1. College of Physics Science and Technology, Baotou Normal University, Baotou, Inner Mongolia 014030, P. R. China
2. Inner Mongolia Key Laboratory of Magnetics and Magnetic Materials, Baotou Teachers College, Baotou, Inner Mongolia 014030, P. R. China
3. Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, P. R. China