We report magnetic susceptibility, isothermal magnetization, and specific-heat measurements on the rare-earth hexaaluminate ${\mathrm{SmMgAl}}_{11}{\mathrm{O}}_{19}$, where ${\mathrm{Sm}}^{3+}$ realizes a strongly quenched Kramers doublet on a triangular lattice with an exceptionally weak net exchange scale. Curie-Weiss analysis yields strongly reduced ground-doublet $g$ factors, $g_{ab}\simeq 0.65$ and $g_c\simeq 0.70$, indicating that the low-temperature response is governed primarily by single-ion physics, with crystal-field splitting and $J$-multiplet mixing renormalizing the ${\mathrm{Sm}}^{3+}$ moment rather than collective exchange. For $H\parallel c$, the specific heat shows no $\lambda$-type anomaly down to 0.35 K, but evolves in field into a two-level Schottky peak whose gap increases linearly with field. The Schottky analysis yields $g_c\simeq 0.62$ and recovers nearly all of $Rln2$ at high fields, supporting an effective $S_{\mathrm{eff}}=\frac{1}{2}$ Kramers doublet description for $T\lesssim 10\mathrm{K}$. Overall, the data place ${\mathrm{SmMgAl}}_{11}{\mathrm{O}}_{19}$ in an extreme weak-exchange, single-ion–dominated regime, with low-temperature deviations from ideal two-level behavior that may reflect a combination of geometric frustration, multiplet mixing, and structural disorder, while no long-range order is observed within our experimental window.

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• Magnetic anisotropy
• Magnetic coupling
• Magnetic interactions
• Magnetic susceptibility
• Magnetism
• Spin-orbit coupling
• Methods in magnetism