3d flat bands and coupled 4f moments in the kagome-honeycomb stacked permanent magnet Sm₂Co₁₇

Hybrid ferromagnets with coupled local-moment and itinerant components are not only interesting due to the rich physics, but also important for applications such as rare earth permanent magnet (REPM). Flat bands generated by destructive interference in special lattices offer additional opportunities to realize strong ferromagnetism. Here, by combining molecular beam epitaxy and angle-resolved photoemission spectroscopy, we report observation of 3d flat bands near the Fermi level (E_F) in a prototypical strong REPM Sm₂Co₁₇, which has a kagome-honeycomb lattice. Two sets of flat bands, one at ~ -300 meV and the other right at E_F, arise from orbital-selective destructive interference in the kagome-honeycomb lattice and strong correlation effects from Co 3d electrons. Our results further unveil that Sm 4f electrons are mostly localized and exhibit an anomalous temperature evolution, due to the 3d-4f interaction and competition with ferromagnetism. Our work provides spectroscopic insight to understand the hybrid ferromagnetism in REPMs. Our study also opens up opportunities to find and tune flat bands in correlated kagome-honeycomb lattices.