Influence of cobalt on the structural and magnetic Inhomogeneities, phase transitions, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Co x O3 ± δ

Abstract

The structure and properties of magnetoresistive ceramics La0.6Sr0.2Mn1.2 − x Co x O3 ± δ (x = 0−0.3) sintered at a temperature of 1200°C are investigated using x-ray diffraction, resistance, and magnetic (χ ac , M, 55Mn NMR) measurements. It is shown that the samples contain the rhombohedral (R $$ \bar 3 $$ c) perovskite (90%) and tetragonal (I41/amd) hausmannite (10%) phases. The lattice parameters of these phases decrease with an increase in the cobalt content x. The real perovskite structure involves point defects (anion and cation vacancies) and nanostructured defects of the cluster type. An analysis of the asymmetrically broadened 55Mn NMR spectra confirms the high-frequency electron-hole exchange between Mn3+ and Mn4+ ions and a local inhomogeneity of their environment by other ions and defects of the point and cluster types. An increase in the Co content leads to an increase in the electrical resistivity, an enhancement of the magnetoresistance (MR) effect, and a decrease in the magnetic susceptibility and the temperatures of the metal-semiconductor (T ms ) and ferromagnetic-paramagnetic (T C) phase transitions due to the suppression of the exchange interaction between Mn3+ and Mn4+ ions by vacancies and clusters. The introduction of cobalt results in a decrease in the ferromagnetic component and the activation energy. The magnetoresistance effect in the vicinity of the phase transition temperatures T ms and T C is associated with the scattering of charge carriers from intracrystallite inhomogeneities of the lattice, and the low-temperature magnetoresistance effect is governed by the tunneling at the intercrystalline boundaries.