1. Ceramics Laboratory, EPFL-Swiss Federal Institute of Technology, Lausanne 1015, Switzerland
2. DPMC-MaNEP, University of Geneva, 24 Quai Ernest
Ansermet, 1211 Geneva
4, Switzerland
3. Peter Grunberg Institute and Ernst Ruska Center for Microscopy and Spectroscopy with Electrons, Research Center Jülich, Jülich 52425, Germany