Intermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi 6 Te 10 .

The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi 2 Te 4 and MnBi 4 Te 7 benchmark the (MnBi 2 Te 4 )(Bi 2 Te 3 ) n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi 2 Te 4 septuple layers (SLs). However, the QAHE realization is complicated in MnBi 2 Te 4 and MnBi 4 Te 7 due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi 2 Te 3 quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi 6 Te 10 (n = 2) with T c ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi 6 Te 10 system as perspective for the QAHE at elevated temperatures.