1. Department of Material Science and Engineering, University of Ioannina, GR 45110 Ioannina, Greece
2. Department of Physics, University of Crete, P.O. Box 2208, GR 71003 Heraklion, Greece
3. Tandem Accelerator Laboratory, INPP, NCSR Demokritos, GR 15310 Ag Paraskevi, Greece
4. Department of Applied Physics, National Technical University of Athens, GR 15780 Athens, Greece
5. Department of Physics, University of Ioannina, GR 45110 Ioannina, Greece