Intrinsic large valley polarization, perpendicular magnetic anisotropy, and piezoelectric polarization in multiferroic two-dimensional ferrovalley semiconductors.

Two-dimensional multiferroic ferrovalley semiconductors are promising candidates for next-generation electronic applications in data encoding and nonvolatile memory technologies. However, materials featuring coupled ferroic orders (ferrovalley, ferromagnetic, and ferroelectric) are scarce. Their small valley polarization and in-plane magnetic anisotropy also pose practical limitations. Here, we propose a symmetry engineering strategy for designing multiferroic ferrovalley semiconductors with enhanced ferroic properties. We utilize spatiotemporal inversion symmetry breaking and strong spin-orbit coupling (SOC) in Janus 5d transition metal dichalcogenides, as well as the M̂zT̂ symmetry breaking in bilayer structures to achieve improved multiple ferroic orders, reversible electrical control of valley polarization, and the anomalous valley Hall effect. We demonstrate the strategy in stable ferrovalley OsClX (X = F, Br, I) monolayers via first-principles calculations. Giant valley polarization (160-280 meV), perpendicular magnetic anisotropy (∼10 meV), high Curie temperatures, and the piezoelectric effect of OsClX (X = F, Br, I) monolayers are explored, and their mechanisms are elucidated through synergistic interactions of spin polarization, electric polarization, and strong SOC. Furthermore, we show that the layer-locked Berry curvature of ferroelectric bilayer OsClBr with AB and BA stacking switched by sliding can result in the unconventional layer-polarized anomalous Hall effect. Our approach applies to diverse two-dimensional Janus ferrovalley systems and is promising for application in valleytronics and magnetic storage devices.