**5. Acknowledgements**

We wish to acknowledge Sonia Melle and J.M. Pastor for all the work done, J.M M. González, J.M. Palomares and F. Pigazo (ICMM) for the VSM magnetometry measurements and J.C. Gómez-Sáez for her proofreading of the English texts. This research has been partially supported by M.E.C. under Project No. FIS2006-12281-C02-02, M.C.I under FIS2009-14008-C02-02, C.A.M under S/0505/MAT/0227 and UNED by 2010V/PUNED/0010.
