1. Institute of Experimental Physics, Slovak Academy of Sciences, 040 01 Kosice, Slovakia
2. Faculty of Aeronautic, Technical University in Kosice, 041 21 Kosice, Slovakia
3. Faculty of Electrical Engineering and Informatics, Technical University in Kosice, 042 00 Kosice, Slovakia
4. Faculty of Mining, Ecology, Process Control and Geotechnology, TU Kosice, Letna 9, 042 00 Kosice, Slovakia
5. Politehnica University of Timisoara, Timisoara 300222, Romania
6. Laboratory of Magnetic Fluids, Romanian Academy - Timisoara Division, Timisoara, Romania