1. School
of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, P. R. China
2. National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem Advanced Materials Co., Ltd., Beijing 100088, P. R. China
3. State
Key Laboratory of Nonferrous Metals and Processes, General Research Institute for Nonferrous Metals, Beijing 100088, P. R. China