Possible Persistence of Multiferroic Order down to Bilayer Limit of van der Waals Material NiI2

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 persists down to the bilayer flake of Nil,. 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 multifunctionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.

Automated summary

This study reveals the possible existence of a multiferroic state with coexisting antiferromagnetic and ferroelectric orders in bilayer flakes of Nil, with a transition temperature of about 20 K. The magnitude and direction of the ferroelectric order arising from the cycloidal spin order were successfully traced using optical second-harmonic generation technique. This observation will inspire extensive efforts to demonstrate multifunctionality in vdW materials, which have been largely explored using heterostructures of singly ferroic materials.