1. Laboratorio MDM, IMM−CNR, Via C. Olivetti 2, 20864 Agrate Brianza (MB), Italy
2. INRiM, NanoFacility, Electromagnetism Division, Strada delle Cacce 91, 10135 Torino, Italy
3. Department of Physics, University of Milan, Via Celoria 16, 20133 Milano, Italy