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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.

The structure, its defects, nanostructural clustering and functional properties of the La0.7Ca0.3-xKxMn1+xO3-δ ceramic with x = 0–0.3 have been investigated by x-ray diffraction, thermogravimetric, SEM, resistance, magnetic, magnetoresistance and dielectric spectroscopy methods. We establish that the lattice parameter of the cubic structure increases when the Ca2+ ions are substituted with ions of K+. We show that the real perovskite structure contains different valence states of manganese MnA2+,MnB3+ and MnB4+ in A- and B-positions, as well as vacancy type point defects, in the form of anionic V(a) and cationic V(c) vacancies. The increase in the content of the superstoichiometric manganese is connected to the nanostructured clustering process. We find that K+ and superstoichiometric manganese cause the increase in relative permittivity e′ and dielectric loss tangent angle tg δ. The experimental phase diagram of the magnetic state has been constructed and allows determining the composition of the magneto...

X-ray diffraction, magnetic, transport and SEM measurements were used to investigate the role of structural and magnetic inhomogeneities in the formation of magneto-transport properties of the La0.6−xSmxSr0.3Mn1.1O3−δ (x=0–0.6) ceramics. Reduction of the a parameter of perovskite structure and change of its symmetry are shown to be due to both replacement of La3+ by Sm3+ and an increase concentration of anion and cation vacancies. Broad asymmetric 55Mn NMR spectra are indicative of the high frequency electron double exchange Mn3+↔Mn4+ and demonstrate the heterogeneity of magnetic and valence Mn-states. An excess manganese is dissolved in perovskite structure statistically without formation of planar antiferromagnetic clusters of Mn2+ ions in the deformed A positions. Constructing phase diagram of “composition–structure–properties” describes a strong correlation between structural, resistive and magnetic properties as well as reflects a special role of structural defects in the formation of magneto-transport properties of the rare-earth manganites.

The influence of substitution of Fe ions for manganese on the structure, phase transitions, magnetoresistance, 55 Mn NMR and 57 Fe Mossbauer spectra in the ceramic La 0.6 Sr 0.3 Mn 1.1 − x Fe x O 3 ( x =0–0.15) samples has been studied by X-ray diffraction, electron microscopy, magnetic, 55 Mn NMR and 57 Fe Mossbauer methods. The real rhombohedral perovskite structure ( R 3 ¯ c ) is established to contain the different valence manganese ions (Mn 3+ and Mn 4+ ), anion and cation vacancies as well as nanostructural clusters with Mn 2+ located in the A-sites. Temperature dependences of the a lattice parameter, a ( T ), demonstrate the anomalies near the Curie temperature, T c . Wide asymmetric 55 Mn NMR spectra confirm the high frequency electron double exchange between Mn 3+ and Mn 4+ ions and irregularity of their surrounding by other ions and defects. According to the Mossbauer spectroscopy data, Fe 3+ ions ( ~ 80 % ) substitute for Mn 3+ and partially Mn 4+ in the B-positions. The rest of Fe 3+ (Fe 2+ ) ions and clusters with Mn 2+ are located in the A-positions. The temperatures of metal–semiconductor and ferromagnet–paramagnet phase transitions are reduced with increasing x , and the magnetic irregularity increases due to the weakening high-frequency Mn 3+ ↔Mn 4+ double exchange by Fe 3+ ions. The amount of ferromagnetic phase is also reduced. The anomalous hysteresis is interpreted as a result of anisotropy of exchange interaction between the ferromagnetic matrix and antiferromagnetic cluster containing Mn A 2 + ions. The phase diagram demonstrates the strong correlated interrelation among magnetic, transport and magnetoresistance properties.

X-ray powder diffraction, resistance, magnetic susceptibility, magnetization, nuclear magnetic resonance (NMR) and magnetoresistance measurement methods were used to study the influence of excess manganese on structure defects and functional properties of (Nd0.7Sr0.3)1−xMn1+xO3 ceramics annealed at 1200 and 1500 °C. It was established that, with an increase in both excess manganese and annealing temperature, the perovskite structure defects, resistance and activation energy increase, while the magnetic susceptibility and fraction of ferromagnetic phase decrease. The occurrence of nanostructured planar clusters is shown to be caused by the presence of excess manganese. Broad asymmetric 55Mn NMR spectra are indicative of both the high-frequency electron Mn3+ ↔ Mn4+ exchange and the uneven distribution of ions and vacancies. The magnetoresistance effect near the Curie temperature increases with increasing annealing temperature and excess manganese content. Magnetic phase diagrams characterizing a strong correlation between the composition, structural defects and functional properties in manganese–neodymium perovskites were constructed.

Detailed studies of manganese zinc ferrites for electronic applications are used to assess the effect of oxygen nonstoichiometry on their structural, electrical, and magnetic properties. The optimal synthesis conditions and nonstoichiometry are determined which ensure the preparation of high-permeability manganese zinc ferrites with low electromagnetic energy losses. The manganese zinc ferrites are shown to be attractive as materials for high-performance magnetoresistive electric current and magnetic field sensors and spintronic devices.

Ceramic samples of manganite perovskites La0.6 − xPrxSr0.3Mn1.1O3 (x = 0−0.6) have been studied using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R\(\bar 3\)c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature Tmi and the ferromagnetic-paramagnetic phase transition temperature Tc with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn3+ ↔ Mn4+. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn2+ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened 55Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn3+ Mn4+ in the B-positions and a local heterogeneity of their surrounding by other ions (La2+, Pr3+, Sr2+) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures Tmi and Tc, and magnetoresistive properties.

The effects of high pressure (3 GPa) and torsional strain on the structure and properties of compacts prepared from nonstoichometric lanthanum-strontium manganite La0.6Sr0.2Mn1.2O3 ± δ (LSMO) nanopowder with a perovskite structure have been studied for the first time using X-ray diffraction, low-temperature Ar adsorption, electron microscopy, and magnetic (χac, 55Mn NMR, and magnetoresistance) measurements. The laws governing the influence of this treatment on the structure, local valence and magnetic states of manganese ions and their inhomogeneous environment (involving other ions and defects), and on the resistivity, microstresses, coercivity, Curie temperature, and magnetoresistance of LSMO nanopowder compacts have been determined. A positive effect of several cycles of torsion under pressure on the magnetoresistance of this material has been observed for the first time.

Magnetoresistive ceramic samples La0.7Ca0.3 − x Sr x MnO3 ± δ sintered at temperatures of 1150 and 1350°C are investigated using X-ray diffraction, microscopic, resistance, and magnetic (χ, 55Mn NMR) measurements. The specific features of the influence of the composition on the type and parameters of the perovskite structure, its imperfection, the porous crystallite structure, the metal-semiconductor and ferromagnetic-paramagnetic phase transitions, the 55Mn NMR spectra, and the magnetoresistance effect are established. The magnetic phase diagram is constructed. The conclusions are drawn regarding the nonuniformity of the distribution of ions and vacancies around manganese involved in the high-frequency electron-hole exchange (Mn3+ ai Mn4+) and the nanostructured separation of the perovskite structure containing anion and cation vacancies, with the concentrations and magnetoresistance effect decreasing and the lattice parameters and phase transition temperatures increasing as calcium is replaced by strontium.

The structure and properties of magnetoresistive ceramics La0.6Sr0.2Mn1.2 − x Co x O3 ± δ (x = 0−0.3) sintered at a temperature of 1200°C are investigated using x-ray diffraction, resistance, and magnetic (χ ac , M, 55Mn NMR) measurements. It is shown that the samples contain the rhombohedral (R $$ \bar 3 $$ c) perovskite (90%) and tetragonal (I41/amd) hausmannite (10%) phases. The lattice parameters of these phases decrease with an increase in the cobalt content x. The real perovskite structure involves point defects (anion and cation vacancies) and nanostructured defects of the cluster type. An analysis of the asymmetrically broadened 55Mn NMR spectra confirms the high-frequency electron-hole exchange between Mn3+ and Mn4+ ions and a local inhomogeneity of their environment by other ions and defects of the point and cluster types. An increase in the Co content leads to an increase in the electrical resistivity, an enhancement of the magnetoresistance (MR) effect, and a decrease in the magnetic susceptibility and the temperatures of the metal-semiconductor (T ms ) and ferromagnetic-paramagnetic (T C) phase transitions due to the suppression of the exchange interaction between Mn3+ and Mn4+ ions by vacancies and clusters. The introduction of cobalt results in a decrease in the ferromagnetic component and the activation energy. The magnetoresistance effect in the vicinity of the phase transition temperatures T ms and T C is associated with the scattering of charge carriers from intracrystallite inhomogeneities of the lattice, and the low-temperature magnetoresistance effect is governed by the tunneling at the intercrystalline boundaries.

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.

The magnetic and transport properties of La1−xMn1+xO3 manganites with excess manganese are studied. It is shown that magnetic and charge ordering heavily depends on the superstoichiometric manganese content, magnetic field, and pressure. The magnetoresistive effect (MRE) is enhanced as the manganese concentration increases. In addition to the paramagnet-ferromagnet transition, the temperature dependences of the magnetization exhibit anomalies at low temperatures in samples with x=0.1–0.4. The magnetization decreases at T

The Mossbauer and X-ray diffraction techniques are used to study the evolution of structure and chemical inhomogeneities in magnesium-zinc ferrites. These are the octahedral short-range environment of a magnetic Fe3+ ion, mesoscopic quasi-planar regions of size 100–200 A (clusters) with locally changed chemical composition and crystallographic order, and the general long-range order of the ferrite matrix. In each of these three inhomogeneities, the high-temperature long-term firing of the Mg-Zn ferrite (T=1280°C, τ=0–8 h) induces transitions (magnetic in the short-range order, concentration in the clusters, and structural in the matrix) within the time period τ=1–2 h. Subsequently, the structure stabilizes with the initial three-scale hierarchy retained.

The La 0.6 Sr 0.2 Mn 1.2− x Cr x O 3 (0⩽ x ⩽0.2) ceramic samples with perovskite-like structure (R 3 c) have been studied using the X-ray diffraction, NMR, magnetic and resistance methods. The 55 Mn NMR spectrum was shown to depend strongly on the Cr/Mn ratio. A single symmetric line centered at 379 MHz was found for La 0.6 Sr 0.2 Mn 1.2 O 3 ( x =0). For La 0.6 Sr 0.2 Mn 1.1 Cr 0.1 O 3 , the 55 Mn NMR spectrum is slightly expanded and shifted to lower frequencies. An analysis of the 55 Mn NMR spectrum for the La 0.6 Sr 0.2 MnCr 0.2 O 3 compound has shown that a wide asymmetrical line can be uniquely expanded into four components with different resonance frequencies. These components arise due to both different ionization states of the manganese ions and a non-equivalence of their crystallographic surrounding. There are manganese ions, whose ionization states are close to (3+) and (4+), as well as Mn ions with intermediate valence which resulted from the high-frequency electron exchange. The Curie temperature and the temperatures of both resistance maximum and magnetoresistance peak were found to decrease, while the value of magnetoresistance increases with increasing Cr content.

Taking into account changes in the Cu charge states, general characteristics and distinctive features of structural transformations typical of high-temperature superconducting and ferroelectric phases in Y-Ba-Cu-Ti-O ceramic samples are studied. It is shown that with substitution of titanium for Y and Cu the concentration of Jahn-Teller ions Cu2+ detected by EPR technique does not rise higher than 10%. According to X-ray spectroscopy data, the Cu charge state lowers from Cu2.2+ to Cu0.5+ with increasing titanium content. It is found by X-ray diffraction studies that with increasing Ti content, the fraction of the layered structure containing (00l) planes of orthorhombic structure decreases and the growth of planes of the (11l) type pertaining to tetragonal structure is stimulated. The structural phase transition from the orthorhombic to tetragonal crystal structure mainly occurs in the range x=0.3–0.5.

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.

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.

The concept of effective coordinate charge is used to show that the relative shift in the K absorption edge of ions of transition metals in simple oxides and ferrites is determined by their valence. Formulas are obtained linking the shortwave shift of the K-edge with the valence of the ions in ferrites having the structure of a spinel. A method is proposed for determining the mean valence of ions of iron and manganese in manganese-bearing ferrites. The method is used to study cooling of commercial MnZn-ferrite with different oxygen partial pressures.

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.

X-ray and Mossbauer spectroscopy is used to investigate the effect of vacuum cooling on the valence of cations of manganese and iron, nonstoichiometry and the degree of inversion of manganese-zinc ferrites. The dependence of the valency of the cations of manganese and iron, the degree of inversion, and the degree of nonstoichiometry on the partial pressure of oxygen produced in the cooling stage is established. An interrelationship is found between the defects and the degree of inversion of manganese-zinc ferrites.

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.