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Title: Improving the functional control of ferroelectrics using insights from atomistic modelling
Author: Chapman, Jacob Bernard John
ISNI:       0000 0004 7429 2819
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2018
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Lead zirconate titanate is a ferroelectric material of considerable interest with a wide range of technological applications. It has been the subject of many experimental and theoretical studies yet there are a number of unsolved questions preventing further miniaturisation and optimisation of this and other ferroelectric materials. Exotic ultra-dense domain morphologies, as an example, offer an exciting avenue for the development of novel nanoelectronics. In this work, large scale molecular dynamics is used to construct a strain-temperature phase diagram of the domain morphology of PbTiO3 ultrathin films. By sampling a wide range of strain values over a temperature range up to the Curie temperature, it is found that epitaxial strain induces the formation of a variety of closure- and in-plane domain morphologies. The local strain and ferroelectric-antiferrodistortive coupling at the film surface vary for the strain mediated transition sequence and this could offer a route for experimental observation of the morphologies. Remarkably, a new nanobubble domain morphology is identified that is stable in the high-temperature regime for compressively strained PbTiO3. It is demonstrated that the formation mechanism of the nanobubble domains morphology is related to the wandering of flux closure domain walls, which is characterised using the hypertoroidal moment. Molecular dynamics calculations, supplemented with electrical measurements from collaborators, are used to provide insight into the microscopic switching properties of near-morphotropic PZT. The simulations and experiments exhibit qualitatively similar hysteretic behaviour of the polarisation at different temperatures, showing widening of the Polarisation - Electric field hysteresis loops, and the decrease of the coercive field towards high temperatures. Remarkably, polarisation switching at low temperatures is shown to occur via a polarisation rotation and growth mechanism that is fundamentally different from the high temperature switching, where nucleation is rate limiting. Analysis of B-cation contributions show that nucleation and switching are facilitated by Zr centred unit cells and, by extension, Ti centred unit cells in Zr-rich environments. Ti-rich clusters in morphotropic PZT, at low temperature, are observed to have suppressed ferroelectric displacements which may incorrectly be perceived as ferroelectrically inactive `dead-layers'. Finally, fundamental insight into the microscopic mechanisms of the ageing processes are provided. From simulations of the prototypical ferroelectric material PbTiO3, it is demonstrated that experimentally observed ageing phenomena can be reproduced from intrinsic interactions of defect-dipoles related to dopant-vacancy associates, even in the absence of extrinsic effects. Variation of the dopant concentration is shown to modify the material's hysteretic response, identifying a universal method to reduce loss and tune the electromechanical properties of inexpensive ceramics for efficient technologies.
Supervisor: Duffy, D. ; Kimmel, A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available