1. International Research Centre Magtop, Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02668 Warsaw, Poland
2. Consiglio Nazionale delle Ricerche CNR-SPIN, UOS Salerno, I-84084 Fisciano, Salerno, Italy
3. Dipartimento di Fisica “E.R. Caianiello”, Università degli Studi di Salerno, I-84084 Fisciano, Salerno, Italy
4. Department of Semiconductor Materials Engineering, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, PL-50370 Wrocław, Poland
5. Faculty of Physics, University of Warsaw, Pasteura 5, PL-02093 Warsaw, Poland