Giant dynamical magnetoelectric coupling in a van der Waals multiferroic

The recent discovery of multiferroicity in few-layer helical van der Waals magnets has raised the prospect of realizing unprecedented magnetoelectric couplings in two dimensions. Nevertheless, the exact nature and strength of these couplings have remained elusive to date. Here, we perform a precision measurement of the dynamical magnetoelectric coupling in the van der Waals multiferroic NiI₂. We evaluate this interaction in resonance with a collective electromagnon mode, capturing the impact of its oscillations on the material’s ferroelectric and magnetic orders. We find that NiI₂ possesses a giant natural optical activity at terahertz frequencies, greatly outclassing other known multiferroics. First-principles calculations reveal that this dynamical magnetoelectric coupling originates from direct interactions between the electronic spin and orbital degrees of freedom, resulting in substantial enhancements over lattice-mediated effects. Our findings highlight the potential for intertwined electronic states to enable unique functionalities in the two-dimensional limit and pave the way for the development of nanoscale magnetoelectric devices operating at terahertz speeds.