Prediction of intrinsic multiferroicity and large valley polarization in a layered Janus material

Two-dimensional (2D) intrinsic multiferroics have attracted considerable attention for the next generation of advanced information technologies. Herein, we report that bilayer Janus FeSCl, a novel 2D system designed by substituting sulfur in monolayer 1T-FeCl2, exhibits a giant spontaneous valley polarization and intrinsic magnetoelectric coupling. This Janus structure exhibits a ground-state bilayer structure that breaks space-inversion symmetry, enabling sliding ferroelectricity. Each monolayer displays robust intralayer ferromagnetic ordering, while the bilayer hosts interlayer antiferromagnetic alignment with opposing magnetic moments. Crucially, ferrovalley-mediated coupling links ferroelectric polarization and antiferromagnetic order, allowing electric-field-driven magnetic reversal. Notably, the direction of the net magnetic moment can be reversed through ferroelectric polarization switching, enabling nonvolatile control of the magnetism. The elucidated mechanisms are generalizable to diverse 2D material families, offering a universal framework for designing atomic-scale multiferroics. This work not only establishes foundational insights into 2D multiferroics but also advances the understanding of coupled charge-spin-valley physics in low-dimensional systems.