Effect of hyperstoichiometric manganese on the structure and transport, magnetic, and magnetoresistance properties of manganite-lanthanum (La$_{0.7}$Ca$_{0.3}$)$_{1-x}$Mn$_{1+x}$O$_3$ perovskites

Ceramic ($\mathrm{La}_{0.7}\mathrm{Ca}_{0.3})_{1-x}\mathrm{Mn}_{1+x}\mathrm{O}_3$ samples are studied by X-ray diffraction, resistive, magnetic, $\mathrm{NMR}^{55}\mathrm{Mn}$, and magnetoresistance methods. The concentration changes of lattice parameter a of the cubic perovskite structure and its average ionic radius are in good agreement if the concentrations of anion and cation vacancies and nanostructured clusters with $\mathrm{Mn}^{2+}$ in the A positions increase with $x$. Phase transition temperatures $T_{ms}$ and $T_c$ weakly depend on $x$, and the electrical resistivity and the activation energy decrease substantially with increasing $x$ due to a change in the imperfection of the perovskite structure. An analysis of the broad asymmetric $\mathrm{NMR}^{55}\mathrm{Mn}$ spectra of the samples indicate a high-frequency $\mathrm{Mn}^{3+}\leftrightarrow\mathrm{Mn}^{4+}$ electron superexchange and nonuniform magnetic and valence states of these ions because of a nonuniform distribution of ions and defects, which decrease the amplitude resonance frequency with increasing $x$. The magnetoresistive (MR) effect near phase-transition temperatures $T_{ms}$ and $T_c$ increases substantially with $x$ and is caused by the effect of a magnetic field on the scattering of charge carriers by intracrystallite nanostructured heterogeneities of an imperfect perovskite structure. The second MR effect is located in the low-temperature range, is related to tunneling through mesostructural crystallite boundaries, and decreases weakly with increasing $x$. A correlation is found between the hyperstoichiometric manganese content, the imperfection of the perovskite structure, and the magnitude of the MR effect.