1. Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu Taiwan 300, Taiwan
2. Institute for Materials Research (IMO), Hasselt University, 3590 Diepenbeek, Belgium
3. Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
4. Department of Physics, Tamkang University, Tamsui, Taiwan 251, Taiwan