Observation of Multiferroicity in Two-Dimensional NiBr₂

Following the recent discovery of monolayer NiI₂ as the first 2D type-II multiferroic, we report the discovery of multiferroicity in monolayer NiBr₂, further expanding this family of materials. Using scanning tunnelling microscopy (STM), we studied NiBr₂ grown epitaxially on highly oriented pyrolytic graphite (HOPG). Below 4 K, STM imaging reveals periodic, stripe-like variations in the local density of states (LDOS) that indicate a multiferroic order driven by an underlying magnetic spin spiral and spin-orbit coupling, a mechanism similar to that observed in NiI₂. We demonstrate strong magnetoelectric coupling by manipulating the multiferroic domain boundaries with external electric fields and further show that the periodicity of the multiferroic stripes can be tuned by applying both external electric and magnetic fields. Unlike NiI₂, NiBr₂ can be synthesized from a single precursor source, which significantly simplifies the growth process. These findings establish monolayer NiBr₂ as a promising and robust platform for the exploration of van der Waals 2D multiferroic materials.