Two-dimensional bipolar ferromagnetic semiconductors from layered antiferromagnets

Bipolar magnetic semiconductors (BMSs) are a class of compounds possessing different valence band maximum (VBM) and conduction band minimum (CBM) in terms of electron spins, which are related to but distinct from half-metals. They allow one to switch one spin current to another by applying external gate voltage or chemical doping, which can meet the requirement of bipolar manipulation in spintronic devices. Designing and searching BMSs are still a challenge. Here we predict that monolayer ${\mathrm{CrPS}}_{4}$ and ${\mathrm{CrPSe}}_{4}$ are bipolar ferromagnetic semiconductors with Curie temperature 58 K and 82 K, respectively. We predicted that they can be exfoliated from their A-type antiferromagnetic bulk form. Electric-field gating could drive ${\mathrm{CrPS}}_{4}$ and ${\mathrm{CrPSe}}_{4}$ into half-metals with reversible spin-polarization directions, where the bipolar doping is relatively easier to access for ${\mathrm{CrPSe}}_{4}$ in experiment than ${\mathrm{CrPS}}_{4}$ for its suitable electron affinity and ionic potential. Furthermore, we highlight the role played by the different magnitude between spin exchange splitting and crystal field splitting in the formation of BMSs. The results presented here may provide new clues in designing or searching BMSs.