Observation of domain structure in non-collinear antiferromagnetic Mn₃Sn thin films by magneto-optical Kerr effect

We perform hysteresis-loop measurement and domain imaging for $(11¯00)$-oriented D0₁₉-Mn_3+xSn_1-x (⁠$−0.11≤x≤0.14$⁠) thin films using the magneto-optical Kerr effect (MOKE) and compare it with the anomalous Hall effect (AHE) measurement. We obtain a large Kerr rotation angle of 10 mdeg, comparable with bulk single-crystal Mn₃Sn. The composition x dependence of AHE and MOKE shows a similar trend, suggesting the same origin, i.e., the non-vanishing Berry curvature in the momentum space. Magnetic domain observation at the saturated state shows that x dependence of AHE and MOKE is explained by the amount of the reversible area that crucially depends on the crystalline structure of the film. Furthermore, in-depth observation of the reversal process reveals that the reversal starts with nucleation of sub-micrometer-scale domains dispersed in the film, followed by domain expansion, where the domain wall preferentially propagates along the $[112¯0]$ direction. Our study provides a basic understanding of the spatial evolution of the reversal of the chiral-spin structure in non-collinear antiferromagnetic thin films.