1. Institute of Physics, Faculty of Sciences, P.J. Šafárik University, Park Angelinum 9, 041 54 Košice, Slovakia
2. Institute of Inorganic Chemistry, Faculty of Sciences, P.J. Šafárik University, Moyzesova 11, 041 54 Košice, Slovakia
3. Department of Condensed Matter Physics, University of Sevilla, P.O. Box 1065, 41080 Sevilla, Spain