The intrinsic antiferromagnetic topological insulator MnBi₂Te₄ provides a very attractive platform for the realization of various magnetic and topological states. In the ground state, the MnBi₂Te₄ thin films with an even number of septuple-layer blocks are axion insulators, but with increasing external magnetic field, they show a transition to quantum anomalous Hall regime, which is accompanied by conversion between collinear and non-collinear magnetization textures.
In this letter we present theoretical investigation of such topological transition. In the continuum approach, we model an effective two-dimensional Hamiltonian for the thin film of a topological insulator with non-collinear magnetization, on the basis of which we obtain the energy spectrum and the Berry curvature. The analysis of topological indices makes it possible to construct a topological phase diagram depending on the parameters of the system and the canting angle. For topologically different regions of the diagram, we describe the edge electronic states on the film side face. In addition, we investigate the spectrum of one-dimensional states on the domain wall separating domains with the opposite canting angles (see the figure). We also discuss the experimental situation in the MnBi₂Te₄ thin films.
The obtained results significantly deepen the understanding of the relationship between band topology and magnetic ordering.

Spectrum of electronic states in a thin film of antiferromagnetic topological insulator containing a domain wall.

V. N. Men’shov & E. V. Chulkov
JETP Letters 117, issue 2 (2023)