[1] Univ Michigan, Dept Elect Engn & Comp Sci, Ann Arbor, MI 48109 USA
[2] Louisiana State Univ, Dept Phys & Astron, Baton Rouge, LA 70803 USA
[3] Chinese Acad Sci, Ningbo Inst Mat Technol & Engn NIMTE, Key Lab Magnet Mat & Devices, Ningbo 315201, Zhejiang, Peoples R China
[4] Chinese Acad Sci, Ningbo Inst Mat Technol & Engn NIMTE, Zhejiang Prov Key Lab Magnet Mat & Applicat Techn, Ningbo 315201, Zhejiang, Peoples R China