1. Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
2. Institut Laue-Langevin, 6 rue Jules Horowitz, B.P. 156, 38042 Grenoble, France, and Biomathematics and Epidemiology, ENVL-TIMC, B.P. 83, 69280 Marcy l'Etoile, France
3. Laue-Langevin Institute, F-38042, Grenoble, France, and L. D. Landau Institute for Theoretical Physics, RAS, 117940 GSP-1, Moscow, Russia
4. Structures et Propriétés d'Architectures Moléculaires, UMR 5819 (CEA, CNRS, UJF), DRFMC/SPrAM, CEA-Grenoble, 38054 Grenoble cedex 9, France