We report dielectric and magnetoelectric studies of single-crystalline ${\mathrm{CeMgAl}}_{11}{\mathrm{O}}_{19}$, a Kramers triangular magnet embedded in a polarizable hexaaluminate lattice. In zero magnetic field, the permittivity ${\varepsilon }^{\prime }\left(T\right)$ follows the Barrett law of a quantum paraelectric down to $\sim 25\mathrm{K}$, below which a broad minimum develops near $3\mathrm{K}$ without evidence of static ferroelectric or magnetic order. Application of magnetic fields up to $9\mathrm{T}$ shifts this minimum to higher temperatures and broadens it, evidencing a tunable magnetoelectric response. The magnetoelectric coupling was characterized using results from magnetization measurements. The anomaly temperature $T^{*}$, extracted from the local minimum of ${\varepsilon }^{\prime }\left(T\right)$, exhibits a linear dependence on the squared magnetization $M^2$, consistent with the biquadratic magnetoelectric coupling allowed in centrosymmetric systems. This magnetoelectric effect, mediated by spin-orbit-entangled Kramers doublets interacting with a frustrated antipolar liquid, establishes ${\mathrm{CeMgAl}}_{11}{\mathrm{O}}_{19}$ as a prototype for exploring quantum magnetoelectricity in frustrated systems.

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• Electrical properties of membranes
• Magnetic coupling
• Magnetic interactions
• Magnetism
• Magneto-dielectric effect
• Spin-orbit coupling