Estimating the cation distributions in the spinel ferrites Cu0.5−xNi0.5ZnxFe2O4 (0.0≤x≤0.5)

Ferrite samples of the composition Cu_0.5−xNi_0.5Zn_xFe₂O₄ (0.0≤x≤0.5) were synthesized by chemical co-precipitation. The samples exhibited a single phase cubic spinel structure, and the saturation magnetization of the samples was found to increase with increasing Zn content. Using a quantum mechanical method proposed by our group, the cation distributions in the samples were estimated. Estimated cation distributions obtained by fitting the magnetic moments of the samples were then used to perform Rietveld fitting for X-ray diffraction patterns. The acceptable error parameters in the Rietveld fitting indicate that the estimated cation distributions in the samples are reasonable.     

Magnetic and Microwave Absorbing Properties of Electrospun Ba(1−x)LaxFe12O19 Nanofibers

Ba_(1−x)La_xFe₁₂O₁₉ (0.00≤x≤0.10) nanofibers were fabricated via the electrospinning technique followed by heat treatment at different temperatures for 2 h. Various characterization methods including scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and microwave vector network analyzer were employed to investigate the morphologies, crystalline phases, magnetic properties, and complex electromagnetic parameters of nanofibers. The SEM images indicate that samples with various values of x are of a continuous fiber-like morphology with an average diameter of 110±20 nm. The XRD patterns show that the main phase is M-type barium hexaferrite without other impurity phases when calcined at 1100 °C. The VSM results show that coercive force (H_c) decreases first and then increases, while saturation magnetization (M_s) reveals an increase at first and then decreases with La³⁺ ions content increase. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the M-type barium hexaferrites. The microwave absorption performance of Ba_0.95La_0.05Fe₁₂O₁₉ nanofibers gets significant improvement: The bandwidth below −10 dB expands from 0 GHz to 12.6 GHz, and the peak value of reflection loss decreases from −9.65 dB to −23.02 dB with the layer thickness of 2.0 mm.     

Nanostructural,magnetic and Mössbauer studies of nanosized Co1−xZnxFe2O4 synthesized by co-precipitation

This work presents a systematic investigation on the structural and magnetic properties of Co_1−xZn_xFe₂O₄ (0.5<x<0.75) nanoparticles synthesized by the chemical co-precipitation method. The X-ray diffraction analysis, the Fourier Transform Infrared (FTIR) and the Vibrating Sample Magnetometer were carried out at room temperature to study the micro-structural and magnetic properties. The X-ray measurements revealed the production of a broad single cubic phase with the crystallite size within the range of 6–10 nm. The FTIR measurements between 400 and 4000 cm⁻¹ confirmed the intrinsic cation vibrations of the spinel structure. The magnetic measurements show that the saturation magnetization and coercivity decrease by increasing the zinc content. Furthermore, the results reveal that the sample with a chemical composition of Co_0.3Zn_0.7Fe₂O₄ exhibits the super-paramagnetic behavior and the Curie point of 97 °C.     

Magnetic properties of magnetic Co1−xMgxFe2O4 spinel by HTSE method

Magnetic properties and exchange-coupling interactions of diluted magnetic spinels A_1−xA′_xB₂X₄, where A and B are magnetic ions, namely Co_1−xMg_xFe₂O₄, were investigated using the high-temperature series expansion method (HTSE) and the distribution method of magnetic cations in the range 0≤x≤1. The magnetic phase diagram and transition temperature versus dilution x were determined using the Padé approximants method along with HTSE. The critical exponent associated with the magnetic susceptibility γ was then deduced. The obtained results are in good agreement with experimental results and critical exponent values are consistent with those suggested by the universality hypothesis.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

Finite size effect on Gd doped CoGdxFe2−xO4 (0.0≤x≤0.5) particles

Nanoparticles of CoGd_xFe_2−xO₄ (where x=0.0, 0.1, 0.3, 0.5) series have been prepared by chemical co-precipitation. The effect of Gd³⁺ ion concentration on crystalline phase, crystallinity, crystallite size, molecular vibrations and magnetic resonance has been investigated in detail. The crystallinity decreases with an increase in Gd³⁺ ion concentration and changes the structural parameters. The spin lattice relaxation has been correlated with the doping ion concentration. Similarly, the superparamagnetic behavior of these particles has been observed with EPR spectroscopy.     

Reentrant spin glass behavior and large initial permeability of Co0.5−xMnxZn0.5Fe2O4

A series of polycrystalline ferrites having nominal chemical composition Co_0.50−xMn_xZn_0.5Fe₂O₄ (0<x<0.4) have been synthesized by the solid-state reaction technique. The XRD analysis confirms single phase cubic spinel structure for all compositions. Lattice constant increases from 0.84195 to 0.84429 nm with the increasing Mn content and obeys Vegard's law. The average grain size increases by increasing both Mn content and sintering temperatures. Room temperature saturation magnetization increases for x=0.1 and decreases for increasing Mn content. The coercivity decreases with increasing Mn content due to the decrease of anisotropy constant. A reentrant spin glass behavior of these samples is observed from the zero field cooled magnetization measurements. The real part of the initial permeability increases by increasing both Mn content and sintering temperatures. This is due to the homogeneous grain growth and densification of the ferrites. The highest initial permeability 137 is observed for x=0.4 sintered at 1573 K on the other hand, the highest relative quality factor (2522) is obtained for the sample Co_0.2Mn_0.3Zn_0.5Fe₂O₄ sintered at 1523 K. The Mn substituted Co_0.50−xMn_xZn_0.5Fe₂O₄ ferrites showed improved magnetic properties.     

Structural and magnetic properties of Co1-xZnxFe2O4 nanoparticles

Co1-xZnxFe2O4 nanoparticles, prepared by the polyvinyl alcohol sol-gel method, have been investigated by x-ray diffraction and MSssbauer spectroscopy. These results are compared with those for the bulk material. The lattice parameters of CoZn ferrite nanoparticles are larger than those of the bulk material. Thermal scanning of MSssbauer measurement shows that the transition temperatures for nanoparticles are higher than those of the bulk material except for the sample CoFe2O4.     

Effect of Li substitution on the magnetic properties of LixMg0.40Ni0.60−2xFe2+xO4 ferrites

Structural and magnetic properties of a series of polycrystalline spinel type ferrites with the nominal chemical composition Li_xMg_0.40Ni_0.60−2xFe_2+xO₄, where x=0.0-0.3 in steps of 0.05, were investigated thoroughly. The formation of spinel structure was confirmed by X-ray diffraction analysis. Lattice constants and average grain diameters increased with increase in Li content. The real part of the initial permeability (μ^/_i) is found to increase not only with increase in Li content up to x=0.25 but also with the increase in sintering temperature up to 1100 °C. However, it decreases for further increase in sintering temperature except for x=0 and 0.05 compositions. The grain size dependent μ^/_i is also observed clearly in this research. From the magnetization as a function of applied magnetic field plots, it is clear that all samples exhibit ferrimagnetic state at room temperature and have a low saturation field. The magnetization obtained is explained with the help of redistribution of cations in the tetrahedral and octahedral sites and spin canting due to weakening of exchange interaction.     

Luminescence properties and mechanism of Zn1.92−xBaxSi1−yTiyO4:0.08Mn phosphors in VUV region

The present commercial phosphor Zn₂SiO₄:Mn²⁺ requires further improvement because of its long decay time. In this work, the co-doping effects of Ba²⁺ and Ti⁴⁺ upon emission intensity and decay time were investigated. Ba²⁺ and Ti⁴⁺ cations have favorable influences on the photoluminescent properties. When doped with appropriate amount of Ba²⁺, the intensity of green emission was increased 12% and the decay time was shortened 18%. When doped with appropriate amount of Ti⁴⁺, the luminescence intensity was enhanced a little, and the decay time was shortened 14%. Ba²⁺ and Ti⁴⁺ were co-doped in Zn₂SiO₄:Mn²⁺ system, the luminescence intensity was enhanced 18%, and the decay time was shortened sharply (about 31%).     

Low-temperature synthesis and growth of superparamagnetic Zn0.5Ni0.5Fe2O4 nanosized particles

Mono-disperse spinel Ni_0.5Zn_0.5Fe₂O₄ nanosized particles have been synthesized via a hydrothermal method at low temperature. X-ray diffraction (XRD), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) analysis indicated that the synthesized nanocrystals were of pure cubic spinel structure with the size about 6-20 nm. The activation energy of grain growth is 35.06 kJ/mol experimented by the Arrhenius equation. A primary experimental model was put forward to shed light on the growth mechanism of crystallined spinel Ni-Zn ferrite nanosized particles under hydrothermal conditions. The magnetic measurements shows that the prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticle possess good superparamagnetic behavior.     

Structural, optical and dielectric studies of NixZn1−xFe2O4 prepared by auto combustion route

Ni_xZn_1−xFe₂O₄ (0≤x≤1) powders were synthesised by the auto combustion method. The derived samples show well defined peaks of cubic spinal structure with space group Fd3m. The lattice parameter calculated increased from 0.8372 nm to 0.8429 nm with raise of Zn content. The average crystallite sizes were determined by using Debye-Scherer method and found to be in the range of 18-23 nm. Microstructural analyses show the regular and uniform grain morphology. Raman analyses demonstrated that the peaks have symmetric and asymmetric stretching as well as symmetric bending. Fourier transform infrared spectroscopy was used to investigate the structure and shows the changes in the tetrahedral and octahedral bond stretching. Photoluminescence measurements indicated intense emission in the wavelength range lie in blue-green region. The composition with x=0.2 showed highest intensity and explained on the basis of disordered cluster model. Dielectric analyses showed frequency sensitive behaviour in the low frequency region and frequency independent characteristics at high frequency side. The composition with x=0.2 showed highest dielectric constant and lowest dielectric loss in the studied frequency range. The ac conductivity showed a power law behaviour and conduction is explained on the basis of hoping mechanism.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

The magnetic properties of oxide spinel Li0.5Fe2.5−2xAlxCrxO4 solid solutions

The exchange interactions (J_BB and J_AB are the intra and the inter-sublattice exchange interactions between neighbouring spins, respectively) are obtained by using the general expressions of canting angle and critical temperature obtained by mean field theory of Li_0.5Fe_2.5−2xAl_xCr_xO₄. The expression of magnetic energy of Li_0.5Fe_2.5−2xAl_xCr_xO₄ is obtained for different spin configurations and dilution x. The saturation magnetisation of Li_0.5Fe_2.5-2xAl_xCr_xO₄ is obtained with different values of dilution x. The magnetic phase diagram of Li_0.5Fe_2.5-2xAl_xCr_xO₄ materials is obtained by high temperature series expansions (HTSEs). The critical exponent associated with the magnetic susceptibility of Li_0.5Fe_2.5−2xAl_xCr_xO₄ is deduced.     

Synthesis and magnetic properties of ferrites spinels MgxCu1−xFe2O4

Polycrystalline Mg_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and magnetic measurements.Using mean field theory and high-temperature series expansions (HTSE), extrapolated with the padé approximants method, the magnetic properties of Mg_1−xCu_xFe₂O₄ have been studied. The nearest neighbor super-exchange interactions for intra-site and inter-site of the Mg_1−xCu_xFe₂O₄ ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_c is calculated as a function of Mg concentration. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements.     

First-principles study of the electronic and magnetic properties of a Nickel-Zinc ferrite: ZnxNi1−xFe2O4

Using first-principles density functional theory within the generalized gradient approximation method, the effect of Zn doping on electronic and magnetic properties of NiFe₂O₄ ferrite spinel has been studied. The crystal structure of the compounds is assigned to a pseudocubic structure and the lattice constant increases as the Zn concentration increases. Our spin-polarized calculations give a half-metallic state for NiFe₂O₄ and a normal metal state for Zn_xNi_1−xFe₂O₄ (0<x≤0.5). Based on the magnetic properties calculations, it is found that the saturation magnetic moment enhances linearly with increase in the Zn content in NiFe₂O₄. The Zn doping in NiFe₂O₄ also induces strong ferrimagnetism since it decreases the magnetic moment of A-sites.     

Structural and magnetic characterization of Co67Fe4Ni2Si15B12

 Soft magnetic properties of Co-based amorphous alloy of the composition Co₆₇Fe₄Ni₂Si₁₅B₁₂ have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272 °C and saturation magnetization is 74 emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490 °C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.     

Magnetic properties and phase diagram of ZnxNi1−xFe2O4: High-temperature series expansions

Using mean field theory and high-temperature series expansions (HTSEs), extrapolated with the Padé approximants method, the effect of Zn doping on magnetic properties of NiFe₂O₄ ferrite spinel has been studied. The nearest neighbour super-exchange interactions for intra-site (J_AA, J_BB) and inter-site (J_AB) of the Zn_xNi_1−xFe₂O₄ ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The paramagnetic Curie-Weiss temperature θ and the Curie temperature T_C are calculated as a function of Zn concentration. The critical exponent γ associated with magnetic susceptibility is calculated. The spin correlation functions intra-plane and inter-plane have been also computed and compared with exchange couplings. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements and Mössbauer spectroscopy.     

Effect of In ions doping on the structural and magnetic properties of Mg0.2Mn0.5Ni0.3InxFe2−xO4 spinel ferrites

Effect of substitution of diamagnetic trivalent indium ions on the composition Mg_0.2Mn_0.5Ni_0.3In_xFe_2−xO₄ with x varying from 0.1 to 0.3 in steps of 0.1 using citrate precursor techniques has been investigated. Single-phase cubic spinel structure of these samples has been confirmed from X-ray diffraction analyses. Micro structural features were examined by TEM images. Lattice constant ‘a’ initially increases up to x = 0.1 and thereafter it decreases with further increase in x. This indicates that variation of ‘a’ with x do not obey Vegard's law. Nonlinear behavior of ‘a’ with x may be due to substitutional effect of larger In³⁺ ions (0.91Å) with smaller Fe³⁺ ions (0.67Å) in Mg-Mn-Ni ferrite. Ferrites have been investigated for their structural and magnetic properties such as variations in lattice constant, saturation magnetization, coercivity, retentivity, initial permeability, magnetic loss and relative loss factor (RLF). Fairly constant value of initial permeability over a wide frequency range (0.075–10 MHz) and low values of relative loss factor of order of 10⁻⁶–10⁻⁵ in same frequency range are main achievement of present investigations. RLF has been reduced by three orders of magnitude as compared to those samples prepared by conventional method. Low values of relative loss factor even at a high frequency indicate that prepared materials may have great potential for use in microwave devices. Possible mechanisms contributing to these properties have been discussed in this paper.     

Structural and magnetic characteristics of Co1−xNix/2Srx/2Fe2O4 nanoparticles

Co_1−xNi_x/2Sr_x/2Fe₂O₄ (x=0–0.5 in steps of 0.1) ferrite nanoparticles have been synthesized at room temperature, without calcination, using a reverse micelle process. The site preference was determined by Mössbauer spectroscopy at 300 K. The hyperfine parameters were obtained, for the whole series of solid solutions. For the X≤0.20 samples, the spectra were fitted with two discrete sextets and for the X>0.20 samples, a magnetic hyperfine field distribution and a doublet were also imposed in the fit procedure. Hysteresis loops were measured using a superconducting quantum interference device magnetometer at 2 K and 300 K. The results indicate that the relative decrease in saturation magnetization of nanoparticles compared to the submicron particles could be attributed to a surface spin termination and disorder. Magnetic dynamics of the nanoparticles was studied by the measurement of ac magnetic susceptibility versus temperature at different frequencies and it is found that the results are well described by the Vogel–Fulcher model.