TY  - UNPB
Y1  - 2025/02/28/
AV  - public
EP  - 245
TI  - Superdomains in ferroelectric thin films
N1  - Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author?s request.
PB  - UCL (University College London)
UR  - https://discovery.ucl.ac.uk/id/eprint/10204785/
N2  - Below a critical temperature, Tc, ferroelectric materials develop a spontaneous electric
polarization, which can be switched by an external electric field. Most ferroelectrics are
also ferroelastic, so that below Tc a spontaneous strain also appears. The resulting longrange
elastic and electric fields are minimized by the formation of domains, regions
of material with uniform order parameter (polarisation or strain) that changes sign or
direction across a domain wall.
When ferroelectrics are grown as epitaxial thin films, the mechanical clamping to the
substrate induces a biaxial stress in the film, which favours the formation of complex,
dense nanodomain structures. Peculiar functional properties of technological interest
emerge depending on the exact domain arrangement and crystallography, which can
be controlled by appropriate processing parameters.
In some cases this leads to a hierarchical arrangement of periodic polytwin structures
into bundles, or ?superdomain?. Different superdomains contain twins of different
crystallographic orientations and sizes. Compared to the more commonly observed
?cellular polytwin? case, the presence of such hierarchical bundles is seen to
enhance the response of the overall domain structure to external stimuli. Deterministic
re-configuration of domain arrangements under applied fields is crucial for the
exploitation of novel functionalities found at domain walls in potential future nanoelectronic
devices, making these systems excellent candidates for the task. Most studies
to date have however focussed on certain consequences of the presence of hierarchical
domain arrangements, without dwelling on the unconventional crystallography of
such structures or the exact nature of its response to applied field or temperature.
In this thesis, the structure of a prototypical superdomain ferroelectric thin film is investigated
using a combination of laboratory and synchrotron diffraction and piezoresponse
force microscopy (PFM) in device-like conditions, and the peculiar crystallography
of superdomains is elucidated both in the as-grown case, as well as following
external excitations (electric field and temperature). Chapter 1 gives a general overview of the fundamental properties of ferroic materials
and domains, while Chapter 2 focusses specifically on perovskite ferroelectric oxide
thin films, introducing the mechanisms that lead to the formation of dense polytwin
structures and superdomains in these systems. Chapter 3 describes the experimental
and data analysis techniques used in this work, providing the tools to correctly interpret
the complicated features of the diffraction patterns and PFM images shown
in this thesis. The complex crystallography of superdomains is investigated in detail
in Chapter 4; their stability is subsequently assessed in Chapter 5, which discusses
the results of a temperature-dependent study. Finally, Chapter 6 demonstrates that reversible
and non-volatile ferroelastic superdomain switching occurs in different ways
depending on the boundary conditions imposed to the system.
ID  - discovery10204785
A1  - Zatterin, Edoardo M.
M1  - Doctoral
ER  -