Structural and magnetic inhomogeneities, phase transitions, $^{55}$Mn nuclear magnetic resonance, and magnetoresistive properties of La$_{0.6-x}$Nd$_x$Sr$_{0.3}$Mn$_{1.1}$O$_{3-\delta}$ ceramics

The structure, lattice imperfection, and properties of ceramic samples La$_{0.6-x}$Nd$_x$Sr$_{0.3}$Mn$_{1.1}$O$_{3-\delta}$ ($x$ = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic ($(\chi_{\mathrm{ac}})$, $^{55}$Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral ($x$ = 0, 0.1, 0.2) and cubic ($x$ = 0.3, 0.4) perovskite structures and the average ionic radii $\bar{R}$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn$^{2+}$ ions in $A$-positions. With an increase in the neodymium concentration $x$, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition $T_{\mathrm{ms}}$ and ferromagnetic-paramagnetic phase transition $T_C$ decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened $^{55}$Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn$^{3+}$–O$^{2-}$–Mn$^{4+}$) $\leftrightarrow$ (Mn$^{4+}$–O$^{2-}$–Mn$^{3+}$) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn$^{3+}$/Mn$^{4+}$ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences $4\pi N\chi_{\mathrm{ac}}(T)$ and the $^{55}$Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.