Absence of Long-Range Order and Magnetic Anisotropy in the Triangular Magnet NdMgAl$_{11}$O$_{19}$

Abstract

We investigated the rare-earth triangular-lattice antiferromagnet NdMgAl$_{11}$O$_{19}$ using single-crystal magnetization (1.8~K $\leq T \leq$ 300~K, $\mu_0 H \leq 7$~T) and specific-heat measurements down to 45~mK. The dc susceptibility confirms a well-isolated Kramers doublet ground state with pronounced Ising-type anisotropy, with $g_c \approx 3.7$ and $g_{ab} \approx 1.45$. Curie--Weiss fits yield weak, anisotropic antiferromagnetic exchange, with $\theta_c = -0.54$~K and $\theta_{ab} = -0.87$~K. Heat-capacity measurements show no long-range magnetic order down to 40~mK, corresponding to a frustration index $f \gtrsim 20$. Instead, $C_m/T$ exhibits a broad maximum near 0.081~K whose magnitude and field evolution are consistent with short-range correlations in an anisotropic triangular lattice. Applied magnetic fields open a Zeeman gap where the specific-heat anomaly follows $\Delta = g \mu_B \mu_0 H$, and $M(H,T)$ is well described by a Brillouin function for an effective $J = 1/2$ moment. The field tuning of the low-temperature entropy manifold allows self-cooling from 1.8~K to 53~mK by adiabatic demagnetisation from a 9~T field. These results identify NdMgAl$_{11}$O$_{19}$ as a nearly ideal weak-exchange triangular magnet with a field-tunable correlated ground state, where two-dimensional crossover effects may emerge from frustrated XXZ interactions.