Two-dimensional ferromagnetism and driven ferroelectricity in van der Waals CuCrP₂S₆

Multiferroic materials have the potential to be applied in novel magnetoelectric devices, for example, high-density non-volatile storage devices. During the last decades, research on multiferroic materials was focused on three-dimensional (3D) materials. However, 3D materials suffer from dangling bonds and quantum tunneling in nano-scale thin films. Two-dimensional (2D) materials might provide an elegant solution to these problems, and thus are highly in demand. Using first-principles calculations, we predicted ferromagnetism and electric-field-driving ferroelectricity in the monolayer and even in the few-layers of CuCrP2S6. Although the total energy of the ferroelectric phase of the 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 freedom 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 our experiment, we observed the out-of-plane ferroelectricity in few-layer CuCrP2S6 (approximately 13 nm thick) at room temperature. 2D ferromagnetism of few-layers is inferred from the magnetic hysteresis loops of the massively stacked nanosheets at 10 K. The experimental observations support our calculations very well. Our findings may provide a series of 2D materials for further device applications.