  Contribution of d-electron to ferroelectricity of type-II multiferroics
causes strong magneto-electric coupling and distinguishes them from the
conventional type-I multiferroics. However, their therein polarization is too
small because the ferroelectricity is merely a derivative from the magnetic
order. Here we report a new class of multiferroic materials, monolayer VOX2 (X
= Cl, Br, and I), which combine the advantages of type-I and type-II
multiferroics. Both ferroelectricity and magnetism arise from the same V
cation, where the filled d-orbital is perpendicular to an a priori
ferroelectric polarization and thus poses no hindrance to ferroelectricity,
indicating a violation of the usual d0 rule. This makes the combination of
large polarizations and strong magneto-electric coupling possible. Our findings
not only add new ferromagnetic-ferroelectric multiferroics, but also point to a
unique mechanism to engineer multiferroics.
