Dynamics and formation of antiferromagnetic textures in MnBi$_2$Te$_4$ single crystal

TL;DR

This work uses direct coherent X-ray imaging to visualize antiferromagnetic textures in MnBi2Te4, an intrinsic AFM topological insulator, directly observing antiphase domain walls and quantifying their width. The AFM order parameter from imaging agrees with neutron scattering data, but domain patterns show strong hysteresis: during cooldown, rapid reconfiguration occurs within about 1 K below the Néel temperature, while warming leaves the textures largely static until order vanishes. The results reveal a complex energy landscape balancing exchange, anisotropy, and domain-wall energies, with domain-wall dynamics playing a critical role in the material's magnetic and topological properties. These insights motivate further high-resolution studies of domain-wall motion and energy scales in MnBi2Te4 to better understand and control AFM-related topological phenomena.

Abstract

We report coherent X-ray imaging of antiferromagnetic (AFM) domains and domain walls in MnBi$_2$Te$_4$, an intrinsic AFM topological insulator. This technique enables direct visualization of domain morphology without reconstruction algorithms, allowing us to resolve antiphase domain walls as distinct dark lines arising from the A-type AFM structure. The wall width is determined to be 550(30) nm, in good agreement with earlier magnetic force microscopy results. The temperature dependence of the AFM order parameter extracted from our images closely follows previous neutron scattering data. Remarkably, however, we find a pronounced hysteresis in the evolution of domains and domain walls: upon cooling, dynamic reorganizations occur within a narrow $\sim$1 K interval below $T_N$, whereas upon warming, the domain configuration remains largely unchanged until AFM order disappears. These findings reveal a complex energy landscape in MnBi$_2$Te$_4$, governed by the interplay of exchange, anisotropy, and domain-wall energies, and underscore the critical role of AFM domain-wall dynamics in shaping its physical properties.

Figures (5)

• Figure 1: (a) Illustration of the AFM structure of MnBi$_2$Te$_4$. (b) Schematic depiction of antiphase AFM domains and the associated domain wall. The magnetic ordering phases in regions A and B differ by 180$^\circ$, representing the natural domain configuration permitted by the AFM structure in (a). The gray region between the two domains denotes the domain wall. (c) direct-CXI measurement of antiphase AFM domains and domain walls in MnBi$_2$Te$_4$, recorded at 20 K. The white bar corresponds to a scale of 20 $\mu$m. (d) Magnified view of a single domain wall. The white bar indicates 1 $\mu$m. The red line marks the trajectory of a one-dimensional cut across the wall. (e) One-dimensional intensity profile across the domain wall. The red solid line represents the experimental data, while the blue dashed line is a Gaussian fit. The extracted full width at half maximum (FWHM) of the wall is 550(30) nm.
• Figure 2: (a) AFM texture of MnBi$_2$Te$_4$ measured at 16 K after the initial cooling. Dark wavy lines correspond to antiphase AFM domain walls. (b) Direct-CXI image measured at 26 K during warming. The dark line indicated by the green arrow originates from surface-related geometrical structures and produces contrast that is independent of temperature. (c) AFM texture measured again at 16 K after recooling, showing a domain morphology distinct from that in (a). The dark lines represent antiphase AFM domain walls. (d) AFM order parameter extracted from the image intensities recorded during the thermal cycling sequence shown in (a)–(c). Red data points correspond to measurements upon warming, while blue data points correspond to those obtained during cooling. The intensity was obtained by integrating the signal over the illuminated area of images represented in (a) - (c). The AFM signal emerges below approximately 25 K. The white bar corresponds to a scale of 20 $\mu$m. The intensity scale displayed alongside panel (b) serves as the reference color scale for panels (a), (b), and (c).
• Figure 3: Direct-CXI images of AFM textures in MnBi$_2$Te$_4$ at selected temperatures during warming. (a) Image recorded well below $T_N$ at $T = 23.0$ K. The features marked by green arrows and asterisks arise from temperature-independent geometrical structures on the sample surface and are consistently observed in all panels (b) - (f), thereby confirming that the same surface region was probed throughout. (b) $T = 24.0$ K, (c) $T = 24.2$ K, (d) $T = 24.4$ K, (e) $T = 24.6$ K, and (f) $T = 24.8$ K just below $T_N$. In these images, the curved line represents antiphase AFM domain wall. The white scale bar corresponds to 20 $\mu$m. The color map used for these images is identical to the color scheme employed in Fig. \ref{['fig2']}
• Figure 4: Direct-CXI images of AFM textures in MnBi$_2$Te$_4$ at selected temperatures during cooling. (a) Image recorded just below $T_N$ at $T = 24.8$ K. The features marked by green arrows and asterisks arise from temperature-independent geometrical structures on the sample surface and are consistently observed in all panels (b) - (f), thereby confirming that the same surface region was probed throughout. (b) $T = 24.6$ K, (c) $T = 24.4$ K, (d) $T = 24.2$ K, (e) $T = 24$ K, and (f) $T = 23$ K. In these images, the dark wavy lines represent antiphase AFM domain walls. The white scale bar corresponds to 20 $\mu$m. The color map used for these images is identical to the color scheme employed in Fig. \ref{['fig2']}
• Figure 5: (a) and (c). Comparison between extracted domain wall features at 24.6 K and 24.8 K during warming and cooling, respectively. Corresponding original images are shown in Fig. \ref{['fig3']} and \ref{['fig4']}. (b) AFM order parameter obtained from Fig. \ref{['fig2']}. (d) Difference between the images taken during warming and cooling. See text for details.