  Ferrovalley materials with spontaneous valley polarization are crucial to
valleytronic application. Based on first-principles calculations, we
demonstrate that two-dimensional (2D) YX$_2$(X= I, Br,and Cl) in 2H structure
constitute a series of promising ferrovalley semiconductors with large
spontaneous valley polarization and high Curie temperature. Our calculations
reveal that YX$_2$ are dynamically and thermally stable 2D ferromagnetic
semiconductors with a Curie temperature above 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 YX$_2$(X = I, Br and
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 YBr$_2$ and YCl$_2$ can be
switched on via applying a moderate compression strain. The present findings
promise YX$_2$ as competitive candidates for the further experimental studies
and practical applications in valleytronics.
