1. CEA, Institut of Nanosciences and Cryogenics, INAC/SPSMS/GT, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France, CEA, LETI-Minatec, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France, CEA-DSM/IRAMIS/SPCSI, CEA-Saclay, 91191 Gif-sur-Yvette, France, and Institut Néel, CNRS and Université Joseph Fourier, B.P. 166, 38042 Grenoble Cedex 09, France