Two-dimensional ferromagnetism with strong magnetic interactions induced by Janus-type hydrogenation in FeSe2

In this paper, using first-principles calculations, we find that single-face (Janus-type) hydrogenation induces a magnetic transition from a ${120}^{\ensuremath{\circ}}$ noncollinear antiferromagnet (AFM) to a robust ferromagnet (FM) with high Curie temperature $({T}_{\mathrm{c}},\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}345\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and large perpendicular magnetic anisotropy (MA, $\ensuremath{\sim}2$ meV) in two-dimensional (2D) $\mathrm{Fe}{\mathrm{Se}}_{2}$. This hydrogenation results in a shift of spin-down states of in-plane orbitals $({d}_{xy}/{d}_{{x}^{2}\ensuremath{-}{\mathrm{y}}^{2}})$ to the Fermi level, which is crucial in inducing the high ${T}_{\mathrm{c}}$ and large perpendicular MA. To clarify the strong FM, we proposed a mechanism, i.e., FM direct exchange interaction of spin-down half-filled in-plane orbitals. This interaction is different from the traditional direct exchange interactions in most 2D magnets, which tend to form AFM coupling. As the inversion symmetry is broken, a significant Dzyaloshinskii-Moriya interaction (DMI; $\ensuremath{\sim}2.6$ meV) is also induced by the Janus-type hydrogenation. It is found that tuning the exchange integral of orbitals can change the sign of its component of the DMI, providing an approach to manipulate the chirality of magnetic order.