1. IMEC, Kapeldreef 75, 3001 Heverlee, Belgium
2. Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium
3. Laboratory of Solid State Physics and Magnetism, KU Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium