1. Janusz Gil Institute of Astronomy, University of Zielona Góra, Licealna 9, PL-65–417, Zielona Góra, Poland
2. Institute of Physics, University of Zielona Góra, Licealna 9, PL-65–417, Zielona Góra, Poland
3. Institute for Condensed Matter Physics of NAS of Ukraine, 1 Svientsitskii Street, 79011 Lviv, Ukraine