Magnetic properties of Zn-substituted Li–Cu ferrites

The magnetic properties of Zn-substituted Li–Cu ferrites having the general formula Li_xCu_0.4Zn_0.6−2xFe_2+xO₄ (where, x=0.0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3) are investigated as a function of Zn content. The X-ray analysis confirms the formation of single-phase of the samples. The lattice parameter increased linearly with Zn content, which is attributed to ionic size differences of the cations involved. The bond lengths R_A and R_B are found to increase with increase in Zn content. The increase of R_A and R_B is attributed to the increase in lattice parameter. The magnetic moment increases with increase in zinc content up to 0.3 and then it decreases with further addition of Zn. The decrease in magnetic moment beyond Zn=0.3 is due to the presence of a triangular spin arrangement on B-site; the three sublattice model suggested by Yafet and Kittle. The initial permeability (μ_i) has been measured, and it is found that it decreases with increase in Zn content. The Curie temperatures also show a decreasing trend.     

Mössbauer spectroscopy of lanthanum and holmium ferrites

PrRh₂Si₂ is highly anisotropic Ising-type antiferromagnetic system. The ordering temperature (T _N~ 68 K) of PrRh₂Si₂ is exceptionally high on the de-Gennes scale in the family of RRh₂Si₂ (R = rare earths). The reason for this anomalous behaviour is not clear. It is believed that the presence of uniaxial anisotropy assists in enhancing the T _N. The Mössbauer study was performed on a 10% Fe-doped PrRh₂Si₂ sample to understand the magnetic coupling between different sites of PrRh₂Si₂. The Mössbauer spectra together with the magnetic susceptibility data suggest that the magnetic coupling in PrRh₂Si₂ is provided mainly by the RKKY interaction between Pr-moments.     

Ferroelectricity from iron valence ordering in rare earth ferrites?

The possibility of multiferroicity arising from charge ordering in LuFe₂O₄ and structurally related rare earth ferrites is reviewed. Recent experimental work on macroscopic indications of ferroelectricity and microscopic determination of coupled spin and charge order indicates that this scenario does not hold. Understanding the origin of the experimentally observed charge and spin order will require further theoretical work. Other aspects of recent research in these materials, such as geometrical frustration effects, possible electric‐field‐induced transitions, or orbital order are also briefly treated.

     

Microwave behavior of ferrites prepared via sol–gel method

Ni-ferrites were prepared at different temperatures via sol–gel method. The electromagnetic properties of these materials, namely permittivity and permeability were measured in the 0.1–13 GHz frequency range. Following a mathematical procedure, microwave absorption diagrams were constructed including the dependence of the microwave absorption of ferrite layer on microwave frequency and layer thickness. The permeability spectra broaden and the microwave absorption improves at 9–10 GHz with increase of annealing temperature.     

High-resistivity nickel–zinc ferrites by the citrate precursor method

Nickel–zinc ferrites of different compositions, Ni_1−xZn_xFe₂O₄ with x=0.2, 0.35, 0.5 and 0.6, have been prepared by a precursor method involving citrate precursors of the concerned metals and mixing them in solution state. Resistivity has been studied as a function of composition and sintering temperature. It is observed that NiZn ferrites prepared by this method have resistivity ⩾10⁸ Ω cm which is higher by atleast two orders of magnitude than that reported (⩽10⁶ Ω cm) for ferrites prepared by the conventional ceramic method. This is attributed to better purity as well as better compositional and microstructural control achievable by the citrate method. High resistivity makes these ferrites suitable particularly for high-frequency applications where eddy current losses are required to be low.     

Dielectric behavior of Cu–Zn ferrites with Si additive

Since ferrites are highly sensitive to the additives present in or added to them, extensive work, to improve the properties of basic ferrites, has been carried out on these aspects. The present paper reports the effects of composition, frequency,and temperature on the dielectric behavior of a series of CuxZn1-xFe2O4 ferrite samples prepared by the usual ceramic technique. In order to improve the properties of the samples, low cost Fe2O3having 0.5 wt.% Si as an additive is selected to introduce into the system. The dielectric constant increases by increasing the Cu content, as the electron exchange of Cu2+= Cu+is responsible for the conduction and the polarization. However, the addition of Si could decrease the dielectric constant as it suppresses the ceramic grain growth and promotes the quality factor at higher frequencies.Dielectric constant ε and loss tangent tan δ of the mixed Cu–Zn ferrite decrease with increasing frequency, attributed to the Maxwell–Wagner polarization, which increases as the temperature increases.     

Structural study of NiXMg1–XFe2O4 ferrites

The ferrite system Ni _x Mg_1-x Fe₂O₄ with 0≤x≤1 was prepared using the usual ceramic technique. The prepared samples were studied by X-ray diffraction and IR spectroscopy. X-ray diffraction analysis proved that all the samples were single-phase with the cubic spinel structure. The lattice constant, radius of the tetrahedral ion, unshared octahedral edge, tetrahedral bond and tetrahedral edge decrease while the bulk and theoretical densities, radius of octahedral ion, octahedral bond and shared octahedral edge increase as nickel ion substitution increases. The positions and intensities of the four bands of IR absorption spectra characterizing ferrites are composition dependent.     

DC conductivity for NixMg1–xFe2O4 ferrites

The DC electrical conductivity was studied as a function of both composition and temperature for the ferrite system Ni_xMg_1–xFe₂O₄ prepared by the usual ceramic technique. The experimental results proved that, the DC electrical conductivity increases as the temperature increases and as the nickel ion content and porosity decrease. The Curie temperature and the activation energies for electrical conduction increase as the nickel ion content increases.     

Influence of Co–Zr substitution on coercivity in Ba ferrites

Polycrystalline BaCo_xZr_xFe_11.5−2xO_18.25 samples with 0?x?0.6$0?x?0.6$ ions per formula units were prepared by modified citrate precursor method with the initial ratio of Ba:Fe equal to 1:11.5. The cationic site preferences of Co²⁺ and Zr⁴⁺ in Co–Zr substituted Ba ferrite were investigated by magnetic measurements and X-ray diffractometer (XRD) analysis. The coercivity H_c was decreasing with increasing Co–Zr substitution. The datum showed that the max coercive force (H_c) was obtained when substitution of 0.2, while the best saturation magnetizations (M_s) was obtained when substitution of 0.4.     

Ni–Zn–Sm nanopowder ferrites: Morphological aspects and magnetic properties

Ni–Zn ferrites have been widely used in components for high-frequency range applications due to their high electrical resistivity, mechanical strength and chemical stability. Ni–Zn ferrite nanopowders doped with samarium with a nominal composition of Ni_0.5Zn_0.5Fe_2−xSm_xO₄ (x=0.0, 0.05, and 0.1 mol) were obtained by combustion synthesis using nitrates and urea as fuel. The morphological aspects of Ni–Zn–Sm ferrite nanopowders were investigated by X-ray diffraction, nitrogen adsorption by BET, sedimentation, scanning electron microscopy and magnetic properties. The results indicated that the Ni–Zn–Sm ferrite nanopowders were composed of soft agglomerates of nanoparticles with a high surface area (55.8–64.8 m²/g), smaller particles (18–20 nm) and nanocrystallite size particles. The addition of samarium resulted in a reduction of all the magnetic parameters evaluated, namely saturation magnetization (24–40 emu/g), remanent magnetization (2.2–3.5 emu/g) and coercive force (99.3–83.3 Oe).     

Auto-combustion synthesis,Mössbauer study and catalytic properties of copper-manganese ferrites

Spinel ferrites with nominal composition Cu _0.5Mn _0.5Fe ₂ O ₄ and different distribution of the ions are obtained by auto-combustion method. Mössbauer spectroscopy, X-ray Diffraction, Thermogravimetry-Differential Scanning Calorimetry, Scanning Electron Microscopy and catalytic test in the reaction of methanol decomposition is used for characterization of synthesized materials. The spectral results evidence that the phase composition, microstructure of the synthesized materials and the cation distribution depend on the preparation conditions. Varying the pH of the initial solution microstructure, ferrite crystallite size, cation oxidation state and distribution of ions in the in the spinel structure could be controlled. The catalytic behaviour of ferrites in the reaction of methanol decomposition also depends on the pH of the initial solution. Reduction transformations of mixed ferrites accompanied with the formation of Hägg carbide χ-Fe ₅ C ₂ were observed by the influence of the reaction medium.     

Influence of Al doping on electrical properties of Ni–Cd nano ferrites

We have reported the structural and electrical properties of nano particles of Al doped Ni_0.2Cd_0.3Fe_2.5O₄ ferrite using X-ray diffraction, dielectric spectroscopy and impedance spectroscopy at room temperature. XRD analysis confirms that the system exhibits polycrystalline single phase cubic spinel structure. The average particle size estimated using Scherrer formula for Lorentzian peak (3 1 1), has been found 5(±) nm. The results obtained show that real (ε′), imaginary (ε″) part of the dielectric constant, loss tangent (tan δ), and ac conductivity (σ_ac) shows normal behaviour with frequency. The dielectric properties and ac conductivity in the samples have been explained on the basis of space charge polarization according to Maxwell–Wagner two-layer model and the Koop’s phenomenological theory. The impedance analysis shows that the value of grain boundary impedance increases with Al doping. The complex impedance spectra of nano particles of Al doped Ni–Cd ferrite have been analyzed and explained using the Cole–Cole expression.     

Properties of Cr-substituted M-type barium ferrites prepared by nitrate–citrate gel-autocombustion process

The Cr-substituted M-type barium hexaferrites, BaFe_12−xCr_xO₁₉, with x=0.0–0.8$x=0.0–0.8$ have been successfully prepared by nitrate–citrate auto-combustion process using citric acid as a fuel/reductant and nitrates as oxidants. The resulting precursors were calcined at 1100 °C for 1 h and followed by sintering at 1200 °C for 12 h in oxygen atmosphere. The ferrites were systematically investigated by using powder X-ray diffractometer (XRD), magnetic hysteresis recorder, Mössbauer spectrometer, and scanning electron microscope (SEM). The XRD data show the formation of pure magnetoplumbite phase without any other impurity phases. Both a and c lattice parameters calculated by the Rietveld method systematically decrease with increasing Cr content. The effects of Cr³⁺ ions on the barium ferrites were reported and discussed in detail. The site preference of Cr³⁺ and magnetic properties of the ferrites have been studied using Mössbauer spectra and hystereses. The results show that the magnetic properties are closely related to the distributions of Cr³⁺ ions on the five crystallographic sites. The saturation magnetization systematically decreases, however, the coercivity increases with Cr concentration. The magnetization and Mössbauer results indicate that the Cr³⁺ ions preferentially occupy the 2a, 12k, and 4f_VI sites. The average size of hexagonal platelets obtained by SEM photographs tends to decrease with respect to Cr content.     

Cation distribution by Rietveld,spectral and magnetic studies of chromium-substituted nickel ferrites

Ultrafine crystals of chromium-substituted nickel ferrite were prepared by wet chemical co-precipitation method using sulphates of respective metal ions. Formation of these materials has been confirmed by X-ray powder diffraction method. The fine crystal nature of these materials is evidenced from scanning electron microscope (SEM). Cation distribution has been investigated using X-ray diffraction technique. Cation distribution indicates that chromium occupy octahedral site for all the values of composition x. The saturation magnetization and magneton number both are decreasing with increase of chromium concentration x. The decrease in saturation magnetization and magneton number is attributed to the substitution of the Cr³⁺ ions. Curie temperature (T _C ) from susceptibility plot is found to decrease with Cr concentration x. Curie temperature of all the compositions are also obtained theoretically and it agrees with observed Curie temperature.     

Structural and magnetic properties of nano-crystalline Ni–Zn ferrites synthesized using egg-white precursor

Nano-crystalline nickel–zinc ferrites of different compositions; Ni_1−xZn_xFe₂O₄ (x=0.0–1.0) were prepared by a precursor method involving egg-white and metal nitrates. An appropriate mechanism for the egg-white-metal complexation was suggested. Differential thermal analysis-thermogravimetry, X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometer and AC-magnetic susceptibility measurements were carried out to investigate chemical, structural and magnetic aspects of Ni–Zn ferrites. XRD confirmed the formation of spinel cubic structure. The average crystallite size was calculated using line broadening in XRD patterns. Structural parameters like lattice constant, X-ray density, bond lengths and inter-cationic distance were determined from XRD data. TEM showed agglomerated particles with average size agreed well with that estimated using XRD. FT-IR spectra confirm the formation of spinel structure and further lends support to the proposed cation distribution. Zn-content was found to have a significant influence on the magnetic properties of the system. The changes in the magnetic properties can be attributed to the influence of the cationic stoichiometry and their occupancy in the specific sites.     

Could Mg content control the conduction mechanism of Ba Co Zn-W-type hexagonal ferrites?

Electrical properties as a function of composition, frequency and temperature for a series of W-type hexagonal ferrites with the general formula BaCoZn_1−xMg_xFe₁₆O₂₇; 0≤x≤0.6 prepared using the conventional ceramic method were studied. These samples are semiconductor-like materials, where the ac conductivity increases with increasing temperature. The results show that the conduction mechanism depends on the Mg²⁺ substitution. The transition temperature (T_σ) increases with increasing Mg content and gives a hump at x=0.5; after that T_σ decreases again. Both the ac conductivity and dielectric constant vary with Mg content and reach the highest value at x=0.5, due to the highest value of the ratio of Fe²⁺/Fe³⁺ at x=0.5. The peak value of the dielectric constant depends on the Mg content x.     

Magnetic relaxation spectrum of ferrites Mn x Fe3−x O4+γ

The initial permeability disaccommodation in ferritesMn _x Fe₃–xO₄₊ , 0·5x1, was studied in a temperature range around –200°C to +180°C. Four separate bands were found in the relaxation spectrum of these ferrites.

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Mn _x Fe_3–x O₄₊

Mn _x Fe_3–x O₄₊ , 0,5x1, –200°C +180°C. .

     

Fine structure and inverse population in the MnKβ1 spectra of MnxZnyFezO4 ferrites

The electronic structure of Mn_xZn_yFe_zO₄ single-crystal ferrites was investigated. The fine structure of their X-ray spectra was found to correlate with the populations of quantum states. A scheme is suggested for the evolution of the electronic structure with strengthening exchange interaction of valence electrons as the chemical composition of the system becomes more complex. The metal-insulator phase transition is possible in the range of compositions corresponding to densities from ρ=5.097 g/cm³ to ρ=5.125 g/cm³, where manganese changes its charge state.