1. Contribution from the National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea, Department of Physics and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Korea, Department of Physics and Pohang Light Source, Pohang University of Science and Technology, Pohang 790-784, Korea, and School of Physics and CSCMR, Seoul National University, Seoul 151-747, Korea