1. Centre for Research of Adaptive Nanostructures and Nanodevices, School of Physics, Trinity College Dublin, Dublin 2, Ireland, Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany, Laboratoire National des Champs Magnétiques Pulsés, 143, Ave. de Rangueil, F-31400 Toulouse, France, and High Field Magnet Laboratory, Institute of Molecules and Materials, Radboud University Nijmegen, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands