Investigating the fabrication and dipole characteristics of complex ferroelectric nanoshapes

Lead Research Organisation: Queen's University Belfast
Department Name: Sch of Mathematics and Physics

Abstract

The traditional view of the ordering of polarisation or magnetisation in both ferroelectrics and ferromagnets is that local dipoles or magnetic moments are arranged into neat rows and columns, and that boundaries between neatly arranged groups must strictly conform to the crystallography of the host material (conventional stripe domains). However, recent experimental research in three-dimensionally size-constrained soft ferromagnets has revealed the existence of completely different domain states which form into vortices. As with many aspects of behaviour in ferromagnetism, analogous properties in the behaviour of the electrical polarisation in ferroelectrics is often seen, and recent modelling strongly suggests that such vortex domain states should also exist in ferroelectrics. Differences in the energetics between ferromagnets and ferroelectrics means that such unusual behaviour is only expected to dominate whenever ferroelectric dimensions are reduced to the order of ~10 nm. The creation of such small structures and the characterisation of their domain states represents a serious challenge to experimentalists involved in ferroelectric research and yet the potential for new discovery is immense. Further, simple vortex structures may only be the tip of the ice-berg, as much more exotic domain patterns have been postulated: for example some theorists have suggested the possibility of an electrostatic solenoid-analogue. Given the research performed to date, and the postulations made by theorists, the creation of three-dimensionally constrained nanostructures in ferroelectrics, and the subsequent analysis of their domain characteristics, clearly represents an exciting and challenging problem. This project will address this area of research by combining expertise in nanoscale ferroelectric fabrication with specialist characterisation techniques such as electron holography, second-harmonic near field optics, nano-Raman spectroscopy and scanning probe microscopy. The programme builds on an already established successful collaboration between ferroelectric activities in Queen's University Belfast and Cambridge, and this is augmented by international experts in specifically chosen characterisation techniques.

Publications

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Byrne D (2008) Ordered arrays of lead zirconium titanate nanorings. in Nanotechnology

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Burns S (2016) Nanostructuring Ferroelectrics via Focused Ion Beam Methodologies in Advanced Functional Materials

 
Description A great deal of novel research was undertaken during this project; however, the take-home message is simple: that domain patterns definitely change in response to exterior morphology in nanodots, forming beautiful ordered patterns.

In detail, key specific findings are as follows:
(i) when ferroelectric nanodots are cooled through the Curie Temperature in air, then either flux-closure patterns form, or bands of "superdomains" create effective 180 degree polarisation stripes to screen depolarising fields. These insights were published in detail in Nano Letters (for PZT nanodots in 2011) and in APL (for BTO nanodots in 2011);
(ii) when nanodots or nanobars of ferroelectric are cooled through the Curie Temperature in vacuum under the electron beam in the TEM, then flux closure patterns do NOT form; instead, highly ordered domain patterns which appear to have some kind of radially symmetric polarisation occur (published in PRL in 2013 and in Nano Lett in 2009), although the centre of the radial pattern shifts as morphology is altered in a way that can be understood as a shape-induced phase transition in the domain patterns (published in PRB in 2011);
(iii) complex flux-closure and quadrupole dipole chains can spontaneously appear during cooling in nanoshapes or near the edges of single crystal lamellae in both BTO and in PZN-PT. These fascinating structures were mapped using both TEM and PFM and findings published in Nano Letters (2010 and 2013).
Exploitation Route The possibility of using ferroelectrics in both memory applications and in complex nanoelectronic devices makes the understanding of the way that domains behave at the nanoscale absolutely critical. The discoveries made in this grant represent some of the clearest nanoscale domain behaviour information ever obtained.
Sectors Digital/Communication/Information Technologies (including Software)

Electronics

 
Description EPSRC
Amount £324,494 (GBP)
Funding ID EP/H047093/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 12/2010 
End 11/2013
 
Description EPSRC
Amount £16,812 (GBP)
Funding ID EP/H04339X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2011 
End 07/2011
 
Description Leverhulme Trust
Amount £120,694 (GBP)
Funding ID F/00 203/V 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2010 
End 02/2013
 
Description Underpinning Multi-User Equipment
Amount £1,826,476 (GBP)
Funding ID EP/P030246/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2017 
End 03/2018