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

TL;DR

NdMgAl11O19 emerges as a weak-exchange, geometrically frustrated triangular-lattice magnet with a well-isolated Kramers doublet ground state and pronounced easy-axis anisotropy. Through single-crystal magnetization, specific-heat measurements, EPR, and point-charge crystal-field calculations, the study finds no long-range order down to 40 mK, a broad low-temperature specific-heat feature indicating persistent short-range correlations, and a field-tunable ground-state consistent with an effective $J_{eff}=1/2$ pseudospin; Zeeman splitting follows a linear dependence with $g_{eff} \approx 3.86$. The magnetic interactions are weak and anisotropic, describable by an easy-plane XXZ exchange with $J \approx -0.50$ K and $D = J_z - J \approx -0.14$ K, suggesting potential 2D-XY/BKT-type physics or a quantum-disordered ground state. The material also demonstrates substantial adiabatic-demagnetization cooling, highlighting its potential as a magnetocaloric refrigerant and a platform for exploring frustrated magnetism in a clean, single-crystal triangular lattice.

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, $μ_0 H \leq 7$~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 $θ_c = -0.54$~K and $θ_{ab} = -0.87$~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 $Δ= g μ_B μ_0 H$, 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.

Figures (8)

• Figure 1: The magnetoplumbite structure of NdMgAl$_{11}$O$_{19}$ (a), with magnetic Nd$^{3+}$ triangular lattice inter-planar (b) and intra-planar (c) separation distances. (d) A representation of the point-charge-approximation calculation of crystal electric field splitting of the Nd$^{3+}$$^4$I$_{9/2}$ free ion electronic levels, resulting in a Kramers' doublet ground-state, separated from the first excited doublet level by 2.174 meV (25.2 K).
• Figure 2: Magnetic susceptibility measured with an external magnetic field $\mu_oH = 0.1T$, applied parallel to the $c$-axis, and perpendicular to the $c$-axis (a). Corresponding inverse susceptibility with CW fits at low temperatures. The solid red lines represent the Curie-Weiss fitting of the inverse susceptibility, described in the text.
• Figure 3: Isothermal magnetization as a function of the field for fields applied along the $c$-axis (a) and in the $ab$-plane (b), with the Brillouin fit shown as solid lines.
• Figure 4: (a) Total specific heat ($C_{\mathrm{tot}}$), non-magnetic (phonon) contribution ($C_p$), and the resulting magnetic specific heat ($C_m$) obtained by subtraction, shown as functions of temperature in zero magnetic field. (b) Magnetic specific heat $C_m(T)$ measured in various magnetic fields. Solid lines show fits to the two-level Schottky model, while dashed lines indicate extrapolated curves inferred from the Schottky analysis. (c) Field dependence of $\eta$, the fraction of ions contributing to the Schottky anomaly; inset: extracted Schottky gap $\Delta$ as a function of magnetic field. (d) Magnetic entropy $S_m(T)$. The semi-transparent gray shaded area below the dashed magenta line highlights the entropy deficit attributed to Nd deficiency, while the transparent gray shaded area between the dashed orange and dashed magenta lines marks the residual entropy remaining below $40$ mK.
• Figure 5: Self-cooling curve for a pressed disc of NdMgAl$_{11}$O$_{19}$ powder during adiabatic demagnetisation from 9 T at a start temperature of 1.8 K.