1. Institute for Materials Science, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany
2. Institute of Physics, P. J. Safarik University, Park Angelinum 9, 041 54 Kosice, Slovakia
3. Laboratoire de Physique des Solides, Université Paris-Sud, CNRS UMR 8502, 91405 Orsay, France