First-principles predictions of room-temperature ferromagnetism in orthorhombic MnX₂ (X = O, S) monolayers

Two-dimensional ferromagnets with high spin-polarization at ambient temperature are of considerable interest because they might be useful for making nanoscale spintronic devices. We report that even though bulk phases of MnO₂ are generally antiferromagnetic with low ordering temperatures, the corresponding MnO₂ and MnS₂ monolayers are ferromagnetic, and MnS₂ is a high temperature half metallic ferromagnet. Based on first-principles calculations, we find that the MnO₂ monolayer is an intrinsic ferromagnetic semiconductor with a Curie temperature T_C of ∼300 K, while the half-metallic MnS₂ monolayer has a remarkably high T_C of ∼1150 K. Both compounds have substantial magnetocrystalline anisotropy, out of plane in the case of MnO₂ monolayers, and in plane along the b-axis of orthorhombic MnS₂ monolayer. Interestingly, a metal-insulator phase transition occurs in the MnS₂ monolayer when the applied biaxial strain is beyond -2%. Tuning near this metal-insulator transition offers additional possibilities for devices. The present work shows that MnX₂ (X = O, S) monolayers have the properties required for ultrathin nano-spintronic devices.