Magnetic flat band in metallic kagome lattice FeSn.

Magnetic metals support both collective spin-wave and quasiparticle spin-flip excitations, also called Stoner excitations. In three-dimensional (3D) metallic ferromagnets, the quasiparticle bands are broad and spin waves are well-defined only at long wavelengths, disappearing when they enter the Stoner continuum at intermediate spin-wave momenta. In 2D metallic kagome lattice materials, destructive interference of electronic hopping pathways around the kagome bracket can produce nearly localized electrons, and thus electronic bands that are flat in momentum space. However, the influence of these flat bands on spin-wave and Stoner excitations is unknown. Here we use inelastic neutron scattering (INS) to demonstrate the coexistence of well-defined conventional spin waves, and a narrow band of spin-excitations that cannot be described by a simple spin-wave model in magnetic kagome metal FeSn. We interpret these dispersionless excitations as local rotations of the localized flat-band spins relative to the itinerant spins. Our discovery of narrow localized spin-excitations in FeSn demonstrates that metallic kagome lattices combine localized and itinerant magnetism in a way that opens up new strategies to design and control magnetic properties of materials using lattice-driven band structure engineering.