Intertwined Topological and Magnetic Orders in Atomically Thin Chern Insulator MnBi 2 Te 4 .

MnBi ₂ Te ₄ , a van der Waals magnet, is an emergent platform for exploring Chern insulator physics. Its layered antiferromagnetic order was predicted to enable even-odd layer number dependent topological states. Furthermore, it becomes a Chern insulator when all spins are aligned by an applied magnetic field. However, the evolution of the bulk electronic structure as the magnetic state is continuously tuned and its dependence on layer number remains unexplored. Here, employing multimodal probes, we establish one-to-one correspondence between bulk electronic structure, magnetic state, topological order, and layer thickness in atomically thin MnBi ₂ Te ₄ devices. As the magnetic state is tuned through the canted magnetic phase, we observe a band crossing, i.e., the closing and reopening of the bulk band gap, corresponding to the concurrent topological phase transition in both even- and odd-layer-number devices. Our findings shed new light on the interplay between band topology and magnetic order in this newly discovered topological magnet.