1. a Institute für Angewandte Physik der Universität , D–4400 , Münster , F.R. , Germany
2. d Department of Physics , Garhwal University , Srinagar , Uttar Pradesh , India
3. b Department of Physics , Garhwal University , Srinagar , Garhwal , India
4. c Department of Physics , Government Postgraduate College , Rampur , Uttar Pradesh , India