Using density functional theory, we revisit the magnetic properties of a recently proposed family of noncentrosymmetric two-dimensional magnetic materials, chromium chalcohalide monolayers, $\mathrm{Cr}XY$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$; $Y=\mathrm{Cl},\mathrm{Br},\mathrm{I}$). These systems consist of three atomic planes stacked in the $X$–Cr–$Y$ sequence, which breaks inversion symmetry, giving rise to their designation as Janus monolayers. We consider both $1T$ and $1H$ structural polymorphs of $\mathrm{Cr}XY$, beginning with an assessment of their dynamical stability via phonon spectra calculations. All systems, except for the $1H\text{−}\mathrm{CrTe}Y$ variants, are found to be dynamically stable. Among the two polymorphs, the $1T$ phase is consistently more favorable, with energy gains exceeding 0.55 eV per formula unit. Our total-energy calculations reveal that all dynamically stable $\mathrm{Cr}XY$ monolayers exhibit ferromagnetic coupling. However, robust out-of-plane magnetic anisotropy is observed only in the CrSI and CrSeI compositions for both $1T$ and $1H$ structures. The perpendicular magnetic anisotropy results from a constructive interplay between single-ion and anisotropic exchange contributions that overcome the dipole–dipole interaction. We further quantify the Dzyaloshinskii-Moriya interaction (DMI) in CrSI and CrSeI for both polymorphs and reveal a weak-to-moderate DMI strength as compared to the isotropic exchange interaction term. Finally, for systems exhibiting ferromagnetic coupling and perpendicular magnetic anisotropy, the exchange and anisotropy parameters derived from density functional theory calculations are employed as inputs for large-scale atomistic spin dynamics simulations to probe the temperature evolution of real-space magnetic structures. The calculated Curie temperatures are at least 210 K for $1T\text{−}\mathrm{CrSI}$, 235–260 K for $1H\text{−}\mathrm{CrSeI}$, and 370–410 K for $1H\text{−}\mathrm{CrSI}$. In contrast, the sizable DMI in $1T\text{−}\mathrm{CrSeI}$ results in a wormlike domain ground state at zero external field and enables the stabilization of skyrmions under a perpendicular magnetic field. We then discuss and illustrate how $\mathrm{Cr}XY$ monolayers can be used to tune the Bychkov-Rashba split surface state of BiTeI or to enhance magnetism of thin films of the intrinsic antiferromagnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$.

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• Magnetic anisotropy
• Magnetic coupling
• Magnetic interactions
• Magnetic order
• Magnetism
• Transition temperature
• Janus materials
• Density functional theory
• Landau-Lifschitz-Gilbert equation