Novokhatska A. The effect of excess manganese on the formation of the structure and magnetoresistive properties of doped manganites

The thesis is devoted to the study of the evolution of the structure and magnetotransport properties of nanoscale powder and ceramic doped manganites due to the content of excess manganese and the sintering temperature. It is found for the first time that the presence of excess manganese promotes the production of nanosized manganites with ferromagnetic ordering in crystallites with dimensions > 10 nm, which is half the size of manganites without excess manganese. According to the results of a study of the microstructure of ceramic manganites sintered in the temperature range 800-1500 °C, it is established that in manganites with excess manganese the formation of ceramics occurs at a sintering temperature of 1000 °C. It is found that the presence of excess manganese leads to a change in the mechanism of grain growth at sintering temperatures above 1000 °C, during which a layered substructure of the grain of manganite ceramics is formed. The appearance of the grain substructure is accompanied by the conservation of the phase composition of the manganite phase, the uniform distribution of the doped elements in the grain. This leads to an increase in the ferromagnetic homogeneity of the material, a decrease in the resistivity by 1-4 order, and a growth in the magnetoresistive effect by 2-3 times in comparison with manganites without excess manganese. A qualitative model of conduction channels in ceramic manganites is proposed depending on the grain structure.