1. Siemens AG, ZFE BT MR 11, Postfach 3220, D-8520 Erlangen, FRG
2. Dept. of Physics, Nara Medical University Kashihara, Nara 634, Japan
3. Dept. of Physics, Okayama University, 3–1–1 Tsushima-Naka, Okayama 700, Japan
4. Physik-Department E12, TU München, D-8046 Garching, FRG