Hadjimichael, Marios; (2019) Ferroelectric domains in lead titanate heterostructures. Doctoral thesis (Ph.D), UCL (University College London).

Abstract

This thesis focuses on the properties of thin films and multilayers of lead titanate (PbTiO3), an archetypal ferroelectric. Using off-axis radiofrequency magnetron sputtering, PbTiO3 thin films and superlattices are fabricated on different substrates, allowing control of the domain structures by tailoring electrostatic and mechanical boundary conditions. The domain structures and the macroscopic properties of these materials are then characterised using laboratory and synchrotron X-ray diffraction, piezoresponse force microscopy and dielectric impedance spectroscopy. The thesis initially focuses on synchrotron X-ray diffraction studies of ferroelectric domains in PbTiO3/SrTiO3 superlattices, periodic repetitions of ferroelectric PbTiO3 and dielectric SrTiO3 bilayers. It studies how the orientations of ferroelectric domain walls behave as a function of temperature across the ferroelectric phase transition and how they behave locally as a function of local strains and strain gradients (probed with a nanofocused X-ray beam), results that give more insight into their complex nature. The focus then turns to ferroelectric/metal superlattices (comprised of PbTiO3 and metallic SrRuO3 layers). It’s shown that even though the SrRuO3 layers remain metallic down to about four unit cells, the finite screening length at the interface between the metal and the ferroelectric creates a depolarising field which forces the PbTiO3 layers to adopt a polydomain configuration. These systems then allow us to study the macroscopic properties (e.g. electric permittivity) of ultrathin PbTiO3 layers, while at the same time making it possible to create novel domain configurations.

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