Structure, phase transitions, 55 Mn NMR, magnetic and magnetotransport properties of the magnetoresistance La 0.9−x Ag x Mn 1.1 O 3−δ ceramics

Abstract

Abstract Bulk polycrystalline La0.9−хAgхMn1.1O3−δ samples have been prepared by the conventional solid-state reaction method for x = 0–0.5. The samples have been investigated by X-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR, SEM and magnetoresistive methods. The X-ray data shows that the samples are single phase for x ≤ 0.2 and have a rhombohedral R 3 ¯ c perovskite structure. For x ≥ 0.25, a small amount of the second phase of Mn3O4 has been observed. The lattice parameter of the R 3 ¯ c structure increases with x as A cations of La3+ are replaced by larger Ag+ ions. The molar formulas of a real defect perovskite structure have been determined. The real structure contains anion V(a) and cation V(c) vacancies as well as variable valence MnB4+, MnB3+ and MnA2+ ions. The increase in Ag+ concentration induces a change in the Mn3+/Mn4+ ratio, increases defectiveness, raises phase transition temperatures, non-monotonically decreases the content of a ferromagnetic phase and reduces resistivity. Broad asymmetric 55Mn NMR spectra have confirmed an electron double Mn3+↔O2−↔Mn4+ exchange, heterogeneity of the near surrounding of manganese as well as a significant influence of V(a) and V(c) vacancies on the transfer and localization of the charge during such a double exchange. The “Composition-Defectiveness-Properties” phase diagram plotted experimentally, characterizes a strong interrelation between the defect structure, magnetic and magnetotransport properties of the magnetoresistance La0.9−хAgхMn1.1O3−δ ceramics. Besides it allows to prepare manganites with desirable functional properties.