  Multiferroicity allows magnetism to be controlled using electric fields or
vice versa, which has gained tremendous interest in both fundamental research
and device applications. A reduced dimensionality of multiferroic materials is
highly desired for device miniaturization, but the coexistence of
ferroelectricity and magnetism at the two-dimensional limit is still debated.
Here, we used a NbSe2 substrate to break both the C3 rotational and inversion
symmetries in monolayer VCl3 and thus introduced exceptional in-plane
ferroelectricity into a two-dimensional magnet. Scanning tunnelling
spectroscopy directly visualized ferroelectric domains and manipulated their
domain boundaries in monolayer VCl3, where coexisting antiferromagnetic order
with canted magnetic moments was verified by vibrating sample magnetometer
measurements. Our density functional theory calculations highlight the crucial
role that highly directional interfacial Cl-Se interactions play in breaking
the symmetries and thus in introducing in-plane ferroelectricity, which was
further verified by examining an ML-VCl3/graphene sample. Our work demonstrates
an approach to manipulate the ferroelectric states in monolayered magnets
through van der Waals interfacial interactions.
