Magnetic, dielectric and complex impedance spectroscopic studies of nanocrystalline Cr substituted Li-ferrite

Nanocrystalline Li_0.5Fe_2.5−xCr_xO₄ (2.5≤x≥0) ferrites were prepared by a sol-gel autocombustion route. X-ray diffraction was employed to confirm the cubic spinel phase formation of the ferrites. The lattice parameter decreases with increase in Cr content. The saturation magnetization, coercivity and remanance were studied as a function of Cr content. The dielectric constant and dielectric loss were measured as a function of frequency in the frequency range 20 Hz-1 MHz. Frequency dependence of dielectric constant shows dielectric dispersion due to the Maxwell-Wagner type of interfacial polarization. In order to understand the conduction mechanism, complex impedance measurements were carried out. The substitution of chromium plays an important role in changing the dielectric and magnetic properties of lithium ferrites.     

Magnetic and dielectric properties of low vanadium doped nickel-zinc-copper ferrites

The new spinel-type of general formula Ni_0.6+xZn_0.2Cu_0.2V_xFe_2−2xO₄ with 0.0≤x≤0.25 was synthesized by the usual ceramic method. Structure of the prepared ferrites was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Room temperature magnetic hysteresis loops were measured using magnetic field strength up to 6 kOe. Saturation magnetization (M_s) increased with vanadium content up to x=0.05 and then decreased. Variation of (M_s) as a function of x is explained in terms of cation redistribution between A and B sublattices. Coercive force (H_c), remanent induction (B_r) and squareness of the hysteresis loop (B_r/M_s) as functions of x are presented. Dielectric permittivity (ε′, ε″) and dielectric loss tangent (tan δ) were measured as functions of frequency and temperature. These parameters were found to be strongly dependent on V₂O₅ concentration. The variation of dielectric loss tangent with frequency at different temperatures shows abnormal behavior, where more than one relaxation peaks were observed at low and high temperatures. This behavior could be attributed to the collective contribution of two types of carriers (p and n) to polarization.     

Enhancement of magnetic and dielectric properties of low temperature sintered NiCuZn ferrite by Bi2O3-CuO additives

With a series of 1.0 wt%Bi$_{2}$O$_{3}$-$x$ wt% CuO ($x =0.0$, 0.2, 0.4, 0.6, and 0.8) serving as sintering additives, Ni$_{0.23}$Cu$_{0.32}$Zn$_{0.45}$Fe$_{2}$O$_{4}$ ferrites are successfully synthesized at a low temperature (900 $^\circ$C) by using the solid state reaction method. The effects of the additives on the phase formation, magnetic and dielectric properties as well as the structural and gyromagnetic properties are investigated. The x-ray diffraction (XRD) results indicate that the added Bi$_{2}$O$_{3}$-CuO can lower the synthesis temperature significantly without the appearing of the second phase. The scanning electron microscope (SEM) images confirm that Bi$_{2}$O$_{3}$ is an important factor that determines the sintering behaviors, while CuO affects the grain size and densification. With CuO content $x=0.4$ or 0.6, the sample shows high saturation magnetization, low coercivity, high real part of magnetic permeability, dielectric permittivity, and small ferromagnetic resonance linewidth ($\Delta H$). The NiCuZn ferrites are a promising new generation of high-performance microwave devices, such as phase shifters and isolators.     

Study of dielectric and ac impedance properties of Ti doped Mn ferrites

We have reported dielectric and ac impedance properties of Ti doped Mn_1+xFe_2−2xO₄ (0x0.5) ferrites prepared by solid-state reaction method, using dielectric and impedance spectroscopy in the frequency range of 42 Hz–5 MHz, between the temperatures (300K–473K). The dielectric constant and dielectric loss (tan δ) decreases with increasing frequency but these parameters increase with increasing temperature. The dielectric loss tangent curves exhibit dielectric relaxation peaks at high frequencies (3.6 kHz–5 MHz), which are attributed to the coincidence of the frequency of charge hopping between the localized charge states and the external field. The dielectric properties have been explained on the basis of space charge polarization according to Maxwell–Wagner’s two-layer model and the hopping of charge between Fe²⁺ and Fe³⁺ as well as between Mn³⁺ and Mn²⁺ ions at B-sites. The complex impedance analysis has been used to separate grain and grain boundary in studied samples. Two semicircles corresponding to grain and grain boundary have been observed at low temperature, while only one semicircle has been seen at high temperatures. The resistance of grain and grain boundary both increase with Ti⁴⁺ doping.     

Densification and magnetic properties of low-fire NiCuZn ferrites

The mixing of (Ni_0.38Cu_0.12Zn_0.50)Fe₂O₄ powders with Bi₂O₃ was performed using the solid-state mixing as well as wet chemical coating processes such as ammonia precipitation coating, urea precipitation coating, and solution coating. Ferrites prepared from the wet chemical coating processes could be densified at a lower sintering temperature without significant impact on the microstructural evolution compared with that prepared by solid-state mixing. In addition, samples prepared from the wet chemical coating process have a higher B_r and B_s and a lower H_c compared with that from solid-state mixing. Considering both the effects of sintering temperature and sintered density (>95% T.D.), ferrites with 1.5 wt% Bi₂O₃ addition by ammonia precipitation coating sintered at 900°C can provide the best permeability and quality factor (191 and 68.2, respectively) among all the cases studied.     

Elastic and thermodynamic properties of AVO3 (A=Sr, Pb) perovskites

We have investigated the elastic and thermodynamic properties for the perovskite type metavanadate SrVO₃ and the multiferroic PbVO₃, probably for the first time by the means of a Modified Rigid Ion Model (MRIM). We present the elastic constants (C_11,C_12,C₄₄) and other elastic properties like Bulk modulus (B), Young′s modulus (E), shear modulus (G), Poisson′s ratio (σ) and wave velocity (υ_l, υ_s, υ_m). Besides we have reported the thermodynamic properties molecular force constant (f), Reststrahlen frequency (ν), cohesive energy (?), Debye temperature (θ_D) and Gruneisen parameter (γ). We have also computed the variation of heat capacity (C_P) and there by volume thermal expansion coefficient (α) in a wide temperature range. We found that the computed properties reproduce well with the available data in literature. To our knowledge some of the properties are reported for the first time.     

Phase formation, grain growth and magnetic properties of NiCuZn ferrites

We studied the effects of iron-deficient, stoichiometric and iron-excessive compositions on the phase formation, crystal structure, grain growth and magnetic property of NiCuZn ferrites. As the Fe₂O₃ ratio increased from iron-deficiency 47.0 mol% to iron-excess 54.0 mol%, the X-ray diffraction peaks initially shifted towards lower angle and then moved to higher angle. Correspondingly, an initial increase in lattice parameter followed by a subsequent decrease was observed. The lattice parameter showed a maximum 8.396 Å when the Fe₂O₃ ratio was 49.0 mol%. When the system was iron-deficient, ZnO phase was detected in addition to the spinel phase. However, equimolar and iron-excessive compositions exhibited a single spinel phase. As the content of Fe₂O₃ increased, the grain size, density, saturation induction and initial permeability first increased and then decreased. Core losses at 50 kHz and 150 mT, however, changed in the opposite way. Finally, NiCuZn ferrite with an equimolar composition (50.0 mol%) showed the highest initial permeability (1467), highest saturation induction (361 mT) and lowest core losses (234 kW/m³).     

Influence of Nd:YAG laser irradiation on AC impedance and dielectric properties of lithium ferrite

Polycrystalline samples of Li_0.5Fe_2.5O₄ spinel ferrite have been synthesized by a standard ceramic technique. The samples were irradiated with an Nd:YAG laser to study the effect of laser irradiation on the structural, dielectric and AC impedance properties. The X-ray diffraction results show the formation of a disordered cubic structure after irradiation. The dielectric constant (?′), dielectric loss (?″) and the loss tangent (tan δ ) were measured at room temperature as a function of frequency (f=20 Hz–1 MHz) for the irradiated and unirradiated samples of Li_0.5Fe_2.5O₄ spinel ferrite. The dielectric constant of the irradiated samples is decreased in magnitude compared to the unirradiated samples. It was found that laser irradiation increases the polarization and the resistivity of the samples as a result of electronic rearrangement and lattice defects. The AC conductivity of the samples was derived from the dielectric constant and loss tangent data. The change in AC conductivity is attributed to the creation of lattice vacancies after laser irradiation. The AC impedance analysis was used to separate the grain and grain boundary of the Li_0.5Fe_2.5O₄ spinel ferrite.     

Influence of annealing temperature on the formation, microstructure and magnetic properties of spinel nanocrystalline cobalt ferrites

Nanocrystalline cobalt ferrites were synthesized by a simple, general, one-step sol–gel auto-combustion method. An interpretation based on the measurement of the adiabatic flame temperature and the amounts of gas evolved during reaction had been proposed for the nature of combustion. The influence of annealing temperatures on the magnetic properties was investigated. The microstructure was characterized by means of X-ray diffractometer (XRD) and transmission electron microscopy (TEM). It was found that the particle size and magnetic properties of the as-prepared ferrite samples showed strong dependence on the annealing temperature. The coercivity initially increased and then decreased with increasing annealing temperature whereas the particle size and saturation magnetization continuously increased.     

Structural,dielectric and magnetic properties of cobalt based spinel ferrites

CoYb_xFe_2-xO₄ (x = 0.00, 0.025, 0.05, 0.075, 0.10) spinel ferrites were synthesized by co-precipitation technique. Structural, dielectric and magnetic properties were measured. X-ray diffraction analysis showed that all the prepared spinel ferrites possessed cubic spinel structure. Dielectric constant, AC conductivity and dielectric loss decreased with the addition of rare earth ions. The impedance analysis explained the role of grains and grain boundaries with in prepared samples. Cole-Cole plots helped to measure the values of grains and grain boundary's resistance. The magnetic properties proved the soft nature of these ferrites. Saturation magnetization and remanence decreased while coercivity was enhanced with the addition of ytterbium concentration. All these parameters suggested that these prepared samples might be suitable for high frequency applications.     

Structural and magnetic properties of Li-Cu mixed spinel ferrites

Li_0.5−x/2Cu_xFe_2.5−x/2O₄ (where x=0.0-1.0) ferrites have been prepared by solid-state reaction. X-ray diffraction was used to study the structure of the above investigated ferrites at various sintering temperatures. Samples were sintered at 1000, 1100 and 1200 °C for 3 h in the atmosphere. For the sintering temperature of 1000 °C, Li_0.5−x/2Cu_xFe_2.5−x/2O₄ undergoes cubic to tetragonal transformation for higher Cu content. However, for the sintering temperature of 1100 and 1200 °C, X-ray diffraction patterns are mainly characterized by fcc structure, though presence of tetragonal distortion was found by other temperature dependence of initial permeability curves. The lattice parameter, X-ray density and bulk density were calculated for different compositions. Curie temperature was measured from the temperature dependence of initial permeability curves. Curie temperatures of Li-Cu mixed ferrites were found to decrease with the increase in Cu²⁺ content due to the reduction of A-B interaction. As mentioned earlier, temperature dependence of initial permeability curves was characterized by tetragonal deformation for the samples containing higher at% of Cu. The complex initial permeability has been studied for different samples. The B-H loops were measured at constant frequency, f=1200 Hz, at room temperature (298 K). Coercivity and hysteresis loss were estimated for different Cu contents.     

Improvement of sintering,nonlinear electrical,and dielectric properties of ZnO-based varistors doped with TiO_2

The effects of TiO_2 on sintering and nonlinear electrical properties of(98.5-x)ZnO–0.5MnO_2–0.5Co_2O_3-0.5Bi_2O_(3–x)TiO_2(x = 0.3,0.5,0.7,0.9 mol%) ceramic varistors prepared by the ceramic technique are investigated in this work.The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages.The optimum sintering temperature is found to be 1200℃,at which each of the samples shows a maximum firing shrinkage and minimum water absorption.Also minimum water absorption appears in a sample of x = 0.9 mol%.Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering.The crystal size of ZnO decreases with increasing TiO_2 doping.The addition of TiO_2 improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO_2,further addition negatively affects it.A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz–200 kHz.The temperature and composition dependences of the dielectric constant and AC conductivity are also studied.The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.     

BaCoxZn2-xFe16O27六角铁氧体电子结构与介电特性的第一性原理研究

提出了W型六角铁氧体BaCo_xZn_2-xFe₁₆O₂₇的晶体结构模型,并通过基于密度泛函理论框架下Hubbard参数U修正的广义梯度近似密度泛函理论方法研究了该材料的基态电子结构、磁性和静电介电性特性.Co和Zn共掺杂引起BaFe₁₈O₂₇的导电性从半金属转换到半导体.BaCo_xZn_2-xFe₁₆O₂₇的能隙随x增加而增加,品格常数和原胞磁矩随之而变小.介电常数计算表明,BaCo_xZn_2-xFe₁₆O₂₇的静电介电常数随x增加而增加,在6.2—7.2范围而且显示各向异性.Bom电荷计算分析表明Co和Zn本身极化对材料的介电常数和其各向异性影响不大.     

Elastic properties of lead-phosphate glasses doped with samarium trioxide

A series of ternary lead-phosphate glasses doped with samarium trioxide, xSm₂O₃-(40−x)PbO-60 P₂O₅ (where x=0.1 to 0.5 mol%) have been prepared by using conventional melt quenching method and their elastic properties have been studied at room temperature. Results from the studies show that both longitudinal and transverse sound velocities decrease by increasing the Sm₂O₃ content. Elastic moduli, Poisson's ratios decrease with increase of Sm₂O₃ composition up to 0.3 mol% and then there is a slight increasing trend observed at 0.4 mol%. The densities of these glasses have been measured by using displacement method. The variations of these parameters have been discussed with respect to samarium trioxide concentration.     

Study of dielectric and impedance properties of Mn ferrites

The paper reports on the effect of Al substitution on the structural and electrical properties of bulk ferrite series of basic composition MnFe_2−2xAl_2xO₄ (0.0≤x≤0.5) synthesized using solid state reaction method. XRD analysis confirms that all the samples exhibit single phase cubic spinel structure excluding presence of any secondary phase. The dielectric constant shows a normal behaviour with frequency, whereas the loss tangent exhibits an anomalous behaviour with frequency for all compositions. Variation of dielectric properties and ac conductivity with frequency reveals that the dispersion is due to Maxwell-Wagner type of interfacial polarization in general and hopping of charge between Fe⁺² and Fe⁺³ as well as between Mn⁺² and Mn⁺³ ions at octahedral sites. The complex impedance plane spectra shows the presence of two semicircles up to x=0.2, and only one semicircle for the higher values of x. The analysis of the data shows that the resistive and capacitive properties of the Mn ferrite are mainly due to processes associated with grain and grain boundaries.     

尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)[B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和[B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的[B]位.     

Effect of TiO2 crystalline composition on the dielectric properties of TiO2/P(VDF‐TrFE) composites

In order to study the effect of the TiO₂ particle crystalline composition (with different proportions of rutile and anatase crystals) on the dielectric properties of the composite, titanium dioxide (TiO₂) particles and TiO₂/poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)] composites were synthesized by a reflux method and the solution route, respectively. The results indicated that the optimum TiO₂ particle crystalline composition is anatase content of 37% and rutile content of 63% for dielectric‐constant modifier applications. Furthermore, a dielectric constant of 25.7 with dielectric loss of 0.17 at 100 Hz at room temperature were obtained in the composite with 40 wt% TiO₂ particles. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis, microstructure, dielectric and magnetic properties of Cu substituted Ni-Li ferrites

Cu substituted Ni-Li spinel ferrites were prepared by a conventional sol-gel auto-combustion method. The structure, surface morphology, dielectric and magnetic properties were investigated by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, impedance spectroscopy and vibrating sample magnetometer, respectively. X-ray diffraction studies reveal the single phase spinel structure of the ferrites and the crystallite size varies from 23 to 35 nm. Incorporation of Cu in the Ni-Li ferrites increases the grain size. The dielectric parameters such as ε´, ε′′, loss tan δ and ac conductivity (σ_ac) have been measured for the annealed samples in the temperature range from 35 to 200 °C and over the frequency range from 10¹ to 10⁷ Hz. The saturation magnetization and coercivity show a dependence on the composition and microstructure. The values of saturation magnetization vary from 25.6 to 33.6 emu/g with increase in x for samples annealed at 600 °C. The values of the coercivity increase from 170 to 203 Oe with increase in x.     

Magnetic properties and local structure studies of Zn doped ferrites

A series of nano-particle zinc-doped ferrites samples Zn_xFe_3 − xO₄ (x = 0, 0.17, 0.33, 0.50, and 0.67) were prepared with the hydrothermal method, and the effects of Zn doping on the magnetic properties and local structures of the samples were studied. By increasing zinc content from 0 to 0.33, the saturated magnetization M_s measured with the vibrating sample magnetometry (VSM) method increases, while it decreases by further Zn doping. The experimental results of extended X-ray absorption fine structure (EXAFS) show that most of dopant Zn atoms occupy A site, and only a few Fe atoms at B site are substituted by Zn atoms. The Neel's two-sublattice model and the molecular field analysis of Yafet–Kittel (Y–K) spin-ordering using the three-sublattice model are compared to explain the experimental phenomena. The Y–K angle between the magnetic moments at A and B sublattices is found changing due to the decrease of A–O–B exchange interaction, which shows that the magnetic moments at B site are no longer rigidly parallel to the few remained magnetic moments at A site.     

Effect of Pr doping on magnetic and dielectric properties of Ni-Zn ferrites by “one-step synthesis”

Pr³⁺-doped Ni-Zn ferrites with a nominal composition of Ni_0.5Zn_0.5Pr_xFe_2−xO₄ (where x=0-0.08) were prepared by a one-step synthesis. The magnetic and dielectric properties of the as-prepared Ni-Zn ferrites were investigated. X-ray diffraction data indicated that, after doping, all samples consisted of the main spinel phase in combination of a small amount of a foreign PrFeO₃ phase. The lattice constants of the ferrites initially increased after Pr³⁺ doping, but then became smaller with additional Pr³⁺ doping. The addition of Pr³⁺ resulted in a reduction of grain size and an increase of density and densification of the as-prepared samples. Magnetic measurement revealed that the saturation magnetization of the as-prepared ferrites, M_s, decreased, while the coercivity, H_c, increased with increasing substitution level, x, and the Curie temperature, T_c, kept a rather high value, fluctuating between 308 and 320 °C. Both the real and imaginary parts of permeability of the ferrites decreased slightly after Pr³⁺ doping. However, the natural resonance frequency shifted towards higher frequency from 13.07 to 36.17 MHz after the addition of Pr³⁺, driving the magnetic permeability to much higher frequency, reaching the highest value (36.17 MHz) when x=0.04. Introduction of Pr³⁺ ions into the Ni-Zn ferrite reduced the values of the dielectric loss tangent, especially in the frequency range of 1-400 MHz. However, the magnitude of dielectric loss of the samples doped with different amounts of Pr³⁺ raised little.