Janus Engineering of Polar Magnetism and Valley Splitting in CuCrP2X6 (X: S, Se) Monolayers.

We investigate the interplay of multiferroicity and structural asymmetry in two-dimensional CuCrP2X6 (X: S, Se) and the Janus CuCrP2S3Se3 monolayer using density functional theory. While pristine CuCrP2S6 exhibits an antiferroelectric ground state with in-plane magnetic anisotropy, complete or partial substitution of S by Se fundamentally alters both lattice polarity and magnetic behavior. In particular, the Janus CuCrP2S3Se3 structure stabilizes a ferroelectric phase with a finite out-of-plane dipole moment arising from asymmetric Cu+ displacement. This polarity, combined with enhanced spin-orbit coupling, drives a transition to robust out-of-plane magnetic anisotropy and induces sizable spontaneous valley splitting at the K and K' points. All studied monolayers are predicted to be ferromagnetic semiconductors with composition-dependent band gaps and exchange interactions. The coupling between ferroelectric polarization and magnetic order in the Janus system suggests a route toward electric-field control of magnetism in a single-phase material. These findings highlight Janus engineering of transition metal phosphorus trichalcogenides as an effective strategy to realize intrinsically coupled ferroic and valleytronic functionalities for next-generation (opto)spintronic applications.