1. Contribution from the Max-Planck-Institute for Solid State Research, Heisenbergstr.1, D-70569 Stuttgart, Germany, Institute of Technical Physics, University of Erlangen, Erwin-Rommel Strasse 1, D-91058 Erlangen, Germany, School of Physics and Condensed Matter Research Institute, Seoul National University, Seoul, 151-747, Korea, and Physics and Astronomy Department, Michigan State University, East Lansing, Michigan 48824-2320