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

Rare earth permanent magnets (REPMs) with both localized moments and itinerant conduction bands are not only important for fundamental research but also have significant technological applications. In particular, Sm₂Co₁₇ is a prototypical high-temperture REPM, where the Co atoms form a kagome-honeycomb stacked lattice. Here we report the synthesis of epitaxial Sm₂Co₁₇ films using molecular beam epitaxy and measurements of their momentum-resolved electronic structure from in-situ angle-resolved photoemission spectroscopy. Our results unveil two flat bands from Co 3d orbitals near the Fermi level (E_F), one at ∼ −300 meV and another right at E_F, which arise from orbital-selective destructive interference and strong electron correlations, respectively. In addition, our results reveal that Sm 4f states are far away from E_F (hence mostly localized) and exhibit an anomalous temperature dependence, caused by the 3d-4f magnetic coupling. Our findings provide direct spectroscopic insights to understand the strong uniaxial ferromagnetism in Sm₂Co₁₇ (and REPMs alike). Our work also opens avenues to explore flat-band physics near E_F and emergent phenomena in correlated kagome-honeycomb lattices.