High-temperature ferromagnetism and half-metallicity in hole-doped Janus OM2S(M=Ga,In,andTl) monolayers

Realizing tunable and stable magnetic states in two-dimensional (2D) materials has been an attractive issue in current materials and physics fields. To date, however, the reported magnetic 2D materials mostly have low Curie transition temperature. In this paper, we theoretically propose a class of Janus group-III monochalcogenide $\mathrm{O}{M}_{2}\mathrm{S}(M=\mathrm{Ga},\mathrm{In},\phantom{\rule{0.16em}{0ex}}\text{and}\phantom{\rule{0.16em}{0ex}}\mathrm{Tl})$ monolayers. Among them, ${\mathrm{OTl}}_{2}\mathrm{S}$ possesses the sombrerolike valence band edge (SVBE) and a Van Hove singularity of the density of states, indicating a possible ferromagnetic transition upon hole doping based on the Stoner criteria. In contrast, the SVBE can be formed in ${\mathrm{OGa}}_{2}\mathrm{S}$ and ${\mathrm{OIn}}_{2}\mathrm{S}$ via tensile strain. Also, the SVBE is responsible for the intraband Lifshitz transition in these monolayers. Ferromagnetism and half-metallicity can be introduced in Janus $\mathrm{O}{M}_{2}\mathrm{S}$ monolayers by injecting hole carriers. At the mean field level, the highest Curie transition temperature ${T}_{\mathrm{C}}$ is estimated to be 627.5 K in freestanding ${\mathrm{OGa}}_{2}\mathrm{S}$, which is higher than those of GaSe and $M\mathrm{O}(M=\mathrm{Ga}\phantom{\rule{0.16em}{0ex}}\text{and}\phantom{\rule{0.16em}{0ex}}\mathrm{In})$ monolayers. The origin of the higher ${T}_{\mathrm{C}}$ is attributed to the unique sombrerolike band dispersion. We further show that the magnetic properties of ${\mathrm{OGa}}_{2}\mathrm{S}$ are sensitive to doped hole density and applied strain, implying a potential tunability in experiment. Finally, the feasibility of introducing the magnetic states through $p$-type dopants and monovacancy defect is explored. In this paper, we not only provide a class of nonmagnetic 2D materials for investigating the high-temperature ferromagnetism and half-metallicity but also widen the application potentials of group-III monochalcogenides.