Tunability of Type-II Multiferroicity in Monolayer Nickel Dihalides

The recent discovery of type-II multiferroicity in monolayer NiI2 indicated a new pathway for intrinsic magnetoelectric coupling in the two-dimensional limit. However, determining whether this phenomenon is a unique anomaly or a general, chemically tunable property of the material class remains unresolved. Here, we demonstrate the universality of type-II multiferroicity in the transition metal dihalides by visualizing the ferroelectric order in monolayer NiBr2. 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 NiBr2 is energetically less robust than in its iodide counterpart, consistent with modified superexchange interactions and the reduced spin-orbit coupling. Our results establish the transition metal dihalides as a versatile platform where the stability of magnetoelectric phases can be engineered through chemical substitution.近期在单层碘化镍(II)中发现的II型多铁性（type-II multiferroicity），为二维极限下的本征磁电耦合指明了全新的研究路径。然而，该现象究竟是独特的反常行为，还是该材料体系普遍存在、可通过化学手段调控的特性，目前仍未得到解答。本研究通过可视化单层溴化镍(II)中的铁电序，证明了过渡金属二卤化物中II型多铁性的普适性。本研究借助扫描隧道显微镜（STM），解析了原子级分辨率的铁电畴结构，并通过双向调控实验证实了其磁电起源：通过电场实现磁序的重新取向，以及利用外磁场抑制电极化。此外，我们发现溴化镍(II)中的多铁态在能量稳定性上弱于其碘化物对应体系，这与改性后的超交换相互作用以及减弱的自旋轨道耦合效应相一致。本研究结果证实，过渡金属二卤化物可作为一类多功能研究平台，其磁电相的稳定性可通过化学取代进行精准调控。