1. Institute for Low Temperature and Structure Research, P.O. Box 1410, 50-950 Wrocław 2, Poland
2. Forschungszentrum Karlsruhe, Institut für Festkörphysik, P.O. Box 3640, D-76021 Karlsruhe, Germany
3. Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland