Intermixing-driven surface and bulk ferromagnetism in the quantum anomalous Hall candidate MnBi${}_6$Te${}_{10}$

Abstract

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 (SL). 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 SL. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SL with an increasing number $n$ of Bi${}_2$Te${}_3$ layers. However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, we demonstrate robust FM properties in MnBi${}_6$Te${}_{10}$ ($n=2$) with $T_C\approx 12$ K and establish their origin in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. Our measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. Our investigation thus consolidates the MnBi${}_6$Te${}_{10}$ system as perspective for the QAHE at elevated temperatures.