Multiferroic metallic monolayer Cu(CrSe2)2†

The two-dimensional (2D) Cu(CrSe₂)₂ monolayer stands out for its combined ferromagnetic (FM), ferroelectric (FE), and metallic properties, marking itself as a prominent 2D multiferroic metal. This work studies those properties and the relevant physics, using density functional calculations, Monte Carlo simulations, and ab initio molecular dynamics. Our results show that Cu(CrSe₂)₂ monolayer is in the Cr³⁺ t³_2g state with S = 3/2 and Cu¹⁺ 3d¹⁰ with S = 0. A ligand hole in the Se 4p orbitals gives rise to metallic behavior and enhances the FM coupling between the local Cr³⁺S = 3/2 spins. The observed in-plane magnetic anisotropy primarily arises from exchange anisotropy, which is associated with the Cr–Se–Cr itinerant ferromagnetism. In contrast, both single-ion anisotropy and shape magnetic anisotropy contribute negligibly. The Dzyaloshinskii–Moriya interaction is also quite weak, only about 3% of the intralayer exchange parameters. Our Monte Carlo simulations show a FM Curie temperature (T_C) of 190 K. Moreover, the monolayer exhibits a vertical FE polarization of 1.79 pC m⁻¹ and a FE polarization switching barrier of 182 meV f.u.⁻¹, and the FE state remains stable above room temperature as shown by ab initio molecular dynamics simulations. Furthermore, a magnetoelectric coupling is partially manifested by a magnetization rotation from in-plane to out-of-plane associated with a FE-to-paraelectric transition. The magnetization rotation can also be induced by either hole or electron doping, and the hole doping increases the T_C up to 238 K. In addition, tensile strain reduces the FE polarization but enhances T_C to 290 K, while a compressive strain gives an opposite effect. Therefore, the multiferroic metallic Cu(CrSe₂)₂ monolayer may be explored for advanced multifunctional electronic devices.