1. Laboratory
of Solid State Physics and Magnetism, KU Leuven, Celestijnenlaan
200D, BE-3001 Leuven, Belgium
2. imec, Kapeldreef 75, BE-3001 Leuven, Belgium
3. Institute
for Materials Research, Hasselt University, Wetenschapspark 1, BE-3590 Diepenbeek, Belgium