Possible persistence of multiferroic order down to bilayer limit of van der Waals material NiI$_{2}$

TL;DR

This study shows that multiferroic order, involving both magnetic and electric properties, can persist down to bilayer thickness in NiI$_{2}$, revealing potential for two-dimensional multiferroic materials.

Contribution

It demonstrates the possible persistence of multiferroic order in NiI$_{2}$ down to bilayer thickness using optical second-harmonic generation.

Findings

Multiferroic order persists in bilayer NiI$_{2}$.

Transition temperature decreases from 58 K to 20 K in bilayer.

Optical second-harmonic generation effectively traces ferroelectric order.

Abstract

Realizing a state of matter in two dimensions has repeatedly proven a novel route of discovering new physical phenomena. Van der Waals (vdW) materials have been at the center of these now extensive research activities. They offer a natural way of producing a monolayer of matter simply by mechanical exfoliation. This work demonstrates that the possible multiferroic state with coexisting antiferromagnetic and ferroelectric orders possibly persists down to the bilayer flake of NiI$_{2}$. By exploiting the optical second-harmonic generation technique, both magnitude and direction of the ferroelectric order, arising from the cycloidal spin order, are successfully traced. The possible multiferroic state's transition temperature decreases from 58 K for the bulk to about 20 K for the bilayer. Our observation will spur extensive efforts to demonstrate multi-functionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.