Dielectric Signatures of Crystal-Field and Low-Temperature Correlated Dynamics in NdMgAl11O19

We report dielectric spectroscopy of single-crystalline NdMgAl11O19, a magnetoplumbite hexaaluminate in which localized Nd^3+ moments coexist with a polarizable AlO5 bipyramidal network. The real part of the permittivity, ε'_c(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 ε'_c(T) is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of Δ= 25.85 ± 0.32 K consistent with the lowest Nd^3+ crystal-electric-field (CEF) splitting. Below ∼ 30 K, the dielectric response becomes strongly frequency and magnetic-field dependent. Isothermal ε_c'(H) measurements reveal a reproducible low-field crossover near μ_0H_c ≃ 0.85 T, which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. 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