Topological phenomena in magnetic topological insulator thin films*

Quantum anomalous Hall effect (QAHE), which is related to the momentum-space Berry phase, was experimentally realized in Cr-doped (Bi,Sb)₂Te₃ (BST) and V-doped BST thin films. The full quantization of anomalous Hall effect (AHE) in these two systems requires extremely low temperature (<50mK)[1][2]. Magnetic inhomogeneity has been proposed as one of the main reasons that limit the temperature for the realization of QAHE. In the first part, we will present direct visualization of long-range ferromagnetism in the Cr,V co-doped BST films, by using the MFM and in-situ transport measurement[3]. The magnetization reversal process reveals typical ferromagnetic domain behavior, i.e. domain nucleation and domain wall propagation. In the second part, we will discuss topological Hall effect(THE), a real-space Berry phase effect that originates from the non-coplanar spin textures[4]. Intrinsic THE was revealed at different temperatures in the magnetic topological insulator heterostructure Sb₂Te₃/V-doped Sb₂Te₃, after a back-gate voltage was applied to zero the AHE. The high-temperature THE persists above the Curie temperature, which originates from the spin chirality fluctuations[5]. At temperatures well below the Curie temperature, THE emerges around the coercive fields, when the ferromagnetic bubble domains form. The low-temperature THE likely results from the static topological charges at the chiral domain walls.

[1] C.-Z. Chang et al., Science 340, 167 (2013).
[2] C.-Z. Chang et al., Nature Materials 14, 473–477 (2015).
[3] W. Wang et al., Nature Physics 14, 791 (2018).
[4] Y. Taguchi et al., Science 291, 2573 (2001).
[5] W. Wang et al., Nature Materials 18, 1054 (2019).