1. Institute of Physics, Faculty of Sciences, P.J. Safarik University, Park Angelinum 9, 041 54 Košice, Slovakia
2. Material Physics Department, Faculty of Chemistry, University of the Basque Country, UPV/EHU, Plaza Elhuyar, 2, 20018, San Sebastian, Spain
3. Institute of Material Research, SAS, Watsonova 47, 040 01 Košice, Slovakia
4. IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
5. Dpto. de Física Aplicada, EUPDS, UPV/EHU, 200018, San Sebastian, Spain