1. Center for Memory and Recording Research, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0401, United States
2. Department of Physics and Astronomy, Uppsala University, P.O. Box 516, 75120 Uppsala, Sweden
3. Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic