Dielectric Slowing Down toward the Magnetically Ordered State in a Polar Conductor (EDT-TTFVO)_2FeBr_4(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We have measured the dielectric response as a function of both frequency (f=0.3Hz-100kHz) and electric bias field for a quasi-one-dimensional, polar conductor (EDT-TTFVO)_2FeBr_4 undergoing a ferrimagnetic-like transition at T_C〜0.9K. Around T^*=15K, 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 GaBr_4 salt, the polarizations may be ascribed to both π and σ electrons localized on the polar donor. At T_C < T < 7K, the complex dielectric constant clearly exhibits a Debye relaxation, and the relaxation time follows a critical behavior as (T-T_0)^<-n_τ> (n_τ=6.5) toward T_0=0.75±0.15K 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.
社団法人日本物理学会の論文
2007-09-15