E. S. Ivanova

Specific features in the ferroelectric properties of Sr0.75Ba0.25Nb2O6 crystals

The dielectric properties of strontium-barium niobate crystals with a strontium concentration of 0.75, which corresponds to the most smeared ferroelectric phase transition, were measured. The dielectric hysteresis loops and the polarization relaxation are shown to exhibit clearly pronounced features characteristic of crystals with structural disorder and a broad distribution of a random internal electric field. The measurements were performed in dc, slowly varying (quasi-static), and pulsed electric fields. The experimental data obtained are used to construct the distribution function for the coercive field in the crystal bulk.

Scanning probe microscopy investigation of ferroelectric properties of barium strontium niobate crystals

The ferroelectric properties of relaxor ferroelectric materials SrxBa1 − xNb2O6 of different compositions have been investigated using scanning probe microscopy and macroscopic polarization methods. An analysis of the results obtained from microscopic and macroscopic measurements of the hysteresis loops has demonstrated that a frequency dependence of the coercive field Ec is observed at frequencies f in the range from 0.03 to 250 Hz, so that the coercive field Ec regularly decreases with decreasing frequency. The observation of the kinetics of domains under a direct-current voltage applied to the probe of the atomic force microscope has revealed a lateral motion of domain walls in response to electric fields E ≪ Ec. This result qualitatively explains a slow polarization relaxation occurring for a giant time in SrxBa1 − xNb2O6.

Polarization relaxation in a ferroelectric crystal having different states of the domain structure and surface

Slow polarization relaxation of a ferroelectric in a weak electric field is investigated in triglycine sulfate crystals with different states of the domain structure and surface. An automated setup is described that allows one to register relaxation with record accuracy. It is shown that under conditions of little change in the “degree of metastability” of the structure, the variation of the polarization ΔP with time t follows the law ΔP=C/(1+t/ta)n in all cases, where the parameters C, a, and n depend on the state of the structure and the surface. We present a phenomenological analysis of the experimental data based on the assumption that the nuclei are independent and contribute additively to the total polarization of the crystal, and construct spectra of the energy barriers of the domain walls. Aspects of the transformation of the spectra with variation of the nature of the domain structure, depth of surface relief, and magnitude of the external electric field are elucidated.

Change in dielectric properties of triglycine sulfate in a constant magnetic field

The effect of a constant magnetic field on the dielectric properties of triglycine sulfate crystals has been investigated. It has been shown that, after the magnetic treatment of the crystal (2 T, 20 min), the hysteresis loop becomes narrower; i.e., the coercive field decreases, and the dielectric permittivity changes in the region of the phase transition. It has been found that the observed effect is anisotropic with respect to the orientation of the crystal in a magnetic field and occurs when the vector of magnetic induction is perpendicular to the polar axis of the crystal. The relative orientation of the magnetic field and the domain structure determines its sign. The doping of the crystal with chromium makes the effect more pronounced and leads to a change in the kinetics of the magnetically stimulated increase in the dielectric permittivity.

Dielectric Hysteresis Loops of Photosensitive Relaxor Ferroelectrics

We have studied the effects of illumination by a photoactive light on the polarization switching of strontium-barium niobate crystals in quasi-static periodic electric field. The illumination results in an increase of the polarization amplitude and of the most probable coercive field. The origin of this phenomenon is discussed in terms of screening of depolarizing fields and of internal random fields presenting within the bulk of a relaxor ferroelectric with photoinduced free charge carriers.

Suppression of ferroelectricity by low uniaxial stresses in some crystals with successive phase transitions

Abstract Study of disappearance of ferroelectricity on compressing the tetramethylammonium tetrachlorozincate, tetrachlorocobaltate and thiourea crystals by uniaxial stresses [sgrave] is presented. The phase [sgrave], T - diagrams are plotted, the critical values of [sgrave] being anomalously small. The effect is shown to be essentially anisotro-pic and nonlinear. The data published recently are shortly reviewed and some new results are presented.

On the permittivity of barium strontium niobate, a photosensitive relaxor ferroelectric

The permittivity ɛ of the photosensitive relaxor ferroelectric Sr0.61Ba0.39Nb2O6 doped with La and Ce is studied both in the dark and under illumination with a power density of up to 0.22 W/cm2 in the wavelength range 400–500 nm of the absorption spectrum of the crystal. The permittivity ɛ is measured at frequencies of 1 kHz and 1 MHz, as well as under a dc electrical bias of 2 kV/cm at different temperatures. It is found that the permittivity ɛ exhibits a photoinduced increase (photodielectric effect), which reaches a maximum in the temperature range of the maximum permittivity and is proportional to the illumination power density. The specific features of the observed photodielectric effect and the possible influence of the screening of random internal nonuniform electric fields by photoinduced carriers on this phenomenon are discussed.

Two forms of polarization relaxation in polydomain ferroelectrics in an electric field

The characteristic features of the polarization and depolarization kinetics of polydomain ferroelectrics with square and narrow “extended” dielectric hysteresis loops are investigated for the model of TGS and Rb2ZnCl4 crystals. It is shown that for the second crystal, in contrast to the first crystal, the local free energy is asymmetric relative to the direction of polarization, the coercive field does not have a definite value, and only part of the crystal volume participates in the slow thermoactivational relaxation. The slow relaxation follows a universal empirical power law in all cases. The distribution functions of the relaxation times in crystals are constructed on the basis of experimental data, and comparative estimates are made of the relaxation parameters and the parameters of the energy barriers for domain walls.

Magnetic memory effects in triglycine sulfate ferroelectric crystals

The effect of a magnetic field on the processes of relaxation of the defect structure relaxation in a triglycine sulfate (TGS) ferroelectric (nonmagnetic) crystal has been observed for the first time. The atomic-force microscopy study has shown that the application of a static weak magnetic field (2 T, 20 min) significantly changes the size distribution of defect nanoclusters characteristic of TGS. Previously known macroscopic aftereffects of the magnetic field in TGS (slow relaxation of the dielectric susceptibility, symmetrization of P–E dielectric hysteresis loops, etc.) can be explained by the redistribution of pinning centers of domain walls caused by the magnetically induced reconfiguration of the defect structure.

Influence of the electrical conductivity on dielectric characteristics of a triglycine sulfate crystal over a wide frequency range

The dielectric spectra of ferroelectric triglycine sulfate crystals were measured in the frequency range from 10 to 1011 Hz and analyzed based on the concept of a significant contribution of the conductive component to the dielectric response of ferroelectrics. Images of lenticular ferroelectric domains in sulfate triglycine crystals were obtained using scanning spreading resistance atomic force microscopy, and background currents (10−14 A) and currents at domain walls (10−12 A) were measured. It was shown that the inclusion of the contribution from the electrical conductivity to the dielectric spectra is in good agreement with the experimental data and provides additional information on the temperature dynamics of the domain structure.