Tunable magnetocrystalline anisotropy and valley polarization in an intrinsic ferromagnetic Janus 2H -VTeSe monolayer

Two-dimensional Janus monolayers with specular asymmetry exhibit excellent physicochemical properties and are a good candidate for optoelectronic, valley electronic, and nanoelectronics devices. Based on the density functional theory, the Janus $2H$-VTeSe monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.324 eV and exhibits a good thermodynamic and kinetic stability, in-plane magnetocrystal anisotropy, large spontaneous valley polarization of 155 meV, and high Curie temperature (${T}_{c}$) of 380 K. The biaxial strain ($\ensuremath{-}6%<\ensuremath{\varepsilon}<6%$) can effectively tune the Janus $2H$-VTeSe monolayer from the bipolar magnetic semiconductor phase to half-semiconductor, spin gapless semiconductor, and half-metallic phases. Moreover, the magnetocrystal anisotropy energy (MAE) is modulated by the strain from 0.54 meV ($\ensuremath{\varepsilon}=\ensuremath{-}6%$) to 1.32 meV ($\ensuremath{\varepsilon}=\ensuremath{-}2%$), and the easy [100] and hard [001] magnetic axes could be switched from each other by charge carrier doping. The calculated valley optical response of the Janus $2H$-VTeSe monolayer exhibits a valley-selective circular dichroism. Due to the broken inversion and time-reversal symmetry, the valley polarization and Berry curvature can be continuously tuned by applying biaxial strains (modulation range 24.2%), external electric field (modulation range 2%), and varying the magnetization angle (0 to 180 degrees). The Janus $2H$-VTeSe monolayer possesses intrinsic ferromagnetic ordering, large spontaneous valley polarization, high ${T}_{c}$ of 380 K, and considerable MAE of 1.15 meV, giving it a potential application in the two-dimensional spintronic devices.