1. a Institute of Materials Chemistry, Brno University of Technology , Purkynova 118, CZ-61200 Brno, Czech Republic
2. b Department of Physics , University of Pardubice , Studentska 84, 532 10 Pardubice, Czech Republic
3. c Institute of Physics, Academy of Sciences , Cukrovarnicka 10, CZ-162 53 Prague 6, Czech Republic