Direct visualization of surface spin-flip transition in MnBi4Te7

TL;DR

This study visualizes the surface spin-flip transition in MnBi4Te7 using cryogenic magnetic force microscopy, revealing a surface-specific transition at lower fields and supporting potential for quantized transport in ultra-thin layers.

Contribution

First direct visualization of surface spin-flip transition in MnBi4Te7, demonstrating surface-specific magnetic behavior and supporting applications in topological magnetism.

Findings

Surface magnetic layer undergoes a first-order spin-flip transition at lower fields than the bulk.

Magnetic contrast confirms antiferromagnetic order persists at the surface.

Enhanced surface order parameter suggests robust ferromagnetism in single-layer limit.

Abstract

We report direct visualization of spin-flip transition of the surface layer in antiferromagnet MnBi4Te7, a natural superlattice of alternating MnBi2Te4 and Bi2Te3 layers, using cryogenic magnetic force microscopy (MFM). The observation of magnetic contrast across domain walls and step edges confirms that the antiferromagnetic order persists to the surface layers. The magnetic field dependence of the MFM images reveals that the surface magnetic layer undergoes a first-order spin-flip transition at a magnetic field that is lower than the bulk transition, in excellent agreement with a revised Mills' model. Our analysis indicates an enhancement of the order parameter in the surface magnetic layer, implying robust ferromagnetism in the single-layer limit. The direct visualization of surface spin-flip transition not only opens up exploration of surface metamagnetic transitions in layered antiferromagnets, but also provides experimental support for realizing quantized transport in ultra-thin films of MnBi4Te7 and other natural superlattice topological magnets.