Eugene A. Eliseev

Direct imaging of the spatial and energy distribution of nucleation centres in ferroelectric materials

Macroscopic ferroelectric polarization switching, similar to other first-order phase transitions, is controlled by nucleation centres. Despite 50 years of extensive theoretical and experimental effort, the microstructural origins of the Landauer paradox, that is, the experimentally observed low values of coercive fields in ferroelectrics corresponding to implausibly large nucleation activation energies, are still a mystery. Here, we develop an approach to visualize the nucleation centres controlling polarization switching processes with nanometre resolution, determine their spatial and energy distribution and correlate them to local microstructure. The random-bond and random-field components of the disorder potential are extracted from positive and negative nucleation biases. Observation of enhanced nucleation activity at the 90 composite function domain wall boundaries and intersections combined with phase-field modelling identifies them as a class of nucleation centres that control switching in structural-defect-free materials.

Observation of a periodic array of flux-closure quadrants in strained ferroelectric PbTiO3 films

Getting closure in ferroelectric films Ferroelectric materials have a spontaneous electric polarization that can be manipulated for applications. The polarization is usually not uniform throughout the material, and for nanosized ferroelectrics, polarization can be quite complex. Using scanning transmission electron microscopy, Tang et al. found that in thin films of the ferroelectric PbTiO3, the polarization vector rotated in space, forming a closed loop, the so-called flux closure. The flux closure structures formed an array, with the period dependent on the width of the thin film, and caused the buildup of considerable strain within the crystal lattice of the material Science, this issue p. 547 Scanning transmission electron microscopy is used to observe closed polarization loops in the ferroelectric PbTiO3. Nanoscale ferroelectrics are expected to exhibit various exotic domain configurations, such as the full flux-closure pattern that is well known in ferromagnetic materials. Here we observe not only the atomic morphology of the flux-closure quadrant but also a periodic array of flux closures in ferroelectric PbTiO3 films, mediated by tensile strain on a GdScO3 substrate. Using aberration-corrected scanning transmission electron microscopy, we directly visualize an alternating array of clockwise and counterclockwise flux closures, whose periodicity depends on the PbTiO3 film thickness. In the vicinity of the core, the strain is sufficient to rupture the lattice, with strain gradients up to 109 per meter. Engineering strain at the nanoscale may facilitate the development of nanoscale ferroelectric devices.

Local probing of ionic diffusion by electrochemical strain microscopy: Spatial resolution and signal formation mechanisms

Electrochemical insertion-deintercalation reactions are typically associated with significant change in molar volume of the host compound. This strong coupling between ionic currents and strains underpins image formation mechanisms in electrochemical strain microscopy (ESM), and allows exploring the tip-induced electrochemical processes locally. Here we analyze the signal formation mechanism in ESM, and develop the analytical description of operation in frequency and time domains. The ESM spectroscopic modes are compared to classical electrochemical methods including potentiostatic and galvanostatic intermittent titration, and electrochemical impedance spectroscopy. This analysis illustrates the feasibility of spatially resolved studies of Li-ion dynamics on the sub-10-nm level using electromechanical detection.

Direct observation of ferroelectric field effect and vacancy-controlled screening at the BiFeO3/LaxSr1−xMnO3 interface

The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.

Tunable Metallic Conductance in Ferroelectric Nanodomains

Metallic conductance in charged ferroelectric domain walls was predicted more than 40 years ago as the first example of an electronically active homointerface in a nonconductive material. Despite decades of research on oxide interfaces and ferroic systems, the metal-insulator transition induced solely by polarization charges without any additional chemical modification has consistently eluded the experimental realm. Here we show that a localized insulator-metal transition can be repeatedly induced within an insulating ferroelectric lead-zirconate titanate, merely by switching its polarization at the nanoscale. This surprising effect is traced to tilted boundaries of ferroelectric nanodomains, that act as localized homointerfaces within the perovskite lattice, with inherently tunable carrier density. Metallic conductance is unique to nanodomains, while the conductivity of extended domain walls and domain surfaces is thermally activated. Foreseeing future applications, we demonstrate that a continuum of nonvolatile metallic states across decades of conductance can be encoded in the size of ferroelectric nanodomains using electric field.

Phase transitions induced by confinement of ferroic nanoparticles

A general approach for considering primary ferroic (ferroelectric, ferromagnetic, ferroelastic) nanoparticle phase transitions was proposed in phenomenological theory framework. The surface stress, order parameter gradient, and striction, as well as depolarization, demagnetization, and de-elastification effects, were included into the free energy. The strong intrinsic surface stress under the curved nanoparticle surface was shown to play the important role in the shift of transition temperature (if any) up to the appearance of a new ordered phase absent in the bulk ferroic. Euler-Lagrange equations obtained after the Landau-Ginzburg-Devonshire free energy minimization were solved by direct variational method. This leads to the conventional form of the free energy with renormalized coefficients depending on nanoparticle sizes, surface stress, and electrostriction tensor values, and so opens the way for polar property calculations by algebraic transformations. Surface piezoeffect causes built-in electric field that induces an electretlike polar state and smears the phase transition point. The approximate analytical expression for the size-induced ferroelectric transition temperature dependence on cylindrical or spherical nanoparticle sizes, polarization gradient coefficient, correlation radius, intrinsic surface stress, and electrostriction coefficient was derived. Under the favorable conditions (radius of

The depolarization field effect on the thin ferroelectric films properties

5\char21{}50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}

Atomic-scale evolution of modulated phases at the ferroelectric–antiferroelectric morphotropic phase boundary controlled by flexoelectric interaction

and compressive surface stress), spatial confinement induces a ferroelectric phase in incipient ferroelectric nanowires and nanospheres. The prediction of size-induced ferroelectricity in

Electromechanical detection in scanning probe microscopy : Tip models and materials contrast

\mathrm{K}\mathrm{Ta}{\mathrm{O}}_{3}

Interplay of Octahedral Tilts and Polar Order in BiFeO3 Films

nanorods with radius less than