[1] Max Planck Inst Mikrostrukturphys, D-06120 Halle An Der Saale, Germany
[2] Univ Tecn Lisboa, Dept Phys, Inst Super Tecn, P-1049001 Lisbon, Portugal
[3] Univ Tecn Lisboa, CFIF, Inst Super Tecn, P-1049001 Lisbon, Portugal
[4] CNRS, Lab Louis Neel, F-38042 Grenoble, France