Magnetic properties of anion-deficient La1-xBaxMnO3-x/2 (0 <= x <= 0.30) manganites

An experimental investigation is performed of the crystal structure and magnetic and electrical properties of anion-deficient compositions of La1-x3+Bax2+Mn3+O3-x/22- (0less than or equal toxless than or equal to0.30) 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 (0less than or equal toxless than or equal to0.10) 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 similar to 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 (0less than or equal toxless than or equal to0.15) 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 La1-x3+Bax2+Mn3+O3-x/22- (0less than or equal toxless than or equal to0.30) 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. (C) 2003 MAIK Nauka/Interperiodica.