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MnBi₂Te₄ is a promising candidate of antiferromagnetic (AFM) topological insulator which might host fascinating quantized anomalous Hall effect or axion insulator state in few molecular layers^1,2. These quantum phenomena are induced by the mass gap on Dirac surface states opened by the A-type AFM order (alternating ferromagnetic layers)³ with uniaxial anisotropy in MnBi₂Te₄. However, recent high resolution angle resolved photoemission spectroscopy studies reveal gapless surface states, indicating paramagnetic or non-uniaxial A-type AFM orders^4–7. Yet there is no direct evidence of such surface magnetism. Recently we demonstrated magnetic imaging of AFM domain walls in MnBi2Te4 family using our homemade cryogenic magnetic force microscope (MFM)⁸. In this talk, we will present MFM studies of MnBi2Te4 single crystals to address the nature of surface magnetism.

References
1. Otrokov, M. M. et al. PRL 122, 107202 (2019).
2. Gong, Y. et al. CPL 36, 076801 (2019).
3. Yan, J.-Q. et al. PRM 3, 064202 (2019).
4. Li, H. et al. arXiv:1907.06491 (2019).
5. Swatek, P. et al. arXiv:1907.09596 (2019).
6. Hao, Y.-J. et al. arXiv:1907.03722 (2019).
7. Chen, Y. J. et al. arXiv:1907.05119 (2019).
8. Sass, P. M. et al. arXiv:1910.06488 (2019).