Abtract: In this talk, I will discuss how antiferromagnetic (AFM) domains and their boundaries—known as domain walls—form and evolve in MnBi2Te4, the first experimentally discovered antiferromagnetic topological insulator. To probe these magnetic textures, we employed a full-field coherent soft X-ray imaging technique developed in 2018, which allows direct visualization of AFM domain structures in real space without the need for computational reconstruction.

Using this approach, we observed that MnBi2Te4 hosts characteristic “antiphase” domain walls, which appear as dark lines in the images due to the phase reversal between adjacent AFM layers in its A-type magnetic structure. The typical domain-wall width is about 550 nm, consistent with earlier magnetic-force-microscopy measurements. The temperature dependence of the AFM order parameter extracted from these images matches previous neutron-scattering results.

Most intriguingly, we discovered that the domains exhibit strong hysteresis near the Néel temperature: during cooling, domain walls reorganize rapidly within a narrow ~1 K range below TN, while during warming the domain configuration remains largely unchanged until the AFM order vanishes. These findings suggest that MnBi2Te4 possesses a complex magnetic energy landscape governed by competing exchange, anisotropy, and domain-wall energies, and that the motion and stability of AFM domain walls play a decisive role in its physical behavior and potentially its topological properties.