**Acknowledgements**

The authors would like to express their sincerest gratitude and indebtedness to Maximo Siu Li of São Carlos Physical Institute for providing the PL measurements, the LMA-IQ for FEG– SEM facilities and the Brazilian research funding agencies CNPq and FAPESP-CEPID/CDMF 2013/07296-2 for granting the financial support for this research project.
