1. ABB Corporate Research Center, 940 Main Campus Drive, Raleigh, North Carolina 27606, USA
2. Clausthal University of Technology, Faculty of Natural and Materials Science, Arnold-Sommerfeld-Str. 6, D-38678 Clausthal-Zellerfeld, Germany
3. Institute of Physics, Faculty of Science, P.J. Šafárik Univesity, Park Angelinum 9, 04154 Košice, Slovakia