Probing Type-II Multiferroicity in Monolayer NiBr₂

The recent discovery of type-II multiferroicity in monolayer ${\mathrm{NiI}}_2$ indicated a new pathway for intrinsic magnetoelectric coupling in the two-dimensional limit. However, whether this phenomenon is a unique feature of ${\mathrm{NiI}}_2$ or a chemically tunable property of the broader material class has remained unresolved. Here, we demonstrate that type-II multiferroicity in the nickel dihalides is tunable through halide ligand substitution by visualizing the ferroelectric order in monolayer ${\mathrm{NiBr}}_2$ . Using scanning tunneling microscopy (STM), we resolve atomic-scale ferroelectric domains and confirm their magnetoelectric origin through reciprocal manipulation experiments: reorienting magnetic order via electric fields and suppressing the electric polarization with external magnetic fields. Furthermore, we find that the multiferroic state in ${\mathrm{NiBr}}_2$ is energetically less robust than in its iodide counterpart, consistent with modified superexchange interactions and the reduced spin-orbit coupling (SOC). Our results establish the nickel dihalides as a versatile platform where the stability of magnetoelectric phases can be engineered through chemical substitution.