1. Department of Mechanics, Mathematics and Management, Politecnico di Bari, v.le Japigia 182, I-70126 Bari, Italy
2. Physics Department M. Merlin, CNR Institute for Photonics and Nanotechnologies U.O.S. Bari, via Amendola 173, I-70126 Bari, Italy
3. Peter Grünberg Institut-1, FZ-Jülich, 52425 Jülich, Germany