Observation of Magnon Damping Minimum Induced by Kondo Coupling in a van der Waals Ferromagnet Fe_3-xGeTe_2

In heavy-fermion systems with f electrons, there is an intricate interplay between Kondo screening and magnetic correlations, which can give rise to various exotic phases. Recently, similar interplay appears to also occur in d-electron systems, but the underlying mechanism remains elusive. Here, using inelastic neutron scattering, we investigate the temperature evolution of the low-energy spin waves in a metallic van der Waals ferromagnet Fe_3-xGeTe_2 (Curie temperature T_ C∼160 K), where the Kondo-lattice behavior emerges in the ferromagnetic phase below a characteristic temperature T^*∼90 K. We observe that the magnon damping constant diverges at both low and high temperatures, exhibiting a minimum coincidentally around T^*. Such an observation is analogous to the resistivity minimum as due to the single-impurity Kondo effect. This unusual behavior is described by a formula that combines logarithmic and power terms, representing the dominant contributions from Kondo screening and thermal fluctuations, respectively. Furthermore, we find that the magnon damping increases with momentum below T_ C. These findings can be explained by considering spin-flip electron-magnon scattering, which serves as a magnonic analog of the Kondo-impurity scattering, and thus provides a measure of the Kondo coupling through magnons. Our results provide critical insights into how Kondo coupling manifests itself in a system with magnetic ordering and shed light on the coexistence of and interplay between magnetic order and Kondo effect in itinerant 3d-electron systems.