Topological Spin Textures in Janus Monolayers of Chromium Trihalides Cr(I,X)₃

Topological magnetic states are promising for ultra-dense memory and logic devices. Recent progresses in two-dimensional magnets encourage the idea to realize topological states, such as skrmions and merons, in freestanding monolayers. However, monolayers such as CrI₃ lack Dzyaloshinskii-Moriya interactions (DMI) and thus do not naturally exhibit skyrmions/merons but rather a ferromagnetic state. Here we propose the fabrication of Cr(I,X)₃ Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and top-layer Br or Cl atoms. By performing first-principles calculations and Monte-Carlo simulations, we identify strong enough DMI, which leads to not only helical cycloid phases, but also to topologically nontrivial states, such as the intrinsic domain wall skyrmions in Cr(I,Br)₃ and the magnetic-field-induced bimerons in Cr(I,Cl)₃. Microscopic origins of such spin textures are revealed as well.