1. Contribution from the National Nanotechnology Laboratory of CNR-INFM, Distretto Tecnologico ISUFI, 73100 Lecce, Italy, TASC-INFM-CNR National Laboratory, Area Science Park, Basovizza, 34012 Trieste, Italy, CNR-Istituto di Cristallografia (IC), via Amendola 122/O, 70126 Bari, Italy, and LNMO-INSA, 135 avenue de Rangueil, Toulouse, France