Universality of Type-II Multiferroicity in Monolayer Nickel Dihalides

TL;DR

This study demonstrates that type-II multiferroicity is a universal property in monolayer transition metal dihalides, confirmed through atomic-scale imaging and reciprocal magnetic-electric control, and shows tunability via chemical substitution.

Contribution

It provides the first evidence that type-II multiferroicity is a general, chemically tunable phenomenon in monolayer transition metal dihalides, extending beyond NiI2.

Findings

Visualized ferroelectric domains in NiBr2 monolayer.

Confirmed magnetoelectric coupling via electric and magnetic field manipulation.

Showed multiferroic state stability varies with chemical composition.

Abstract

The recent discovery of type-II multiferroicity in monolayer NiI${_2}$ 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 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 NiBr${_2}$ 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.