Prediction of high Curie-temperature intrinsic ferromagnetic semiconductors and quantum anomalous Hall states in XBr3 (X = Cu, Ag, Au) monolayers

Abstract

Two-dimensional magnetic materials have recently attracted much attention owing to their potential applications in the design of spintronic devices. Based on density-functional theory calculations, we build three monolayer transition-metal tribromides (XBr₃, X = Cu, Ag, Au), which are found to be a series of stable two-dimensional intrinsic ferromagnetic (FM) semiconductors. Their easy axes are all along out-of-plane directions with large magnetic anisotropy energies (1.478 meV, 1.598 meV, and 0.386 meV per transition-metal atom for CuBr₃, AgBr₃, and AuBr₃, respectively). Very high Curie temperature (149 K) is predicted for CuBr₃ based on a Heisenberg model, much larger than that of the monolayer CrI₃. The FM states of XBr₃ (X = Cu, Ag, Au), dominated by super-exchange coupling between the e_g(d_z², d_x²−y²) states of two neighbor X atoms (mediated by Br 4p states), are all robust against tensile strain with certain strengths. Interestingly, AgBr₃ and AuBr₃ are bipolar FM semiconductors. The band gaps for the three built monolayers exhibit different trends in variation as a function of the strain, which can be attributed to the competition of the magnetic exchange, band dispersion, and spin–orbit coupling (SOC). A topologically nontrivial quantum anomalous Hall (QAH) state is obtained in the carrier-doped monolayer CuBr₃. Our results provide an excellent class of experimentally feasible materials for the intrinsic FM semiconductors, which can facilitate the development of spintronic and microelectronic devices.