1. Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
2. Center for Memory and Recording Research, University of California San Diego, San Diego, California 92093, USA
3. Department of Physics, School of Sciences, National Institute of Technology, Andhra Pradesh 534102, India
4. Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, CZ 121 16 Prague, Czech Republic
5. Department of Physics and Astronomy, Uppsala University, P. O. Box 516, SE-75120 Uppsala, Sweden
6. Department of Molecular Sciences and Nanosystems, Ca' Foscari University of Venice, 30172 Venice, Italy