1. Department
of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, P. R. China
2. Department
of Physics, Pukyong National University, Busan 608-737, Korea
3. Department
of Physics, Changwon National University, Changwon 641-773, Korea
4. Department
of Electronic Material Engineering, Silla University, Busan 617-736, Korea