Antiferrmagnetic topological insulators are promising material platform for realizing quantum anomalous Hall or axion insulator states in the thin-film limit [1–3]. Thus, it is imperative to understand the surface magnetism that would play a critical role in thin films. In this talk, I will present our recent cryogenic magnetic force microscopy (MFM) studies of the surface spin-flip transition in topological antiferromagnet MnBi4Te7, a natural superlattice of alternating MnBi2Te4 and Bi2Te3 layers . Our MFM results confirm that the A-type 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 suggests no reduction of the order parameter in the surface magnetic layer, implying robust ferromagnetism in the single-layer limit. Combined with our previous discovery of surface spin-flop transition in MnBi2Te4 , our work not only opens up exploration of surface metamagnetic transitions in layered antiferromagnets, but also provides experimental support for realizing quantized transport in ultrathin films of MnBi4Te7 and other natural superlattice topological magnets.
 M. M. Otrokov, I. P. Rusinov, M. Blanco-Rey, M. Hoffmann, A. Y. Vyazovskaya, S. V. Eremeev, A. Ernst, P. M. Echenique, A. Arnau, and E. V. Chulkov, Unique Thickness-Dependent Properties of the van Der Waals Interlayer Antiferromagnet MnBi2Te4, Phys. Rev. Lett. 122, 107202 (2019).
 Y. Deng, Y. Yu, M. Z. Shi, J. Wang, X. H. Chen, and Y. Zhang, Quantized Anomalous Hall Effect in Intrinsic Magnetic Topological Insulator MnBi2Te4, Science (80-. ). 367, 895 (2020).
 C. Liu, Y. Y. Wang, H. Li, Y. Wu, Y. Li, J. Li, K. He, Y. Xu, J. Zhang, and Y. Y. Wang, Robust Axion Insulator and Chern Insulator Phases in a Two-Dimensional Antiferromagnetic Topological Insulator, Nat. Mater. 19, 522 (2020).
 W. Ge, J. Kim, Y. Chan, D. Vanderbilt, J. Yan, and W. Wu, Direct Visualization of Surface Spin-Flip Transition in MnBi4Te7, Phys. Rev. Lett. 129, 107204 (2022).
 P. M. Sass, J. Kim, D. Vanderbilt, J. Yan, and W. Wu, Robust A-Type Order and Spin-Flop Transition on the Surface of the Antiferromagnetic Topological Insulator MnBi2Te4, Phys. Rev. Lett. 125, 037201 (2020).