Defect driven flexo-chemical coupling in thin ferroelectric films

Abstract

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.

25 pages, 9 figures

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• Melting of Spatially Modulated Phases in La-doped BiFeO3 at Surfaces and Surface-Domain Wall Junctions
• Adsorption of ions from aqueous solutions by ferroelectric nanoparticles
• Mesoscopic structure of mixed type domain walls in multiaxial ferroelectrics
• Nontrivial temperature behavior of the carrier concentration in the nanostructure "graphene channel on ferroelectric substrate with domain walls"
• Flexoelectric Polarization Enhancement in Paraelectric $\mathrm{BaHf}{\mathrm{O}}_3$ via Strain Gradient Engineering
• Control of domain states in rhombohedral PZT films via misfit strains and surface charges
• Intrinsic structural instabilities of domain walls driven by gradient couplings: meandering anferrodistortive-ferroelectric domain walls in BiFeO3
• Control of polarization hysteresis temperature behavior by interfacial screening in thin ferroelectric films