Structure, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − y Cr y O3 ± δ

Abstract

The structure and properties of lanthanum strontium manganite perovskites La0.6Sr0.2Mn1.2 − y Cr y O3 ± δ (y = 0–0.3) sintered at 1430°C have been studied by X-ray, resistive, and magnetic (χac and 55Mn NMR) methods. The parameter of the rhombohedrally distorted (R \( \bar 3 \) c) perovskite structure decreases with increasing y. The real perovskite structure contains point (anion and cation vacancies) and cluster-type nanostructure defects. The analysis of asymmetrically broadened 55Mn NMR spectra has confirmed the high-frequency electron-hole exchange Mn3+ ↔ Mn4+ and local inhomogeneity of their surrounding by other ions and point and cluster-type defects. An increase in the Cr content leads to an increase in the resistivity and the magnetoresistive effect and a decrease in the metal-semiconductor and ferromagnetic-paramagnetic phase transition temperatures (T ms and T c ) due to the distortion of the exchange interactions Mn3+ ↔ Mn4+ by chromium ions, vacancies, and clusters. Introduction of Cr decreases the ferromagnetic component and increases the activation energy. The magnetoresistive effect near T ms and T c is caused by scattering of charge carriers from intercrystallite nanostructure inhomogeneities of the lattice, and the low-temperature effect is associated with the tunneling on mesostructural intercrystallite boundaries.