Effect of hyperstoichiometric manganese on the structure and transport, magnetic, and magnetoresistance properties of manganite-lanthanum (La0.7Ca0.3)1 − x Mn1 + x O3 perovskites

Abstract

Ceramic (La0.7Ca0.3)1 − xMn1 + xO3 samples are studied by X-ray diffraction, resistive, magnetic, 55Mn NMR, 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 Mn2+ in the A positions increase with x. Phase transition temperatures Tms and Tc 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 55Mn NMR spectra of the samples indicate a high-frequency Mn3+↔ Mn4+ 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 Tms and Tc 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.