Spin order and excitations in the collinear kagome antiferromagnet Y 0.5 Fe 3 Sn 3

Kagome lattices host rich magnetic, topological, and transport phenomena owing to their geometric frustration and strong spin correlations. Here we present a systematic neutron scattering study of the kagome metal Y 0.5 Fe 3 Sn 3, which orders into a collinear c-axis antiferromagnetic (AF) state below T N = 551 K. An additional anomalous Hall effect (AHE) contribution beyond the conventional magnetization-scaling response is observed well below T N under an in-plane magnetic field B ab. Elastic neutron scattering demonstrates that the Fe moments preserve their collinear c-axis AF alignment down to 1.5 K at zero field. Inelastic neutron scattering reveals a distinctive Y-shape spin excitation dispersion exhibiting a two-gap structure below 10 meV, likely arising from two nondegenerate spin-wave branches. Polarized neutron measurements further show that the low-temperature spin excitations are strongly anisotropic: the in-plane component M ab dominates while the c-axis component M c is nearly absent. Upon warming, M c gradually grows markedly but remains much weaker than M ab even at 300 K. We propose that the unconventional AHE originates from a nonzero scalar spin chirality induced by field-driven spin canting either in the static or dynamic configurations due to gapless excitations. These findings establish the collinear AF kagome metal Y 0.5 Fe 3 Sn 3 as a platform for chirality-driven AHE without spontaneous spin reorientation.