Spin Waves in a Bilayer Kagome Ferromagnet Fe₃Sn₂

Magnetic metals with kagome structure can host various topologically non-trivial spin or electronic states, providing an extraordinary platform for studying the fundamental physics of quantum materials. The metallic kagome ferromagnet Fe₃Sn₂, which has large spin-orbital coupling, shows considerable interplay between magnetism and non-trivial electronic states. The material is a host of anomalous bulk properties, including a first order spin reorientation transition(<i>1</i>), an anomalous planar Hall effect(<i>2</i>), and field tunable electronic states(<i>3, 4</i>). Recently, anomalous quasiparticles showing marginal Fermi liquid behavior and fractionalization have been observed in this system(<i>5</i>). Using Magnetic Circular Dichroism (MCD) Resonant Inelastic X-ray Scattering (RIXS) and X-ray absorption spectroscopy (XAS), we discover a flat spin wave band with large (compared to elemental iron) orbital moments in Fe₃Sn₂(6). The flat mode energy is consistent with the high Curie temperature (~ 640 K) as well as the strong acoustic mode dispersion. Our results unveil that the defining units of this very popular topological metal are a triangular lattice of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of interactions of topological bosons (spin waves) and fermions (electrons) with the very specific target of explaining boson lifetimes.<br/><br/><b>Reference</b><br/><br/>1. K. Heritage<i> et al.</i>, Images of a First Order Spin Reorientation Phase Transition in a Metallic Kagome Ferromagnet. <i>Advanced Functional Materials</i> <b>30</b>, (2020).<br/>2. N. Kumar, Y. Soh, Y. Wang, J. Li, Y. Xiong, Anomalour Planar Hall Effect in a kagome ferromagnet. <i>arXiv:2005.14237</i>, (2020).<br/>3. M. Yao<i> et al.</i>, Switchable Weyl nodes in topological Kagome ferromagnet Fe3Sn2. <i>ArXiv e-prints</i>. 2018.<br/>4. N. Kumar, Y. Soh, Y. Wang, J. Li, X. Y., Tuning the electronic band structure in a kagome ferromagnetic metal via magnetization. <i>Phys Rev B</i> <b>106</b>, 045120 (2022).<br/>5. S. A. Ekahana<i> et al.</i>, Anomalous electrons in a metallic kagome ferromagnet. <i>Nature</i> <b>627</b>, 19 (2024).<br/>6. W. Zhang <i> et al.</i>, Spin waves in a ferromagnetic topological metal. 2023 (https://doi.org/10.48550/arXiv.2302.01457).