1. Institute of Physics, P.J. Safarik University, Park Angelinum 9, 041 54 Kosice, Slovakia
2. CP M-TIP, P.J. Safarik University, Park Angelinum 9, 041 54 Kosice, Slovakia
3. Institute of Materials Science, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany