1. Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-187 Poznań, Poland
2. W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrocław 2, Poland
3. International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 50-539 Wrocław, Poland