1. Institute of Physics, University of Silesia, 4 Uniwersytecka, 40‐007 Katowice, Poland
2. Institute of Materials Science, University of Silesia, 12 Bankowa, 40‐007 Katowice, Poland
3. Laboratoire de Physique de l'Etat Condencé, Universite du Maine, 72085 Le Mans, France