1. Department of Engineering Physics, École Polytechnique de Montréal, Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec H3C 3A7, Canada
2. Peter Grünberg Institute 9 and JARA-FIT, Forschungszentrum Juelich, 52425 Juelich, Germany
3. CEA, LETI, Minatec Campus, 17 rue des Martyrs, 38054 Grenoble, France