Coexistence of a Gapless Topological State and Surface Ferromagnetism in MnBi₂Te₄

The recent discovery of the intrinsic magnetic topological insulator MnBi₂Te₄ has sparked considerable interest because of the novel quantum properties that can arise as a result of the coupling between the topological and magnetic properties. However, the inability of the magnetic ordering to open a gap at the Dirac point, in contradiction to theoretical calculation, has led to speculation as to whether the magnetic ordering extends to the surface region. We use both static and time resolved ARPES to show that bulk magnetism does extend to the surface of MnBi₂Te₄. Our results have important implications for understanding the fundamental properties of the topological state and offer a clue for understanding its low quantum anomalous hall temperature.

D. Nevola et al. PRL 125, 117205, (2020).