1. Dpto. Fisica Materia Condensada C3, Instituto Nicolas Cabrera (INC), Condensed Matter Physics Institute (IFIMAC), Universidad Autonoma de Madrid, Madrid 28049, Spain
2. SPINTEC, UMR 8191, CEA/CNRS/UJF & G-INP, INAC, 38054 Grenoble Cedex, France
3. Hitachi Global Storage Technologies, San Jose, California 95135, USA