[1] Univ Complutense Madrid, RENFE, Inst Magnetismo Aplicado, Madrid 28230, Spain
[2] CSIC, Inst Ciencia Mat Madrid, E-28049 Madrid, Spain
[3] EUITT UPM, Dept Fis Aplicada Telecomunicac, Madrid 28031, Spain
[4] IMA, Unidad Asociada ICMM, Madrid 28230, Spain