Influence of oxygen vacancies on the magnetic and electrical properties of La $\mathsf{_{1-x}}$ Sr $\mathsf{_{x}}$ MnO $\mathsf{_{3-x/2}}$ manganites

The crystal structure, magnetization and electrical transport depending on the temperature and magnetic field for the doped stoichiometric ${\rm La}_{1-x}^{3 + } {\rm Sr}_x^{2 + } {\rm Mn}_{1-x}^{3 + } {\rm Mn}_x^{4 + } {\rm O}_3^{2-} $ as well as anion-deficient ${\rm La}_{1-x}^{3 + } {\rm Sr}_x^{2 + } {\rm Mn}^{3 + }{\rm O}_{3-x/2}^{2-} $ ( $0\le x \le 0.30$ ) ortomanganite systems have been experimentally studied. It is established that the stochiometric samples in the region of the $0 \le x \le 0.125$ are an ${\rm O}'$ -orthorhombic perovskites whereas in the $0.175 \le x \le 0.30$ - 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 ( $0.175 > x \le 0.30$ ) 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 ${\rm La}_{1-x}^{3 + } {\rm Sr}_x^{2 + } {\rm Mn}^{3 + }{\rm O}_{3-x/2}^{2-} $ system have been established by the magnetic measurements and compared with those for the stoichiometric ${\rm La}_{1-x}^{3 + } {\rm Sr}_x^{2 + } {\rm Mn}_{1-x}^{3 + } {\rm Mn}_x^{4 + } {\rm O}_3^{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. Copyright Springer-Verlag Berlin/Heidelberg 2004