First-Principles Calculations of Room-Temperature Ferromagnetic Half-Metallicity and Quantum Anomalous Effect in 2D NbX₂ (X = S, Se, Te) Monolayers: Implications for Spintronic Nanodevices

The development of two-dimensional (2D) ferromagnetic half-metals with 100% spin polarization provides a broad platform for exploring the nanoscales. Herein, based on first-principles calculations, we report one type of 2D transition metal chalcogenides, NbX2 (X = S, Se, Te), with high thermodynamic stability. It is found that NbS2, NbSe2, and NbTe2 monolayers are robust ferromagnetic half-metals with above room-temperature Curie temperature and considerable band gaps in the semiconducting spin channels (larger than 1.0 eV). In particular, both NbSe2 and NbTe2 monolayers are turned to be Quantum Anomalous Hall (QAH) metals with a nonzero Chern number of |C| = 1 on the basis of the spin orbit coupling (SOC) effect. In addition, applying biaxial tensile strains in NbSe2 and NbTe2 can induce a transition from ferromagnetic half-metals to ferromagnetic semiconductors. Our study proposes that the NbX2 (X = S, Se, Te) monolayers are potential candidates in room-temperature spintronics devices.