Anomalous oxygen annealing effects on La0.7Ca0.3MnO3 films grown on MgO substrates

Epitaxial La0.7Ca0.3MnO3 thin films were grown on MgO (001) substrates by p ulsed laser deposition. The as-grown films exhibit a metal-insulator transi tion at temperatures ranging from 200 to 230 K. Surprisingly, the transitio n temperature of postannealed films under O-2 at 900 degreesC decreases by more than 100 K, featured with the colossal magnetoresistance. The O-2 anne aled films also show thermal hysteresis in resistivity and nonlinear I-V be havior. These results are interpreted in terms of Mn deficiency induced by large structural difference between film and substrate, and also discussed in viewpoint of a percolative phase transition. (C) 2001 American Institute of Physics.