Imperfection of the nanostructure, phase transitions, 55Mn NMR, and magnetoresistive properties of La 0.7 3+ Ca 0.3 − x 2+ Sr x 2+ MnO3 ± δ ceramics

Abstract

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.