1. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, United Kingdom
2. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Department of Materials, Imperial College London, London, United Kingdom
4. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich GmbH, Jülich, Germany