arXiv subjects

Anna N. Morozovska

Publications and source records attributed to Anna N. Morozovska.

At least 163 records · Page 9Linked to original sources

arXiv2015

Finite size effects in ferroelectric-semiconductor thin films under open-circuited electric boundary conditions

General features of finite size effects in the ferroelectric-semiconductor film under open-circuited electric boundary conditions are analyzed using Landau-Ginzburg-Devonshire theory and continuum media electrostatics. The temperature dependence of the film critical thickness, spontaneous polarization and depolarization field profiles of the open-circuited films are found to be significantly different compared to the short-circuited ones. In particular, we predict the re-entrant type transition boundary between the mono-domain and poly-domain ferroelectric states due to reduced internal screening efficiency and analyzed possible experimental scenarios created by this mechanism. This analysis is relevant to the behavior of the free-standing ferroelectric films and scanning-probe microscopy based experiments on free ferroelectric surfaces.

cond-mat.mtrl-sci↗

arXiv2014

Rotomagnetic couplings influence on the magnetic properties of antiferrodistortive antiferromagnets

Within the framework of Landau-Ginzburg-Devonshire (LGD) phenomenological theory we consider the possibility to control properties of paraelectric antiferromagnets via biquadratic rotomagnetic coupling with and without external magnetic and electric field application. We use EuTiO3 as a prototype with relatively well-known material parameters. Surprisingly strong influence of this coupling practically on all the properties without external fields was obtained in the temperature region with coexistence of antiferromagnetic and antiferrodistorted phases i.e. in multiferroic state. In particular, the observed Neel temperature TN (5.5 K) was shown to be defined by rotomagnetic coupling, while without this coupling TN appeared to be much higher (26 K). For small or high enough rotomagnetic coupling constant value the antiferromagnetic phase transition order appeared to be the second or the first order respectively. The essential influence of rotomagnetic coupling on the form and value of magnetic and dielectric permittivity was also forecasted. The rotomagnetic coupling along with rotoelectric one opens the additional way to control the form of the phase diagrams by application of external magnetic or electric field. The critical value of the electric field (for antiferromagnetic to ferromagnetic phase transition) appeared essentially smaller than the one calculated without rotomagnetic coupling that can be important for applications.

cond-mat.mtrl-sci↗

arXiv2014

New room temperature multiferroics on the base of single-phase nanostructured perovskites

The theoretical description of the nanostructured Pb(Fe1/2Ta1/2)x(Zr1/2Ti1/2)1-xO3 (PFTx-PZT(1-x)) and Pb(Fe1/2Nb1/2)x(Zr1/2Ti1/2)1-xO3 (PFNx-PZT(1-x)) intriguing ferromagnetic, ferroelectric and magnetoelectric properties at temperatures higher than 100 K are absent to date. The goal of this work is to propose the theoretical description of the physical nature and the mechanisms of the aforementioned properties, including room temperature ferromagnetism, phase diagram dependence on the composition x with a special attention to the multiferroic properties at room temperature, including anomalous large value of magnetoelectric coefficient. The comparison of the developed theory with experiments establishing the boundaries between paraelectric, ferroelectric, paramagnetic, antiferromagnetic, ferromagnetic and magnetoelectric phases, as well as the characteristic features of ferroelectric domain switching by magnetic field are performed and discussed. The experimentally established absence of ferromagnetic phase in PFN, PFT and in the solid solution of PFN with PbTiO3 (PFNx-PT(1-x)) was explained in the framework of the proposed theory.

cond-mat.mtrl-sci↗

arXiv2014

Electrostrictive and electrostatic responses in contact mode voltage modulated Scanning Probe Microscopies

Electromechanical response of solids underpins image formation mechanism of several scanning probe microscopy techniques including the piezoresponse force microscopy (PFM) and electrochemical strain microscopy (ESM). While the theory of linear piezoelectric and ionic responses are well developed, the contributions of quadratic effects including electrostriction and capacitive tip-surface forces to measured signal remain poorly understood. Here we analyze the electrostrictive and capacitive contributions to the PFM and ESM signals and discuss the implications of the dielectric tip-surface gap on these interactions.

cond-mat.mtrl-sci↗

arXiv2014

Humidity effects on tip-induced polarization switching in lithium niobate

Interest to ferroelectric materials has been increased significantly in last decades due to development of new generation of nonlinear optical and data storage devices. Scanning probe microscopy (SPM) can be used both for study of domain structures with nanoscale spatial resolution and for writing the isolated nanodomains by local application of the electric field. Tip-induced switching in the ambient still needs experimental investigations and theoretical explorations. Here we studied influence of the value of relative humidity in the SPM chamber on the process of tip-induced polarization switching. This phenomenon was attributed to existing of the water meniscus between tip and the sample surface in humid atmosphere. Presented results are important for further complex investigations of the ferroelectric materials and their applications.

cond-mat.mtrl-sci↗

arXiv2013

Non-Ising Character of a Ferroelectric Wall Arises from a Flexoelectric Effect

Using the classic ferroelectric BaTiO$_3$ as an example, we show that the 180 degree ferroelectric domain wall, long considered to be of Ising-type and charge neural, contains both Bloch and Neel type polarization components, and is thus charged. The bound charge density at the wall may reach as high as 10$^6$~10$^7$ Cm$^{-3}$. It is demonstrated that the flexoelectric effect arising from stress inhomogeneity is responsible for the additional Bloch and Neel polarization components. The magnitudes of these additional components are determined by the competing depolarization and flexoelectric fields.

cond-mat.mtrl-sci↗

arXiv2013

Space charge dynamics in solid electrolytes with steric effect and Vegard stresses: resistive switching and ferroelectric-like hysteresis of electromechanical response

We performed self-consistent modelling of electrotransport and electromechanical response of solid electrolyte thin films allowing for steric effects of mobile charged defects (ions, protons or vacancies), electron degeneration and Vegard stresses. We establish correlations between the features of the space-charge dynamics, current-voltage and bending-voltage curves in the wide frequency range of applied electric voltage. The pronounced ferroelectric-like hysteresis of bending-voltage loops and current maxima on double hysteresis current-voltage loops appear for the electron-open electrodes. The double hysteresis loop with pronounced humps indicates the resistance switching of memristor-type. The switching occurs due to the strong coupling between electronic and ionic subsystem. The sharp meta-stable maximum of the electron density appears near one open electrode and moves to another one during the periodic change of applied voltage. Our results can explain the nature and correlation of electrical and mechanical memory effects in thin films of solid electrolytes. The analytical expression proving that the electrically induced bending of solid electrolyte films can be detected by interferometric methods is derived.

cond-mat.mtrl-sci↗

arXiv2013

Ferroelectric domain triggers the charge modulation in semiconductors

We consider a typical heterostructure domain patterned ferroelectric film/ultra thin dielectric layer/ semiconductor, where the semiconductor can be an electrolyte, paraelectric or multi layered graphene. Unexpectedly we have found that the space charge modulation profile and amplitude in the semiconductor, that screens the spontaneous polarization of a 180-degree domain structure of ferroelectric, depends on the domain structure period, dielectric layer thickness and semiconductor screening radius in a rather non-trivial nonlinear way. Multiple size effects appearance and manifestation are defined by the relationship between these three parameters. In addition, we show that the concept of effective gap can be introduced in a simple way only for a single domain limit. Obtained analytical results open the way for understanding of current AFM maps of contaminated ferroelectric surfaces in ambient atmosphere as well as explore the possibilities of conductivity control in ultra-thin semiconductor layers.

cond-mat.mtrl-sci↗

arXiv2013

Defect thermodynamics and kinetics in thin strained ferroelectric films: the interplay of possible mechanisms

We present a theoretical description of the influence of misfit strain on mobile defects dynamics in thin strained ferroelectric films. Self-consistent solutions obtained by coupling the Poissons equation for electric potential with continuity equations for mobile donor and electron concentrations and time-dependent Landau-Ginzburg-Devonshire equations reveal that the Vegard mechanism (chemical pressure) leads to the redistribution of both charged and electro-neutral defects in order to decrease the effective stress in the film. Internal electric fields, both built-in and depolarization ones, lead to a strong accumulation of screening space charges (charged defects and electrons) near the film interfaces. Importantly, the corresponding screening length is governed by the misfit strain and Vegard coefficient. Mobile defects dynamics, kinetics of polarization and electric current reversal are defined by the complex interplay between the donor, electron and phonon relaxation times, misfit strain, finite size effect and Vegard stresses.

cond-mat.mtrl-sci↗

arXiv2013

Universal emergence of spatially-modulated structures induced by flexo-antiferrodistortive coupling in multiferroics

We proved the existence of a universal flexo-antiferrodistortive coupling as a necessary complement to the well-known flexoelectric coupling. The coupling is universal for all antiferrodistortive systems and can lead to the formation of incommensurate, spatially-modulated phases in multiferroics. Our analysis can provide a self-consistent mesoscopic explanation for a broad range of modulated domain structures observed experimentally in multiferroics.

cond-mat.mtrl-sci↗

arXiv2013

Novel multiferroics with ferromagnetic phase induced in paraelectric antiferromagnets by electric field application

The phase diagram of a quantum paraelectric antiferromagnet EuTiO3 under an external electric field was calculated using Landau-Ginzburg-Devonshire theory. It was shown that the application of an external electric field E leads to the appearance of a ferromagnetic phase due to the magnetoelectric coupling. In particular, electric field application decreases the transition temperature TAFM to antiferromagnetic (AFM) phase and induces ferromagnetic (FM) phase, so that at some E field larger than the critical field (Ecr), TFM becomes higher than TAFM and the FM phase appears. Note that Ecr increases and magnetization decreases as the temperature increases. The value of the critical field Ecr = 0.40*10^6 V/cm we calculated appeared close to the value Ecr = 0.5*10^6 V/cm obtained recently by Ryan et al. with the help of density functional theory for EuTiO3 film under a compressive strain produced by substrate. At the fields E >0.83*10^6 V/cm, AFM disappears for all considered temperatures and so FM becomes the only stable magnetic phase. We find that ferromagnetic phase can be induced by an E-field in other paraelectric antiferromagnet oxides with a positive AFM-type magnetoelectric (ME) coupling coefficient and negative FM-type ME coupling coefficient. In particular, the critical E-field was estimated for another paraelectric antiferromagnet Sr0.7Ba0.3MnO3 as 0.2x10^5V/cm at 0 K. Analysis of the dependence of magnetization and antimagnetization on the external electric field and the polarization induced by the field, which yields the magnetoelectric coupling, is reported. The results show the possibility to control multiferroicity, including the FM and AFM phases, with help of an electric field application.

cond-mat.mtrl-sci↗

arXiv2013

Low-symmetry monoclinic phase stabilized by oxygen octahedra rotations in thin strained EuxSr1-xTiO3 films

Using Landau-Ginzburg-Devonshire theory and phase field modeling, we explore the complex interplay between the long-range structural order parameter (oxygen octahedron rotations) and polarization in EuxSr1-xTiO3 thin epitaxial films. In biaxially tensile strained films, we discover the presence of a low symmetry monoclinic phase with in-plane ferroelectric polarization that is stabilized by antiferrodistortive oxygen octahedra tilts. The monoclinic phase is stable in a wide temperature range, and is characterized by the large number of energetically equivalent polar and structural twin domains, which stimulates easy twinning of the film and thus enhances its effective piezoelectric response. The flexoelectric coupling and rotostriction give rise to additional spontaneous polarization, piezo- and pyro-electricity in the ferroelastic twin boundaries.

cond-mat.mtrl-sci↗

arXiv2013

Effective piezoelectric response of twin walls in ferroelectrics

The effective piezoelectric coefficients of twin walls in tetragonal ferroelectric are calculated in the framework of decoupling approximation and Landau-Ginzburg-Devonshire theory allowing for polarization gradient terms, electrostriction and flexoelectric coupling. Using an example of piezoelectric response of a1-a2 twins to a homogeneous electric field, we show that the response is almost independent on the flexoelectric coupling, but is very sensitive to the values of polarization gradient coefficients. This behavior originates from the strong coupling between local dielectric susceptibility and the gradient coefficients. The enhancement of piezoelectric response from 10% up to a factor of 103 times is predicted. The local electromechanical response of the domain walls can thus provide information on the gradient terms in GLD expansion and pinning mechanisms of the ferroelectric domain walls. The observability of these effects by the piezoresponse force microscopy of electroded structures and impact on the functional properties of the systems with dense domain structures is analyzed

cond-mat.mtrl-sci↗

arXiv2012

Pyroelectric origin of the carrier density modulation at graphene-ferroelectric interface

Using continuous approximation we study the static and high-frequency heat dissipation in multi-layer graphene on a ferroelectric. We demonstrate that the Joule heating effect, caused by a high-frequency ac electric current in graphene, creates a pronounced temperature gradient in a ferroelectric substrate. The pyroelectric effect transforms the gradient into the spontaneous polarization gradient.. Therefore, the high-frequency depolarizing electric field occurs and penetrates in the multi-layer graphene. Free charges in graphene immediately screen the electric field and thus their density oscillates at high-frequency. Performed calculations had proved that the pyroelectric effect can modify essentially the free carrier density at the graphene-ferroelectric interface and consequently the conductivity of multi-layer graphene channel. So, pyroelectric mechanism can be critical for understanding of the complex physical thermal and electrical processes taking place across and along graphene-ferroelectric interfaces at terahertz frequencies.

cond-mat.mtrl-sci↗

arXiv2012

New multiferroics based on EuxSr1-xTiO3 nanotubes and nanowires

Using Landau-Ginzburg-Devonshire theory we study the complex interplay between structural antiferrodistortive order parameter (oxygen octahedron rotations), polarization and magnetization in EuxSr1-xTiO3 nanotubes and nanowires. We calculated EuxSr1-xTiO3 bulk and nanosystem phase diagrams, which include the antiferrodistortive, ferroelectric, ferromagnetic and antiferromagnetic phases. Our calculation shows the presence of "triple" antiferrodistortive-ferroelectric-ferromagnetic phases for EuxSr1-xTiO3 nanowires and nanotubes. presents interest for fundamental study and can be important for their potential applications. Since the triple phase exists at low temperatures (<10 K) with high polarization and magnetization values (~50 muC/cm2 and ~0.5 MA/m2). Therefore, the strong coupling between structural distortions, polarization and magnetization suggest the EuxSr1-xTiO3 nanosystems as strong candidates for possible miltiferroic applications.

cond-mat.mtrl-sci↗

arXiv2012

Correlation Between Structure And C-Afm Contrast Of 180-Degree Domain Walls In Rhombohedral Bati03

Using Landau-Ginzburg-Devonshire theory we describe 180-degree domain wall structure, intrinsic energy and carrier accumulation in rhombohedral phase of BaTiO3 as a function of the wall orientation and flexoelectric coupling strength. Two types of domain wall structures (phases of the wall) exist depending on the wall orientation. The low-energy 'achiral' phase occurs in the vicinity of the {110} wall orientation and has odd polarization profile invariant with respect to inversion about the wall center. The second 'chiral' phase occurs around {211} wall orientations and corresponds to mixed parity domain walls that may be of left-handed or right-handed chirality. The transformation between the phases is abrupt, accompanied with 20-30% change of the domain wall thickness and can happen at fixed wall orientation with temperature change. We suggest that the phase transition may be detected through domain wall thickness change or by c-AFM. The structure of the domain wall is correlated to its conductivity through polarization component normal to the domain wall, which causes free carriers accumulation. Depending on the temperature and flexoelectric coupling strength relative conductivity of the wall becomes at least one order of magnitude higher than in the single-domain region, creating c-AFM contrast enhancement pronounced and detectable.

cond-mat.mtrl-sci↗

arXiv2012

Anisotropic conductivity of uncharged domain walls in BiFeO3

Experimental observations suggest that nominally uncharged, as-grown domain walls in ferroelectric thin films can be conductive, yet comprehensive theoretical models to explain this behavior are lacking. Here, Landau theory is used to evolve an analytical treatment of the anisotropic carrier accumulation by nominally uncharged domain walls in multiferroic BiFeO3. Strong angular dependence of the carrier accumulation by 180-degree domain walls originates from local band bending via angle-dependent electrostriction and flexoelectric coupling mechanisms. Theoretical results are in qualitative agreement with experimental data, and provide a Landau-Ginzburg-Devonshire counterpart that is consistent with recent first principles calculations. These studies suggest that a significantly more diverse range of domain wall structures could possess novel electronic properties than previously believed. Similarly, emergent electronic behaviors at ferroic walls are typically underpinned by multiple mechanisms, necessitating first-principle studies of corresponding coupling parameters.

cond-mat.mtrl-sci↗

arXiv2012

Conductivity of twin walls - surface junctions in ferroelastics: interplay of deformation potential, octahedral rotations, improper ferroelectricity and flexoelectric coupling

Electronic and structural phenomena at the twin domain wall-surface junctions in the ferroelastic materials are analyzed. Carriers accumulation caused by the strain-induced band structure changes originated via the deformation potential mechanism, structural order parameter gradient, rotostriction and flexoelectric coupling is explored. Approximate analytical results show that inhomogeneous elastic strains, which exist in the vicinity of the twin walls - surface junctions due to the rotostriction coupling, decrease the local band gap via the deformation potential and flexoelectric coupling mechanisms. This is the direct mechanism of the twin walls static conductivity in ferroelastics and, by extension, in multiferroics and ferroelectrics. On the other hand, flexoelectric and rotostriction coupling leads to the appearance of the improper polarization and electric fields proportional to the structural order parameter gradient in the vicinity of the twin walls - surface junctions. The "flexo-roto" fields leading to the carrier accumulation are considered as indirect mechanism of the twin walls conductivity. Comparison of the direct and indirect mechanisms illustrates complex range of phenomena directly responsible for domain walls static conductivity in materials with multiple order parameters.

cond-mat.mtrl-sci↗