Role of columnar grain size in magnetization of La0.8MnO3 thin films grown by pulsed laser deposition

La0.8MnO3 thin films have been deposited on (100) SrTiO3 substrates at different substrate temperatures by a pulsed laser deposition method. Electronic transport measurements show that a higher substrate temperature results in lower resistivity and higher insulator-metal transition temperature. Transmission electron microscope studies reveal that all the films exhibit a feature of columnar structure with the grain size decreasing with substrate temperature. We argue that the columnar grain size strongly affects the ferromagnetic transition temperature and, in turn, dominates the resistivity behavior. Based on this point, other effects, such as of annealing and film thickness, on the electronic properties are also discussed.