Multiferroicity in few-layer van der Waals NiI2

Van der Waals (vdW) materials have been discovered new physical phenomena and building block for optoelectronic or spintronics devices. The vdW material NiI2 was reported that ferroelectricity and antiferromagnetism coexisted in bulk [1]. It has been unknown, however, whether two physical properties remain in the two-dimensional limit. In this work, we demonstrate that the multiferroic state exists in the few-layer NiI2 and persist down to bi-layer. We observed the second harmonic generation (SHG) signal in exfoliated sample. The signal originated from the ferroelectric order, arising from the cycloidal spin order. The transition temperature of multiferroic state decreases from 58 K in the bulk to about 20 K in the bi-layer. Our work proves the multi-functionality in few-layer vdW material.
[1] Kurumaji, T. et. al, PRB ,87 (1) , 014429 (2013).