Direct evidence of the antiparallel magnetization alignment in a quantum anomalous Hall heterostructure for realizing axion insulator state*

Quantum anomalous Hall (QAH) sandwich with antiparallel magnetization alignment was proposed to realize the axion insulator state, and was reported to be realized in two recent experimental works. [3,4] The observed zero Hall conductance (σ_xy) plateau these heterostructures was interpreted as an evidence of antiparallel magnetization alignment between the top and bottom magnetic TI layers. However, no direct evidence of this antiparallel magnetization alignment was presented. Herein, we will present cryogenic magnetic force microscopy (MFM) studies of magnetic domain behavior in QAH sandwich heterostructures in which two compositionally different magnetic TI layers are separated by an undoped TI layer. [5] Our MFM results reveal sequential magnetization reversals of the top and bottom layers at different coercive fields, providing a direct evidence of the antiparallel magnetization alignment that is necessary for realizing axion insulator states.

[1] M. Mogi et al., Nat. Mater. 16, 516 (2017).
[2] M. Mogi et al., Sci. Adv. , 3, eaao1669 (2017).
[3] D. Xiao et al., arXiv 1710.00471 (2017).