[1] CNRS, Inst Neel, F-38042 Grenoble 09, France
[2] Univ Grenoble 1, F-38042 Grenoble 09, France
[3] LPMMC, Grenoble, France
[4] LPTMC, Paris, France
[5] Karlsruhe Inst Technol, Inst Nanotechnol, D-76344 Eggenstein Leopoldshafen, Germany
[6] CNRS, Inst Phys & Chim Mat Strasbourg, F-67034 Strasbourg, France