${\mathrm{SmAgSb}}_2$, a layered magnetic semimetal in the tetragonal $RT{\mathrm{Sb}}_2$ family ($R=\mathrm{Y}$, Sc, rare earth; $T$ = transition metal), is known to exhibit extremely large magnetoresistance (XMR) below its antiferromagnetic (AFM) transition temperature. In this work, we present a comprehensive investigation combining magnetotransport measurements, density functional theory calculations accounting for electron correlation, and angle-resolved photoemission spectroscopy. Our results reveal a complex electronic structure characterized by a multiband Fermi surface and intricate magnetic ground states. We demonstrate that simple two-band models, previously employed in the literature, fail to consistently describe the observed transport phenomena. Notably, we report an XMR of approximately $25200\%$ at 2 K under a 14 T magnetic field, significantly exceeding earlier reports for this material family and rivaling the performance of prominent nonmagnetic XMR systems. This pronounced enhancement below $T_N$ suggests that the XMR originates from a combination of multiband electron-hole compensation and enhanced magnetic scattering in this correlated AFM semimetal.

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• Electrical properties
• Electronic structure
• Magnetism
• Transport phenomena
• Topological materials
• Angle-resolved photoemission spectroscopy