1. B.Verkin Institute for Low Temperature Physics and Engineering of NAS of Ukraine 1 , 47 Nauky Ave., 61103 Kharkiv, Ukraine
2. Department of Experimental Physics, Comenius University 2 , Mlynská dolina, 84248 Bratislava, Slovakia
3. Institute of Physics, Slovak Academy of Sciences 3 , Dúbravská cesta, Bratislava, Slovakia