%0 Thesis
%9 Doctoral
%A Zatterin, Edoardo M.
%B Physics and Astronomy
%D 2025
%F discovery:10204785
%I UCL (University College London)
%P 245
%T Superdomains in ferroelectric thin films
%U https://discovery.ucl.ac.uk/id/eprint/10204785/
%X 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.
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