Brunier, Alan (2019) Functionalities at ferroic domain walls. PhD thesis, University of Warwick.

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Abstract

Over the last decade domain walls in ferroelectric and multiferroic materials have attracted significant research interest. This has been driven by the discovery that these interfaces can exhibit exotic functional properties, including but not limited to enhanced electrical conductivity, and has been enabled by the development of advanced experimental characterisation techniques including atomic-resolution transmission electron microscopy and polarisation-sensitive atomic force microscopy methods which enable the polarisation at these nanoscale interfaces to be effectively probed. Much of the early work focussed on determining the fundamental physical properties of domain walls, but now more research is being directed towards engineering useful devices where the domain walls are the active element. The recent demonstration that it is possible to manufacture prototype ferroelectric domain wall memory devices which are compatible with the requirements of industry has only furthered this.

The objective of this thesis is to further understanding of the properties of domain walls, most often directly using a device geometry, with the aim of obtaining information that can be used to engineer future novel devices. The materials studied here, multiferroic BiFeO3 and ferroelectrics PbZr0.2Ti0.8O3 and BaTiO3, are among those that have attracted most research interest due to having properties which are especially attractive from the perspective of device engineering.

First the domain wall motion in Pt/PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 thin films is studied using piezoelectric force microscopy to probe nanoscale domain structure and an advanced low-field dielectric spectroscopy method to probe the bulk behaviour of capacitor devices. It has been demonstrated that this combined approach allows information about the mechanisms and energetics of the domain wall motion to be obtained. Values of these quantities for PbZr0.2Ti0.8O3 are presented.

Next the evolution of the domain pattern and domain wall architecture in BiFeO3/SrRuO3/DyScO3 capacitor devices during ferroelectric switching was studied. By correlating the composition of domain walls in the device with the low temperature electrical behaviour of the device it was possible to determine that the low temperature conductivity of 109◦ walls is higher than 71◦ domain walls. The effective resistivity of each type of domain wall was determined. This is the first report of through-electrode piezoelectric force microscopy being used to gain accurate information about the in-plane polarisation in capacitor devices, and demonstrates that this type of investigation offers a way to carry out electrical and magnetotransport characterisation of conductive nanoscale domain walls in systems which are too insulating to measure at low temperature with scanning probe microscopy or in-plane device geometries. Further than this, the findings in this work elucidate many observations which are already reported in the literature.

Following this, a time-resolved study of the electric field driven ferroelectric switching in (001)pc oriented BiFeO3 is presented. The process of ferroelectric switching was found to consist of two steps. First the polarisation rotates in-plane and is suppressed, before being flipped to its new orientation. This is a more complicated picture than the previous studies of switching in BiFeO3 have predicted. Furthermore, it was shown that slow processes like the creep motion of domain walls and the collective motion of the extended oxygen octahedral structures have a key role in mediating the timescale of the switching process. These factors are highly dependent on the strain imposed on the ferroelectric BiFeO3 layer by the substrate.

By similar time-resolved synchrotron methods it was found that it is possible to enhance the process of polarisation reversal in metal-ferroelectric-metal by generating an ultrashort compressive strain wave which propagates through the ferroelectric layer during ferroelectric switching. The compressive strain wave is launched by illuminating a conductive oxide electrode layer with an ultrashort infrared laser pulse. Improving the timescale of ferroelectric switching is an attractive goal for researchers who work to design devices for applications which require fast data processing. This method is demonstrated to be a possible method by which one can break the speed limit that domain wall velocity imposes on the ferroelectric switching process. This effect is showcased in both BiFeO3 and BaTiO3 capacitor devices.

Finally a structural investigation of a complex structure in a BiFeO3/NdScO3 thin film is presented. The structure stabilised here is unlike anything which has been reported for this system before, and is consistent with what can reasonably be expected near the predicted tensile strain-induced phase boundary in BiFeO3. Detailed structural measurements by x-ray diffraction and transmission electron microscopy reveal that despite being dominated by the expected MB phase (with mainly 109◦ stripe domains), the sample includes many regions where the polarisation has a significantly larger than expected in-plane component. This indicates the presence of an orthorhombic or orthorhombic-like phase. Even within domains there is a clear polarisation instability. Temperature dependant measurements of the photovoltaic effect in this sample show a behaviour which is different from that of phase pure and essentially unstrained 109◦ stripe domain BiFeO3 stripe domain films and also predicts that a phase transition to a from a mixed phase to a single phase structure occurs at around 200 K.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Domain structure, Ferromagnetic materials, Thin films -- Electric properties, Thin films -- Magnetic properties
Official Date:	August 2019
Dates:	Date Event August 2019 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Alexe, M. (Marin)
Format of File:	pdf
Extent:	xx, 165 leaves : illustrations (chiefly colour)
Language:	eng

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