Strain-modulated ferroelectricity and ferromagnetism in two-dimensional multiferroics of CuCrP2X6 (X = S, Se)

Abstract

Two-dimensional (2D) multiferroics hold promise for application in field-effect transistors and non-volatile memories. However, extremely rare 2D multiferroics with coexisting ferroelectric (FE) and ferromagnetic (FM) orderings are reported, which almost all exhibit small FE polarizations and low Curie temperatures (T_c). Based on first-principles calculations, 2D multiferroics CuCrP₂X₆ (X = S, Se) are found to have significant out-of-plane FE polarizations of 1.25 and 1.11 μC cm⁻², respectively. Remarkably, the band gap type of monolayer CuCrP₂S₆ transitions from indirect to direct under the critical tensile strain of 2%, optimizing the electronic structure properties. Meanwhile, the CuCrP₂X₆ monolayers exhibit robust FE polarizations and moderately increased energy barriers under tensile strain, which effectively stabilizes the FE state. Additionally, strain engineering improves t_2g–e_g orbital hybridization and enhances the exchange interactions between Cr³⁺ cations by decreasing the band gap, which effectively increases the T_c to ∼70 K. These results demonstrate the critical role of strain in the ferroelectricity and ferromagnetism in CuCrP₂X₆ monolayers, illustrating the potential applications of CuCrP₂X₆ in next-generation nonvolatile and advanced electronic devices.