The influence of oxygen deficiency on the magnetic and electric properties of La0.70Ba0.30MnO3-γ (0≤γ≤0.30) manganite with a perovskite structure

The crystal structure and the magnetic and electric properties of La0.70Ba0.30MnO3 - gamma manganite (0 less than or equal to gamma less than or equal to 0.30) with a perovskite structure were studied experimentally depending on the concentration of oxygen vacancies. The stoichiometric La0.70Ba0.30MnO3 compound (gamma = 0) had cubic unit cell symmetry, which did not change as oxygen deficiency increased up to gamma = 0.30. A decrease in the content of oxygen in the compound under study caused the occurrence of several sequential magnetic phase transitions in the ground state, from the ferromagnetic state at gamma = 0 through the cluster spin glass state (gamma = 0.15) to the antiferromagnetic state (gamma = 0.30) with the presence of a small ferromagnetic component. The specific electric resistance grew to become activation in character at gamma = 0.11, and the metal-semiconductor transition disappeared as oxygen deficiency increased. The magnetoresistance of anion-deficient compositions included (1) magnetoresistance close to the temperature of the transition to the magnetically ordered state and (2) low-temperature magnetoresistance. The magnetoresistance peak at T-C disappeared as gamma increased (gamma = 0.11), whereas the low-temperature magnetoresistance component first increased to attain a maximum of about 34% at gamma = 0.15 and then decreased. The results of experimental studies were used to construct a magnetic phase diagram. These results could be interpreted within the framework of superexchange magnetic ordering theory. The suggestion was made that Mn3+-O-Mn3+ indirect exchange interactions were positive in the orbitally disordered phase only when manganese was in octahedral coordination, whereas these interactions became negative if at least one of the Mn3+ ions was five-coordinate or had a smaller coordination number. (C) 2002 MAIK "Nauka/Interperiodica".