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

摘要

Ceramic samples of La_(1-x)~3Sr_x~(2+)MnO_3~(2-) (x = 0, 0.1, 0.15, 0.2, 0.3, 0.4), sintered at different temperatures (1150, 1200, 1350, and 1500 deg 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 (R3-bar c) distortion (alpha) both decrease with increasing content of Sr~(2+) in La_(1-x)Sr_xMnO_(3+ - delta) (x = 0-0.4). 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 onion (V~(a)) and cation (in A and B positions) vacancies, average valency of manganese (omega), 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 ( approx 80 percent), and second, the formation of V~(a) vacancies (approx 20 percent), 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 <