Large spontaneous valley polarization and high magnetic transition temperature in stable two-dimensional ferrovalley YX2(X=I, Br, and

Ferrovalley materials with spontaneous valley polarization are crucial to valleytronic application. Based on first-principles calculations, we demonstrate that two-dimensional (2D) $\mathrm{Y}{X}_{2}(X=\mathrm{I}, \mathrm{Br}, \text{and} \mathrm{Cl})$ in a 2H structure constitutes a series of promising ferrovalley semiconductors with large spontaneous valley polarization and high magnetic transition temperature. Our calculations reveal that $\mathrm{Y}{X}_{2}$ are dynamically, energetically, thermally and mechanically stable 2D ferromagnetic semiconductors with a magnetic transition temperature about 200 K. Due to the natural noncentrosymmetric structure, intrinsic ferromagnetic order and strong spin orbital coupling, the large spontaneous valley polarizations of 108.98, 57.70, and 22.35 meV are also predicted in single-layer $\mathrm{Y}{X}_{2}(X=\mathrm{I}, \mathrm{Br}, \text{and} \mathrm{Cl})$, respectively. The anomalous valley Hall effect is also proposed based on the valley contrasting Berry curvature. Moreover, the ferromagnetism and valley polarization are found to be effectively tuning by applying a biaxial strain. Interestingly, the suppressed valley physics of ${\mathrm{YBr}}_{2}$ and ${\mathrm{YCl}}_{2}$ can be switched on via applying a moderate compression strain. The present findings promise $\mathrm{Y}{X}_{2}$ as competitive candidates for the further experimental studies and practical applications in valleytronics.