Effect of oxygen vacancies on structural, electrical and magnetic properties of La_0.67Sr_0.33CoO₃ thin films

The microstructure and chemical expansion of annealed and unannealed La_0.67Sr_0.33CoO₃ (LSCO) thin films were investigated using high-resolution transmission electron microscopy (HRTEM). In both annealed and unannealed LSCO thin films, the modulated structure in La/Sr planes (A-sites) was discovered, which is ascribed to the cation ordering. However, the oxygen vacancy ordering induced modulated structure in Co-O planes (B-sites) only appeared in the vacuum-annealed LSCO thin films. In addition, the anisotropic chemical expansion was only observed in the vacuum-annealed LSCO films and the anisotropic elongation is associated with the modulated structures in the Co-O planes. What's more, the electrical and magnetic properties of the LSCO thin films were studied before and after annealing in the vacuum. The ferromagnetic-metallic LSCO thin films turned into a non-magnetic insulator after the annealing in vacuum, which is attributed to appearance of the oxygen vacancy ordering in the vacuum-annealed LSCO films.