REPOSITORY > RESULTS

Abstract

Ferroelectric domain walls (DWs) are nano-to-atomic-scale interfaces that separate
domains of different polar order. What makes DWs so interesting is their dynamic nature
in an electric field, which offers an exciting potential for nanoscale and atomic-scale
applications, but, also, more generally, the dynamics of DWs is linked to a fundamental
understanding of ferroelectricity. The aim of this PhD thesis was to study DWs in leadfree
ferroelectric bulk systems under static and dynamic conditions on the local scale by
(scanning)-transmission electron microscopy-(S)TEM.
In the first part of the thesis a comparative study between step-like uncharged and
charged DWs in polycrystalline BiFeO3 is performed in terms of strain and structure. Our
results show that uncharged DWs exhibit a higher and more concentrated strain than their
charged counterparts. The lower strain of charged DWs is associated with a wider transition
(i.e. change in directions and magnitude) of polarization vector across the DWs. All the
step-like DWs reported here, regardless of their charge state, exhibit a comparable number
of Bi vacancies and present a non-Ising behavior.
The second part presents a dynamic study of the atomic-level response of zigzag DWs
in BiFeO3 single crystals in a subcoercive field regime. The atomic-level movement of a
single DW was observed. One of the foremost results is that, on the atomic scale, the
movement of the wall under an electric field is decoupled from the strain field marking the
initial position of the wall. We could explain this by the segregation of defects with a
relatively low mobility, such as clusters of oxygen vacancies. Furthermore, the triangular
tip of the zigzag DW is pinned, but a short-range change in its properties occurs:
asymmetry is induced in the wall plane leading to strain, bound charge and a possible
redistribution of the Bi-vacancies.
The last part of the thesis presents the results for the kinetics of domains in (K,Na)NbO3
single crystals. The dynamics of the mobile, needle-like DWs is influenced by other
immobile walls, which act as random bound defects. Domain growth and coalescence may
cease at a certain voltage with a subsequent increase in the number of DWs due to splitting
of the needles and the formation of nanodomains. Pinning events and domain-domain
interactions between two orthogonal, needlelike domains are directly probed. Our results
suggest that this interaction is mediated by strain.