1. School of Physics/FOCAS, Dublin Institute of Technology, Dublin 8, Republic of Ireland, Laboratoire de Physique Cristalline, IMN, BP 32229, 44322 Nantes, France, Department of Physics, University of Dublin, Dublin 2, Republic of Ireland, Deparment of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, and Groupe de Dynamique des Phases Condensees, Universite Montpellier, Montpellier, France