Within the rapidly expanding family of two-dimensional (2D) materials, 2D ferromagnetic (FM) Janus semiconductors are attracting increasing attention because of their particular potential as emergent multifunctional devices. However, the low Curie temperatures of the experimentally available semiconductors hinder progress. Robust FM superexchange interactions are a prerequisite to achieve high Curie temperatures. Experimentally, 2D FM semiconductors with Janus structures are usually synthesized from non-Janus structures through the so-called Janus engineering strategy. Along synthetic routes of Janus engineering, there is a complex interplay between the two distinct kinds of effects, i.e., lattice size and ligand electronegativity. To unravel the effects of Janus engineering, we computationally study the superexchange interactions in $2H\text{−}\mathrm{Fe}XY$ ($X,Y$= Cl, Br, I) semiconductors. By quantitatively separating the two distinct kinds of effects using strength indicators for superexchange interactions, we find that decreasing lattice size and ligand atoms with smaller electronegativity values can strengthen FM superexchange interactions. The summarized guidelines for enhancing Curie temperatures are useful for the rational design of 2D FM Janus semiconductors for high Curie temperatures by strengthening FM superexchange interactions.

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• Exchange interaction
• Ferromagnetism
• Magnetic interactions
• Magnetism
• 2-dimensional systems
• Janus materials
• Magnetic semiconductors
• Density functional theory
• Electronic structure
• First-principles calculations