Prylypko S. Characteristics of manufacturing of nanocrystalline manganites and their electrical and magnetic properties

The thesis is devoted to manufacturing of nanocrystalline manganites and to establish the relationship between their crystallite size and properties. It is found that the cold isostatic pressing intensifies the synthesis of lanthanum manganite, reduces its temperature and inhibits the growth of crystallites. The obtained results allowed to develop a technique for manufacturing of manganites in the nanocrystalline state with average crystallite sizes down to 6 nm. For samples of La0,7Mn1,3O3±d and (La0,65Sr0,35)0,8Mn1,2O3±d with crystallite sizes from 6 to 200 nm was experimentally demonstrated that the coercive field in manganites reaches its maximum when the size of the crystallites is about 70 nm and completely disappears for lanthanum manganite crystallites with ~6 nm due to reaching the superparamagnetic state. As the crystallite size of the samples decreases, the resistivity and coercive force increase, the tunneling magnetoresistance decrease and the Curie temperature changes. It was established that the density and electromagnetic properties of manganite ceramics depend on the initial size of the crystallites of the powder. The measurements suggest that the grain boundaries can play a key role in the properties of ceramics, and the structure of grain boundaries caused by the characteristics of sintering of nanocrystalline powder compacts.