Robust second-order topological insulator in 2D van der Waals magnet CrI₃

CrI₃ offers an intriguing platform for exploring fundamental physics and the innovative design of spintronics devices in two-dimensional (2D) magnets, and moreover has been instrumental in the study of topological physics. However, the 2D CrI₃ monolayer and bilayers have long been thought to be topologically trivial. Here we uncover a hidden facet of the band topology of 2D CrI₃ by showing that both the CrI₃ monolayer and bilayers are second-order topological insulators (SOTIs) with a nonzero second Stiefel-Whitney number w₂ = 1. Furthermore, the topologically nontrivial nature can be explicitly confirmed via the emergence of floating edge states and in-gap corner states. Remarkably, in contrast to most known magnetic topological states, we put forward that the SOTIs in 2D CrI₃ monolayer and bilayers are highly robust against magnetic transitions, which remain intact under both ferromagnetic and antiferromagnetic configurations. These interesting predictions not only provide a comprehensive understanding of the band topology of 2D CrI₃ but also offer a favorable platform to realize magnetic SOTIs for spintronics applications.