1. Physics Department, Graduate School of Science, Tohoku University, Sendai 980-77
2. European Commission, JRC, Institute for Transuranium Elements, Postfach 2340, D-76125, Karlsruhe, Germany
3. Institut Max von Laue-Paul Langevin, Grenoble, Cedex 9, France
4. Center for Low Temperature Science, Tohoku University, Sendai 980-77