1. Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesser Strasse 15, D-76131, Karlsruhe, Germany
2. Institute of Nanotechnology, Karlsruhe Institute of Technology, Postfach 3640, D-76021, Karlsruhe, Germany
3. Institut Néel, CNRS & University J. Fourier, BP 166, 38042, Grenoble, Cedex 9, France