Vacancy-induced enhancement of magnetic interactions in (Ca, Na)-doped lanthanum manganites

Structural, electric, and magnetic properties of the bulk polycrystalline samples with nominal composition La 0.7 Ca 0.3 − x Na x MnO 3 ( x = 0 − 0.10 ) have been investigated in the work. As follows from the results of chemical and x-ray structural analyses, the increase in x brings about the rise in the number of structural vacancies in both lanthanum and oxygen sublattices. The influence of the variation of the vacancy number on peculiar features of the transition from ferromagnetic metallic state to paramagnetic state with activated conductivity has been studied in detail. The conclusion is made that the rise in the number of structural vacancies gives rise not only to the enhancement of magnetic inhomogeneity and broadening of the magnetic transition, but also to the increase in the temperature of magnetic transition. It is demonstrated that the changes in magnetic parameters also result in the increase of the temperatures of resistivity and magnetoresistance peaks as well as in the change of the character of the temperature dependence of electric resistance.