Direct electrical measurement of polarization in monolayer $\mathrm{NiI_2}$
In plain words
Type-II multiferroics are materials whose spiral spin order itself creates electric polarization. A single atomic layer of $\mathrm{NiI_2}$ has been shown to host the spin spiral, but its electric polarization has not been measured directly.
Precise statement
Single-layer $\mathrm{NiI_2}$ shows a canted spin spiral with charge modulation in spin-polarized STM. Measure the spin-induced polarization $P$ and its onset temperature $T_{c}$ directly, by electrical (pyroelectric or switching current) or piezoresponse probes, and test whether $P$ reverses with the spiral chirality. An answer is the measured $P(T)$ and $T_{c}$ with the chirality dependence.
What would settle it
Pyroelectric, switching-current or piezoresponse measurement of P(T) on a characterized monolayer, combined with spin-polarized STM of its spiral chirality.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Song et al. (Nature 2022) was questioned by Jiang et al. (Nature 2023, Matters Arising); STM detected electromagnons (arXiv:2510.08253) and vectorial spin-polarized STM resolved the spiral (Wang et al., PRL 2026, arXiv:2604.06959); no direct electrical measurement of P in the monolayer was found.
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