Simultaneous Development\nof Antiferromagnetism and\nLocal Symmetry Breaking in a Kagome Magnet (Co<sub>0.45</sub>Fe<sub>0.55</sub>)Sn

CoSn\nand FeSn, two kagome-lattice metals, have recently\nattracted\nsignificant attention as hosts of electronic flat bands and emergent\nphysical properties. However, current understandings of their physical\nproperties are limited to knowledge of the average crystal structure.\nHere, we report the Fe-doping induced coemergence of the antiferromagentic\n(AFM) order and local symmetry breaking in (Co<sub>0.45</sub>Fe<sub>0.55</sub>)Sn. Rietveld analysis on the neutron and synchrotron X-ray\ndiffraction data indicates A-type antiferromagnetic order with the\nmoment pointing perpendicular to the kagome layers, associated with\nthe anomaly in the MSn(1)<sub>2</sub>Sn(2)<sub>4</sub> (M = Co/Fe)\noctahedral distortion and the lattice constant <i>c</i>.\nReverse Monte Carlo (RMC) modeling of the synchrotron X-ray total\nscattering results captured the subtle local orthorhombic distortion\ninvolving off-axis displacements of Sn(2). Our results indicate that\nthe stable hexagonal lattice above <i>T</i><sub><i>N</i></sub> becomes unstable once the A-type AFM order is formed\nbelow <i>T</i><sub><i>N</i></sub>. We argue that\nthe local symmetry breaking has a magnetic origin, since the spatially\nvaried M–Sn(2) bond lengths arise from out-of-plane magnetic\nexchange coupling <i>J</i><sub><i>c</i></sub> via\nthe exchange pathway M–Sn(2)–M. Our study provides comprehensive\ninformation on the crystal structure in both long-range scale and\nlocal scale, unveiling unique coupling between AFM order, octahedral\ndistortion, and hidden local symmetry breaking.