Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of [La.sub.0.6-x][Pr.sub.x][Sr.sub.0.3][Mn.sub.1.1][O.sub.3]

Ceramic samples of manganite perovskites [La.sub.0.6-x][Pr.sub.x][Sr.sub.0.3][Mn.sub.1.1][O.sub.3] (x = 0-0.6) have been studied using the X-ray diffraction, resistive, magnetic ([χ.sub.ac], [sup.55]Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R[bar.3]c (x = 0-0.3) to orthorhombic Pbnm (x = 0.4-0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature [T.sub.mi] and the ferromagnetic-paramagnetic phase transition temperature [T.sub.c] with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange [Mn.sup.3+] [left and right arrow] [Mn.sup.4+]. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with [Mn.sup.2+] in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened [sup.55]Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions [Mn.sup.3+] [left and right arrow] [Mn.sup.4+] in the B-positions and a local heterogeneity of their surrounding by other ions ([La.sup.2+], [Pr.sup.3+], [Sr.sup.2+]) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures [T.sub.mi] and [T.sub.c], and magnetoresistive properties.