1. Department of Physics University of Paderborn; Warburger Str. 100 33098 Paderborn Germany
2. Peter Grünberg Institut Forschungszentrum Jülich; 52428 Jülich Germany
3. Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons Forschungszentrum Jülich; 52428 Jülich Germany