Spin-Orientation-Dependent Topological States in Two-Dimensional Antiferromagnetic NiTl ₂ S ₄ Monolayers

The topological states of matter arising from the nontrivial magnetic configuration provide a better understanding of physical properties and functionalities of solid materials. Such studies benefit from the active control of spin orientation in any solid, which is known to take place rarely in the two-dimensional (2D) limit. Here we demonstrate by the first-principles calculations that spin-orientation-dependent topological states can appear in the geometrically frustrated monolayer antiferromagnet (AFM). Different topological states including the quantum anomalous Hall (QAH) effect and time-reversal-symmetry (TRS) broken quantum spin Hall (QSH) effect can be obtained by changing the spin orientation in the NiTl ₂ S ₄ monolayer. Remarkably, the dilated nc-AFM NiTl ₂ S ₄ monolayer gives birth to the QAH effect with the hitherto reported largest number of quantized conducting channels (Chern number C = -4) in 2D materials. Interestingly, under tunable chemical potential, the nc-AFM NiTl ₂ S ₄ monolayer hosts a novel state supporting the coexistence of QAH and TRS broken QSH effects with a Chern number of C = 3 and a spin Chern number of Cs = 1. This work manifests a promising concept and material realization of topological spintronics in 2D antiferromagnets by manipulating their spin degree of freedom.