Effects of ac-field amplitude on the dielectric susceptibility of relaxors

The thermally activated flips of the local spontaneous polarization in relaxors were simulated to investigate the effects of the applied-ac-field amplitude on the dielectric susceptibility. It was observed that the susceptibility increases with increasing the amplitude at low temperatures. At high temperatures, the susceptibility experiences a plateau and then drops. The maximum in the temperature dependence of susceptibility shifts to lower temperatures when the amplitude increases. A similarity was found between the effects of the amplitude and frequency on the susceptibility.Comment: 8 pages, 7 figures, Phys. Rev. B (in July 1st

Dielectric nonlinearity of relaxor ferroelectric ceramics at low ac drives

Dielectric nonlinear response of (PbMg$_{1/3}$Nb$_{2/3}$O$_3$)$_{0.9}$(PbTiO$_3$)$_{0.1}$ (0.9PMN-0.1PT) relaxor ceramics was investigated under different ac drive voltages. It was observed that: (i) the dielectric permittivity is independent on ac field amplitude at high temperatures; (ii) with increasing ac drive, the permittivity maximum increases, and the temperature of the maximum shifts to lower temperature; (iii) the nonlinear effect is weakened when the measurement frequency increases. The influences of increasing ac drive were found to be similar to that of decreasing frequency. It is believed that the dielectric nonlinearities of relaxors at low drives can be explained by the phase transition theory of ergodic space shrinking in succession. A Monte Carlo simulation was performed on the flips of micro polarizations at low ac drives to verify the theory.Comment: Submitted to J. Phys.: Cond. Matte

Dynamics of relaxor ferroelectrics

We study a dynamic model of relaxor ferroelectrics based on the spherical random-bond---random-field model and the Langevin equations of motion. The solution to these equations is obtained in the long-time limit where the system reaches an equilibrium state in the presence of random local electric fields. The complex dynamic linear and third-order nonlinear susceptibilities $\chi_1(\omega)$ and $\chi_3(\omega)$, respectively, are calculated as functions of frequency and temperature. In analogy with the static case, the dynamic model predicts a narrow frequency dependent peak in $\chi_3(T,\omega)$, which mimics a transition into a glass-like state.Comment: 15 pages, Revtex plus 5 eps figure

Coexistence of the Critical Slowing Down and Glassy Freezing in Relaxor Ferroelectrics

We have developed a dynamical model for the dielectric response in relaxor ferroelectrics which explicitly takes into account the coexistence of the critical slowing down and glassy freezing. The application of the model to the experiment in PMN allowed for the reconstruction of the nonequilibrium spin glass state order parameter and its comparison with the results of recent NMR experiment (Blinc et al., Phys. Rev. Lett. 83, No. 2 (1999)). It is shown that the degree of the local freezing is rather small even at temperatures where the field-cooled permittivity exceeds the frequency dependent permittivity by an order of magnitude. This observation indicates the significant role of the critical slowing down (accompanying the glass freezing) in the system dynamics. Also the theory predicts an important interrelationship between the frequency dependent permittivity and the zero-field-cooled permittivity, which proved to be consistent with the experiment in PMN (A. Levstik et. al., Phys. Rev. B 57, 11204 (1998))

Crossover in a non-analytical behaviour of dielectric non-linearity in PbMg1/3Nb2/3O3 relaxor ferroelectric

The dielectric response of the relaxer ferroelectric PbMg1/3Nb2/3O3 (PMN) is found to be a non-analytical function of the ac field, with the absolute value of the non-linear component of the polarization given by \P-nl\ alpha E-m(gamma(omega,T)). The dependence on the temperature, T, and frequency, omega, of the exponent gamma manifests itself in the form of a crossover in gamma(T) from approximately 2 to 3 upon cooling, whose position depends upon the frequency of the applied ac held. From the comparison of y(w, T) with the linear complex dielectric permittivity, epsilon(l)* (omega, T), of PMN, the following can be correlated: (i) gamma approximate to 2 in the regime of the quasi-static response, and y starts deviating from 2 simultaneously with the onset of the frequency dispersion; and (ii) gamma approximate to 3 when the spectrum of relaxation times in PMN becomes flat. These findings can be understood in terms of the change in the type of the motion of the interphase boundaries of the microscopic polar regions existing in PMN, namely in the change of the scale on which the interphase boundaries can move

Mechanism of polarization response in the ergodic phase of a relaxor ferroelectric

It is shown that the effects of large de bias and ac driving fields on the dielectric permittivity of single crystals of PbMg1/3Nb2/3O3 (PMN) relaxer ferroelectric exhibit very different behavior, including anisotropy and sign of the effect, which is traced up to the high-temperature interval in the ergodic phase. The observed phenomena strongly suggest that in the ergodic phase of PMN the dielectric response to the ac field is controlled by a side-way motion of the interphase boundaries of the polar regions, rather than by the thermally activated reorientations of the local spontaneous polarization

Does freezing in PbMg1/3Nb2/3O3 relaxor manifest itself in nonlinear dielectric susceptibility?

A possible anomaly in the temperature dependence of nonlinear dielectric susceptibility chi(nl) of relaxor ferroelectrics related with the freezing phase transition was investigated. First, based on the phenomenological approach, the anomaly in the chi(nl) was analyzed, including its shape, sign, and crystalline anisotropy. Second, the theoretical results were applied to the analysis of experimental data on nonlinear dielectric permittivity of single crystals of PbMg1/3Nb2/3O3 (PMN) relaxor. It was concluded that in contrast to earlier publications, there is no evidence for the anomaly in chi(nl)(T) of PMN related with the freezing transition. (C) 1999 American Institute of Physics

A "breathing" model for the polarization response of relaxer ferroelectrics

The present paper proposes a model of relaxer ferroelectrics, which describes the polarization response of these materials in terms of vibration of the interphase boundary between the polar regions and nonpolar matrix. The model is formulated and is applied to explain various features of the polarization response of a "classical" relaxer, PbMg1/3Nb2/3O3, such as the relaxation dispersion of the linear response, its temperature dependence, and the nonlinear effects due to large ac and/or de fields

Dielectric non-linearity and the nature of polarization response of PbMg1/3Nb2/3O3 relaxor ferroelectric

We investigate the non-linear dielectric properties of PbMg1/3Nb2/3O3 (PMN) single crystals and present the following experimental evidence: (a) the absence of the anomaly in the temperature dependence of non-linear coefficients in the relationship between the electric field and polarization, E(P); (b) large difference between the "ac" and "dc" non-linear effects; (c) non-analytical behavior of non-linear dielectric response and the crossover in its field exponents. The fact (a) clearly evidences against the glassy behavior of PMN. The results (b) and (c) can be understood using the model of the interphase boundary motion in PMN