1. São Carlos Institute of Physics, University of São Paulo, PO Box 369, 13560-970 São Carlos, São Paulo, Brazil
2. Université de Toulouse (UPS), CEMES-CNRS, 118 Route de Narbonne, 31062 Toulouse, France
3. Departamento de Física, Universidade Federal de São Carlos, 13565-905 São Carlos, São Paulo, Brazil
4. Centre for Emerging Device Technologies, Department of Engineering Physics, McMaster University, Hamilton, Ontario L8S 4L7, Canada