Dielectric signatures of crystal-field and low-temperature correlated dynamics in NdMgAl11O19

We report dielectric spectroscopy of single-crystalline \ce{NdMgAl11O19}, a magnetoplumbite hexaaluminate in which localized \ce{Nd^{3+}} moments coexist with a polarizable \ce{AlO5} bipyramidal network. The real part of the permittivity, ${\epsilon }_c^{\mathrm{\prime }}(T)$, measured along the crystallographic $c$ axis, increases as the temperature is lowered from 275~K to 30~K and is frequency-independent between 4~Hz and 50~kHz. At lower temperatures, a frequency-dependent decrease in permittivity is observed, followed by a further upturn below 2~K. The high-frequency ${\epsilon }_c^{\mathrm{\prime }}(T)$ is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of $\mathrm{\Delta }=25.85\pm 0.32$~K consistent with the lowest \ce{Nd^{3+}} crystal-electric-field (CEF) splitting. Below $\sim 30$~K, the dielectric response becomes strongly frequency and magnetic-field dependent. Isothermal ${\epsilon }_c^{\mathrm{\prime }}(H)$ measurements reveal a reproducible low-field crossover near ${\mu }_0{H}_c\simeq 0.85$~T, which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. \ce{NdMgAl11O19} thus provides a Kramers reference system in which dielectric signatures of the excited-state CEF manifold can be distinguished from those of the field-tuned, correlation-dominated ground-state doublet sector in a centrosymmetric frustrated magnetoplumbite host