Interfacial coupling induced critical thickness for the ferroelectric bistability of two-dimensional ferromagnet/ferroelectric van der Waals heterostructures

The discovery of two-dimensional (2D) ferroic materials has stimulated substantial efforts in developing emergent functionalities by synthesizing van der Waals (vdW) heterostructures, and one promising effect is the nonvolatile electrical control of magnetism in magnetoelectric (ME) heterostructures consisting of coupled 2D vdW ferromagnetic and ferroelectric layers. In this paper, it is proposed that the asymmetric interfacial coupling in such heterostructures may seriously distort the ferroelectric double-well potential, thus destabilizing the ferroelectricity. We investigate this consequence in $\mathrm{F}{\mathrm{e}}_{3}\mathrm{GeT}{\mathrm{e}}_{2}/\ensuremath{\alpha}\ensuremath{-}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ vdW heterostructure using the first-principles calculations. It is revealed that one of the two potential wells for ferroelectric monolayer $\ensuremath{\alpha}\ensuremath{-}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ is suppressed by the asymmetric interfacial coupling between electric polarization and the built-in electric field induced by intrinsic charge transfer, while the ferroelectric bistability can be recovered when the $\ensuremath{\alpha}\ensuremath{-}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ layer is thicker than three unit cells. Therefore, this work presents an alternative mechanism of critical thickness for the 2D ferroelectricity in ME vdW heterostructures.