Bipolar ferromagnetic semiconductors and doping-tuned room-temperature half-metallicity in monolayer MoX3 (X=Cl, Br,

Two-dimensional magnetic materials have great potential applications in designing spintronics devices. Here, electronic structures and magnetic properties of monolayer molybdenum trihalides $\mathrm{Mo}{X}_{3}$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ are systematically investigated by using first-principles calculations with hybrid functional HSE06. Our calculations show that the magnetic ground state of the monolayer ${\mathrm{MoI}}_{3}$ is ferromagnetic (FM) with a Curie temperature of 24 K, while the monolayer $\mathrm{Mo}{\mathrm{Cl}}_{3}$ and $\mathrm{Mo}{\mathrm{Br}}_{3}$ are $\mathrm{N}\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}\mathrm{el}$ antiferromagnetic. The monolayer $\mathrm{Mo}{\mathrm{Cl}}_{3}$ and $\mathrm{Mo}{\mathrm{Br}}_{3}$ can be tuned into FM phase with a tensile strain. The monolayer ${\mathrm{MoI}}_{3}$ has a large magnetic anisotropy energy (MAE) of 1.051 meV/Mo with an out-of-plane easy axis. The Curie temperature and MAE of the monolayer ${\mathrm{MoI}}_{3}$ can be increased by about $50%$ with a tensile strain of $3%$. Remarkably, the monolayer ${\mathrm{MoI}}_{3}$ and the tensile strained $\mathrm{Mo}{\mathrm{Cl}}_{3}$ and $\mathrm{Mo}{\mathrm{Br}}_{3}$ are found to be bipolar ferromagnetic semiconductors. The Curie temperature of the monolayer ${\mathrm{MoI}}_{3}$ can be increased up to room temperature by carrier doping. For the monolayer $\mathrm{Mo}{\mathrm{Br}}_{3}$, a magnetic phase transition from the antiferromagnetic to FM can be triggered by an electron doping. In addition, the room-temperature half-metallic states can be achieved in the carrier-doped monolayer ${\mathrm{MoI}}_{3}$ and tensile strained $\mathrm{Mo}{\mathrm{Br}}_{3}$. The topological properties for the valence bands of the monolayer $\mathrm{Mo}{X}_{3}$ are also studied. Chern insulating states are obtained in the tensile strained monolayer $\mathrm{Mo}{\mathrm{Cl}}_{3}$ and $\mathrm{Mo}{\mathrm{Br}}_{3}$. Our paper shows that the monolayer $\mathrm{Mo}{X}_{3}$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ are promising candidates for exploring two-dimensional magnetism and spintronics in experiments.