Dielectric Slowing Down toward the Magnetically Ordered State in a Polar Conductor (EDT-TTFVO)₂FeBr₄

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We have measured the dielectric response as a function of both frequency (f=0.3 Hz–100 kHz) and electric bias field for a quasi-one-dimensional, polar conductor (EDT-TTFVO)₂FeBr₄ undergoing a ferrimagnetic-like transition at T_C∼0.9 K. Around T^*=15 K, the dielectric constant at low f exhibits a peak that shifts to higher T at higher f, being reminiscent of relaxor ferroelectrics with glassy polarizations. Since this dielectric anomaly is also present in the nonmagnetic isostructural GaBr₄ salt, the polarizations may be ascribed to both π and σ electrons localized on the polar donor. At T_C<T<7 K, the complex dielectric constant clearly exhibits a Debye relaxation, and the relaxation time follows a critical behavior as (T−T₀)^−n_τ (n_τ=6.5) toward T₀=0.75±0.15 K just very close to T_C. This phenomenon of the dielectric-slowing-down toward the magnetically ordered state is discussed as a novel type of multifunctional coupling in the short-range ordered state between the possible electric polarization in the distorted tetrahedral anion and the centered d spin of 5/2, the latter of which, eventually at T_C, may induce the magnetic order with π spins of 1/2 disproportionately localized on the polar donors.