Van Hung, Nguyen, Department of Physics, Hanoi University of Science. 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Varga, R., Institute of Physics, Faculty of Science, University of Pavol Jozef Safarik, Park Angelinum 9, 041 54 Kosice, Slovakia Varga, R., RVmagnetics, a. s., Hodkovce 21, 04421 Košice, Slovakia
