structure (space group P2₁/n). Magnetic measurements show that Y₂MnCoO₆ becomes ferromagnetic (FM) below T _C ~75 K. More importantly, the temperature dependence of magnetization (zero field cooled and field cooled behavior) and the M(H) behavior are indicative of spin-glass-like behavior of Y₂MnCoO₆, which may be caused by competition between the FM and antiferromagnetic interaction. Dielectric relaxor behavior at low temperature is also observed; this arises from dipolar effects associated with the charge ordering of Co²⁺ and Mn⁴⁺.     

Effect of Sn Content on Structure and Properties Near the Morphotropic Phase Boundary in a PbSnO3-PbZrO3-PbTiO3 Ternary System

Piezoelectric ceramics in the PbSnO₃-PbZrO₃-PbTiO₃ (PZST) ternary system with pure perovskite structure were synthesized by a conventional solid state reaction method with the compositions near the morphotropic phase boundary (MPB). The influences of Sn content on dielectric, piezoelectric, and ferroelectric properties of Pb(Zr _x Sn _y Ti_1?x?y )O₃ ceramics were investigated. The experimental results showed that, with increasing the Sn content along the Ti = 0.48 line, the optimized sintering temperature, tetragonality, and Curie temperature (T _C) decreased monotonically. The polarization–electric field hysteresis loops show a pinning effect in some low Sn compositions. Frequency dependence of the pinning effect was studied accompanied with current–electric field curve analysis. The PZST compositions near MPB show promising advantages for practical applications.     

Improving the Magnetic Properties of Li-Zn-Ti Ferrite by Doping with H3BO3-Bi2O3-SiO2-ZnO Glass for LTCC Technology

Li-Zn-Ti ferrite doped with 0.5 wt.% to 16 wt.% H₃BO₃-Bi₂O₃-SiO₂-ZnO (BBSZ) glass was synthesized using a low-temperature ceramic sintering process. Selected parameters of saturation induction (B _S), coercivity (H _C), Curie temperature (T _C), and complex permeability spectra were measured as functions of doping content, and their relationships with ferrite density and microstructure are discussed. It was found that Li-Zn-Ti ferrite can be fired at low temperature (900°C) with BBSZ glass content varying from 0.5 wt.% to 2 wt.%. The real permeability increased from 80 to 190 in the frequency range from 1 MHz to 3 MHz, the saturation induction B _S increased from 105 mT to 150 mT at 1 kHz, whereas the coercivity H _C decreased from 165 A/m to 65 A/m at 1 kHz and the Curie temperature T _C slightly declined from 155°C to 143°C. These results confirm that this new ferrite material could be used in low-temperature cofired ceramic (LTCC) devices.     

A Giant Electrocaloric Effect in Nanoscale Antiferroelectric and Ferroelectric Phases Coexisting in a Relaxor Pb0.8Ba0.2ZrO3 Thin Film at Room Temperature

Recently, large electrocaloric effects (ECE) in antiferroelectric sol‐gel PbZr_0.95Ti_0.05O₃ thin films and in ferroelectric polymer P(VDF‐TrFE)55/45 thin films were observed near the ferroelectric Curie temperatures of these materials (495 K and 353 K, respectively). Here a giant ECE (ΔT = 45.3 K and ΔS = 46.9 J K^?1 kg^?1 at 598 kV cm^?1) is obtained in relaxor ferroelectric Pb_0.8Ba_0.2ZrO₃ (PBZ) thin films fabricated on Pt(111)/TiO_x/SiO₂/Si substrates using a sol‐gel method. Nanoscale antiferroelectric (AFE) and ferroelectric (FE) phases coexist at room temperature (290 K) rather than at the Curie temperature (408 K) of the material. The giant ECE in such a system is attributed to the coexistence of AFE and FE phases and a field‐induced nanoscale AFE to FE phase transition. The giant ECE of the thin film makes this a promising material for applications in cooling systems near room temperature.     

Effect of thermal annealing on optical and photoelectric properties of microcrystalline hydrogenated silicon films

The effect of thermal annealing in the temperature range T _a=300–600°C of films of microcrystal-line hydrogenated silicon (μc-Si:H) lightly doped with boron on the spectral dependences of the absorption coefficient (α) at photon energies hν=0.8–2.0 eV, dark conductivity (σ_d), and photoconductivity (Δσ_ph) was studied at room temperature. With increasing annealing temperature, a nonmonotonic variation of α (at hν<1.2 eV), σ_d, and Δσ_ph was observed. The data obtained are attributed to a change in the concentration of electrically active impurities and formation of defects, caused by hydrogen effusion and bond restructuring at high annealing temperatures.     

Effect of the magnetic phase transition on the charge transport in layered semiconductor ferromagnets TlCrS<Subscript>2</Subscript> and TlCrSe<Subscript>2</Subscript>

TlCrS₂ and TlCrSe₂ crystals were synthesized by solid-state reaction. X-ray diffraction analysis showed that TlCrS₂ and TlCrSe₂ compounds crystallize in the hexagonal crystal system with lattice parameters a = 3.538 Å, c = 21.962 Å, c/a ≈ 6.207, z = 3; a = 3.6999 Å, c = 22.6901 Å, c/a ≈ 6.133, z = 3; and X-ray densities ρ _x = 6.705 and 6.209 g/cm³, respectively. Magnetic and electric studies in a temperature range of 77–400 K showed that TlCrS₂ and TlCrSe₂ are semiconductor ferromagnets. Rather large deviations of the experimental effective magnetic moment of TlCrS₂ (3.26 μ_B) and TlCrSe₂ (3.05 μ_B) from the theoretical one (3.85 μ_B) are attributed to two-dimensional magnetic ordering in the paramagnetic region of strongly layered ferromagnets TlCrS₂ and TlCrSe₂. The effect of the magnetic phase’s transition on the charge transport in TlCrS₂ and TlCrSe₂ is detected.     

Structural Characterization and Magnetic and Electrical Properties of La0.7Ca0.3MnO3–La1.5Sr0.5NiO4 Nanocomposites

Nanocomposite samples of (1 ? x)La_0.7Ca_0.3MnO₃ + xLa_1.5Sr_0.5NiO₄ (x = 0 to 0.3) were synthesized by a combination of the mechanical milling and solid-state reaction methods. X-ray diffraction analyses and magnetic measurements indicated that no reaction occurred between La_0.7Ca_0.3MnO₃ (LCMO) and La_1.5Sr_0.5NiO₄ (LSNO). The Curie temperature (T _C) was almost independent of x, while the metal–insulator transition temperature (T _MI) shifted from 251 K for x = 0.0 to 65 K for x = 0.2. The samples with x ≥ 0.25 exhibited insulating behavior in the temperature range from 30 K to 300 K. Addition of LSNO substantially increased the resistivity of the composites. This is attributed to enhanced magnetic disorder at LCMO grain boundaries due to the addition of LSNO. The temperature dependence of the resistivity, ρ(T), could be described by the phenomenological percolation model of phase segregation. Fitting the experimental ρ(T) data in the temperature range of 30 K to 300 K indicated that the activation energy of the composites increases as a function of the LSNO doping concentration (x).     

Effects of Annealing Temperature on the Electric Properties of 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3 Ferroelectric Thin Film

0.94(Na_0.5Bi_0.5)TiO₃–0.06BaTiO₃ (NBT–BT6) ferroelectric thin films have been fabricated on Pt–Ti–SiO₂–Si(100) substrate by metal–organic decomposition. The effects of annealing temperature (650–800°C) on the microstructure, and the piezoelectric, ferroelectric, and dielectric properties of the thin films were studied in detail. The residual stress was evaluated by the orientation average method to clarify its dependence on annealing temperature and grain size, and it was correlated with the electric properties to understand the mechanism of piezoelectric enhancement. Among the thin films, NBT–BT6 thin film annealed at 750°C has the largest effective piezoelectric coefficient, 95.1 pm/V, remnant polarization, 49.7 μC/cm², spontaneous polarization, 105.2 μC/cm², and dielectric constant, 504, and the lowest dielectric loss, 0.05, and tensile residual stress, 24.5 MPa. For the NBT–BT6 thin film annealed at 750°C, a wide temperature range, 183–210°C, around the phase transition temperature (T _m) was observed in the dielectric temperature plots, and the diffusion coefficients (γ) were quantitatively assessed as 1.6, 1.78, and 1.6. Piezoelectric performance is discussed on the basis of the dispersion phase transition and residual stress.     

Radiative recombination in Ge<Superscript>+</Superscript>-implanted SiO<Subscript>2</Subscript> films annealed under hydrostatic pressure

Photoluminescence (PL) spectra and PL excitation spectra were recorded at room temperature from SiO₂ films implanted with Ge⁺ ions and annealed at temperature T _a =450–1100°C under hydrostatic pressure P=12 kbar. The emergence of features in the violet and green bands of the PL and PL excitation spectra correlates with the formation of hydrostatically strained Ge nanocrystals. The shift of the PL bands to higher energies, which occurs as the annealing temperature is raised to T _a ≥800°C, can be attributed to a shift of the energy levels related to the radiative recombination centers, which is caused by the increasing deformation potential. The observed PL is accounted for by the enhanced probability of direct radiative transitions in Ge nanocrystals with an X-like conduction band.     

Colossal magnetoresistance in Sol-Gel derived epitaxial thin film of Co-doped La1−xCaxMnOx

Electrical, magnetic, and magnetotransport properties of sol-gel derived epitaxial thin films of Co-doped La_1?xCa_xMnO₃ are reported. The epitaxial thin films, deposited using metal-salt routed sol-gel processing, show excellent quality of epitaxy (FWHM=0.3°). Their surface roughness is about 30Å and average grain size is 500Å. The thin films exhibit the typical behavior of colossal magnetoresistive oxide, manifesting paramagnetic semiconductor to ferromagnetic metal transition near magnetic transition. The doping of Co reduces electrical conductivity, Curie temperature (T_c) and saturation magnetization (M_s). However, the peak magnetoresistance ratio does not show a monotonous change with increasing Co content. These results are interpreted by spin-disorder scattering, magnetic inhomogeneity, and lattice distortion.     

The Effect of Heat Treatment on Structural and Multiferroic Properties of Phase-Pure BiFeO3

Phase-pure multiferroic BiFeO₃ (BFO) was prepared by the coprecipitation technique using diverse precursors bismuth oxide at temperature as low as 400°C. The dependence of structural, microstructural, thermal, electrical (AC and DC), and magnetic properties on sintering temperature was systematically investigated. Uniaxially pressed samples (Ø8 mm) were sintered in air at 500°C to 800°C for 4 h. X-ray diffraction analysis was used to determine the amorphous and perovskite nature of as-synthesized and calcined/sintered samples, respectively. The crystallite size of sintered powders increased from 47 nm to 67 nm. Scanning electron microscopy showed grain growth during sintering, which improved intergranular connectivity and decreased porosity in the samples. The ferroelectric to paraelectric Curie transition temperature (T _C) of pure BFO powder was detected by differential scanning calorimetry analysis and found to be 820°C ± 1°C. The samples exhibited high AC resistivity and dielectric constant, and low loss tangent values. The samples exhibited weak ferromagnetic behavior with an unsaturated magnetization versus magnetic field hysteresis loop at room temperature. Ferroelectric behavior and variation in remnant polarization and coercivity were observed from polarization versus electric field loops. Enhanced capacitance in the magnetic field revealed the magnetoelectric effect in the samples.     

On the growth,structure, and surface morphology of epitaxial CdTe films

The structure and surface morphology of epitaxial CdTe films grown on glassy substrates with and without compensation with an additional Te vapor source during growth are studied. The optimal conditions of the production of structurally perfect epitaxial films with a pure smooth surface with no inclusions of another phase (T _so = 1000–1100 K, T _su = 570–670 K) are determined. It is established that, on glassy substrates, the epitaxial films grow via the (111) plane of the face-centered cubic (fcc) lattice with the parameter a = 6.481 Å. By varying the temperature of the main and compensating sources, CdTe films with n- and p-type conductivity are produced.     

Temperature dependent electrical and dielectric properties of a metal/Dy2O3/n-GaAs (MOS) structure

This paper describes the structural properties, electrical and dielectric characteristics of thin Dy₂O₃ layer deposited on the n-GaAs substrate by electron beam deposition under ultra vacuum. Structural and morphological characterizations are investigated by atomic force microscopy (AFM) and X-ray diffraction measurements (XRD). The XRD shows that the elaborated Dy₂O₃ oxide has a cubic structure. The electrical and dielectric properties of Co/Au/Dy₂O₃/n-GaAs structure were studied in the temperature range of 80–500 K. The conductance and capacitance measurements were performed as a function of bias voltage and frequency. The dielectric constant (ε′), dielectric loss (ε″) and dielectric loss tangent (tanδ) of the structure are obtained from capacitance–voltage (C–V) and conductance–voltage (G/ω–V) measurements. These parameters are found to be strong functions of temperature and bias voltage. A strong negative capacitance (NC) phenomenon has been observed in C–V; hence ε′–V plots for each temperature value take negative values. The following behavior of the C and ε′ in the forward bias region has been explained with the minority-carrier injection and relaxation theory. From DC conductance study, electronic conduction is found to be dominated by thermally activated hopping at high temperature. Activation energy is deduced from the variation of conductance with temperature. The interface state density (N_ss) of the structure is of the order 1.13×10¹³ eV⁻¹ cm⁻².