Switchable polarization in 2D antiferromagnetic CuFe2X4 (X=S, Se)

The synthesis of two-dimensional AgCr2S4 has generated significant interest in AM2X4 type non-van der Waals materials. AM2X4 monolayer breaks the central inversion symmetry of the system by intercalation, resulting in intrinsic multiferroicity. Based on recent experimental achievements of the FeS2 monolayer, we predict two intrinsic multiferroic materials: CuFe2S4 and CuFe2Se4. Utilizing first-principles calculations, we demonstrate their ferroelectric and ferromagnetic properties. Our findings indicate that strain can switch the direction of polarization due to the different displacements of the ionic center and the center of electron density. CuFe2X4 (X = S, Se) monolayers are metal with antiferromagnetic coupling in the perpendicular direction under ambient conditions. The spontaneous polarizations for CuFe2S4 and CuFe2Se4 monolayers are 1.73 and 1.42 pC/m, respectively. Applying a compressive strain exceeding 3% can reverse the polarization direction, while a tensile strain of 4% enhances the polarization of CuFe2S4 to 2.65 pC/m. In addition, the strain effectively influences the magnetic ground state, the direction of the easy axis, and the contribution of different orbital hybridization. These findings help us understand the ferromagnetic and ferroelectric mechanisms in the CuFe2X4 monolayers and expand the family of 2D multiferroic materials.