1. Dipartimento di Fisica Informatica e Matematica, Università di Modena e Reggio Emilia, via G Campi 213/a, I-41125 Modena, Italy
2. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C) and Peter Grünberg Institute (PGI), Forschungszentrum Jülich, D-52425 Jülich, Germany
3. CNR-IMM, Via P. Gobetti 101, I-40129 Bologna, Italy
4. CNR-Istituto Nanoscienze, Centro S3, Via G Campi 213/a, I-41125 Modena, Italy
5. Department of Physics, University of Ottawa, 25 Templeton, Ottawa, Ontario K1N 6N5 Canada
6. Department of Physics, Institute for Advanced Studies in Basic Sciences, 45137-66731 Zanjan, Iran
7. CNR-IMEM Parco Area delle Scienze 37/A, I-43124 Parma, Italy