Glassy freezing in relaxor ferroelectric lead magnesium niobate

The freezing process in lead magnesium niobate (PMN) has been investigated by measurements of the frequency-dependent complex dielectric constant and its third harmonic component. The linear complex dielectric susceptibility was analyzed by a temperature-frequency plot in order to determine the temperature dependence of the dielectric relaxation spectrum and to identify the freezing temperature. It was found that both the shape of the relaxation spectrum and its temperature behavior in the PMN relaxer show remarkable similarities to dipolar glasses, i.e., the longest relaxation time diverges according to the Vogel-Fulcher law, while the bulk of the distribution of relaxation times remains finite even below the freezing temperature. The frequency and the temperature dependence of the third harmonic susceptibility, similar to the behavior observed in linear dielectric response, indicate that the same underlying relaxation spectrum and therefore the same slowing-down mechanism is controlling both linear and nonlinear dynamic response. The observed splitting between the field-cooled and zero-field-cooled dielectric constant-comparable to the one obtained in spin glasses-effectively demonstrates the occurrence of typical glassy nonergodic behavior in the vicinity of the transition temperature where the ferroelectric phase would appear above a threshold electric field.