Silcheva G. Influence of doping on the defectiveness of structure, the phase transitions and the magnetoresistive properties of manganite-lanthanum perovskites with super-stoichiometric manganese

The thesis is devoted to determining the laws behind the influence of super-stoichiometric manganese in the manganite-lanthanum perovskites doped into A- and B- sublattices on the defectiveness of structure, the phase transitions and the magnetoresistive properties. The relation between the defectiveness of manganite-lanthanum perovskite structure and the temperatures of phase transitions and laws governing the influence of doping in A- positions and the substitution of super-stoichiometric manganese in the B- positions by the magnetic and transport properties is established experimentally and computationally. It is found that the substitution of A- sublattice in manganitoperovskites of La0.6A0.2Mn1.2O3±d with ions K+, Ca2+, Pb2+, Ba2, Sr2+ leads to different changes in the defectiveness of perovskite structure, which correlate with the temperatures of "metal-semiconductor" phase transitions of Tms and "ferro-paramagnetic" of Tc. The anomalous temperature dependences of lattice parameter near the magnetic and electric transport phase transitions, explained by changes in the superposition of the different-valence ions of manganese and their spin states, are discovered. The substitution of super-stoichiometric manganese in the B- sublattice by ions Cr, Fe, Co, Ni leads to decrease in temperatures of phase transitions "metal-semiconductor" of Tms and "ferro-paramagnetic" Tc, due to infringement of high-frequency exchange interaction Mn3 + - Mn4 +. The method NMR of 55Mn confirmed the high-frequency electron-hole exchange between Mn3+ - Mn4+ (in the B-position) and the heterogeneity in surrounding ions and defects. Magnetic phase diagrams are built on the basis of magnetic studies. Ferromagnetic and paramagnetic regions are determined, as well as the intermediate region, in which the local ordering of magnetic moment occurs, and the temperature of the maximum speed of this ordering coincides with the temperature of magnetoresistance effect (Tp).