Magnetic imaging of domain walls in antiferromagnetic topological insulator MnBi₄Te₇

MnBi₂Te₄ has been established as the first tangible candidate for an antiferromagnetic topological insulator (AFM-TI). Recent first principle calculations and ARPES measurements suggest that natural superlattice compounds (MnBi₂Te₄)(Bi₂Te₃)_n (n = 1, 2) are also AFM-TI. With n layers of Bi₂Te₃ inserted between MnBi₂Te₄ layers, the interlayer antiferromagnetic coupling reduces dramatically so that the metamagnetic transition becomes a spin-flip transition. Previously we discovered surface spin-flip transition preceding the bulk one using cryogenic magnetic force microscopy (MFM) [1]. In this talk, I will present our recent MFM results which uncover seemingly complex antiferromagnetic domain states via minor-loop field cycling into the bulk spin flip transition. By analyzing the surface spin-flip transition of the unconventional domains, we infer the formation of shallow antiphase domains induced by the minor-loop field annealing. The combination of the stray fields from vertical and horizontal domain walls of shallow antiferromagnetic domains causes the complex appearance of antiferromagnetic domain pattern in magnetic imaging.

[1] W. Ge, J. Kim, Y-T Chan, D. Vanderbilt, J-Q Yan, W. Wu, PRL, 129, 107204 (2022).