1. Department FIM, University of Modena and Reggio Emilia, via G. Campi 213/a, Modena 41125, Italy
2. CNR-Institute of Nanoscience-S3, via G. Campi 213/a, Modena 41125, Italy
3. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, 52425, Jülich, Germany
4. Department of Physics and Astronomy, University of Bologna, viale B. Pichat 6/2, Bologna 40127, Italy