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Title: Domain topologies in nanoscale single-crystal ferroelectrics
Author: McGilly, L. J.
ISNI:       0000 0004 2720 9461
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2012
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An investigation into domain states' that form spontaneously innanoscale single-crystal structures of BaTi03 and PbZr(042)Ti(o58P3 on cooling through the Curie temperature has been conducted. Through the use of Piezoresponse Force Microscopy (PFM) and Transmission Electron Microscopy (TEM) a hierarchy of structure existing over distinct length scales has been observed revealing large-scale 'superdomains' which are composed of finer-scale 90° stripe 'subdomains'. "The nature of the formation of these superdomains is investigated by considering a higher-level polar ordering to perform roles usually associated with subdomains. In BaTi03 " these superdomains can be considered to possess a resultant polarisation orientated along (110)pseudocubic directions; using PFM to map the resultant polarisations demonstrated that 60°, 90° and 180° superdomain structures can form. Characteristics of thesuperdomains appear to have more in common with an orthorhombic symmetry than with the fine-scale parent tetragonal state. This is rationalised from the orientation of the resultant polarisation and the observed superdomain variants which are generally only seen for orthorhombic systems. Detailed arialysis of the superdomain boundaries reveals that they typically occupy crystallographically defined directions. Complex topologies of domain walls are also seen at superdomain boundaries occasionally including interlinking chains of flux-closure and quadrupole states. For the 180° superdomains composed of a-a subdomains observed in BaTi03 single- crystal free-standing dots, produced by Focused Ion Beam (FIB) milling, their formation is thought to be a response to depolarising fields that should be present in the plane of the structure. This is determined from superdomain scaling adherence to a Landau-Kittel-type relation. A free energy analysis is used to estimate the 180° domain wall energy density to be 20 ± 2 mlm" and to explain the features of the Landau-Kittel-type scaling. Nanoscale structures of wires and dots were produced by means of FIB milling of single-crystal lamellar sheets of PbZr(042)Ti(o58P3' Subsequent investigation through use of TEM revealed complex domain structures that displayed superdomain features. For PbZr(042)Ti(o.58P3 single-crystal nanodots, in the majority of cases, 90° stripe domains were found to form into four distinct 'bundles' or quadrants. Detailed analysis of the dipole orientations in the system was undertaken and led to the conclusion that resultant polarisations, associated with the four quadrant domain bundles, form into a closed loop. This 'polarisation closure' pattern appears to be a highly stable, equilibrium state.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available