Effect of oxygen annealing on the magnetoresistance in La0.7Ca0.3MnO3 epitaxial films

Epitaxially c-axis oriented thin films of La0.7Ca0.3MnO3 (LCMO) have been f abricated on (001) SrTiO3 and (001) LaAlO3 substrates by pulsed laser-ablat ed deposition technique. The physical properties of as-grown films are corr elated with the structural nature of the substrate. The O-2-annealed films grown on both substrates exhibit higher metal-insulator (M-I) transition te mperatures (T-MI) and larger magnetoresistance (MR) ratios than the corresp onding as-grown films. On the other hand, the Ar-annealed films display enh anced MR without any noticeable change in T-MI We suggest that increase in T-MI is mainly ascribed to oxygen incorporation whereas enhancement of MR i s largely associated with microstructural change induced by thermal anneali ng. Structural and transport data reveal that post-annealing with O-2 not o nly saturates the anion defect sites but also relieves structural strain, r esulting in the enhancement of T-MI and large MR.