Maxim V. Silibin

Flexocoupling impact on size effects of piezoresponse and conductance in mixed-type ferroelectric semiconductors under applied pressure

We explore the role of flexoelectric effect in functional properties of nanoscale ferroelectric films with mixed electronic-ionic conductivity. Using a coupled Ginzburg-Landau model, we calculate spontaneous polarization, effective piezoresponse, elastic strain and compliance, carrier concentration, and piezoconductance as a function of thickness and applied pressure. In the absence of flexoelectric coupling, the studied physical quantities manifest well-explored size-induced phase transitions, including transition to paraelectric phase below critical thickness. Similarly, in the absence of external pressure flexoelectric coupling affects properties of these films only weakly. However, the combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase and induces the electretlike state with irreversible spontaneous polarization below critical thickness. Interestingly, the built-in field leads to noticeable increase of the average strain and elastic compliance in this thickness range. We further illustrate that the changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high- or low-conductivity states, i.e., the nonvolatile piezoresistive switching, in which the swing can be controlled by the film thickness and flexoelectric coupling. The obtained theoretical results can be of fundamental interest for ferroic systems, and can provide a theoretical model for explanation of a set of recent experimental results on resistive switching and transient polar states in these systems.

Surface and finite size effects impact on the phase diagrams, polar, and dielectric properties of (Sr,Bi)Ta2O9 ferroelectric nanoparticles

In the framework of the thermodynamic approach Landau-Ginzburg-Devonshire (LGD) combined with the equations of electrostatics, we investigated the effect of polarization surface screening on finite size effects of the phase diagrams, polar, and dielectric properties of ferroelectric nanoparticles of different shapes. We obtained and analyzed the analytical results for the dependences of the ferroelectric phase transition temperature, critical size, spontaneous polarization, and thermodynamic coercive field on the shape and size of the nanoparticles. The pronounced size effect of these characteristics on the scaling parameter, the ratio of the particle characteristic size to the length of the surface screening, was revealed. Also our modeling predicts a significant impact of the flexo-chemical effect (that is a joint action of flexoelectric effect and chemical pressure) on the temperature of phase transition, polar, and dielectric properties of nanoparticles when their chemical composition deviates from th...

Flexocoupling impact on the generalized susceptibility and soft phonon modes in the ordered phase of ferroics

The impact of the flexoelectric effect on the generalized susceptibility and soft phonon dispersion is not well known in the long-range-ordered phases of ferroics. Within the Landau-Ginzburg-Devonshire approach we obtained analytical expressions for the generalized susceptibility and phonon dispersion relations in the ferroelectric phase. The joint action of the static and dynamic flexoelectric effects induces nondiagonal components of the generalized susceptibility, whose amplitude is proportional to the convolution of the spontaneous polarization with the flexocoupling constants. The flexocoupling essentially broadens the k spectrum of the generalized susceptibility and leads to an additional “pushing away” of the optical and acoustic soft mode phonon branches. The degeneracy of the transverse optical and acoustic modes disappears in the ferroelectric phase in comparison with the paraelectric phase due to the joint action of flexoelectric coupling and ferroelectric nonlinearity. Lastly, the results obtained might be mainly important for theoretical analyses of a broad spectrum of experimental data, including neutron and Brillouin scattering.

Thermodynamic potential and phase diagram for multiferroic bismuth ferrite (BiFeO 3 )

We construct a Landau–Ginzburg thermodynamic potential, and the corresponding phase diagram for pristine and slightly doped bismuth ferrite, a ferroelectric antiferromagnet at room temperature. The potential is developed based on new X-ray and neutron diffraction experiments complementing available data. We demonstrate that a strong biquadratic antiferrodistortive-type coupling is the key to a quantitative description of Bi1−xLaxFeO3 multiferroic phase diagram including the temperature stability of the antiferromagnetic, ferroelectric, and antiferrodistortive phases, as well as for the prediction of novel intermediate structural phases. Furthermore, we show that “rotomagnetic” antiferrodistortive–antiferromagnetic coupling is very important to describe the ferroelectric polarization and antiferrodistortive tilt behavior in the R3c phase of BiFeO3. The Landau–Ginzburg thermodynamic potential is able to describe the sequence of serial and trigger-type phase transitions, the temperature-dependent behavior of the order parameters, and the corresponding susceptibilities to external stimuli. It can also be employed to predict the corresponding ferroelectric and antiferrodistortive properties of Bi1−xLaxFeO3 thin films and nanoparticles by incorporating the gradient and surface energy terms that are strongly dependent on the shape, size, and preparation method.Multiferroics: The potential of bismuth ferriteA theoretical approach for describing the complex phase diagram of the multiferroic bismuth ferrite has been developed. Multiferroics are materials that exhibit multiple types of ferroic ordering, such as ferromagnetism and ferroelectricity, simultaneously. Bismuth ferrite is perhaps one of the best known of these as it exhibits multiferroicity at room temperature, making it useful for a range of applications, but the underlying physical mechanisms responsible for its multiferroic properties remains somewhat unclear. An international team of researchers led by Dmitry Karpinsky from the Scientific-Practical Materials Research Centre of NAS of Belarus and the Moscow Institute of Electronic Technology use existing experimental data to construct a Landau-Ginzbur-like thermodynamic potential that can not only provide a quantitatively description of bismuth ferrites known behavior, but also predicts new intermediate phases.

Self-consistent modelling of electrochemical strain microscopy in mixed ionic-electronic conductors: Nonlinear and dynamic regimes

The frequency dependent Electrochemical Strain Microscopy (ESM) response of mixed ionic-electronic conductors is analyzed within the framework of Fermi-Dirac statistics and the Vegard law, accounting for steric effects from mobile donors. The emergence of dynamic charge waves and nonlinear deformation of the surface in response to bias applied to the tip-surface junction is numerically explored. The 2D maps of the strain and concentration distributions across the mixed ionic-electronic conductor and bias-induced surface displacements are calculated. The obtained numerical results can be applied to quantify the ESM response of Li-based solid electrolytes, materials with resistive switching, and electroactive ferroelectric polymers, which are of potential interest for flexible and high-density non-volatile memory devices.

Polarization reversal in organic-inorganic ferroelectric composites: Modeling and experiment

Tailoring of ferroelectric properties of copolymer P(VDF-TrFE) by incorporation of ceramic inclusions in the polymer matrix is promising for advanced applications in sensorics. We have observed experimentally that in composites of P(VDF-TrFE) with barium-doped lead zirconate titanate (BPZT), the remanent polarization increases, while the coercive field substantially decreases in comparison with the pure polymer samples. Results of simulation in framework of the modified Weiss model have shown that the changes of the hysteresis loops characteristics are due to increase of the dielectric susceptibility of the composite as compared to pure PVDF-TrFE. This originates from the strong dispersion of the mean field constant α, which describes the feedback of the polarization on the electric field at the location of the dipoles and is closely related with the local increase of composite susceptibility in the vicinity the BPZT inclusions. This phenomenon effectively becomes macro-scale due to the long-range nature of the inhomogeneous elastic and electric fields occurring at the interfaces between the matrix and inclusions.

Magnetic properties and magnetoresistance in La0.5Sr0.5Co1−xMexO3 (Me = Cr, Ga, Ti, Fe) cobaltites

Magnetic and magnetotransport properties of stoichiometric cobaltites La0.5Sr0.5Co1−xMexO3 (Me = Cr, Ga, Ti, Fe, x ≤ 0.25) have been investigated by neutron powder diffraction, magnetization and electrical measurements in fields up to 14 T. It is shown that doping with Fe up to x = 0.2 slightly increases magnetization herewith the Curie point decreases. The chromium doping leads to dramatic decrease of magnetization and the Curie point, associated coherent magnetic contribution in NPD patterns quickly decreases with doping and becomes nearly negligible for x = 0.2 compound. The substitution with diamagnetic Ga and Ti ions decreases the magnetization to a lesser extent than in the chromium doped compound. The Cr, Ti and Ga substitution leads to a strong increase in magnetoresistance at low temperature as compared with undoped cobaltite. The obtained results indicate that the magnetic interactions between Co and Fe are positive whereas those between Co and Cr ions are negative. Enhancement of magnetoresistance is attributed to the magnetic field induced transition from local antiferromagnetic order to ferromagnetic one.

Local ferroelectric properties in polyvinylidene fluoride/barium lead zirconate titanate nanocomposites: Interface effect

The local piezoelectric properties of ferroelectric composites consisting of P(VDF-TrFE) copolymer matrix with barium lead zirconate titanate ceramic inclusions were addressed both experimentally using piezoresponse force microscopy technique and theoretically applying the Landau-Ginzburg-Devonshire formalism. A transient region with a width of approximately 40u2009nm has been found at the interface between the two constituents. It is shown that the piezoresponse in the vicinity of the interface is strongly affected by inhomogeneous stresses originating from an incompatibility of thermal expansion coefficients of PVDF and lead zirconate titanate.

Polarization Effect on Dielectric Response of Ferroelectric Copolymer P(VDF-TrFE)

Abstract The temperature dependence of the dielectric permittivity has been determined on poly(vinylidene fluoride-triflouroethylene) copolymer films for both virgin and preliminary polarized states. It was found that the polarization process led to considerable changes in the dielectric response of the copolymer. A change in the dielectric permittivity, the shift of the maximum of temperature dependence and an almost unchanged relaxor-like behavior were observed.

Effect of the structure and thermoelastic properties of components on the average stress in anodic aluminum oxide having pores filled with metallic nanowires

We have assessed the average stress induced by the thermal expansion mismatch between the components of composites consisting of metallic nanowires and an anodic aluminum oxide matrix and performed model calculations for composites containing tin, zinc, and indium nanocrystals.