1. Electronics and Photonics Research Institute (ESPRIT), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
2. Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble, France
3. Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan
4. Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan
5. Faculty of Science and Technology and MESA+ Institute of Nanotechnology, University of Twente, 7500 AE, Enschede, The Netherlands