1. Institute of Physics, Szczecin University of Technology, al. Piastów 17, 70-310 Szczecin, Poland
2. Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784, Athens, Greece
3. Institute of Material Engineering, Szczecin University of Technology, al. Piastów 17, 70-310 Szczecin, Poland
4. Applied Physics Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784, Athens, Greece