Universality of Type-II Multiferroicity in Monolayer Nickel Dihalides

TL;DR

This work addresses whether type-II multiferroicity is a general, tunable feature of the 2D transition metal dihalide family by examining monolayer NiBr2. Using STM/STS, it images ferroelectric stripes arising from a non-collinear spin-spiral and demonstrates reciprocal magnetoelectric coupling through electric-field control of magnetic order and magnetic-field suppression of polarization, with a clear link between ferroelectric order and the spin texture. NiBr2 exhibits a larger spin-spiral wavelength, λ ≈ 56a (≈ 15.1a), and reduced stability (Tc ≈ 5.5 K, Bc ≈ 4 T) compared with NiI2, due to weaker spin-orbit coupling and modified superexchange. Collectively, the results establish nickel dihalides as a chemically tunable platform for engineering magnetoelectric phases at the atomic scale, enabling programmable low-power spintronic functionality via halide substitution.

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.

Figures (3)

• Figure 1: Characterization of monolayer NiBr$_2$. a Overview topography of monolayer NiBr$_2$ island grown on HOPG (2V, 30pA, scale bar 100nm). The inset shows a representative d$I$/d$V$ spectrum taken on a monolayer NiBr$_2$ island. We attribute the small peak at -2.8V to be the onset of valence band, although are unable to confirm due to junction instabilities around this voltage range. b Zoom-in image of monolayer NiBr$_2$ taken at 0.3K showing the appearance of stripe-like pattern when scanning within conduction band (740mV, 20pA, scale bar 10nm). c-d Electric manipulation of the domains; a NiBr$_2$ island before bias sweep (740mV, 20pA, scale bar 20nm) and after bias sweep (740mV, 10pA, scale bar 20nm). The circled polarons highlight that the scans have been taken at the same area.
• Figure 2: Atomic-scale imaging and spectroscopy of the multiferroic stripes. a High-resolution topography of multiferroic stripes on monolayer NiBr$_2$ acquired within the conduction band (750mV, 10pA; scale bar: 5nm). b Topography of the same area taken within the bandgap, where both the atomic lattice and multiferroic stripes are simultaneously resolved (250mV, 10pA; scale bar: 5nm). c FFT of image b. Blue circles mark the reciprocal lattice points of the NiBr$_2$ unit cell, yellow circles indicate the multiferroic modulation, and red circles correspond to the moiré pattern arising from the lattice mismatch between HOPG and NiBr$_2$. d Spectroscopic line profile of the conduction band (CB) acquired across the multiferroic stripes. A modulation is visible between 0.5V and 0.6V. e Individual point spectra taken at the positions indicated by vertical lines in d. A modulation of the LDOS, a shift of the CB maximum, and contrast inversions are observed between 0.4V and 0.6V.
• Figure 3: Manipulation of multiferroicity through temperature and magnetic field. a-b Temperature dependence measurement; Scans taken at 3.5 K (740mV, 20pA, scale bar 10nm) and at 5.5 K (740mV, 20pA, scale bar 10nm). c-d Magnetic field dependence; Scans taken at 1.5 T (760mV, 30pA, scale bar 10nm) and at 4.0 T (760mV, 30pA, scale bar 10nm). The red (a-b) and blue (c-d) circles show stationery defects, showing that the scans have been taken at same areas.