Two-dimensional (2D) semiconducting multiferroics that can effectively couple magnetic and polarization $\left(P\right)$ orders have great interest for both fundamental research and technological applications in nanoscale, which are, however, rare in nature. In this paper, we propose a general mechanism to realize semiconducting 2D multiferroics via van der Waals (vdW) heterojunction engineering, as demonstrated in a typical heterostructure consisting of magnetic bilayer $\mathrm{Cr}{\mathrm{I}}_3\left(bi\text{-Cr}{\mathrm{I}}_3\right)$ and ferroelectric monolayer ${\mathrm{In}}_2{\mathrm{Se}}_3$. Interestingly, the novel indirect orbital coupling between $\mathrm{Se}4p$ and $\mathrm{Cr}3d$ orbitals, intermediated by the interfacial $\mathrm{I}5p$ orbitals, is 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 ${\mathrm{In}}_2{\mathrm{Se}}_3$, the realization of opposite magnetic orders in opposite $P$ configurations can eventually result in the novel multiferroicity in $bi\text{−}{\mathrm{CrI}}_3/{\mathrm{In}}_2{\mathrm{Se}}_3$. Finally, we demonstrate that our mechanism can generally be applied to design other vdW multiferroics even with tunable layer thickness.

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• Magnetoelectric effect
• Bilayer films
• Magnetic semiconductors
• Multiferroics
• First-principles calculations