P. V. Yudin

Fundamentals of flexoelectricity in solids

The flexoelectric effect is the response of electric polarization to a mechanical strain gradient. It can be viewed as a higher-order effect with respect to piezoelectricity, which is the response of polarization to strain itself. However, at the nanoscale, where large strain gradients are expected, the flexoelectric effect becomes appreciable. Besides, in contrast to the piezoelectric effect, flexoelectricity is allowed by symmetry in any material. Due to these qualities flexoelectricity has attracted growing interest during the past decade. Presently, its role in the physics of dielectrics and semiconductors is widely recognized and the effect is viewed as promising for practical applications. On the other hand, the available theoretical and experimental results are rather contradictory, attesting to a limited understanding in the field. This review paper presents a critical analysis of the current knowledge on the flexoelectricity in common solids, excluding organic materials and liquid crystals.

Controlling domain wall motion in ferroelectric thin films

Domain walls in ferroic materials have attracted significant interest in recent years, in particular because of the unique properties that can be found in their vicinity. However, to fully harness their potential as nanoscale functional entities, it is essential to achieve reliable and precise control of their nucleation, location, number and velocity. Here, using piezoresponse force microscopy, we show the control and manipulation of domain walls in ferroelectric thin films of Pb(Zr,Ti)O₃ with Pt top electrodes. This high-level control presents an excellent opportunity to demonstrate the versatility and flexibility of ferroelectric domain walls. Their position can be controlled by the tuning of voltage pulses, and multiple domain walls can be nucleated and handled in a reproducible fashion. The system is accurately described by analogy to the classical Stefan problem, which has been used previously to describe many diverse systems and is here applied to electric circuits. This study is a step towards the realization of domain wall nanoelectronics utilizing ferroelectric thin films.

Controlled stripes of ultrafine ferroelectric domains

In the pursuit of ferroic-based (nano)electronics, it is essential to minutely control domain patterns and domain switching. The ability to control domain width, orientation and position is a prerequisite for circuitry based on fine domains. Here, we develop the underlying theory towards growth of ultra-fine domain patterns, substantiate the theory by numerical modelling of practical situations and implement the gained understanding using the most widely applied ferroelectric, Pb(Zr,Ti)O3, demonstrating controlled stripes of 10 nm wide domains that extend in one direction along tens of micrometres. The observed electrical conductivity along these thin domains embedded in the otherwise insulating film confirms their potential for electronic applications.

Upper bounds for flexoelectric coefficients in ferroelectrics

Flexoelectric effect is the response of electric polarization to the mechanical strain gradient. At the nano-scale, where large strain gradients are expected, the flexoelectric effect becomes appreciable and may substitute piezoelectric effect in centrosymmetric materials. These features make flexoelectricity of growing interest during the last decade. At the same time, the available theoretical and experimental results are rather contradictory. In particular, experimentally measured flexoelectric coefficients in some ferroelectric materials largely exceed theoretically predicted values. Here, we determine the upper limits for the magnitude of the static bulk contribution to the flexoelectric effect in ferroelectrics, the contribution which was customarily considered as the dominating one. The magnitude of the upper bounds obtained suggests that the anomalously high flexoelectric coupling documented for perovskite ceramics can hardly be attributed to a manifestation of the static bulk effect.

Room temperature concurrent formation of ultra-dense arrays of ferroelectric domain walls

Properties of ferroelectric domain walls are attractive for future nano- and optoelectronics. An important element is the potential to electrically erase/rewrite domain walls inside working devices. Dense domain wall patterns, formed upon cooling through the ferroelectric phase transition, were demonstrated. However, room temperature domain wall writing is done with a cantilever tip, one domain stripe at a time, and reduction of the inter-wall distance is limited by the tip diameter. Here, we show, at room temperature, controlled formation of arrays of domain walls with sub-tip-diameter spacing (i.e., inter-wall distance down to approximate to 10 nm). Each array contains 100-200 concurrently formed walls. Array rewriting is confirmed. The method is demonstrated in several materials. Dense domain pattern formation through a continuous electrode, practical for potential device applications, is also demonstrated. A quantitative theory of the phenomenon is provided

Impact of Strain Effects on the Stability of Ising Walls in Ferroelectrics

The stability of Ising 180-degree domain walls with respect to appearance of chirality is analytically studied in tetragonal perovskite-type ferroelectrics taking into account electro-mechanical coupling. It is shown that the widely used approximation neglecting the elastic effects may lead to qualitatively wrong results. Sufficient condition for the stability of Ising walls is formulated only in terms of elastic and electrostrictive properties, regardless the correlation energy. We demonstrate that elasticity stabilizes Ising walls in tetragonal Pb(Zr1-cTic)O3 and makes chiral walls unfavorable for any composition of the material (factor c), while nonelastic model predicts chirality of 180-degree walls near the morphotropic boundary.

Charged Domain Walls in Ferroelectrics

Charged Domain Walls (CDWs) in ferroelectrics are compositionally homogeneous interfaces, some of which display metallic-like conductivity and can be created, displaced and erased inside a monolith of nominally insulating materials. Such CDWs are promising electronic elements for reconfigurable nanoelectronics. This chapter introduces types of CDWs, their theoretically predicted and experimentally observed properties, and methods of their artificial engineering.

Thick domain walls in non-magnetic ferroics

ABSTRACT Recent developments in the physics of non-magnetic ferroics identified possible functionality of domain walls in these materials, which specifies the current interest to the internal structure of the walls. In this context, the wall thickness itself can be regarded as an important parameter of walls as functional elements, raising the following questions. How thick can ferroelectric domain walls be? To what extent can their thickness be controlled? In answering these questions, we discuss mechanisms controlling the wall thickness in non-magnetic ferroics for different types of the walls. Besides, we provide an overview of the recent results on anomalously thick ferroelectric domain walls.

Erratum: “Upper bounds for flexoelectric coefficients in ferroelectrics” [Appl. Phys. Lett. 104, 082913 (2014)]

Reference EPFL-ARTICLE-212631doi:10.1063/1.4919883View record in Web of Science Record created on 2015-09-28, modified on 2017-11-27

Moving antiphase boundaries using an external electric field

Antiphase boundaries (APBs) are unique domain walls that may demonstrate switchable polarization in otherwise non-ferroelectric materials such as SrTiO3 and PbZrO3. The current study explores the possibility of displacing such domain walls at the nanoscale. We suggest the possibility of manipulating APBs using the inhomogeneous electric field of an Atomic Force Microscopy (AFM) tip with an applied voltage placed in their proximity. The displacement is studied as a function of applied voltage, film thickness, and initial separation of the AFM tip from the APB. It is established, for example, that for films with thickness of 15 nm, an APB may be attracted under the tip with a voltage of 25 V from initial separation of 30 nm. We have also demonstrated that the displacement is appreciably retained after the voltage is removed, rendering it favorable for potential applications.