1. CEA, LETI, MINATEC, F38054 Grenoble, France, CEA, INAC/SP2M/L_sim, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France, Institut Néel, CNRS and Université Joseph Fourier, B.P. 166, 38042 Grenoble Cedex 09, France, and Institute for Materials Science, TU Dresden, D-01062 Dresden, Germany