Magnetic and dielectric properties of nanophase manganese-substituted lithium ferrite

Nanocrystalline manganese-substituted lithium ferrites viz. Li_0.5Fe_2.5−xMn_xO₄ (2.5≤x≥0) were prepared by sol-gel autocombustion method. X-ray diffraction analysis confirmed that as the concentration of manganese increases the cubic phase changes to the tetragonal phase. The variation of saturation magnetization was studied as a function of manganese content. All the compositions indicate that they are ferrimagnetic in nature. The dielectric constant, dielectric loss tangent and ac conductivity of all samples were measured at room temperature as a function of frequency. These parameters decrease with increase in frequency for all of the samples. The substitution of manganese plays an important role in changing the structural and magnetic properties of these ferrites. The compositional variation of dielectric constant and d.c. resistivity shows an inverse trend of variation with each other.     

Magnetic properties of substituted strontium ferrite nanoparticles and thin films

SrFe_12−x(Zr_0.5Mg_0.5)_xO₁₉ nanoparticles and thin films with x=0-2.5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). Structural and magnetic characteristics of synthesized samples were studied employing x-rays diffraction (XRD), transmission electron microscopy (TEM), magnetic susceptometer, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). TEM micrographs display that the narrow size distribution of ferrite nanoparticles with average particle size of 50 nm were fabricated. Fitting obtained data of effective magnetic susceptibility by Vogel-Fulcher law confirms the existence of strong magnetic interaction among fine particles. XRD patterns and FE-SEM micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. AFM micrographs exhibited that the surface roughness increases with an increase in Zr-Mg content. It was found from the VSM graphs that with an increase in substitution contents the coercivity decreases, while the saturation of magnetization increases. The Henkle plots confirms the existence of exchange coupling among nano-grain in ferrite thin films.     

Magnetic and dielectric studies of nanocrystalline zinc substituted Cu-Mn ferrites

Ferrites with the general formula Cu_1−xZn_xFeMnO₄ (where 0≤x≤1) were prepared through a citrate gel auto-combustion route. Structural characterizations carried out by X-ray diffraction reveal that the lattice constant increases with increase in zinc content. Transmission electron microscopic measurements confirm the nanoscale nature of the particles. Room temperature saturation magnetization was measured as a function of zinc concentration. The saturation magnetization increases up to x=0.25 and then decreases as zinc concentration increases. Dielectric permittivity, dielectric loss tangent, ac conductivity and complex dielectric impedance were studied in the frequency range 20 Hz-1 MHz. The results indicated a usual dielectric dispersion due to the Maxwell-Wagner type of interfacial polarization. Dielectric loss showed similar behavior as dielectric permittivity. The ac conductivity increased linearly with frequency. Complex impedance spectroscopic studies confirmed that conduction in the samples is via grain boundaries. In general, substitution of zinc plays an important role in changing the structural, electrical and magnetic properties of these ferrites.     

Influence of sintering time on structural,magnetic and electrical properties of Si–Ca added Sr-hexa ferrites

Sr-hexa ferrites with the addition of Si (0.5 wt%) and Ca (0.5 wt%) have been prepared by solid-state reaction method with sintering time variation ranging from 2 to 10 h. The structural characterization of the samples confirmed the major phase of Sr-hexa ferrite. Average grain size was found within the range of 1–4 μm. Vicker hardness increased from 512 to 1187H_v. The coercivity and remanence had the ranges from 596 to 4255 Oe and 324 to 516 G, respectively. The DC electrical resistivity measurements were carried out by two-probe method as a function of temperature from 303 to 723 K. The room temperature DC resistivity increased from 1.67×10⁶ to 2.89×10⁸ Ω cm in turn the activation energy also increased from 0.314 to 0.495 eV. The DC electrical resistivity decreased while drift mobility increased with the rise in temperature, ensuring the semi-conducting behavior. Dielectric properties were studied as a function of frequency in the range of 80 Hz to 1 MHz at room temperature.     

Magnetic properties of cobalt substituted M-type barium hexaferrite prepared by co-precipitation

The co-precipitation and solid state methods were used in the synthesis of barium hexaferrite (BaM). Phase pure BaM was obtained with 1, 2, 3, 5, 10, 15, 20 and 30 wt% cobalt oxide (Co₃O₄). The addition of Co^2+/3+ ions to the BaM increased the permeability and magnetic loss tangent to a value of 3.5 at 5% and reduced to 1 at 30% doping. With increased Co doping, M_s was reduced from 87-58 emu/g, M_r increased from 11 to 40 emu/g with 3–5 wt% Co and 9 emu/g for 30% doping. H_c sharply increased from 540 to 2200 Oe with a reduction to 280 Oe at 10 K with increasing temperature to 300 K. T_c increased from 740 to 750 K for 30% Co doping. DTA–TGA studies of green body showed decarboxilation to occur at around 825 °C and the transformation of residual Co₃O₄ to Co₂O₃ at around 577 °C. The XRD data confirmed the Co ions substituting into Fe sites until a 10–15% doping level where the structure altered to W-type hexaferrite. The densities of the compounds varied with doping to a maximum of 4.45 g/cm³.     

Chemical synthesis and studies on structural and magnetic properties of fine grained nickel cadmium ferrites

The cadmium substituted nickel ferrites Ni_1−xCd_x Fe₂O₄ (x=0.2, 0.4, 0.6) has been prepared by the self-propagating auto combustion method. Phase identification of the samples was studied by XRD patterns which are characteristics of cubic spinel structure ferrites. As the content of cadmium in the ferrites increases the lattice parameter. The tetrahedral and octahedral vibrations in the samples were studied by Fourier Transform Infrared (FT-IR) spectra. The SEM study reveals the fine-grained nature of ferrites with little agglomeration. The decrease in the drift mobility is found with increase in cadmium content. DC resistivity indicates the semiconducting nature of the samples. The Super Quantum Interference Device measurements were taken to study the magnetic properties of the samples.     

Correlation between site preference and magnetic properties of substituted strontium ferrite thin films

SrFe_12−x(Sn_0.5Zn_0.5)_xO₁₉ thin films with x=0−5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). The site preference and magnetic properties of Zn-Sn substituted strontium ferrite thin films were studied using ⁵⁷Fe Mössbauer spectroscopy and magnetic measurements. Mössbauer spectra displayed that the Zn-Sn ions preferentially occupy the 2b and 4f₂ sites. The preference for these sites is responsible for the anomalous increase in the magnetization at high Zn-Sn substitutions. X-ray diffraction (XRD) patterns and field emission scanning electron microscope (FE-SEM) micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. Vibrating sample magnetometer (VSM) was employed to probe magnetic properties of samples. The maximum saturation of magnetization and coercivity at perpendicular direction were 265 emu/g and 6.3 kOe, respectively. It was found that the complex susceptibility has linear variation with static magnetic field.     

Effect of rare earth substitutions on some physical properties of Mn-Zn ferrite studied by positron annihilation lifetime spectroscopy

Mn-Zn ferrite substituted with rare earth ions have been investigated by means of positron annihilation lifetime spectroscopy (PALS). The variations of lifetime parameters τ_av, I₂, and κ with ionic radius of rare earth ions, grain size and electrical resistivity for all samples have been studied. The intergranular pores increase with increasing the ionic radius of rare earth ions and grain size of the samples.     

Effect of annealing conditions on structural and magnetic properties of laser ablated copper ferrite thin films

The effect of annealing conditions on structural and magnetic properties of copper ferrite thin films on (100) Si substrates was examined in detail. After deposition, the ferrite thin films were post-annealed in vacuum and in oxygen atmosphere for several hours. It is found that the crystal structure of CuFe₂O₄ thin films changed drastically depending on different heating process. A maximum magnetization was achieved in the film that was vacuum annealed and it decreased remarkably after oxygen annealing.     

Effect of magnesium substitution on dielectric and magnetic properties of Ni-Zn ferrite

The dielectric and magnetic properties of Mg incorporated Ni-Zn spinel ferrites have been investigated. Ni_0.5−xZn_0.5Mg_xFe₂O₄ ferrites have been prepared by sol-gel auto-combustion technique. The as prepared ferrites were annealed at 673, 873 and 1073 K. The X-ray diffraction studies reveal the spinel structure of annealed ferrites. The TEM results are in agreement with XRD results. FTIR study has also been carried out to get insight into the structure of these ferrites. The dielectric measurements show that the dielectric constant (ε′), dielectric loss (tan δ) and conductivity (σ_ac) increase on incorporation of Mg in the Ni-Zn ferrite. ε′, tan δ and σ_ac also show dependence on temperature, frequency of external applied electric field and microstructure of the samples. The magnetic moment measurements reveal that the saturation magnetization (M_s) increases and coercivity (H_c) decreases with the increase in concentration of Mg²⁺ ions. M_s and H_c also show dependence on the annealing temperature.     

Remarkable influence on the dielectric and magnetic properties of lithium ferrite by Ti and Zn substitution

The effect of Zn and Ti substitution on the magnetic and electrical properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in steps of 0.05) +0.5wt% Bi₂O₃ prepared by a standard ceramic technique has been investigated. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 700 K in the frequency range from 100 Hz to 2 MHz have been analysed. The variation of the real part of dielectric constant (ε^′) and loss tangent (tanδ) with frequency and temperature has been studied; it follows the Maxwell–Wagner model based on the interfacial polarization in consonance with the Koops phenomenological theory. It is found that the permittivity of zinc and titanium substituted lithium ferrite improves and shows a maximum value ( 1.5×10⁵) at 100 Hz for the x=0.25 sample. The dielectric transition temperature (T_d) depends on the concentration of Ti and Zn in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄. The saturation magnetization and Curie temperature both decrease with increase in the concentration of Ti and Zn in the ferrite.     

Effect of Cd on the structural, magnetic and electrical properties of nanostructured Mn-Zn ferrite

Nanoparticles of Mn_0.5Zn_0.5−xCd_xFe₂O₄ (x=0.0, 0.1, 0.2 and 0.3) have been synthesized by a chemical co-precipitation method. The lattice constant increases with increasing Cd content. X-ray calculations indicate that there is deviation in the cation distribution in the nanostructured spinel ferrite. The dielectric constant and dielectric loss decrease for the samples with Cd content up to x=0.2. However the dielectric constant rises for x=0.3. This is due to an increase in the hopping process at the octahedral (B sites). The dielectric constant increases with increase in temperature, indicating a thermally activated hopping process. The DC resistivity increases with Cd content up to x=0.2 and decreases for Cd content x=0.3. The maximum magnetization of all the samples decreases with increase in Cd content.     

Comparison study of some structural and magnetic properties of nano-structured and bulk Li-Ni-Zn ferrite samples

Nanoparticles of Li_0.1(Ni_1−xZn_x)_0.8Fe_2.1O₄ with x varying from 0.0 to 1.0 were prepared by chemical co-precipitation method. A part of these samples was sintered at 1200 °C for 2 h to obtain bulk samples via increase in particle sizes; the other part was left as-prepared. Structural and magnetic properties comparison studies were carried out between the as-prepared nanoparticle samples and their bulk counterparts. The X-ray diffraction and infrared spectroscopy were used to characterize the samples. The XRD pattern of the samples provides evidence of single phase formation of spinel structure with cubic symmetry. The lattice constant, and X-ray density were calculated from the XRD data. The density of the samples was measured and consequently the porosity was calculated too. In the present work, the effect of nano-structured particle size and Zn concentration on parameters such as bond length and vibration frequency are discussed with the help of the IR data. The B-H loops of all samples obtained by using vibrating sample magnetometer (VSM) are displayed. The reduction of saturation magnetization of the nano-structured samples in comparison with their bulk counterparts and the effect of Zn addition on saturation magnetization of all the samples are discussed.     

镉取代对纳米镍铁氧体结构和磁性的影响

采用湿化学共沉淀法合成了Ni_1-xCd_xFe₂O₄ (0≤x≤0.5)的混合铁氧体,利用X射线衍射对其结构和晶相进行表征．结果表明：随着镉Cd浓度的增加晶格参数逐渐增大．扫描电子显微镜研究微观结构,TG/DTA研究了硫酸共沉淀物．揭示了在650 oC合成铁氧体同时进行分解．测量了纳米粒子的磁化强度和交流磁化率．结果表明,Ni_1-xCd_xFe₂O₄混合铁氧体的磁化率、居里温度和有效磁矩随着镉含量的增加下降。     

Phase transformation and electrical resistivity of tetracyanoethylene under pressure

This paper reports the phase transformation behaviour of tetracyanoethylene (TCNE) under pressure as revealed by AC electrical resistivity, its time evolution and X-ray diffraction studies. An irreversible transformation from monoclinic to cubic phase occurs at 2.1±0.1 GPa and is indicated by a sharp resistivity drop at this pressure. The time evolution of resistivity studies indicate that this transformation occurs via an intermediate phase having resistivity higher than either of the two crystalline phases. Finally, the kinetics of phase transformations obtained by time evolution of resistivity is compared with the X-ray studies on the pressure quenched TCNE.     

Structural,thermal, electrical and magnetic properties of pure and 50% La doped BiFeO3 ceramics

Polycrystalline ceramic samples of pure and 50% La substituted BiFeO₃ have been prepared by standard solid state reaction method using high purity oxides and carbonates. The formation of the single phase compound as well as its chemical analysis has been checked by X-ray diffraction and energy dispersive X-ray microanalysis (EDAX) techniques. A better agreement between observed and calculated X-ray powder diffraction patterns was obtained by performing the Rietveld refinement with a structural model using the non-centrosymmetric space group R3c. The lattice parameters in both the cases have been refined but the over-all structure remains the same. The microstructural studies have been carried out using scanning electron microscopy (SEM). Modulated differential scanning calorimetry (MDSC) has been used to detect the Neel/transition temperature in the compounds. The activation energies calculated from log σ vs 1/T curve are 0.81 eV and 1.13 eV respectively. Vibrating sample magnetometer (VSM) has been used to study the magnetic behaviour of the compounds. It has been observed that by 50% La substitution the insulating behaviour of the material has been improved and showing the antiferromagnetic to weak ferromagnetic behaviour.     

Screening effect of Ti ions on the electrical conductivity and thermoelectric power of Mg ferrite

The samples Mg_1+xTi_xFe_2−2xO₄ were prepared in a single phase spinel structure as indicated from X-ray analysis. The preference of Mg²⁺ ions to the octahedral site decreases the ratio of the normal spinel in the investigated ferrite where the Mg²⁺ increases on the expense of the Fe³⁺ ions on the same site. The increase in the conductivity was found to be due to thermally activated mobility of charge carriers. The mobility data enhances the use of Verway model of conductivity which depends on the electron exchange between iron ions of different valences located on the same crystallographic sites. The existence of Ti⁴⁺ ions on the octahedral site screens the polarization and decreases the conductivity of the samples. Peculiar behavior was obtained for Ti content of 0.7 and 0.8 due to the presence of secondary phases.     

Effect of Nd substitution on structural and electrical properties of nanocrystalline zinc ferrite

Polycrystalline soft ferrite samples with general formula ZnNd_xFe_2−xO₄ (where x=0, 0.01, 0.02 and 0.03) were synthesized by oxalate co-precipitation method. The samples were characterized by XRD and SEM techniques. The single phase cubic spinel structure of all the samples was confirmed by XRD. The lattice constant and grain size of the samples are found to decrease with increase in Nd³⁺ content. Room temperature DC resistivity of the Nd³⁺ substituted zinc ferrites is 10² times higher than that of zinc ferrite. The dielectric constant (ε′) and dielectric loss (tan δ) of all the samples were measured in the frequency range 20 Hz-1 MHz. The dielectric behaviour is attributed to the Maxwell-Wagner type interfacial polarization. The dielectric loss of the samples is found to decrease with increase in Nd³⁺ content. High resistivity and low dielectric loss makes these ferrites particularly suitable for high frequency applications.     

Cu-doped ZnO nanoparticles: Synthesis, structural and electrical properties

Pure and Cu doped ZnO nanopowders (5, 10, 15, 20, 25 and 30 at% Cu) have been synthesized using co-precipitation method. Transmission Electron Microscopic analysis has shown the morphology of ZnO nanopowders to be quasi-spherical. Powder X-ray Diffraction studies have revealed the systematic doping of Cu into the ZnO lattice up to 10% Cu, though the peaks corresponding to CuO in 10% Cu are negligibly very small. Beyond this level, there was segregation of a secondary phase corresponding to the formation of CuO. Fourier Transform Infrared spectra have shown a broad absorption band at ∼490 cm⁻¹ for all the samples, which corresponds to the stretching vibration of Zn-O bond. DC electrical resistivity has been found to decrease with increasing Cu content. The activation energy has also been observed to decrease with copper doping i.e. from ∼0.67 eV for pure ZnO to ∼0.41 eV for 30 at% Cu doped ZnO.     

Dependence of thermal conductivity on electrical resistivity in Bismuth-Tellurides

The p-type (Bi_0.25Sb_0.75)₂Te₃ doped with 3-12 wt% excess Te alone and n-type Bi₂(Te_0.94Se_0.06)₃ codoped with 0.017-0.026 wt% Te and 0.068-0.102 wt% I were prepared by the Bridgman method, to produce intentionally polycrystalline. Some of the as-grown specimens were annealed, in order to prepare specimens with much different ρ. These polycrystalline specimens have almost the same degree of alignment of the c plane parallel to the freezing direction. The electrical rersistivity ρ and thermal conductivity κ were measured at 298 K along the freezing direction and κ was plotted as a function of ρ. As a result, the lattice components κ_ph obtained by subtracting the electronic component κ_el from the observed κ were found to decrease almost linearly with a decrease of ρ in both p- and n-type specimens, where κ_el was calculated using Wiedemann-Franz law. This tendency is consistent with the conventional result that κ_ph becomes negligible small in metals. The significant decrease in κ_ph with decrease in ρ is considered to be caused predominantly by the phonon scattering due to dopants. The relationship between κ_ph and ρ was first clarified in the intermediate region between the metal and insulator.