Influence of composition and sintering temperature on density of structure defects, phase transitions, and properties of magneto-resistive strontium-doped ceramic of manganate-lanthanum perovskites

Abstract

Ceramic samples of La 31−x Sr 2+ x MnO 2− 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 3 c) distortion (α) both decrease with increasing content of Sr 2+ in 4). The metal − semiconductor (T ms ) and ferromagnetic − paramagnetic (T c ) phase transition temperatures and magnetoresistive peak temperature (T p ) all increase, while the activation energy (E a ) 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 Sr 2+ doping, charge compensation is achieved by two mechanisms, first,the Mn 3+ → Mn 4+ 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 T p and the lowfield tunneling magnetoresistive effect increases slightly. It is established that the concentration dependence of the intracrystaline peak magnetoresistive effect at T p is opposite to the concentration dependence of the intercrystalline magnetoresistive effect in the semiconductor region for T << T p .