1. Institut Néel, CNRS and Université Joseph Fourier, 38042 Grenoble, France
2. Fraunhofer Institut Angewandte Festkörperphysik, Tullastrasse 72, 79108 Freiburg, Germany
3. School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, United Kingdom
4. Institut Universitaire de France, 103 Boulevard Saint-Michel, 75005 Paris, France