Ferroelectricity and ferromagnetism in a VO I2 monolayer: Role of the Dzyaloshinskii-Moriya interaction

Multiferroics with intrinsic ferromagnetism and ferroelectricity are highly desired but rather rare, while most ferroelectric magnets are antiferromagnetic. A recent theoretical work [Tan, Phys. Rev. B 99, 195434 (2019)10.1103/PhysRevB.99.195434] predicted that oxyhalides VOX2 (X: halogen) monolayers are two-dimensional multiferroics by violating the empirical d0 rule. Most interestingly, the member VOI2 are predicted to exhibit spontaneous ferromagnetism and ferroelectricity. In this work, we extend the previous study on the structure and magnetism of VOI2 monolayer by using density-functional theory and Monte Carlo simulation. The presence of the heavy element iodine with a strong spin-orbit coupling gives rise to an effective Dzyaloshinskii-Moriya interaction in the polar structure, which favors a short-period spiral magnetic structure.. Another interesting result is that the on-site Coulomb interaction can strongly suppress the polar distortion thus leading to a ferromagnetic metallic state. Therefore, the VOI2 monolayer is either a ferroelectric insulator with spiral magnetism or a ferromagnetic metal, instead of a ferromagnetic ferroelectric system. Our study highlights the key physical role of the Dzyaloshinskii-Moriya interaction.