1. Radboud University, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
2. HGST, A Western Digital Company, San Jose, California 95135, USA
3. Institute of Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany
4. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden–Rossendorf, 01328 Dresden, Germany