1. Peter Gruenberg Institute; Forschungszentrum Juelich GmbH and JARA-FIT; 52425 Juelich Germany
2. Institute of Physical Chemistry; RWTH Aachen University; 52056 Aachen Germany
3. Institute of Electronic Materials; IWE2; RWTH Aachen University; 52056 Aachen Germany
4. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons; Forschungszentrum Juelich GmbH and RWTH Aachen University; 52425 Juelich Germany