1. School of Mechanical and Automotive Engineering, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, P. R. China
2. Research and Development Department, Dongyang Dingfeng Magnetics Co. Ltd., Dongyang City, Zhejiang 322100, P. R. China
3. College of materials and chemistry, China Jiliang University, Hangzhou, Zhejiang 310018, P. R. China