Influence of oxygen vacancies on the magnetic and electrical properties of La1-xSrxMnO3-x/2 manganites
Résumé
The crystal structure, magnetization and electrical transport depending on the temperature and magnetic field for the doped stoichiometric La 1-x 3+Srx 2+Mn1-x 3+Mnx 4+O3 2- as well as anion-deficient La1-x 3+Srx 2+Mn 3+O3-x/2 2- (0 ≤ x ≤ 0.30) ortomanganite systems have been experimentally studied. It is established that the stochiometric samples in the region of the 0 ≤ x ≤ 0.125 are an O'-orthorhombic perovskites whereas in the 0.175 ≤ x ≤ 0.30 - a rhombohedric. For the anion-deficient system the symmetry type of the unit cell is similar to the stoichiometric one. As a doping level increases the samples in the ground state undergo a number of the magnetic transitions. It is assumed that the samples with the large amount of oxygen vacancies are a cluster spin glasses (0.175 < x ≤ 0.30) and temperature of the magnetic moment freezing is ∼40 K. All the anion-deficient samples are semiconductors and show considerable magnetoresistance over a wide temperature range with a peak for the x = 0.175 only. Concentration dependences of the spontaneous magnetization and magnetic ordering temperature for the anion-deficient La1-x 3+Srx 2+Mn3+O3-x/2 2- system have been established by the magnetic measurements and compared with those for the stoichiometric La1-x 3+Sr x 2+Mn1-x 3+Mnx 4+O3 2- one. The magnetic propeprties of the anion-deficient samples may be interpreted on the base of the superexchange interaction and phase separation (chemical disorder) models.