Y.X.Li, G.Z.Xu,J.P.Qu, F.G.Liu and B.W. Wang Hebei University of Technology, Tianjin 300130, China C. Zhao Tianjin University, Tianjin 300072, China S.X. Gao Hebei Normal University,Shijiazhuang 050091, China C.C. Tang, G.H.Wu, J.H.Wang, J.Du and W.S.Zhan State Key Laboratory for Magnetism, Institute of Physics, CAS, Beijing 100080, China