Single Crystal Growth and Magnetotransport Properties of 4 <i>f</i> Electron Antiferromagnet SmGaSi

We report the single‐crystal growth and physical properties of SmGaSi, a member of the rare‐earth gallium‐silicide family. The system adopts the centrosymmetric I 4 1 / amd ‐type structure, with Ga and Si sharing the same crystallographic site in equal proportion. Magnetization measurements reveal antiferromagnetic order at T N = 11 K, followed by a subtle spin‐reorientation transition around 7.8 K, and a small anisotropy between the [100] and [001] axes. Electrical resistivity shows metallic behavior down to 2 K with a low residual resistivity ratio of 1.9, indicative of site‐disorder scattering. Transverse magnetoresistance exhibits an unusual cusp‐like field dependence, deviating from classical B 2 scaling, attributed to weak antilocalization arising from strong spin–orbit coupling, and could indicate nontrivial Berry curvature effects. By fitting the magnetoconductance to a modified Hikami–Larkin–Nagaoka model, we extract an effective coherence length of 22 nm at 2 K and an anomalously large channel‐count parameter, consistent with multiple 3D conduction pathways. The phase coherence length diminishes with increasing temperature following combined electron–electron and electron–phonon scattering. Our results establish SmGaSi as an antiferromagnet exhibiting spin–orbit–driven quantum interference, positioning it as a promising platform for exploring such physics in rare‐earth intermetallics and opening new pathways to investigate the largely unexplored ternary rare‐earth gallides.