Two-dimensional (2D) magnetic materials provide a unique platform for exploring quantum phases from magnetic order in reduced dimensions. While there have been extensive studies on 2D magnetic materials based on $3d$ electrons, experimental studies on $4f$-electron counterparts are far fewer, particularly on their electronic structure. In this study, we report the successful synthesis of ultrathin ${\mathrm{SmC}}_6$ films using molecular beam epitaxy. Utilizing in situ angle-resolved photoemission spectroscopy (ARPES), we uncover a large band splitting in the valence bands, which we attribute to the ferromagnetic order driven by exchange couplings between Sm $4f$ moments and conduction electrons. Despite the small magnetic moment of Sm, the observed splitting is comparable to those of Eu- and Gd-based systems with much larger local moments. Interestingly, the surface state also exhibits splitting with similar magnitude and can be eliminated by overannealing, while the valence bands with ferromagnetic-like splittings remain robust. Our work provides spectroscopic insight to understand the electronic origin of magnetic order in Sm-based compounds. Our study also offers a platform to study 2D magnetic materials based on $4f$ electrons.

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• Electronic structure
• Magnetism
• Rare-earth magnetic materials
• Thin films
• Angle-resolved photoemission spectroscopy
• Molecular beam epitaxy