Nature of magnetic exchange interactions in the kagome antiferromagnets FeGe and FeSn

Magnetic exchange interactions (MEIs) in kagome magnets exhibit rich features due to the interplay of charge, spin, orbital and lattice degrees of freedom, giving rise to a variety of exotic quantum states. Through first-principles calculations, we systematically investigate the MEIs in kagome antiferromagnets FeGe and FeSn. While the antiferromagnetic order originates from the interlayer coupling between neighboring kagome layers, Fe atoms within each kagome layer couple ferromagnetically, driven by the competition between ferromagnetically favorable direct MEIs and antiferromagnetically favorable Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The stronger direct MEIs but weaker RKKY interactions in FeGe result in a substantially higher Néel temperature with respect to FeSn. Interestingly, the nearest neighboring exchange energy in both materials approximately linearly depends on the Fe-Fe bond length, so that moderate compressive strain can significantly enhance their Néel temperatures.