Origin of metal-insulator transition temperature enhancement in underdoped lanthanum manganite films

The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal-insulator transitions. Their transition temperatures T-MI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the T-MI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that T-MI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films. (C) 2003 American Institute of Physics.