1. Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
2. Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India
3. Department of Physics, Kharagpur College, Kharagpur 721305, India
4. Department of Physics, Indian Institute of Technology, Kharagpur 721302, India
5. Institut Laue-Langevin, Boite Postale 156, 38042 Grenoble Cedex, France