1. Institute of Physics, Faculty of Sciences, P.J. Safarik University, Park Angelinum 9, 041 54 Kosice, Slovakia
2. Institute of Inorganic Chemistry, Fac. Sci., UPJS, Moyzesova 11, 041 54 Kosice, Slovakia
3. Institute of Experimental Physics SAS, Watsonova 47, 040 01 Kosice, Slovakia
4. Departamento de Fisica de Materiales, Universidad del Pais Vasco, 20009 Sab Sebastian, Spain