1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden
2. TDK-Micronas GmbH, Hans-Bunte-Strasse 19, D-79108 Freiburg, Germany
3. Department of Physics, Chemistry and Biology, Linkoping University, 581 83 Linköping, Sweden
4. National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom