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.