[Prof. Je-Geun Park] Cross-probing van der Waals multiferroics (published in Nature Reviews Physics)
Multiferroic materials, with coupled electric and magnetic orders, hold the potential to enable low-power memory and logic devices by replacing electrical currents with electric fields for magnetic switching. Reducing the dimensionality of multiferroics, from 3D to 2D, has opened new frontiers in this already burgeoning field by enabling unprecedented control over magnetoelectric coupling that is beyond the reach of bulk counterparts, owing to reduced electrostatic screening and enhanced quantum fluctuations at the atomically thin limit. A notable example is a few-layer NiI2 multiferroic, which exhibits coexisting chiral spin textures and (anti)ferroelectricity, accompanied by strong magnetoelectric coupling; by simultaneously breaking time-reversal and spatial-inversion symmetry, NiI2 shows new physical properties such as unique mutual control of chiral magnetism and polarization with giant magnetoelectric coupling, ultrafast electromagnon excitations and odd-parity p-wave magnetism. However, there are experimental challenges in unambiguously identifying intrinsic multiferroicity. This Review critically examines the current landscape of 2D multiferroic materials, with an emphasis on experimental methodologies rather than material taxonomy. We assess different conventional probes for magnetism and ferroelectricity and identify key limitations in single-technique approaches. We emphasize the importance of magneto-opto-electric cross-correlative measurements in establishing intrinsic multiferroic behaviour and outline future research directions for both fundamental studies and device applications.