V. P. Pashchenko

Influence of temperature, magnetic field, and high hydrostatic pressure on the resistivity and magnetoresistance in La0.9Mn1.1O3±δ ceramics and laser-deposited films

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.

Nanoclustering in (Nd0.7Sr0.3)1 − xMn1 + xO3 ± δ solid solutions

The structure of (Nd0.7Sr0.3)1 − xMn1 + xO3 ± δ solid solutions with x = 0–0.2 (Nd: Sr atomic ratio maintained constant at 2.33) annealed at temperatures from 1200 to 1500°C has been studied by X-ray diffraction. The results demonstrate that, as the annealing temperature is raised, the atomic order in the stoichiometric (x = 0) materials is stable to changes in the homogeneity of the solid solution. In the case of the nonstoichiometric manganites with x = 0.15–0.20, an increase in annealing temperature is accompanied by the development of chemical and topological ordering processes in the matrix structure and cluster growth. The cluster size ranges from 100 to 250 Å, and the largest percentage of clusters (3.5–5.5%) consist of the matrix phase. The fraction of clusters of binary composition, such as MnO-MnO2, is 0.5–2.0%.

Microscopic magnetic and transport properties of La0.7Pb0.3−xSnxMnO3, 0 ≤ x ≤ 0.3: magnetoresistance and 55Mn, 139La MNR measurements

Abstract 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.

Imperfection of the nanostructure, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.73+Ca0.3 − x2+Srx2+MnO3 ± δ ceramics

Magnetoresistive ceramic samples La0.7Ca0.3 − xSrxMnO3 ± 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.

Influence of cobalt on the structural and magnetic Inhomogeneities, phase transitions, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Co x O3 ± δ

AbstractThe structure and properties of magnetoresistive ceramics La0.6Sr0.2Mn1.2 − xCoxO3 ± δ (x = 0−0.3) sintered at a temperature of 1200°C are investigated using x-ray diffraction, resistance, and magnetic (χac, M, 55Mn NMR) measurements. It is shown that the samples contain the rhombohedral (R

Effect of hyperstoichiometric manganese on the structure and transport, magnetic, and magnetoresistance properties of manganite-lanthanum (La0.7Ca0.3)1 − xMn1 + xO3 perovskites

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Spin-wave resonance in the La0.7Mn1.3O3−δ film

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 (Tms) and ferromagnetic-paramagnetic (TC) 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 Tms and TC 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.

Magnetocaloric effect in (La0.6Ca0.4)0.9Mn1.1O3

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...