95 research outputs found
Stochastic model of dispersive multi-step polarization switching in ferroelectrics due to spatial electric field distribution
A stochastic model for polarization switching in tetragonal ferroelectric
ceramics is introduced, which includes sequential 90{\deg}- and parallel
180{\deg}-switching processes and accounts for the dispersion of characteristic
switching times due to a nonuniform spatial distribution of the applied field.
It presents merging of the recent multistep stochastic mechanism (MSM) with the
earlier nucleation limited switching (NLS) and inhomogeneous field mechanism
(IFM) models. The new model provides a much better description of simultaneous
polarization and strain responses over a wide time window and a deeper insight
into the microscopic switching mechanisms, as is exemplarily shown by
comparison with measurements on lead zirconate titanate.Comment: 11 pages, 3 figure
Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder
A self-consistent stochastic model of domain structure formation in a
uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to
a low-temperature ferroelectric phase, is developed with an account of the
applied electric field and the feedback effect via local depolarization fields.
Both polarization and field components are considered as Gauss random
variables. A system of integro-differential equations for correlation functions
of all involved variables is derived and solved analytically and numerically.
Phase diagram in terms of the average value and dispersion of polarization
reveals different possible equilibrium states and available final single-domain
and multi-domain states. The time-dependent evolution of the average
polarization and dispersion discloses a bifurcation behavior and the
temperature-dependent value of the electric field, deciding between the
single-domain and multi-domain final states, which can be interpreted as the
coercive field. Analytical and numerical results for the time-dependent
correlation length and correlation functions exhibit plausible agreement with
available experimental data.Comment: 33 pages, 10 figure
Tuning the Polar States of Ferroelectric Films via Surface Charges and Flexoelectricity
Using the self-consistent Landau-Ginzburg-Devonshire approach we simulate and
analyze the spontaneous formation of the domain structure in thin ferroelectric
films covered with the surface screening charge of the specific nature
(Bardeen-type surface states). Hence we consider the competition between the
screening and the domain formation as alternative ways to reduce the
electrostatic energy and reveal unusual peculiarities of distributions of
polarization, electric and elastic fields conditioned by the surface screening
length and the flexocoupling strength. We have established that the critical
thickness of the film and its transition temperature to a paraelectric phase
strongly depend on the Bardeen screening length, while the flexocoupling
affects the polarization rotation and closure domain structure and induces
ribbon-like nano-scale domains in the film depth far from the top open surface.
Hence the joint action of the surface screening (originating from e.g. the
adsorption of ambient ions or surface states) and flexocoupling may remarkably
modify polar and electromechanical properties of thin ferroelectric films.Comment: 33 pages, 5 figure
Defect driven flexo-chemical coupling in thin ferroelectric films
Using Landau-Ginzburg-Devonshire theory, we considered the impact of the
flexoelectro-chemical coupling on the size effects inpolar properties and phase
transitions of thin ferroelectric films with a layer of elastic defects. We
investigated a typical case, when defects fill a thin layer below the top film
surface with a constant concentration creating an additional gradient of
elastic fields. The defective surface of the film is not covered with an
electrode, but instead with an ultra-thin layer of ambient screening charges,
characterized by a surface screening length. This geometry is typical for the
scanning probe piezoelectric force microscopy. Obtained results revealed an
unexpectedly strong effect of the joint action of Vegard stresses and
flexoelectric effect (shortly flexo-chemical coupling) on the ferroelectric
transition temperature, distribution of the spontaneous polarization and
elastic fields, domain wall structure and period in thin PbTiO3 films
containing a layer of elastic defects. A nontrivial result is the
ferroelectricity persisting at film thicknesses below 4 nm, temperatures lower
than 350 K and relatively high surface screening length (~0.1 nm). The origin
of this phenomenon is the re-building of the domain structure in the film
(namely the cross-over from c-domain stripes to a-type closure domains) when
its thickness decreases below 4 nm, conditioned by the flexoelectric coupling
and facilitated by negative Vegard effect. For positive Vegard effect, thicker
films exhibit the appearance of pronounced maxima on the thickness dependence
of the transition temperature, whose position and height can be controlled by
the defect type and concentration. The revealed features may have important
implications for miniaturization of ferroelectric-based devices.Comment: 25 pages, 9 figure
Multi-step stochastic mechanism of polarization reversal in rhombohedral ferroelectrics
A stochastic model for the field-driven polarization reversal in rhombohedral
ferroelectrics is developed, providing a description of their temporal
electromechanical response. Application of the model to simultaneous
measurements of polarization and strain kinetics in a rhombohedral Pb(Zr,Ti)O3
ceramic over a wide time window allows identification of preferable switching
paths, fractions of individual switching processes, and their activation
fields. Complementary, the phenomenological Landau-Ginzburg-Devenshire theory
is used to analyze the impact of external field and stress on switching
barriers showing that residual mechanical stress may promote the fast
switching.Comment: 33 pages, 9 figure
Space-charge mechanism of aging in ferroelectrics: an exactly solvable two-dimensional model
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.Comment: 8 pages, 4 figures. accepted to Physical Review
Dynamic scaling properties of multistep polarization response in ferroelectrics
Ferroelectrics are multifunctional smart materials finding applications in
sensor technology, micromechanical actuation, digital information storage etc.
Their most fundamental property is the ability of polarization switching under
applied electric field. In particular, understanding of switching kinetics is
essential for digital information storage. In this regard, scaling properties
of the temporal polarization response are well-known for 180{\deg}-switching
processes in ferroelectrics characterized by a unique field-dependent local
switching time. Unexpectedly, these properties were now observed in multiaxial
polycrystalline ferroelectrics, exhibiting a number of parallel and sequential
non-180{\deg}-switching processes with distinct switching times. This behaviour
can be explained by a combination of the multistep stochastic mechanism and the
inhomogeneous field mechanism models of polarization reversal. Scaling
properties are predicted for polycrystalline ferroelectrics of tetragonal,
rhombohedral and orthorhombic symmetries and exemplarily demonstrated by
measurements of polarization kinetics in (K,Na)NbO3-based ferroelectric ceramic
over a timescale of 7 orders of magnitude. Dynamic scaling properties allow
insight into the microscopic switching mechanisms, on the one hand, and into
statistical material characteristics, on the other hand, providing thereby the
description of temporal polarization with high accuracy. The gained deeper
insight into the mechanisms of multistep polarization switching is crucial for
future ultrafast and multilevel digital information storage.Comment: 22 pages, 3 figure
Mesoscopic theory of defect ordering-disordering transitions in thin oxide films
Ordering of mobile defects in functional materials can give rise to
fundamentally new phases possessing ferroic and multiferroic functionalities.
Here we develop the Landau theory for strain induced ordering of defects (e.g.
oxygen vacancies) in thin oxide films, considering both the ordering and
wavelength of possible instabilities. Using derived analytical expressions for
the energies of various defect-ordered states, we calculated and analyzed phase
diagrams dependence on the film-substrate mismatch strain, concentration of
defects, and Vegard coefficients. Obtained results open possibilities to create
and control superstructures of ordered defects in thin oxide films by selecting
the appropriate substrate and defect concentration.Comment: 30 pages, 5 figures, 1 appendi
Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O-3-x(Ba0.7Ca0.3)TiO3 ferroelectrics
Analysis of polarization switching dynamics by means of the inhomogeneous field mechanism model allows insight into the microscopic mechanism of reversed polarization domain nucleation. For all chemical compositions studied, two distinct field regions of nucleation are established. In the high-field region, the activation energy barrier is found to be inversely proportional to the local field according to the Merz law. In contrast, the barriers in the low-field region exhibit a linear field dependence with a minimum in the compositional region of phase instability, which can explain the corresponding peak ferroelectric properties.open7
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