A mechanism of point defect migration triggered by local depolarization fields is shown to explain some still inexplicable features of aging in acceptor doped ferroelectrics. A drift-diffusion model of the coupled charged defect transport and electrostatic field relaxation within a two-dimensional domain configuration is treated numerically and analytically. Numerical results are given for the emerging internal bias field of about 1 kV/mm which levels off at dopant concentrations well below 1 mol%; the fact, long ago known experimentally but still not explained. For higher defect concentrations a closed solution of the model equations in the drift approximation as well as an explicit formula for the internal bias field is derived revealing the plausible time, temperature and concentration dependencies of aging. The results are compared to those due to the mechanism of orientational reordering of defect dipoles.
The dynamic dielectric susceptibility and elastic compliance of the relaxor ferroelectric PLZT 9/65/35 have been measured under different cooling and heating protocols, in order to study aging and memory. The memory of multiple aging stages has been found (several dips in the susceptibility curves on heating), as in spin glass systems below the glass transition. Remarkably, in PLZT the memory is retained also above the freezing temperature deduced from the dynamic susceptibilities. The results are discussed in the light of the existing models of aging and memory in spin and dipolar glasses.
The relaxor ferroelectric (PbMn$_{1/3}$Nb$_{2/3}$O$_3$)$_{1-x}$(PbTiO$_3$)$_{x}$, $x=0.1$, (PMN/PT(90/10)) is found to exhibit several regimes of complicated aging behavior. Just below the susceptibility peak there is a regime exhibiting rejuvenation but little memory. At lower temperature, there is a regime with mainly cumulative aging, expected for simple domain-growth. At still lower temperature, there is a regime with both rejuvenation and memory, reminiscent of spin glasses. PMN/PT (88/12) is also found to exhibit some of these aging regimes. This qualitative aging behavior is reminiscent…
The anomalously large dielectric aging in ferroelectric partially deuterated potassium dihydrogen phosphate (DKDP) is found to have multiple distinct mechanisms. Two components cause decreases in dielectric response over a limited range of fields around the aging field. A large fraction of this aging occurs on time scales of ~1000s after a field change, as expected for a hydrogen/deuterium diffusion mechanism. A slower component can give almost complete loss of domain-wall dielectric response at the aging field after weeks of aging. There is also a particularly unusual aging in which the diele…
Anomalously large dielectric aging is found in the high-susceptibility plateau ferroelectric regime of ~95% deuterated potassium dihydrogen phosphate (DKDP). Much less aging is found in non-deuterated KDP. Optical images of the DKDP domain structure show no dramatic change during aging. Small changes in electric field restore the pre-aged susceptibility, but the previous aging almost recovers after returning to the aging field. Susceptibility vs. field can show memory of at least two prior aging fields. Rectifying non-linear susceptibility develops for fields slightly above or below a prior ag…
Point defect migration is considered as a mechanism for aging in ferroelectrics. Numerical results are given for the coupled problems of point defect migration and electrostatic energy relaxation in a 2D domain configuration. The peak values of the clamping pressure at domain walls are in the range of $10^6$ Pa, which corresponds to macroscopically observed coercive stresses in perovskite ferroelectrics. The effect is compared to mechanisms involving orientational reordering of defect dipoles in the bulk of domains. Domain clamping is significantly stronger in the drift mechanism than in the o…
Migration of charged point defects triggered by the local random depolarization field is shown to plausibly explain aging of poled ferroelectric ceramics providing reasonable time and acceptor concentration dependences of the emerging internal bias field. The theory is based on the evaluation of the energy of the local depolarization field caused by mismatch of the polarizations of neighbor grains. The kinetics of charge migration assumes presence of mobile oxygen vacancies in the material due to the intentional or unintentional acceptor doping. Satisfactory agreement of the theory with experi…
Motion of ferroelectric domain walls greatly contributes to the macroscopic dielectric and piezoelectric response of ferroelectric materials. The domain wall motion through the ferroelectric material is however hindered by pinning on crystal defects which substantially reduces these contributions. Here, using thermodynamic models based on the Landau-Ginzburg-Devonshire theory, we find a relation between microscopic reversible motion of non-ferroelastic 180-degree domain walls interacting with a periodic array of pinning centers and the nonlinear macroscopic permittivity. We show, that the reve…
Materials undergoing antiferroelectric/ferroelectric (AFE/FE) transitions are studied for possible applications that exploit the large volume, charge and entropy differences between the two states, such as electrocaloric cooling, energy storage, electromechanical actuators. Though certain compositions of PZT codoped with La and Sn may withstand millions of electrically induced AFE/FE cycles, in other cases few thermally induced cycles and room temperature aging may cause noticeable changes in the material properties. This is particularly evident in the elastic moduli, which at room temperature…
An evidence that the dielectric ageing in the polydomain Pb(Zr0.45Ti0.55)O3 thin films is controlled by progressive pinning of 180 domain walls is presented. To provide such a conclusion, we use a general method, which is based on the study of the time evolution of the nonlinear, but anhysteretic, dielectric response of the ferroelectric to a weak electric field. A thermodynamic model of the ferroelectric system where the dielectric response is controlled by bending movements of pinned 180 domain walls is developed. Within this model, the nonlinear permittivity of the ferroelectric is expresse…
The kinetic theory of switching processes in crystalline ferroelectric materials under the influence of a variable external electric field is formulated. The basic equations are derived and their exact analytical solution at arbitrary time-dependent external field are obtained. Connection between the hysteresis loops shape and variable external electric field parameters is investigated numerically. The numerical results were found to be in excellent agreement with the experiments. PACS: 77.80.Fm, 77.80.Dj, 77.80.-e Keywords: ferroelectrics, switching processes, hysteresis curves, polarization
An exactly solvable lattice model with infinite-range potential is applied to uniaxial ferroelectrics. Asymptotically exact expression for free energy as a function of an order parameter at any temperatures is obtained. Effect of thermal expansion of lattice unit cell is taken into account. The free energy expansion in powers of order parameter in the vicinity of critical point is presented. Corrections to Landau expansion are obtained. In particular, it is shown that summand with external field contains a contribution of higher powers over order parameter. Keywords: Lattice model, Free energy…
Typical ferroelectrics possess a large spontaneous polarization Ps but simultaneously a large remnant polarization Pr as well, resulting in an inferior energy storage density.A mechanism that can reduce the Pr while maintain the Ps is demanded to enhance the energy storage property of ferroelectrics.In the present study, it is shown that after acceptor doping and aging treatment, the domain switching in ferroelectrics becomes reversible, giving rise to a pinched double hysteresis loop. The pinched loop with a large Ps and a small Pr thus results in an enhanced energy storage density. The physi…
We report on antiferroelectriclike double polarization hysteresis loops in multiferroic HoMnO3 thin films below the ferroelectric Curie temperature. This intriguing phenomenon is attributed to the domain pinning by defect dipoles which were introduced unintentionally during film growth process. Electron paramagnetic resonance suggests the existence of Fe1+ defects in thin films and first principles calculations reveal that the defect dipoles would be composed of oxygen vacancy and Fe1+ defect. We discuss migration of charged point defects during film growth process and formation of defect dipo…
Evolution of a stripe array of polarization domains triggered by the oxygen vacancy migration in an acceptor doped ferroelectric is investigated in a self-consistent manner. A comprehensive model based on the Landau-Ginzburg-Devonshire approach includes semiconductor features due to the presence of electrons and holes, and effects of electrostriction and flexoelectricity especially significant near the free surface and domain walls. A domain array spontaneously formed in the absence of an external field is shown to undergo a reconstruction in the course of the gradual oxygen vacancy migration …
It has been recently shown that the memory of multiple aging stages, a phenomenon considered possible only below the glass transition of some glassy systems, appears also above that temperature range in the relaxor ferroelectric (Pb/La)(Zr/Ti)O_3 (PLZT). Doubts exist whether memory at such high temperature is intrinsic of the glassy relaxor state or is rather due to migration of mobile defects. It is shown that the memory in the electric susceptibility and elastic compliance of PLZT 9/65/35 is not enhanced but depressed by mobile defects like O vacancies, H defects and mobile charges resulting…
The actual mechanism of polarization switching in ferroelectrics remains a puzzle for many decades, since the usually estimated barrier for nucleation and growth is insurmountable ("paradox of the coercive field"). To analyze the mechanisms of the nucleation we consider the exactly solvable case of a ferroelectric film with a "dead" layer at the interface with electrodes. The classical nucleation is easier in this case but still impossible, since the calculated barrier is huge. We have found that the {\em interaction} between the nuclei is, however, long range, hence one has to study an {\em e…
The formation and field-induced switching of defect dipoles in acceptor doped lead titanate is described by a kinetic model representing an extension of the well established Arlt-Neumann model [Ferroelectrics {\bf 76}, 303 (1987)]. Energy barriers for defect association and reorientation of oxygen vacancy-dopant (Cu and Fe) complexes are obtained from first-principles calculations and serve as input data for the kinetic coefficients in the rate equation model. The numerical solution of the model describes the time evolution of the oxygen vacancy distribution at different temperatures and dopan…
