1. Institute of Materials for Electronics and Magnetism, Nanoscience-Trento-FBK Division, 38123 Povo, TN, Italy
2. Istituto Nazionale di Fisica Nucleare, TIFPA, 38123 Povo, TN, Italy
3. Physics Division, School of Science and Technology, University of Camerino, 62032 Camerino, MC, Italy
4. INFN, Sezione di Perugia, 06123 Perugia, PG, Italy
5. Department of Microelectronics and Computer Engineering, ECTM, Delft University of Technology, Feldmanweg 17, 2628 CT Delft, The Netherlands
6. CNR-INO, L.go Enrico Fermi 6, 50125 Firenze, FI, Italy