Coexistence of multiple Weyl fermions and quantum anomalous Hall effect in 2D half-metallic Cr2NT2

Topological state in two-dimensional (2D) magnetic system has attracted widespread attention due to its potential application in spintronics and quantum computing fields. As a novel topological states of matter, Weyl semimetals with multiple topological phases have been studied. However, realizing multiple topological phases in 2D magnetic system and studying their entanglement still face great challenges. In this work, using first-principles calculations, we predict that 2D monolayer Cr2NT2 (T =-OH,-F, =O) has multiple Weyl fermions (WFs) and large spin-down gap. Ferromagnetic ground states of monolayer Cr2NT2 meet the dynamic, thermal and mechanical stabilities. Without spin-orbit coupling (SOC), these WPs are fully spin-polarized. When considering SOC, apart from WP3 in Cr2NO2, all WPs are opened the gap, and the chiral edge states are observed in Cr2NT2. In particular, there is a quantum anomalous Hall effect in Cr2NF2. Further, a biaxial strain in the a -b plane leads to the emergence of rich topological phases without SOC. In addtion, an effective four-band tight-binding model is constructed to clarify the origin of WFs in the non-trivial band topology. Finally, we calculate the Berry curvature and abnormal Hall conductivity of Cr2NT2, and find that they are a non-zero energy points around the Fermi level, indicating that monolayer Cr2NT2 are promising candidates for spin-tronic applications. Therefore, it is necessary to explore 2D magnetic materials with multiple types of WFs and QAH phases.