1. FIM Department, University of Modena and Reggio Emilia, 41125 Modena, Italy
2. Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom
3. CNR-Nanoscience Institute, S3 Center, 41125 Modena, Italy
4. CNR-Institute for Microelectronics and Microsystems, 40129 Bologna, Italy
5. Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
6. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany