Probing Spin-Excitations in an Ultra-Thin van der Waals Helimagnet*

Materials with non-collinear magnetic structures host many unique phenomena, including multiferroic, magnetoelectric and topological phases. Among them, the discovery of non-collinear magnetism in the van der Waals (vdW) nickel dihalides (NiX₂, X = Br, I) down to the few- and single-atomic-layer limit paves the way for the realization of these properties in 2D devices/heterostructures. In these materials, geometric frustration leads to a complex magnetic phase diagram where distinct magnetic states are determined by a delicate balance of intra-layer exchange interactions. However, accessing these interactions in the 2D limit is experimentally challenging due to the limited thickness. In this work, we address these challenges and present measurements of the dispersive magnetic excitations in NiI₂ in the bulk and ultra-thin limit using resonant inelastic X-ray scattering (RIXS). We uncover the dispersion of a single-magnon branch in the bulk which shows a clear correspondence to the incommensurate helimagnetic order. Additional measurements performed on a two-layer sample reveal dispersive spin excitations with marked renormalization with respect to the bulk. Based on these results, we assess the evolution of the intra-layer exchange interactions and the helimagnetic order in the 2D limit of NiI₂. More broadly, these measurements provide information regarding the nature of finite-momentum spin excitations in layered 2D magnets, which have thus far eluded direct observation, and establish RIXS as an ideal technique to investigate complex magnetic phases in vdW materials.