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