1. Dipartimento di Fisica, Università di Parma, Via G. Usberti 7/a, 43100 Parma, Italy, Institut Laue Langevin, BP 156, 6, rue Jules Horowitz, 38042 Grenoble Cedex 9, France, Physical Chemistry Laboratory, ETH-Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland, and Laboratorium für Festkörperphysik, ETH-Zurich, Schafmattstrasse 16, CH-8093 Zurich, Switzerland