  Multiferroic materials are potential to be applied in novel magnetoelectric
devices, for example, high-density non-volatile storage. Last decades, research
on multiferroic materials was focused on three-dimensional (3D) materials.
However, 3D materials suffer from the dangling bonds and quantum tunneling in
the nano-scale thin films. Two-dimensional (2D) materials might provide an
elegant solution to these problems, and thus are highly on demand. Using
first-principles calculations, we predict ferromagnetism and driven
ferroelectricity in the monolayer and even a few-layers of CuCrP2S6. Although
the total energy of the ferroelectric phase of monolayer is higher than that of
the antiferroelectric phase, the ferroelectric phases can be realized by
applying a large electric field. Besides the degrees of freedoms in the common
multiferroic materials, the valley degree of freedom is also polarized
according to our calculations. The spins, electric dipoles and valleys are
coupled with each other as shown in the computational results. In experiment,
we observe the out-of-plane ferroelectricity in a few-layer CuCrP2S6
(approximately 13 nm thick) at room temperature. 2D ferromagnetism of
few-layers is inferred from magnetic hysteresis loops of the massively stacked
nanosheets at 10 K. The experimental observations support our calculation very
well. Our findings may provide a series of 2D materials for further device
applications.
