1. JARA-FIT and 2nd Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany.
2. Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich, 52425 Jülich, Germany.
3. JARA–Institute for Quantum Information, RWTH Aachen University, 52056 Aachen, Germany.
4. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305–0044, Japan.