Dielectric signatures of crystal-field and low-temperature correlated dynamics in NdMgAl 11 O 19

We report dielectric spectroscopy of single-crystalline NdMgAl 11 O 19 , a magnetoplumbite hexaaluminate in which localized Nd 3 + moments coexist with a polarizable AlO 5 bipyramidal network. The real part of the permittivity ɛ 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 ɛ c ′ ( T ) is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of Δ = 25.9 ( 3 ) 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 ɛ c ′ ( H ) measurements reveal a reproducible low-field crossover near μ 0 H c ≃ 0.85 T , which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. NdMgAl 11 O 19 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 in the centrosymmetric frustrated magnetoplumbite host.