Simultaneous Development of Antiferromagnetism and Local Symmetry Breaking in a Kagome Magnet (Co_0.45Fe_0.55)Sn

CoSn and FeSn, two kagome-lattice metals, have recently attracted significant attention as hosts of electronic flat bands and emergent physical properties. However, current understandings of their physical properties are limited to knowledge of the average crystal structure. Here, we report the Fe-doping induced coemergence of the antiferromagentic (AFM) order and local symmetry breaking in (Co_0.45Fe_0.55)Sn. Rietveld analysis on the neutron and synchrotron X-ray diffraction data indicates A-type antiferromagnetic order with the moment pointing perpendicular to the kagome layers, associated with the anomaly in the MSn(1)₂Sn(2)₄ (M = Co/Fe) octahedral distortion and the lattice constant c. Reverse Monte Carlo (RMC) modeling of the synchrotron X-ray total scattering results captured the subtle local orthorhombic distortion involving off-axis displacements of Sn(2). Our results indicate that the stable hexagonal lattice above T becomes unstable once the A-type AFM order is formed below T. We argue that the local symmetry breaking has a magnetic origin, since the spatially varied M-Sn(2) bond lengths arise from out-of-plane magnetic exchange coupling J via the exchange pathway M-Sn(2)-M. Our study provides comprehensive information on the crystal structure in both long-range scale and local scale, unveiling unique coupling between AFM order, octahedral distortion, and hidden local symmetry breaking.