1. Christian Doppler Laboratory for Nanoscale Phase Transformations, Center for Surface and Nanoanalytics, Johannes Kepler University Linz 1 , Altenberger Straße 69, 4040 Linz, Austria
2. Institute of Semiconductor and Solid-State Physics, Johannes Kepler University Linz 2 , Altenberger Straße 69, 4040 Linz, Austria
3. Peter Grünberg Institute (PGI-9) and JARA-Fundamentals of Future Information Technologies, Forschungszentrum Jülich 3 , 52428 Jülich, Germany