Abstract:

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 [4]. 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 [5], 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.

Reference:

[1] 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).

[2] 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).

[3] 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).

[4] 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).

[5] 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).