1. Micro-Nano Technologies Research Center, Hunan University, Changsha 410082, and Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China, Physics Department and the Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China, National Amorphous and Nanocrystalline Alloy Engineering Research Center, Central Iron and Steel Research Institute, XueYuan Nan Road No. 76, Beijing 100081, China