Giant valley polarization in Janus 1T-MNY (M = Mo, W; Y = Br, Cl) monolayers: a new class of ferrovalley materials.

Two-dimensional (2D) ferrovalley materials with spontaneous valley polarization have emerged as pivotal platforms for valleytronic applications. Generally, 1T-phase 2D materials are overlooked as ferrovalley candidates due to their intrinsic inversion symmetry or the absence of ferromagnetic order. Using first-principles calculations, we reveal exceptional ferrovalley properties in a series of Janus 1T-MNY (M = Mo, W; Y = Br, Cl) monolayers. All four monolayers exhibit ferromagnetism, with the MoNBr monolayer uniquely demonstrating perpendicular magnetic anisotropy. When spin-orbit coupling (SOC) is taken into account, the MNY monolayers can give rise to giant valley polarization in the conduction band, reaching up to 521.2 meV. Furthermore, these monolayers host valley-contrast Berry curvature, which enables the realization of the anomalous valley Hall effect under an in-plane electric field. Through strain engineering, the magnetic anisotropy of MNY monolayers can be effectively modulated, allowing switching of the easy magnetization orientation from in-plane to out-of-plane, while valley polarization remains robust against strain even under self-doping. Our research provides highly promising material platforms for experimental studies and practical applications in valleytronics.