  Two-dimensional (2D) semiconducting multiferroics that can effectively couple
magnetic and polarization (P) orders have great interest for both fundamental
research and technological applications in nanoscale, which are, however, rare
in nature. In this study, we propose a general mechanism to realize
semiconducting 2D multiferroics via vdW heterojunction engineering, as
demonstrated in a typical heterostructure consisting of magnetic bilayer CrI3
(bi-CrI3) and ferroelectric monolayer In2Se3. Interestingly, the novel indirect
orbital coupling between Se 4p and Cr 3d orbitals, intermediated by the
interfacial I 5p orbitals, are switchable in the opposite P configurations,
resulting in an unexpected mechanism of strong asymmetrical magnetoelectric
coupling. Therefore, along with the noticeable ferroelectric energy barrier
induced by In2Se3, the realization of opposite magnetic orders in opposite P
configurations can eventually result in the novel multiferroicity in
bi-CrI3/In2Se3. Finally, we demonstrate that our mechanism can generally be
applied to design other vdW multiferroics even with tunable layer thickness.
