1. Technische Universität Wien, Institute of Solid State Electronics, Vienna 1040, Austria
2. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, U.K.
3. Université Grenoble Alpes, CEA, IRIG-DEPHY, Grenoble 38054, France
4. Université Grenoble Alpes, CNRS, Institut NEEL UPR2940, Grenoble 38042, France