Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La$_{0.6-x}$Pr$_x$Sr$_{0.3}$Mn$_{1.1}$O$_3$ ($x$ = 0–0.6)

Ceramic samples of manganite perovskites La$_{0.6-x}$Pr$_x$Sr$_{0.3}$Mn$_{1.1}$O$_3$ ($x$ = 0–0.6) have been studied using the X-ray diffraction, resistive, magnetic ($\chi_{\mathrm{ac}}$, $^{55}$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 $R\bar3c$ ($x$ = 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 mi and the ferromagnetic-paramagnetic phase transition temperature $T_c$ with increasing $x$ correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn$^{3+}$ $\leftrightarrow$ Mn$^{4+}$. For compositions with $x$ = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn$^{2+}$ in the $A$-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened $^{55}$Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn$^{3+}$ $\leftrightarrow$ Mn$^{4+}$ in the $B$-positions and a local heterogeneity of their surrounding by other ions (La$^{2+}$, Pr$^{3+}$, Sr$^{2+}$) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures $T_{mi}$ and $T_c$, and magnetoresistive properties.