X-ray imaging of antiferromagnetic octupole domains in Mn$_3$Sn

Abstract

Novel antiferromagnets with broken time reversal symmetry (TRS) have launched a new direction in spintronics research, combining the advantageous dynamical properties of conventional antiferromagnets with the controllability typically associated with ferromagnets. However, antiferromagnetic domains are notoriously challenging to image in real-space. X-ray magnetic circular dichroism (XMCD) offers a route to overcome this difficulty: XMCD contrast may be finite in TRS-breaking antiferromagnets with an appropriate magnetic space group. Here, we exploit this to image the octupole domains in a focused ion beam-fabricated device of the non-collinear antiferromagnet Mn$_3$Sn. Using scanning transmission x-ray microscopy, we spatially resolve the weak pre-edge XMCD contrast (of 0.2%) that is sensitive to $T_z$, achieving a contrast resolution better than 0.02%. We observe hysteretic switching of the octupole order through both the XMCD contrast and the corresponding anomalous Hall effect within the same device. These results confirm the bulk nature of this contrast, and establish XMCD-based microscopy as a powerful real space imaging method for TRS-breaking antiferromagnets, including altermagnets, enabling future studies of their dynamics, switching, and symmetry-tunable phenomena.

Figures (3)

• Figure 1: (a) The hexagonal atomic structure and unit cell is shown. The possible orientations of the octupole moment and the corresponding arrangement of spins within each cluster of 6 Mn atoms is shown. (b) A scanning electron micrograph of the completed focused ion beam fabricated Mn$_3$Sn device. The false colours indicate the extent of the Mn$_3$Sn (blue) and the Au/Pt contact electrodes (yellow). The orientation of the crystal axes are highlighted. (c) The magnetization $M$ of the bulk Mn$_3$Sn crystal plotted as a function of temperature, measured with a $B$ field of 1 mT applied along the three high symmetry axes, as indicated by the color legend. Vertical lines indicate the transitions between the non-collinear antiferromagnetic (AFM), the antiferromagnetic spiral, and the spin glass (SG) phases. (d) The same as (c), but showing the antisymmetrised Hall resistivity $\rho_{yx}$ measured in the FIB device with an applied field of 1 T along the $[2\hat{1}\hat{1}0]$ axis.
• Figure 2: (a) Illustration of the STXM setup, showing the Fresnel zone plate (FZP), order selecting aperture (OSA), and the avalanche photodiode (APD). (b) An XMCD contrast image of the antiferromagnetic octupole domains in the Mn$_3$Sn device at 260 K and with a nominal x-ray energy $E$ of 639.25 eV. (c) Further example XMCD contrast images, acquired at a range of $E$ around the Mn L$_3$ edge. (d) Linescans of the XMCD contrast at each $E$ over the octupole domain indicated by the orange line in the region of interest (ROI) in (b). (e) The average XMCD contrast within the ROI at each energy was extracted, and plotted as a function of $E$ (red circles), where the error is standard error. The absorption measured in a thinner Mn$_3$Sn sample, showing the Mn L$_3$ edge (blue line). Our observed XMCD contrast is in strong agreement with that measured in reference sakamoto_observation_2021 (red-dashed line).
• Figure 3: (a) The XMCD contrast acquired by averaging over the region of interest of the Mn$_3$Sn device (highlighted in Fig. 2(b)), plotted as a function of the applied magnetic field $B$, at 260 K (red) and 220 K (blue). (b) The Hall resistivity $\rho_yx$ acquired as a function of $B$, measured in the Mn$_3$Sn device, at 260 K (red) and 220 K (blue). (c)-(f) Sequential XMCD contrast images of the octupole domain state within the device at a selection of applied $B$, as indicated. The red box indicates the area probed in (a).