1. Institut für Nanotechnologie, Karlsruhe Institute of Technology (KIT), D-76021 Karlsruhe, Germany, Institute of Chemical Research of Catalonia (ICIQ), Av. Països, Catalans 16, E-43007, Tarragona, Spain, 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550, Stuttgart, Germany, and Institut Néel & Université J. Fourier, CNRS, Grenoble 25, Av. des Martyrs, F-38042, Grenoble, France