Second-Order Topological Insulator in Ferromagnetic Monolayer and Antiferromagnetic Bilayer CrSBr

Second-order topological insulators (SOTIs) in 2D materials have attracted significant research interest. Recent theoretical predictions suggest that SOTIs can be achievable in 2D magnetic systems, especially within ferromagnetic (FM) materials. Yet, the quest for suitable 2D antiferromagnetic (AFM) materials capable of hosting magnetic SOTIs remains a challenge. Herein, utilizing first-principles calculations and theoretical analysis, 2D CrSBr is proposed, including monolayer and bilayer forms, as a promising candidate for a magnetic high-order topological insulator. The monolayer exhibits a FM ground state and features quantized fractional corner charge in its spin-up channel in the absence of spin-orbital coupling (SOC), yielding fully spin-polarized corner states. Intriguingly, the bilayer form adopts an AFM ground state while retaining the SOTI properties, with quantized corner charge in both spin channels. Remarkably, the SOTI properties in both monolayer and bilayer structures remain robust against the influence of SOC and symmetry-breaking perturbations. The work not only identifies a tangible material for realizing 2D magnetic SOTIs, encompassing both FM and AFM phases, but also offers a path to explore the distinctive characteristics of SOTIs merged with magnetism.