Magnetic properties of anion-deficient La_1−x Ba_xMnO_3−x/2 (0≤x≤0.30) manganites

Abstract

An experimental investigation is performed of the crystal structure and magnetic and electrical properties of anion-deficient compositions of La ³⁺_1−x Ba ²⁺_x Mn³⁺O ^2/2−_3−x (0≤x≤0.30) which do not contain manganese ions of different vacancies. It is found that all reduced samples are single-phase perovskites with O-orthorhombic (x=0, 0.05), rhombohedral (x= 0.10, 0.15, 0.20, 0.25), and cubic (x=0.30) symmetry of the unit cell. It is observed that systems of the compounds being treated experience a transition from a weakly ferromagnetic (x=0) to a nonuniform ferromagnetic (0≤x≤0.10) state. An increase in the degree of nonstoichiometry with respect to oxygen leads to the emergence of the antiferromagnetic orbitally disordered phase. For compounds with x>0.20, clearly defined properties are observed that are characteristic of cluster spin glass with the freezing temperature of magnetic moments T _F ∼45 K. The maximal amount of the ferromagnetic component is registered for x=0.15. All of the reduced samples are semiconductors. As the substitution level increases, the electrical resistivity (at room temperature) first decreases in magnitude (0≤x≤0.15) and then increases (x>0.15). The magnetoresistance of all reduced samples increases gradually upon transition to a magnetically ordered state and reaches its maximal value at the liquid nitrogen temperature. A hypothetical magnetic phase diagram is constructed for the system of anion-deficient compositions of La ³⁺_1−x Ba ²⁺_x Mn³⁺O ^2/2−_3−x (0≤x≤0.30) being treated. The investigation results contribute to understanding the nature of 180-degree indirect superexchange interactions between ions of trivalent manganese. It is assumed that the Mn³⁺-O-Mn³⁺ superexchange interactions are negative in the orbitally disordered phase in the case of pentahedral coordination of Mn³⁺ ions.