1. Institute of Physics
of Rennes, CNRS-University of Rennes 1, UMR 6251, F-35042 Rennes, France
2. Institut Laue-Langevin, 71 avenue
des Martyrs, F-38000 Grenoble, France
3. Laboratoire
Léon
Brillouin, UMR 12, CEA-CNRS, F-91191 Gif-sur-Yvette, France