1. Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany
2. Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany
3. Institut für Kristallographie, RWTH-Aachen, Jägerstrasse 17/19, D-52056 Aachen, Germany
4. Faculty of Mathematics and Physics, Dep. of Low Temp. Physics, Charles University Prague,
V Holesovickach 8, 180 00 Prague 8, Czech Republic
5. Department of Physics, Faculty of Science, Kagoshima University, 1-21-35 Koorimoto, Kagoshima 890-0065