[1] Inst. fur Schicht und Ionentechnik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
[2] Dept. of Electron. and Elec. Eng., University of Surrey, Guildford GU2 XH, United Kingdom
[3] Lab. de Magnet. de Bretagne, UFR Sciences, Univ. de Bretagne Occidentale, 6, avenue Le Gorgeu, Brest Cedex, 29285, France