Direct visualization of surface spin flip transition in MnBi₄Te₇

The intrinsic antiferromagnetic topological insulator family MnBi_2nTe_3n+1 (MBT) provides an ideal platform to realize exotic quantum transport such as QAHE and Axion insulator state^1,2. Here, we focus to the n=2 member, MnBi₄Te₇, which consists of alternating MnBi₂Te₄ and Bi₂Te₃ layers with smaller interlayer coupling. Using cryogenic magnetic force microscopy, we observed termination-dependent magnetic contrast across both the domain wall and the step confirming the persistence of A-type order at the surface. Furthermore, we discovered a first-order surface spin flip transition in MnBi4Te7, which has never been reported in natural antiferromagnet. Our observation can be explained by a revised Mills’ Model with reduced exchange interaction and enhanced surface magnetization. Direct visualization of surface spin-flip transition not only initiates the exploration of surface transition in potential functional antiferromagnets, but also pave the way for realizing quantized transport in ultra-thin films of MnBi₄Te₇ and other MBT superlattices.

 

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