Strongly Quenched Kramers Doublet Magnetism in SmMgAl11O19

Abstract

We report magnetic susceptibility, isothermal magnetization, and specific-heat measurements on the rare-earth hexaaluminate SmMgAl${}_{11}$O${}_{19}$, where Sm${}^{3+}$ realizes a strongly quenched Kramers doublet on a triangular lattice with an exceptionally weak net exchange scale. The Curie--Weiss analysis yields strongly reduced ground-doublet $g$ factors, $g_{ab}\simeq 0.65$ and $g_c\simeq 0.70$. This indicates that the low-temperature response is governed primarily by single-ion physics, with crystal-field splitting and $J$-multiplet mixing jointly renormalizing the 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 into a well-defined two-level Schottky peak whose gap grows linearly with field, yielding $g_c\simeq 0.62$ and recovering nearly all of $R\ln 2$ at high fields, thereby confirming an effective $S_{\mathrm{eff}}=\frac{1}{2}$ Kramers doublet description for $T\lesssim 10$~K. Together, these results establish SmMgAl${}_{11}$O${}_{19}$ as a weak-exchange, nearly single-ion triangular Kramers magnet in which frustration produce an anisotropic low-field correlated regime without inducing long-range order.