12 research outputs found
Nanoferroics: state of art, gradient driven couplings and advanced applications (Authors' review)
Ferroics and multiferroics are unique objects for fundamental physical
research of complex nonlinear processes and phenomena, which occur in them in
micro and nanoscale. Due to the possibility of their physical properties
control by size effects, nanostructured and nanosized ferroics are among the
most promising for advanced applications in nanoelectronics,
nanoelectromechanics, optoelectronics, nonlinear optics and information
technologies. The review discuss and analyze that the thickness of the strained
films, the size and shape of the ferroic and multiferroic nanoparticles are
unique tools for controlling their phase diagrams, long range order parameters,
magnitude of susceptibility, magnetoelectric coupling and domain structure
characteristics at fixed temperature. Significant influence of the
flexochemical effect on the phase transition temperature, polar and dielectric
properties is revealed for thin films and nanoparticles. Obtained results are
important for understanding of the nonlinear physical processes in nanoferroics
as well as for the advanced applications in nanoelectronics.Comment: 23 pages, 15 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
Fixed Volume Effect on Polar Properties and Phase Diagrams of Ferroelectric Semi-ellipsoidal Nanoparticles
For advanced applications in modern industry it is very important to reduce
the volume of ferroelectric nanoparticles without serious deterioration of
their polar properties. In many practically important cases fixed volume
(rather than fixed size) corresponds to realistic technological conditions of
nanoparticles fabrication. The letter is focused on the theoretical study of
the behavior of ferroelectric polarization, paramagnetoelectric coefficient and
phase diagrams of semi-ellipsoidal nanoparticles with fixed volume V. Our
approach combines the Landau-Ginzburg-Devonshire phenomenology, classical
electrostatics and elasticity theory. Our results show that the size effects of
the phase diagrams and polarization of semi-ellipsoidal BiFeO3 nanoparticles
nontrivially depends on V. These findings provide a path to optimize the polar
properties of nanoparticles by controlling their phase diagrams at a fixed
volume.Comment: 15 pages, 5 figures, we added the section IV. Paramagnetoelectric
(PME) coefficient at fixed volume in this version and changed title and
abstract accordingl
Ferromagnetic-like behavior of Bi0.9La0.1FeO3-KBr nanocomposites
We studied magnetostatic response of the Bi0.9La0.1FeO3-KBr composites
(BLFO-KBr) consisting of nanosized (about 100 nm) ferrite Bi0.9La0.1FeO3 (BLFO)
conjugated with fine grinded ionic conducting KBr. When the fraction of KBr is
rather small (less than 15 wt percent) the magnetic response of the composite
is very weak and similar to that observed for the BLFO (pure KBr matrix without
Bi1-xLaxFeO3 has no magnetic response as anticipated). However, when the
fraction of KBr increases above 15percent, the magnetic response of the
composite changes substantially and the field dependence of magnetization
reveals ferromagnetic-like hysteresis loop with a remanent magnetization about
0.14 emu/g and coercive field about 1.8 Tesla (at room temperature). Nothing
similar to the ferromagnetic-like hysteresis loop can be observed in BLFO
ceramics, which magnetization quasi linearly increases with magnetic field.
Different physical mechanisms were considered to explain the unusual
experimental results for BLFO-KBr nanocomposites, but only those among them,
which are highly sensitive to the interaction of antiferromagnetic
Bi0.9La0.1FeO3 with ionic conductor KBr, can be relevant. An appropriate
mechanism turned out to be ferro-magneto-ionic coupling.Comment: 24 pages, 4 figures in the main text, and supplement with 4 figure
Flexo-Elastic Control Factors of Domain Morphology in Core-Shell Ferroelectric Nanoparticles: Soft and Rigid Shells
International audienceWithin the framework of the Landau-Ginzburg-Devonshire approach we explore the impact of elastic anisotropy, electrostriction, flexoelectric couplings, and mismatch strain on the domain structure morphology in ferroelectric core-shell nanoparticles of spherical shape. We perform finite element modelling (FEM) for multiaxial ferroelectric nanoparticle cores covered with an elastically-isotropic soft or elastically-anisotropic rigid paraelectric shell, with and without mismatch strains. In the case of a core covered with a soft shell, the FEM results show that at room temperature a single polarization vortex with a dipolar kernel can be stable if the electrostriction coupling is weak. With increasing anisotropic electrostriction coupling, the vortex disappears and is replaced by complex flux-closure structures, which are formed in the equatorial plane and transform into an elongated vortex with a central 180-degree domain wall near the core poles. This complex domain morphology develops in the core due to the anisotropic electrostriction, and the flexoelectric coupling leads to an additional curvature and twist of the polarization isosurfaces. In contrast to this, FEM performed for a core covered with a rigid shell shows that, at room temperature, the anisotropic elastic properties of the shell can stabilize vortex-like structures with three flux-closure domains, which gradually cross in the equatorial plane of the core and transform into 120-degree domains near the core poles. The flexoelectric coupling leads to a noticeable curling of the flux-closure domain walls. A mismatch strain compensates the curling of the flux-closure domains in the core confined by the elastically-anisotropic rigid shell. Our analysis reveals different types of topological defects, namely Bloch point structures (BPS) and Ising lines, that form in a ferroelectric core covered with a soft or rigid shell