Multiple Weyl fermions and tunable quantum anomalous Hall effect in 2D half-metal with huge spin-related energy gap

Topological states of matter in two-dimensional (2D) system, especially those combined with magnetism have received special attention currently, because of their new physics and the potential applications in spintronics and nanoscale. At present, realizing multiple topological phases in a single 2D Magnetic material and studying their potential entanglement still face great challenges. Here, we predict a novel 2D topological material namely monolayer EuO2, which can realize multiple types of topological states, namely, type-I Weyl point (WP), type-II WP, critical-type WP, and Quantum anomalous Hall (QAH) effect. Specifically, Without considering Spin-orbit coupling (SOC), these Weyl points perfectly possess the 100% spin-polarization because monolayer EuO2 is a half metal phase with huge spin-related energy gap as large as 6.2 eV. Remarkably, With considering SOC, the topological states in monolayer EuO2 can be tuned by direction of magnetization. Under out-plane magnetization, monolayer EuO2 can realize the QAH effect with single chiral edge state occurring inside the energy gap. Under in-plane magnetization, the system can remain 2D Weyl states at special directions. In other in-plane magnetization, the system shows the in-plane QAH effect with the Chern number of -1 or 1. Therefore, Our work promotes the realization of tunable topological phase in 2D half metals.