1. Key Laboratory of Magnetic Materials and Devices; Ningbo Institute of Materials Technology and Engineering; Chinese Academy of Sciences; Ningbo 315201 PR China
2. Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology; Ningbo Institute of Materials Technology and Engineering; Chinese Academy of Sciences; Ningbo 315201 PR China
3. Department of Electrical Engineering and Computer Science; University of Michigan; Ann Arbor Michigan 48109 USA
4. Information Storage Materials Laboratory; Department of Electrical and Computer Engineering; National University of Singapore; 4 Engineering Drive 3 Singapore 117583 Singapore