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Influence of manganese substitution with 3d-ions on the structure, defectiveness, microstructure as well as resistive, dielectric and magnetoresistive properties of single-phase nonstoichiometric La0.6Sr0.15Bi0.15Mn1.1-xBxO3-δ (B = Cr, Fe, Co, Ni) compositions have been studied. All compositions demonstrate slight lattice parameter change of rhombohedral R 3 ¯ c distorted perovskite structure. The molar formulas of a real perovskite structure have been determined. The real structure is defect and contains cation V(c) and anion V(a) vacancies. The resistivity ρ increases with increase of x for all compositions. There is no “metal-semiconductor” phase transition at temperature Тms for x ≥ 0.20. It has been shown that the composition with B = Cr has the highest relative permittivity at x = 0.3. For the first time, the electron-phonon interaction constant γph has been determined for all the compositions within the framework of small polaron hopping model and it has been found that the strongest change of functional properties is a characteristic of the compositions with B = Fe, for which γph takes the maximum value γph ≈ 7.7.

Ceramic samples of La31−xSr2+xMnO2−3 (x = 0, 0.1, 0.15, 0.2, 0.3, 0.4), sintered at different temperatures (1150, 1200, 1350, and 1500 °C) are investigated by means of methods of x-ray diffraction analysis and resistive and magnetic analysis. It is established that the perovskite structure parameter (a) and degree of its rhombohedric \((R\bar 3c)\) distortion (α) both decrease with increasing content of Sr2+ in La1−xSrxMnO3±δ (x = 0–0.4). The metal-semiconductor (Tms) and ferromagnetic-paramagnetic (Tc) phase transition temperatures and magnetoresistive peak temperature (Tp) all increase, while the activation energy (Ea) decreases. The density of defects in the lattice which contains anion (V(a)) and cation (in A and B positions) vacancies, average valency of manganese (ω), and tolerance factor (t) also increase. It is shown that with Sr2+ doping, charge compensation is achieved by two mechanisms, first, the Mn3+ → Mn4+ transition (∼80%), and second, the formation of V(a) vacancies (∼20%), with the contribution of the second mechanism increasing with increasing sintering temperature. With increasing x the magnetoresistive effect decreases at Tp and the lowfield tunneling magnetoresistive effect increases slightly. It is established that the concentration dependence of the intracrystaline peak magnetoresistive effect at Tp is opposite to the concentration dependence of the intercrystalline magnetoresistive effect in the semiconductor region for T ≪ Tp.

Studies carried out using various methods — x-ray structural and spectral, electromagnetic, gravimetric, nuclear γ-resonance, and nuclear magnetic resonance — revealed that the crystal-chemical structure of mono-and polycrystalline manganese — zinc ferrites used in video tape recorders and color television sets possess defects, with various degrees of localization (point, plane, and cluster), existing simultaneously in both the cation and anion sublattices. Differences in the structure of mono- and polycrystalline specimens were observed, but the specimens were similar with respect to the nature of the clustered defects. A relationship between the defect structure and specified electromagnetic properties of the ferrites was established. It is concluded that more information is needed concerning the actual crystal-chemical structure of ferrites containing defects of different types and dimensions (micro, meso, macro).

Comprehensive study of the structure and properties of ferrite materials prepared from powders with different specific surface (0.4 m2/g

The clustered inhomogeneity observed in ferromagnetic materials deepens our concepts of the actual structure of solids and opens new possibilities for controlling their properties. These investigations were made for the purpose of establishment of the relationship between clusterization and magnetic properties of SrO-nFe{sub 2}O{sub 3}, where 5.4 < n < 6.2, metal oxide magnetically hard strontium ferrites.

Mossbauer, electromagnetic, and gravimetric studies have been used to examine the influence of ferrite powder specific surfaces, the isostatic compacting pressure, and the sintering temperature on a set of physical properties for isotropic barium ferrites. The optimum production parameters have been determined, such that Ssp=103 m2/kg, P=0.6−0.7 GPa and Ts=1240°C. A correlation has been obtained between the microstresses, the fine crystalline structure parameters and the electromagnetic properties for sintered specimens as a function of the process conditions. It has been shown that the Fe57 nuclei states located in the trigonal bipyramid of a magnetic plumbate network of barium hexaferrites are most sensitive to temperature changes. High isostatic pressures together with other optimized process conditions enable the isotropic barium ferrites quality to be increased to that of the 7BI215 belt quality.

Data yielded by x-ray diffraction, magnetic, metallographic, pycnometric, gravimetric, and dilatometric investigations into the kinetics of formation and sintering of ferrites with various crystal structures have enabled the mechanisms of these processes to be established. It is shown that the formation of a new (ferrite) phase is effected on the basis of the lattice of a more stable oxide, with preferential diffusion of cations of the other oxide. The processes of formation and sintering of ferrites taking place under nonequilibrium conditions are satisfactorily described by a nonequilibrium thermodynamics equation. The ferrite synthesis and sintering processes have comparatively small energies of activation, which is due to the high degree of defectiveness of the ferrite crystal lattice.

The degree of oxidation of ferrite powders from a high-quality batch is lower and that of parts sintered from them higher compared with a low-quality batch. Manganese-zinc ferrospinel parts of better quality are characterized by higher degrees of nonstoichiometry and inversion in the presence of manganese and iron cations of different valences. The lower electrical conductivity of these specimens is attributable to a complex character of reactions of cation-anion and electron-hole defects. To obtain high-quality manganese-zinc ferrites, it is necessary to create conditions preventing overoxidation during the stage of coprecipitation and synthesis of ferrite powders and enabling sintered parts to be produced whose spinel phase deviates slightly from stoichiometry in the direction of an excess of oxygen.

The temperature ranges of the physicochemical transformations occurring during heating in compacts from manganese-zinc ferrite powders produced by different methods have been established. The effect of method of preparation of ferrite powders on the character of processes taking place in compacts during heating is demonstrated. The degree of decomposition of the spinel at a given rate of heating is lower in powders produced by the carbonate coprecipitation method than in powders obtained from an oxide mixture. The reason for this is that the structure of carbonate-grade powders has comparatively few defects. Lowering the rate of heating of specimens in the temperature range 550–650‡C increases the degree of decomposition of the spinel, ensures a better reproducibility of properties, and improves the quality of ferrite parts.

Changing the pressure in the hot pressing of manganese-zinc ferrites affects both the composition and state of the solid-phase system and the structural characteristics of specimens. With rise in hot-pressing pressure, the Fe2+ concentration, density, Curie temperature, and degree of inversion increase; there is also a tendency for the initial magnetic permeability to fall and for the crystallite size to grow. An interrelationship is shown to exist between the Fe2+ content of manganese-zinc ferrospinel on the one hand and the degrees of nonstoichiometry and inversion on the other.

The heating and cooling of a Mn-Zn ferrite during sintering are accompanied by solid-phase oxidation-reduction reactions, additional ferritization, and deferritization, which lead to further changes in the volume and density of the solid-phase system. Structural defects forming during the synthesis of a ferrite powder have a marked effect on its sintering and on solid-phase reactions associated with it. The sintering activity of an “oxide” ferrite powder manifests itself in greater shrinkage, higher density, and a larger grain size, and is due to its more defective structure. The sintering of a Mn-Zn ferrite can be arbitrarily divided into three consecutive stages in which superplasticity, diffusion-viscous flow of material, and vacancy pore dissolution are the dominant mechanisms. The healing of pores during the sintering of a ferrite powder is accompanied by a Frenkel' effect, which is due to the difference in the diffusion coefficients of the components forming the ferrite.

In the formulation of new ferrite materials for magnetic head cores starting raw materials and conditions of preparation of ferrite powders must be chosen with regard to the conditions prevailing in the hot-pressing process. The differences in the properties of the two hot-pressed ferrite batches investigated are attributable to the difference in the activities of the ferric oxide grades used (MM2 and SP) in the solid-phase synthesis and sintering reactions. Use of grade MM2 ferric oxide in the preparation of ferrite powders and hot pressing and annealing under optimum conditions (psp = 40 MPa, Thp = 1663°K, τih = 2.4 ksec, Tann = 1613°K and τann = 7.2 ksec) enable specimens to be obtained combining high values of initial magnetic permeability (μin = 10,000) and density.

It has been established that the mechanism of formation of new crystalline structures can be studied by determining the variation of the most probable crystallite size as a function of the ratio between the amounts of the starting phases. It is shown that magnesium and nickel ferrites with spinel structures form on the bases of the thermally-more-stable magnesium and nickel oxide lattices, with preferential diffusion of iron cations. To the initial stages of formation of these ferrites there will correspond a nonstoichiometric solid solution of increased divalent cation concentration. The crystal lattices of such spinel solid solutions are deficient in oxygen and contain interstitial cations.

The particle size distribution of a high-permeability manganese-zinc ferrite powder has a pronounced effect on the properties of hot-pressed parts. Ferrite powders from which high-density specimens with μo≥ 10,000 were produced by hot-pressing were of uniform particle size, with a predominance of particles of 1- to 2-μm size. Hot-pressed specimens obtained from a ferrite powder of nonuniform particle size were characterized by lower density and lower initial magnetic permeability.

High isostatic pressures were found to have a beneficial effect on the sintering and magnetic properties of nickel-zinc ferrite parts. Raising the pressure to 20 kbar: 1) increased the density of specimens by 10%, decreasing the closed and open porosities by 33 and 70%, respectively; 2) increased the initial magnetic permeability and its relative thermal stability by 19%; 3) diminished the relative tangent of losses by 24% in a field of 0.8 A/m and by 17% in a field of 8 A/m.