1. Heisenberg Spin-dynamics Group, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Straße 1, D-76131 Karlsruhe, Germany
2. Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Straße 1, D-76131 Karlsruhe, Germany
3. Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, D-76344, Eggenstein-Leopoldshafen, Germany
4. Department of Quantum Matter Physics, University of Geneva, 1211 Geneva, Switzerland