1. Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province College of Physics and Electronic Engineering Northwest Normal University Lanzhou 730070 China
2. School of Physical Science and Technology & School of Materials and Energy & Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education Lanzhou University Lanzhou 730000 China
3. School of Materials Science & Engineering & Shaanxi Normal University Xi'an 710119 China