Structural Microinhomogeneities, Magnetic and Nonlinear Electrical Properties of Nonstoichiometric Lanthanum-Strontium Manganites Doped with Cerium and Cobalt

Single-phase manganites La0.60Sr0.35Ce0.05Mn0.95Со0.05O3 + γ with different oxygen concentrations having rhombohedral structure have been synthesized and investigated. Variation of oxygen nonstoichiometry index (γ) was achieved by the heat treatment of sintered (initial) samples in atmospheres with oxygen partial pressure of 10–8 Pa, 10–1 Pa, 105 Pa. It has been established that manganite annealed in high vacuum contains single-charged oxygen ions. According to the data of electron microscopy, scanning tunneling microscopy and spectroscopy, obtained samples contain submicron inhomogeneities with different conduction values affecting crystal lattice microdistortions, temperature range of ferromagnetic-paramagnetic transition, and current-voltage characteristics. Magnetization and Curie point of the samples are weakly dependent on the conditions of redox heat treatments due to the competing effects of changes in valence and spin states of manganese and cobalt ions. Manganite sintered at 1523 K in air has nonlinear current–voltage (I–V) characteristics with N-shaped sections, the occurrence of which can be explained by tunneling of charge carriers between coexisting inhomogeneities with different types of conduction. In manganite annealed at 1223 K in oxygen, S-shaped I–V characteristics are observed, formation mechanism of which is destruction of dielectric inclusions in the matrix metal phase of manganite due to local overheating by flowing current. Magnetic field smoothes out the sections of I–V characteristics with negative differential resistance, and at field strength of 9.2 kOe they are not detected.