Title: Spatial and temporal magnetisation dynamics in systems with antiferromagnetic interactions.

Speaker: Dr. Abhishek Nag, Paul Scherrer Institute, Switzerland

Abstract: Haldane spin systems, characterized by integer spins with no long-range magnetic order, hold paramount importance in condensed matter physics as they challenge traditional magnetic behaviors and have been instrumental in advancing our understanding of quantum magnetism. In the first part of my talk, I will unveil a hidden aspect of these integer spin systems: magnetic excitations carrying two units of angular momentum, as opposed to the conventional magnons which have one unit of angular momentum. Typically, higher-order excitations like these remain invisible to spectroscopic techniques governed by selection rules. However, through the application of resonant inelastic X-ray scattering on Y2BaNiO5, we have observed a unique propagation behavior of these excitations along the spin chain. This phenomenon is not limited to the specific material and can be generalized to any magnetic system with S > 1/2. As a result, our work opens up new avenues for exploring quantum many-body interactions in magnetic systems. In the second part of my talk, I will present the results of our ongoing investigations into the type-II multiferroic system, CoCr2O4. We propose that the individual sublattice magnetization dynamics of this ferrimagnetic system can be effectively studied using time-resolved magneto-optical Kerr spectroscopy. Our preliminary findings indicate that the sublattices exhibit distinct magnetization dynamics due to the presence of different exchange interactions in the system, and these dynamics are strongly influenced by magnetocrystalline anisotropy.