Dzyaloshinskii-Moriya interaction and nontrivial spin textures in the Janus semiconductor monolayers VXY (X=Cl, Br, I; Y=S, Se, Te)

We present a density functional theory based study of a two-dimensional ferromagnetic Janus $\mathrm{V}XY$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I}; Y=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ structure. The dynamical and thermal stabilities of all possible $\mathrm{V}XY$ structures consisting of chalcogen and halogen atoms in the $1T$ and $2H$ phases are determined. Among them, only the $1T$ phases of the VBrS, VIS, and VISe structures are found to be stable. These structures show semiconducting properties and demonstrate spin-orbit-induced valley splittings, reaching 0.1 eV in the VISe monolayer. These structures exhibit in-plane easy axes with Curie temperatures around $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ estimated based on the classical Monte Carlo and quantum Green's function techniques. On the other hand, the absence of inversion symmetry in Janus $\mathrm{V}XY$ compounds gives rise to the Dzyaloshinskii-Moriya interaction with the possibility of forming chiral magnetic structures. In the absence of an external magnetic field, Janus $\mathrm{V}XY$ monolayers exhibit magnetic chiral structures at 0 K, evidenced by nonzero $Q$ (topological charge) values from Monte Carlo simulations. By applying an out-of-plane magnetic field of around 0.7 T, we observe the formation of skyrmions. At 5 K, the VIS monolayer demonstrates chiral magnetic structures, while VBrS and VISe display N\'eel-type domain walls. VISe forms a skyrmion lattice at $\ensuremath{\sim}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ which does not persist beyond this value. Since all the structures have an in-plane easy axis, an in-plane magnetic field of around 0.5 T at 0 K gives rise to the formation of bimerons.