1. NiPS Laboratory, INFN and Dipartimento di Fisica e Geologia, University of Perugia, via Pascoli, I-06100 Perugia, Italy
2. SwissScientific Technologies SA, rue du Rhone 59, 1204 Geneva, Switzerland
3. Materials Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60637, USA