1. Department of Physics, VSB-Technical University of Ostrava, Ostrava-Poruba 708 33, Czech Republic
2. CEITEC IPM, Institute of Physics of Materials, AS CR, Brno 616 62, Czech Republic
3. Institute of Physics of Materials, AS CR, Brno 616 62, Czech Republic
4. Institute of Physics, Slovak Academy of Sciences, Bratislava 845 11, Slovakia