Influence of Manganese Ions Environment in Octahedral Sublattice of La0.7Sr0.3Mn0.9Fe0.1 –xMgxO3 + γ Manganites on Their Properties.

This paper presents structural, magnetic, and electrical characteristics of manganites in the system (0 ≤ x ≤ 0.1) synthesized by solid-state reactions. Sintering was performed in air at 1423 K. Samples with stoichiometric oxygen content (γ = 0) were obtained by annealing at 1223 K and partial oxygen pressure of 0.1 Pa. All studied manganites have rhombohedral structure. With an increase in magnesium content, the unit cell volume decreases, and annealed samples have a larger cell volume than the initial (sintered) manganites containing over-stoichiometric oxygen. The ratio of lattice parameters c/a is practically unchanged. Curie point of manganites decreases in general with an increase in magnesium content, showing weakly expressed plateau in the region of 0.025 < x < 0.075. Magnetization and "metal–semiconductor" transition temperature have maximum values at x = 0, then change non-monotonically, and at x > 0.075 fall sharply. Manganite, which contains the largest amount of magnesium, has the highest resistance at temperatures below 180 K and is characterized by the largest width of the temperature interval of "ferromagnetic–paramagnetic" transition, which indicates the existence of magnetic inhomogeneities. Electromagnetic parameters of initial and annealed samples differ slightly. A number of effects and competing factors that determine complicated dependencies of electromagnetic characteristics of manganites in this system on the composition are considered: inhomogeneous distribution of Mg²⁺ and Fe³⁺ ions; increased concentration of Mn⁴⁺ ions and their shielding by magnesium ions; violation of double exchange interaction between Mn⁴⁺ and Mn³⁺ by ions replacing manganese; difference in magnetic moments of the latter; changes in characteristics of magnetic inhomogeneities. [ABSTRACT FROM AUTHOR]