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The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La0.3Ln0.3Sr0.3Mn1.1O3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, 55Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La3+, Pr3+, Nd3+, Sm3+, or Eu3+ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (T ms) and ferromagnet–paramagnet (T C) phase transitions and the increase in electrical resistivity ρ and activation energy E a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d 4(Mn3+)–2p 6(O2–)–3d 3(Mn4+)–V (a)–3d 4(Mn3+). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric 55Mn NMR spectra support the high-frequency electronic double exchange Mn3+(3d 4) ↔ O2–(2p 6) ↔ Mn4+(3d 3) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.

Structure and its defects, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ magnetoresistive ceramics were investigated by x-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR and magnetoresistance methods. It was found that isovalent substitution of lanthanum by europium A-cation of a smaller ionic radius increases the structural imperfection and leads to a symmetry change from the rhombohedrally distorted perovskite structure of R 3 ¯ c symmetry to the pseudocubic type. It was shown that the real structure contains anionic and cationic vacancies, the concentrations of which increases with the Eu concentration and the sintering temperature tann. A decrease in the temperature of the metal–semiconductor Tms and ferromagnetic–paramagnetic TC phase transitions as well as an increase in the resistivity ρ and the activation energy Ea with increasing x are due to an increase in vacancy concentration, which weakens the high-frequency electron double exchange Mn3+ ↔ ...

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.

Ceramic samples of Pr0.6−xNdxSr0.3Mn1.1O3−δ (x = 0−0.6) were studied by x-ray diffraction, resistive, magnetic (χac and 55Mn NMR), magnetoresistive and electron microscopy methods. It was shown that with increasing the concentration x, the type of unit cell distortion changes from orthorhombic (x = 0–0.2) to pseudo-cubic (x = 0.4–0.6), and the imperfection of the structure, which contains anion and cation vacancies, is increased. A decrease in the temperatures of metal–semiconductor (Tms) and ferromagnetic–paramagnetic (TC) phase transitions and an increase in the resistivity and activation energy with increasing x was explained by an increase in the concentration of vacancies, which weakens high-frequency electronic double-exchange Mn3+ ↔ Mn4+. It was found that the compositions with a higher content of neodymium exhibit a transition to the antiferromagnetic state at temperatures below 130 K. Two types of magnetoresistive effects were observed. The magnitude of the first effect, which occurs near the pha...

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.

The structure, lattice imperfection, and properties of ceramic samples La0.6 − x Nd x Sr0.3Mn1.1O3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $$\bar R$$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn2+ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T ms and ferromagnetic-paramagnetic phase transition T C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened 55Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn3+-O2−-Mn4+) ↔ (Mn4+-O2−-Mn3+) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn3+/Mn4+ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχac(T) and the 55Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.

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 lanthanum strontium manganite perovskites La0.6Sr0.2Mn1.2 − xNbxO3 (x = 0–0.3) annealed at temperatures of 1260 and 1500°C have been investigated using the X-ray diffraction, electron microscopic, resistive, magnetoresistive, and magnetic (χac, 55Mn NMR) methods. It has been found that there is a correlation between the increasing unit cell parameter a of the rhombohedral R\(\bar 3\)c structure and the average ionic radius with increasing niobium concentration x and annealing temperature for the case where the lattice contains anion vacancies, cation vacancies, and nanostructured clusters. The observed increase in the electrical resistivity and decrease in the temperatures of metal-semiconductor phase transition Tms and ferromagnetic-paramagnetic phase transition TC with an increase in the niobium concentration x and the annealing temperature have been explained by the decrease in the content of the ferromagnetic phase, as well as by changes in the ratio Mn3+/Mn4+, the oxygen nonstoichiometry, and the concentration of defects weakening the high-frequency electronic exchange of the ions Mn3+ ↔ Mn4+. The presence of nanostructured clusters in the lattice has been confirmed by an anomalous hysteresis associated with the unidirectional exchange anisotropy of the interaction between the ferromagnetic matrix and antiferromagnetic clusters with Mn2+ and Nb3+ in distorted A-positions. An analysis of the asymmetrically broadened 55Mn NMR spectra and their computer decomposition have revealed a high-frequency electronic exchange and an inhomogeneity of the magnetic and valence states of manganese due to the nonuniform distribution of all ions and defects. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures TC and Tms, is caused by scattering at intracrystalline nanostructured heterogeneities of the imperfect perovskite structure, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between the composition, structure, resistive and magnetic properties of rare-earth manganites.

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 structural, resistive, magnetic, and magnetoresistive properties of the La0.6Sr0.3 − xBixMn1.1O3 ceramics have been studied. The substitution of Bi ions for Sr ions increases the lattice parameter of the rhombohedral perovskite structure, decreases the metal-semiconductor and ferromagnet-paramagnet phase transition temperatures and the peak of the magnetoresistive effect, and increases the resistivity, approaching the system to the ferroelectric state. The 55Mn NVR study indicates on the high-frequency Mn3+ ↔ Mn4+ superexchange and heterogeneity of the valence and magnetic states of manganese due to the nonuniformity of distribution of all ions and defects. The phase diagram has been constructed, which shows a strong correlation between the structural, magnetic, and magnetoresistive properties.

Ceramic (La0.7Ca0.3)1 − x Mn1 + x O3 samples are studied by X-ray diffraction, resistive, magnetic, 55Mn NMR, and magnetoresistance methods. The concentration changes of lattice parameter a of the cubic perovskite structure and its average ionic radius are in good agreement if the concentrations of anion and cation vacancies and nanostructured clusters with Mn2+ in the A positions increase with x. Phase transition temperatures T ms and T c weakly depend on x, and the electrical resistivity and the activation energy decrease substantially with increasing x due to a change in the imperfection of the perovskite structure. An analysis of the broad asymmetric 55Mn NMR spectra of the samples indicate a high-frequency Mn3+↔ Mn4+ electron superexchange and nonuniform magnetic and valence states of these ions because of a nonuniform distribution of ions and defects, which decrease the amplitude resonance frequency with increasing x. The magnetoresistive (MR) effect near phase-transition temperatures T ms and T c increases substantially with x and is caused by the effect of a magnetic field on the scattering of charge carriers by intracrystallite nanostructured heterogeneities of an imperfect perovskite structure. The second MR effect is located in the low-temperature range, is related to tunneling through mesostructural crystallite boundaries, and decreases weakly with increasing x. A correlation is found between the hyperstoichiometric manganese content, the imperfection of the perovskite structure, and the magnitude of the MR effect.

The La0.7Pb0.3 − x Sn x MnO3 composition ceramic is studied by X-ray diffraction, resistive, magnetic, electron-microscopic, magnetoresistance, and NMR (55Mn, 139La) methods. The substitution of tin for lead results in structural phase separation into the basic perovskite ( $R\bar 3c$ ) and spinell (Fd3m), phases: La0.7Pb0.3 − x Sn x MnO3 → La0.7 − x Pb0.3 − x MnO3 + 0.5xLa2Sn2O7. Changes in the lattice parameter of the basic perovskite $R\bar 3c$ structure, the electrical resistivity, and the magnetic and magnetoresistance properties are caused by changes in the composition and content of a conducting perovskite ferromagnetic phase, the Mn3+/Mn4+ ratio, and the imperfection of vacancy and cluster types. An in-plane nanostructured cluster is formed by Mn2+ ions located in distorted A-positions. The detected anomalous magnetic hysteresis is induced by the appearance of a unidirectional exchange anisotropy at the boundary of an in-plane antifer-romagnetic cluster coherently joined with a ferromagnetic matrix structure. The broad asymmetric NMR spectra of 55Mn and 139La indicate a high-frequency Mn3+ ↔ Mn4+ superexchange and a nonuniform distribution of ions and defects. The constructed phase diagram characterizes a strong relation between the magnetic and transport properties in rare-earth manganites.

The magnetic and transport properties of anion- and cation-deficient Nd0.52Sr0.48MnO3 films with different thicknesses, as well as of two films from this system grown on different SrTiO3 and LaAlO3 substrates, are studied. Below Curie temperature T C, the films with different thicknesses exhibit phase separation: they represent magnetic clusters (drops) embedded in a nonconducting paramagnetic (at T > T N, where T N is the Neel temperature) or antiferromagnetic (T < T N) matrix. The temperature dependences of the resistivity of the films are well described in terms of the polaron mechanism of conduction. In external magnetic field H = 0.01 T, the drops may reach 15 nm in size. They consist of magnetic polarons with a small radius (1–2 nm). The drops are shown to interact with each other in the films. Because of competition between drop-drop dipole interaction and the magnetic energy, the drops disintegrate into droplets with a size comparable to that of a magnetic polaron in a field of 1 T. An explanation is given for the discrepancy between our results and the frequently observed growth of the drops with a rise in the external magnetic field. As the film gets thicker, the fraction of the ferromagnetic phase grows with thickness nonlinearly. In the film grown on SrTiO3 (compressed by 0.9%), the characteristic Neel and Curie temperatures are lower than in the film grown on LaAlO3. The diameters of ferromagnetic drops (both maximal at H = 0.01 T and minimal at H = 1 T) turn out to be roughly the same as in the films with different thicknesses.

The structure and properties of lanthanum strontium manganite perovskites La0.6Sr0.2Mn1.2 − y Cr y O3 ± δ (y = 0–0.3) sintered at 1430°C have been studied by X-ray, resistive, and magnetic (χac and 55Mn NMR) methods. The parameter of the rhombohedrally distorted (R $$ \bar 3 $$ c) perovskite structure decreases with increasing y. The real perovskite structure contains point (anion and cation vacancies) and cluster-type nanostructure defects. The analysis of asymmetrically broadened 55Mn NMR spectra has confirmed the high-frequency electron-hole exchange Mn3+ ↔ Mn4+ and local inhomogeneity of their surrounding by other ions and point and cluster-type defects. An increase in the Cr content leads to an increase in the resistivity and the magnetoresistive effect and a decrease in the metal-semiconductor and ferromagnetic-paramagnetic phase transition temperatures (T ms and T c ) due to the distortion of the exchange interactions Mn3+ ↔ Mn4+ by chromium ions, vacancies, and clusters. Introduction of Cr decreases the ferromagnetic component and increases the activation energy. The magnetoresistive effect near T ms and T c is caused by scattering of charge carriers from intercrystallite nanostructure inhomogeneities of the lattice, and the low-temperature effect is associated with the tunneling on mesostructural intercrystallite boundaries.

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 main objective of the performed investigations was to enhance sensitivity of a current sensor to weak changes of magnetic field. New design of the sensor of current based on magnetoresistance effect – MRE (MRE = (RH - R0)/R0 , where RH is the resistance in magnetic field and R0 is the resistance without magnetic field) was developed. The sensor was produced in the form of an annular magnet with a gap, in which the (La0.7Sr0.3)0.8Мn1.2О3 manganite film possessing large negative MRE was inserted. Nominal current in a controllable electric circuit can change from a few tenths parts of ampere to a hundred of amperes. The limit detectable change of current value depends on the size of gap in the annular magnet. The operation time of sensor at current overload and short circuit is less than 0.3 sec. These magnetoresistors are thermally stable over the temperature range from (- 50 ° С) to (+ 50 ° С). Proposed sensors based on MRE can be applied in many electrical arrangements and devices.

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.

Amorphous, nanocluster, and self-organizing bilayer Nd0.5Sr0.5MnO3 films are prepared by rf magnetron sputtering. The amorphous film turns out to be a typical paramagnet with free motion of the individual Mn spins, the magnetic properties of which are well described by the Curie-Weiss approximation. The nanocluster film manifests magnetic properties mimicking superparamagnetic particles with a nonclassical magnetic dynamics. The unique shape of the hysteresis loops, which have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, suggests that each particle (nanocluster) is a closure magnetic domain (or magnetic vortex) rather than a single domian. At the same time, a blocked to unblocked transition was observed with increasing temperature, as in the usual superparamagnet. The self-organizing bilayer film demonstrates a negative exchange bias, which is typical for the ferromagnet/antiferromagnet hybrid system in spite of the fact that both layers in our case have a ferro...

The x-ray structural, resistive, magnetic, and Mn55 NMR methods are used to investigate the ceramic magnetoresistive lanthanum manganite perovskites La0.6Sr0.2Mn1,2−xFexO3 (x=0, 0.02, 0.05, and 0.1) annealed at 1170 and 1500°C. It is found that increasing the Fe content decreases the metal-semiconductor phase transition temperature Tms and the Curie temperature TC, and increases the peak of the magnetoresistive effect near these phase transitions and results in growth of the peak at low temperatures, where the magnetoresistive effect is due to tunneling transitions of carriers between crystallites. The wide asymmetric Mn55 NMR spectrum, whose resonance frequency shifts with increasing x to lower frequencies, confirms the high-frequency electron-hole exchange between the Mn3+ and Mn4+ ions and the high degree of lattice imperfections, including not only vacancies but also clusters. The hysteresis in the field dependences of the magnetization at 4.2K is due to a change in the fraction of low-spin Mn2+ in cl...

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 effects of substitution of barium by tin on the phase composition, structural imperfection, and properties of lanthanum manganite perovskites La0.7Ba0.3−xSnxMnO3 (x=0, 0.1, 0.15, 0.2, 0.3) are established by comprehensive studies done by x-ray diffraction, resistive, and magnetic (including NMR on 55Mn and 139La) methods. It is shown that the introduction of Sn leads to the formation of a pyrochloric phase La2Sn2O7, to an increase in the density of lattice defects of the manganese-enriched main lanthanum manganite phase, and to a substantial decrease of the magnetoresistive effect. The smearing of the metal–semiconductor phase transition temperature is explained by an increase in the inhomogeneity and imperfection of the perovskite structure. The low activation energy is confirmed by a high degree of inhomogeneity and imperfection of the crystal lattice of the samples studied. The broad, asymmetric NMR spectra of 55Mn and 139La attest to high-frequency electron exchange between Mn3+ and Mn4+ and noneq...

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

A study is made of the external structure, magnetization, and magnetoresistive effect in ceramic samples of (La0.8Sr0.2)1−xMn1+xO3 annealed at 1150 and 1500 °C. It is shown that the additional annealing at 1500 °C leads to significant changes in the magnetization and increases the magnetoresistive effect by a factor of 2–3, depending on x. An “anomalous” magnetic hysteresis is observed on the initial parts of the magnetization curves in both ceramic and laser-deposited film samples. It is found that the value of the “anomalous” magnetic hysteresis depends on the composition of the samples (on x) and their annealing temperature. A mechanism for the appearance of the “anomalous” hysteresis is proposed, involving the presence and interaction of ferromagnetic and antiferromagnetic phases.

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 effect of Ba content on the phase composition, lattice parameter, Mn-55 NMR spectrum, resistance, and magnetoresistance of La0.6Sr0.4-xBaxMnO3 perovskites was studied. The lattice parameter of ...

The influence of the magnetic field strength (H=0, 2, 4, 6, 8 kOe), high hydrostatic pressures (P=0−1.8 GPa), and temperature (T=77−300 K) on the resistivity ρ, magnetoresistance (ΔR/R0), and phase transition temperatures in ceramic and thin-film samples of the lanthanum manganite La0.9Mn1.1O3±δ is investigated by x-ray-diffraction, magnetic, and resistive methods. It is found that with increasing H and P the resistivity decreases and the temperatures Tms of the metal-semiconductor phase transition and Tp of the magnetoresistance peak increase. The differences in the resistivities, magnetoresistances, and phase transition temperatures in the ceramics and laser-deposited films are explained by their different nonstoichiometry and defect density. The observed linear dependence of ρ and Tms on P suggests that lanthanum manganite ceramics and films could be used as pressure and temperature sensors.

The evolution of the hard and soft modes in the crystal structure of MnxZnyFezO4 manganese-zinc spinel ferrites is investigated theoretically and experimentally (x-ray structure analysis and magnetic measurements). It is shown that tetragonal and rhombohedral distortions of the cubic structure and fluctuations of the soft mode are accompanied by the formation of clusters in the form of quasi-two-dimensional fragments (50–200 and 400–1200 A), which involve families of γ-Mn2O3, γ-Mn3O4, and α-Fe2O3 oxide planes inherently bound to the spinel matrix.

The effects of magnetic fields H=0–8 kOe and high hydrostatic pressures P=0–2.2 GPa on the resistivity ρ, magnetoresistive effect ΔR/R0, metal–semiconductor phase transition temperature Tms, and peak temperature Tp of the magnetoresistive effect are investigated over a wide range of temperatures T=77–350 K in a ceramic target and a laser film of the same cationic compound La0.7Mn1.3O3±δ. Increasing H and P leads to a decrease of the resistivity. The observed difference in ρ, Tms, and Tp between the ceramics and film samples is explained by the difference of their oxygen nonstoichiometry. The magnetoresistive effect decreases with increasing H and decreases with increasing P. The temperatures Tms and Tp of the ceramics and film increase with increasing P, but the effect is stronger in the film. The observation of two phase transitions in the ceramics (the main transition at Tms=250 K and an additional transition at Tms′=210 K) is explained by its mesoscopic inhomogeneity of the cluster type.

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.

The spin-wave resonance (SWR) spectra have been observed in the (001) oriented La 0.7 Mn 1.3 O 3− δ epitaxial films. The spectra consist of a series of well-resolved spin-wave modes clearly seen in the perpendicular external field configuration. It is shown that the spins are completely pinned on both sides of the film surface. Based on a study of the SWR spectra the value of the spin-wave exchange constant, D , is found to be 150 meV A 2 . This value of D correlates well with this one determined for all known until now manganites.

Electrical resistivity and magnetoresistance (MR) measurements of La 0.7 Pb 0.3− x Sn x MnO 3 , 0 ≤ x ≤ 0.3, perovskite oxides have been performed for the first time. We have investigated the polycrystalline patterns and thin films as well. Typically the samples showed fairly large MR (10–15% at H = 1 T) in the temperature range 77–300 K. To find the PbSn-doping effect on the oxide magnetic phase and giant MR, 55 Mn and 139 La nuclear magnetic resonance measurements have been performed. We have found that one can obtain a large MR at room temperature in PbSn-doped LaMnO 3 thin films, avoiding the contamination of the perovskite phase.

Mesoscopic inhomogeneities and their influence on the behavior of the physical properties (optical reflectivity, dielectric constant, and Curie temperature) of a system of lead zirconate-titanate Pb0.85(Li1/2La1/2)0.15(Zr1−yTiy)O3 solid solutions are investigated. It is established that the disruptions of long-range order have a mesoscopic scale and generate ion groups in the form of interlayers of elements of binary-oxide planes coherently embedded in the crystal structure of the host phase. It is shown that the clustering of the structure is manifested in nonlinearity of the physical properties of the lead zirconate-titanate solid solutions, where the most pronounced variations are characteristic of the coexistence region of the antiferroelectric and ferroelectric phases.

Magnetoresistive properties of thin La0.5Pb0.2Ca0.2Y0.1MnO3−δ films obtained by magnetron sputtering technique are studied. A giant decrease in resistance (up to 98%) in magnetic fields H=10 T is observed at a temperature of 175 K. The physical factors responsible for the effect of composition and preparation technique on their magnetotransport properties are discussed. The mechanisms and empirical relations describing the temperature and field dependences of the resistive properties of films are also discussed.

Results of the study of mesoscopic inhomogeneities in the Pb 0.85 (Li1/2 La1/2) 0.15 (Zr1-y Tiy)03 solid solutions system are presented. It is demonstrated that these formations have some peculiarities in solid solutions for which FE and AFE phases coexist

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).

We have observed an anomalous magnetic hysteresis in polycrystalline (ceramic) and single crystal (film) samples of La0.6Sr0.2Mn1.2O3−δ manganites with a perovskite structure. The anomaly is explained by the coexistence and interaction of two magnetic phases (ferromagnetic and antiferromagnetic) in these lanthanumcontaining manganites.

The effects of high hydrostatic pressures (P) and magnetic fields (H) in a broad temperature range (T = 77-325 K) on the electric resistance (R) and magnetoresistance (DeltaR/R-0) was studied in la ...

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.