1. CNR ISMN, Via Gobetti 101, 40129 Bologna, Italy
2. Department of Physics and Astronomy “A. Righi”, University of Bologna, Via Berti-Pichat 6/2, I-40127 Bologna, Italy
3. Materials Science and Engineering Department, National Chung Hsing University, Taichung 402, Taiwan
4. CNR ISMN, Via Salaria km 29.300, 00015 Monterotondo Scalo, Italy
5. Physics Department, Università degli Studi di Milano, Via Celoria 16, 20133 Milan, Italy