Effect of quenching temperature on magnetization and cation distribution in magnesium based UHF ferrites

The paper gives the results of measuring the specific saturation magnetization, cation distribution in the sublattices, and Curie points for alumino-magnesium, magnesium-chromium, and magnesium-manganese UHF-ferrites according to quenching temperature. General formulas were obtained for sublattice cation distribution in magnesium-based mixed ferrite-spinels, and the results of theoretical analysis were compared with experimental findings.     

Measuring the 9-component magnetic permeability tensor of ferrites

The magnetic permeability tensor of ferrite monocrystals can, in the general case, be represented as consisting of nine components [1]. In [2] the 6-component magnetic permeability tensor was obtained for a triaxial ferrite monocrystal with low magnetic anisotropy energy magnetized by a constant field in the crystallographic plane (110). In the general case of magnetization in an arbitrary crystallographic direction the tensor becomes a 9-component tensor [3]. In the context of the increasing application of ferrite monocrystals in microwave engineering the question of measuring such a tensor is of current scientific and practical interest. In the present article we propose a method for measuring the 9-component tensor for ferrite materials at microwave frequencies.     

Effect of porosity on the permeability tensor of polycrystalline ferrites (independent grain approximation)

The effect of porosity on the form of the permeability tensor is calculated using the independent-grain approximation; this procedure is similar to the Schlömann method. The theoretical curve for the resonant-field distribution is approximated by the Lorentz curve using the method of least squares. It is shown that with this approximation, porosity increases the width of the ferromagnetic resonance line for a non-porous material by the width of the Lorentz distribution curve; thus the resonant field shifts toward lower values. Formulas are obtained for the resonant-field shift due to porosity and the broadening in the ferromagnetic resonance line; these formulas differ somewhat from the Schlömann formulas. In order to check the working formulas and the applicability of the independent-grain approximation, measurements were performed on the tensor for magnesium-chromium-copper ferrites with variable porosity and a magnetization on the order of 1200 gauss at a frequency of 4000 mHz. Specimens having the form of longitudinally magnetized circular cylinders were used so that there was no degeneration in uniform precession of magnetization with long spin waves. The observed effect of porosity on the width of the ferromagnetic resonance line (determined by measuring the tensor) was found to be in good quantitative agreement with calculation. The shift in the resonant field due to porosity was negligibly small, which also agreed with calculation. The experimental results show that when there is no degeneration in uniform precession with spin waves, the independent-grain approximation can be used in experiments even when the magnetization and resonant field are approximately equal.Here, we must allow for the static magnetic susceptibility in the formulas for the resonant-field shift and the broadening in the ferromagnetic resonance line.     

Influence of Cd and Cr substitutions on the magnetization and permeability of magnesium ferrites

Magnetization and permeability of polycrystalline ferrites with general formula Cd_xMg_1−xFe_2−yCr_yO₄ (x=0, 0.2, 0.4, 0.6, 0.8, 1.0; y=0, 0.05 and 0.10) were studied. Study of saturation magnetization reveals that the Neel's two-sublattice model exists upto x=0.4, for y=0, 0.05 and 0.1 and a three-sublattice model (YK-model) is predominant for x>0.4 and y=0, 0.05 and 0.10. The saturation magnetization and magnetic moment were found to decrease with the increase in Cr³⁺ contents, which is attributed to the dilution of B–B site interaction. Variation of initial permeability with temperature revealed the long-range ferromagnetic ordering in the compounds with x=0.4. The sample with x?0.4 and y=0, 0.05 and 0.10 showed peaking behavior near Curie temperature, which is attributed to the decrease of anisotropy constant K₁ to zero. Low-frequency dispersion of initial permeability suggests domain wall displacement. Addition of Cd²⁺ resulted in a sharp decrease in Curie temperature. With the addition of Cr³⁺, initial permeability was found to decrease.     

Room temperature static lattice dielectric constant of lead telluride by a microwave cavity-perturbation technique

We have measured the room-temperature static lattice dielectric constant of PbTe using a cavity-perturbation technique. The result, ?_s = 800 ± 220, implies a transverse optic phonon frequency $\text{ω}{}_{\mathrm{<mtext>TO</mtext>}}\text{= 23 ±}{}^4{}_3$ cm^-1 in agreement with values extrapolated from low-temperature magnetoplasma measurements but in disagreement with the commonly-quoted neutron-diffraction result of Cochran et al., ω_TO = 31.7 ± 1.3 cm^-1.     

Analysis of the microwave magnetic permeability spectra of ferrites with hexagonal structure

This paper presents the results of theoretical and experimental investigations of the microwave magnetic permeability spectra of polycrystalline ferrites with hexagonal crystal structure. It is shown that the observed nonmonotone frequency dependence of the permeability is due to the features of the natural ferromagnetic resonance in the presence of a domain structure. The calculations of the contribution of the rotation of the magnetization vector to the permeability performed in the approximation of independent grains adequately describe experimental data. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 119–124, September, 2006.     

Structure, electric and dielectric studies of indium-substituted magnesium copper manganese ferrites

The structure, electric and dielectric properties of In-substituted Mg-Cu-Mn ferrites having the general formula of Mg_0.9Cu_0.1Mn_0.1In_xFe_1.9−xO₄ with 0.0≤x≤0.4 have been studied. X-ray diffraction (XRD) patterns of the samples indicated the formation of single-phase cubic spinel structure up to 0.2 and mixed phase (cubic and tetragonal phase) for samples x≥0.3. The relation of conductivity with temperature revealed a semiconductor to semimetal behavior as In⁺³ concentration increases. Variation in the universal exponent s with temperature indicates the presence of two hopping conduction mechanisms: the correlated barrier hopping (CHB) at low In⁺³ content x≤0.1 and small-polaron (SP) hopping at In⁺³ content x≥0.2. The variation in dielectric permittivity (ε′, ε″) with temperature at different frequencies shows a normal behavior for the studied compounds, while the variation in dielectric loss tangent with frequency at different temperatures shows abnormal behavior with more than relaxation peak. The conduction mechanism used in the present study has been discussed in the light of electron exchange between Fe³⁺ and Fe²⁺ ions and hole hopping between Mn²⁺ and Mn³⁺ ions at the octahedral B-sites.     

Tailoring the temperature characteristics of the magnetic permeability of NiZn ferrites

The effect of cobalt addition on the temperature characteristics of the magnetic permeability of NiZn ferrites was studied and a comparison to the respective behaviour of cobalt in NiCuZn ferrites was examined. Cobalt-doped NiZn and NiCuZn ferrites were manufactured by the ceramic route and sintered under various atmosphere profiles. The chemical and morphological characteristics were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The evaluation of the magnetic performance of the sintered ferrites showed that N₂-rich atmosphere profiles during the top temperature and cooling time of the sintering process favour the temperature stability of the permeability in the case of NiZn ferrites, while preserving the losses at low levels. Two mechanisms that take place at the same time are proposed: the change of the Fe²⁺/Fe³⁺ ratio due to the reduction-promoting atmosphere of N₂ in combination with an increase in magnetocrystalline anisotropy and magnetostriction due to the presence of Co²⁺ suggest a useful method to tailor the temperature factor α_F of NiZn ferrites. However, the method cannot be applied in NiCuZn ferrites, as the reduction Cu²⁺-Cu⁺ taking place under N₂-rich atmospheres enhances secondary re-crystallization phenomena, causing a dramatic increase in losses.     

Effective dielectric and photoelastic tensor components in thin layered superlattices

The effective photoelastic constants of a superlattice composed of thin alternating layers of orthorhombic symmetry are calculated as functions of dielectric, elastic and photoelastic constants of the constituents. In a preliminary step to this calculation, we also obtain the effective dielectric tensor for any symmetry of the layers. It is shown that appropriate combinations of these effective constants are arithmetic averages of the corresponding quantities in the constituents, each layer having a weight equal to its relative thickness.     

Effects of microstructure on the temperature dependence of relative initial permeability of NiCuZn ferrites

In the present work, ferrites with compositions of (Ni_0.27Zn_0.64Cu_x)Fe_1.98O₄ (x=0.1, 0.2) were prepared by conventional ceramic methods. The relationship between the microstructure and the temperature dependence of relative initial permeability was investigated. It was found that intergranular pores in the ferrites generate large demagnetizing fields, reducing the temperature dependence of the effective anisotropy field H_eff, and decreasing the temperature dependence of the relative initial permeability accordingly. However, intragranular pores pin the movement of domain walls. Compared with the permeability induced by domain wall motion, the permeability that resulted from the spin rotation is more sensitive to temperature. As a result, the relative initial permeability of ferrites becomes more sensitive to temperature with increasing micropores within grains.     

The relation between the inner demagnetizing factor of manganese magnesium ferrites and their porosity and permeability

It is shown that the simple relation of direct proportionality between the inner demagnetizing factor and the relative volume of the pores is not sufficient to explain the experimental data obtained on a system of manganese magnesium ferrites. An equation is derived in which the inner demagnetizing factor is directly proportional to the ratio of the relative volume of the pores to the permeability of the sample. The values calculated according to this equation agree well with the experimental results.

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Temperature dependent microwave dielectric constant of TTF-TCNQ and related one-dimensional conductors

The results of an experimental study of the low temperature (4.2–40 K) complex dielectric constant of the organic conductors (TTF)(TCNQ), (TMTTF)(TCNQ), (DSeDTF)(TCNQ), (TSeF)(TCNQ), and the alloy (TTF)_0.97(TSeF)_0.03(TCNQ) are described. The similar features observed in these different systems suggest that a common mechanism is involved.     

Pressure and temperature dependence of the static dielectric constant of KBr

The static dielectric constant and the temperature and pressure derivatives of the static dielectric constant of KBr have been measured at several temperatures between 4·3°K and room temperature. The sample was prepared in the form of a three-terminal parallel-plate capacitor and the dielectric constant was determined from measurements of the capacitance with a high precision bridge. The dielectric constant decreases with temperature as does the magnitude of the temperature dependence and the magnitude of the pressure dependence. The data were used to calculate the fixed-volume temperature derivative of the dielectric constant. This quantity, (? ln ε/?T)_v, exhibiting lattice-anharmonicity effects, decreases slightly from its room temperature value as the temperature is lowered, rises to a maximum value at about 33°K and then decreases rapidly at lower temperatures.     

Bounds for the complex dielectric constant of a two-phase composite

We study the effective dielectric constant of a two-phase composite for the case where the dielectric constants of the two components are complex. Milton has derived a sequence of narrowing bounds in the complex plane limiting the possible values of the effective dielectric constant, assuming that certain geometrical coefficients characterizing the medium are known. We cast Milton's bounds in a different form, making use of auxiliary functions introduced by Bergman. We derive explicit expressions for the bounds up to third order and compare values for the corresponding geometrical coefficient derived for different geometries.     

Low temperature sintering of sub-stoichiometric Ni-Cu-Zn ferrites: Shrinkage, microstructure and permeability

We have studied sub-stoichiometric Ni-Cu-Zn ferrites with iron deficiency (i.e., <50mol% Fe₂O₃) of composition Ni_0.20Cu_0.20Zn_0.60+zFe_2−zO_4−(z/2) with 0≤z≤0.06. The temperature of maximum shrinkage rate is shifted from T=1000 °C for z=0 towards lower temperatures down to T=900 °C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900 °C for z>0 only. Sub-stoichiometric compositions (z>0) do not form single-phase spinel ferrites after sintering at 900 °C, but rather represent mixtures of CuO and a stoichiometric ferrite with slightly modified composition. The formation of small amounts of CuO at grain boundaries is demonstrated by XRD and SEM. The permeability is increased from μ=80 for stoichiometric ferrites (z=0) to μ=660 for z=0.02. The formation of CuO during sintering of sub-stoichiometric ferrites supports densification and is a prerequisite for low temperature firing of multilayer inductors. Addition of 1 wt% Bi₂O₃ as liquid phase sintering aid is required to provide sufficient densification of the stoichiometric ferrite (z=0) at 900 °C. Addition of 0.37 wt% Bi₂O₃ to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900 °C; however, the microstructure formation is dominated by heterogeneous grain growth.     

The dielectric tensor components of anthracene derived from energy loss experiments

Energy loss spectra of anthracene single crystals corresponding to the dielectric tensor elements ? _xx , ? _yy and ? _zz were measured with 60 keV electrons, after having fixed the main axes of the dielectric tensor corresponding to ? _xx and ? _zz . The real and imaginary parts of the dielectric tensor elements are derived from the loss functions by a Kramers-Kronig-analysis. The results are compared with those obtained by other autors and with measurements on graphite.     

Quasi-optical measurement of complex dielectric constant at 300 GHz

A two beam interferometer in the Martin-Puplett configuration is used to determine the complex dielectric constant at 300 GHz of teflon, TPX-plastics, SPECTRALON and paraffin waxes with melting temperatures of 48° C and 72° C, respectively. The design of the quasi-optical system leads to a constant beam diameter at the power detector independent of path delay and frequency. The power detector signal is recorded not only along one period but over about 50 periods. A spectrum estimation routine allows to determine more exactly amplitude and phase angle of the signal. A basic problem is noticed: imperfect detector and source match cause harmonic distortion of the power detector signal. The effects on processing the loss tangent and the invalidation are shown. Finally loss tangent and dielectric constant are determined indirectly by optimizing an equivalent microwave circuit using a commercial available microwave design system to take multiple reflections and losses in consideration.