Structural and magnetic inhomogeneities, phase transitions, $^{55}$Mn NMR, and magnetoresistive properties of La$_{0.6}$Sr$_{0.2}$Mn$_{1.2-x}$Nb$_x$O$_3$ ceramics

Ceramic samples of lanthanum strontium manganite perovskites La$_{0.6}$Sr$_{0.2}$Mn$_{1.2-x}$Nb$_x$O$_3$ ($x$ = 0–0.3) annealed at temperatures of 1260 and 1500$^\circ$C have been investigated using the X-ray diffraction, electron microscopic, resistive, magnetoresistive, and magnetic ($\chi_{\mathrm{ac}}$, $^{55}$Mn NMR) methods. It has been found that there is a correlation between the increasing unit cell parameter $a$ of the rhombohedral $R\bar3c$ structure and the average ionic radius with increasing niobium concentration $x$ and annealing temperature for the case where the lattice contains anion vacancies, cation vacancies, and nanostructured clusters. The observed increase in the electrical resistivity and decrease in the temperatures of metal-semiconductor phase transition $T_{\mathrm{ms}}$ and ferromagnetic-paramagnetic phase transition $T_C$ with an increase in the niobium concentration $x$ and the annealing temperature have been explained by the decrease in the content of the ferromagnetic phase, as well as by changes in the ratio Mn$^{3+}$/Mn$^{4+}$, the oxygen nonstoichiometry, and the concentration of defects weakening the high-frequency electronic exchange of the ions Mn$^{3+}$ $\leftrightarrow$ Mn$^{4+}$. The presence of nanostructured clusters in the lattice has been confirmed by an anomalous hysteresis associated with the unidirectional exchange anisotropy of the interaction between the ferromagnetic matrix and antiferromagnetic clusters with Mn$^{2+}$ and Nb$^{3+}$ in distorted A-positions. An analysis of the asymmetrically broadened $^{55}$Mn NMR spectra and their computer decomposition have revealed a high-frequency electronic exchange and an inhomogeneity of the magnetic and valence states of manganese due to the nonuniform distribution of all ions and defects. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures $T_C$ and $T_{\mathrm{ms}}$, is caused by scattering at intracrystalline nanostructured heterogeneities of the imperfect perovskite structure, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between the composition, structure, resistive and magnetic properties of rare-earth manganites.