Evidence of additional Ferromagnetic Order in Antiferromagnetic Topological Insulator MnBi₆Te₁₀

MnBi₆Te₁₀ belongs to the antiferromagnetic topological insulator (TI) family MnBi₂Te₄(Bi₂Te₃)_n with n = 2. It can be constructed by intercalating two consecutive Bi₂Te₃ quintuple layers (QL) between adjacent septuple layers (SL) of MnBi₂Te₄. In previous studies, Bi₂Te₃ was considered as a nonmagnetic spacer, which reduces the antiferromagnetic exchange coupling between SLs [1,2]. Previous work demonstrated that the ground states of MnBi₂Te₄(Bi₂Te₃)_n become ferromagnetic for n > 2, while it remain as an A-type antiferromagnet with T_N ~ 12K in MnBi₆Te₁₀ (n=2) [1, 3-6]. There exists an extra step feature near zero field in the magnetic hysteresis loop of single crystal MnBi₆Te₁₀ below 6K, which cannot be explained by the conventional bulk spin flip transition [3]. In this talk, we will show evidence of an additional ferromagnetic order in MnBi₆Te₁₀ below 6K. Domains of this ferromagnetic order were visualized using our homemade cryogenic magnetic force microscope. Our results indicate that this “hidden” ferromagnetic order originates from the Bi₂Te₃ QLs, presumably due to the presence of Mn_Bi antisite defects.

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