Non-Arrhenius stretched exponential dielectric relaxation in antiferromagnetic (formula presented) single crystals

Dielectric spectra of (formula presented) single crystals, grown by the chemical vapor transport technique, have been studied at temperatures between 40 and 200 °C and frequencies from 100 Hz to 1 MHz. The shape of the spectra is well parameterized in terms of the temperature-independent Kohlrausch-Williams-Watts (stretched exponential) form, modified with the superimposed low-frequency dispersion and dc component. A deviation of the relaxation time from the Arrhenius law has been observed, which cannot be fitted into the known formulas. This deviation is phenomenologically explained by the screening effect. A formula has been proposed to describe such a complex dielectric relaxation behavior that is expected to exist in many more materials.