Strongly Quenched Kramers Doublet Magnetism in SmMgAl11O19

TL;DR

This study demonstrates that SmMgAl$_{11}$O$_{19}$ realizes a strongly quenched $S_{ ext{eff}}= frac12$ Kramers doublet on a triangular lattice with an exceptionally weak net exchange scale, as evidenced by near-zero $ heta_{ ext{CW}}$ and reduced ground-state moments ($g_{ab}\simeq0.65$, $g_c\simeq0.70$). Crystal-field modeling with LS-mixing reproduces the ground doublet and assigns excited CF levels at $ abla_1=7.34$ meV and $ abla_2=25.01$ meV, with an effective multiplet gap $ abla_{ ext{eff}}\sim59$ meV driving significant $J$-mixing and Van Vleck contributions. Under field along $c$, a Schottky anomaly with a Zeeman gap $ abla(B)$ grows linearly and the entropy approaches $R\ln2$, indicating a crossover to a nearly single-ion regime dominated by a renormalized doublet; at low fields, disorder and weak interactions induce a correlated regime with partial entropy release and non-ideal two-level behavior. Overall, SmMgAl$_{11}$O$_{19}$ offers a crucial nearly single-ion reference point in the LnMgAl$_{11}$O$_{19}$ family, complementary to exchange-driven QSL-like phases in related compounds and guiding future probes of ultra-low-temperature dynamics and entropy in frustrated Sm-based magnets.

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 $λ$-type anomaly down to 0.35~K but evolves into a well-defined two-level Schottky peak whose gap grows linearly with field, yielding $g_c\simeq0.62$ and recovering nearly all of $R\ln2$ at high fields, thereby confirming an effective $S_{\mathrm{eff}}=\tfrac12$ Kramers doublet description for $T\lesssim10$~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.

Figures (6)

• Figure 1: (a) Magnetoplumbite structure of Sm$_{0.941}$MgAl$_{11}$O$_{19}$, with highlighted AlO$_5$ bipyramids and SmO$_{12}$ polyhedra. (b) Triangular layers of Sm$^{3+}$ ions, vertically separated by $r_{\rm inter} = c/2 \approx$ 10.91 $\mathring{\mathrm{A}}$. Fourier difference map projection at $z/c = 0.25$, showing the electronic density difference (c) before and (d) after the inclusion of the off-centered Sm(2) ions on the 6$h$ site. (e) Triangular lattice of Sm(1) and Sm(2) ions projected along the $c$ axis, with the Sm(1)-Sm(1) distance $r_{\rm nn} = 5.58 \mathring{\mathrm{A}}$.
• Figure 2: (a) DC magnetic susceptibility $\chi(T)$ of SmMgAl$_{11}$O$_{19}$ for $H \parallel c$ and $H \parallel ab$ measured in an applied field of $\mu_{0}H = 0.1$ T. (b) Inverse susceptibility $1/\chi(T)$ for both field directions over the full temperature range, together with Curie--Weiss fits including a Van Vleck term (solid black lines). (c) Low-temperature inverse susceptibility for $H \parallel c$ and $H \parallel ab$ in the 1.8--10 K range, highlighting the linear CW+$\chi_{\mathrm{vv}}$ behavior in the $ab$ plane and its absence along $c$. Solid black lines show the corresponding CW+$\chi_{\mathrm{vv}}$ fits.
• Figure 3: (a) Isothermal magnetization $M(H)$ of SmMgAl$_{11}$O$_{19}$ for $H \parallel c$ at several temperatures. (b) Isothermal magnetization $M(H)$ for $H \parallel ab$. Both panels show the weak anisotropy and absence of saturation up to 7 T.
• Figure 4: Fig. 3. Specific heat and Schottky analysis of SmMgAl$_{11}$O$_{19}$. (a) Total specific heat $C_p(T)$ of SmMgAl$_{11}$O$_{19}$ together with the nonmagnetic analogue LaMgAl$_{11}$O$_{19}$, and the magnetic contribution $C_m(T)$ extracted by subtracting the phonon background in zero field. (b) Magnetic specific heat $C_m(T)$ of SmMgAl$_{11}$O$_{19}$ under various applied magnetic fields. Solid curves show two-level Schottky fits for $\mu_0H \ge 1$ T. (c) Magnetic entropy $S_m(T)$ obtained from integrating $C_m/T$. (d) Zeeman gap $\Delta(B)$ and filling fraction $f(B)$ extracted from the Schottky analysis. The solid blue line represents a linear fit to $\Delta(B)$.
• Figure 5: (a) Scaling plot of magnetization for SmMgAl$_{11}$O$_{19}$ with magnetization $M$ plotted against $H/T$ for fields applied along the $c$ axis. The absence of scaling collapse indicates the presence of additional low-energy correlations and internal fields for $H \parallel c$. (b) Scaling of $M$ vs. $H/T$ for $H \parallel ab$. The near-universal collapse of the curves is consistent with predominantly single-ion paramagnetic behavior in the $ab$ plane.