Vortex Domains in Ferroelectric Nanostructures

When an electric field is applied to a material, its electrons and ions are displaced in opposite directions and electric dipoles form within the material. In general, the charges return to their initial positions when the external electric field is switched off afterwards. However, there exists a special class of materials that keeps their electric polarization even long after the external electric field vanished. In analogy to ferromagnetic materials where a once inscribed magnetization is kept, these polarization-conserving materials are called ferroelectrics. Their ability to retain the polarization favours their application as memory devices called ferroelectric random access memories (FRAM). Although ferroelectric capacitors were developed a lot over the years going from 1D thin films to 2D islands, a move to 3D structures is envisaged in the road map for the next years. Recently, special interest was attributed to ring nanostructures as present theories predict the existence of vortex domains in ferroelectrics. Assuming each vortex domain constitutes a carrier of information, this will allow a much denser packaging of individual information carriers in future ferroelectric mass storage devices. For example, theoretical calculations show that a ferroelectric vortex structure with a diameter of 3.2 nm will produce a vortex domain, resulting ultimately in an ultrahigh storage density of 60 Tbits/inch2 (five orders of magnitude larger than current non-volatile FRAM of 0.2 Gbits/inch2) far exceeding the 1 Gbit/inch2 density of typical MagneticRAM. Therefore the tremendous promise for nanotechnology lies at first within direct experimental investigation of dipole vortices in ferroelectrics. A promising way of controlling ferroelectric dipole vortices is by applying a lateral homogeneous electric field to an asymmetric ferroelectric nanoring, as predicted by recent theories. I consider this project to be a big step in fabricating and characterising ferroelectric nanostructures with vortex domains and, more importantly, in investigating their dipole switching. Although major steps were made in theoretical simulations of the properties which these vortex nanostructures will bring, the discovery of dipole vortices in ferroelectrics is still to be accomplished experimentally. I plan to use a Focused Ion Beam microscope for milling thin lamellae of single crystal ferroelectric material and patterning asymmetric ferroelectric nanorings into lamellae. Static imaging of domains will be achieved by Transmission Electron Microscopy. Further on, I will be using Piezoelectric Force Microscopy for switching and actively imagining the asymmetric ferroelectric nanorings. Overall, the novelty this project brings in consists of three major characteristics: (i) using single crystal materials which offer the best material quality; (ii) creating almost free standing nanostructures with no influence from extrinsic or intrinsic factors present in deposited films onto substrates; (iii) studying new and unique asymmetric nanoshapes which will result in more understanding of domain configurations in ferroelectrics. I have studied ferroelectrics for almost 10 years and focused my research on ferroelectric domains during the last four years. I am well accustomed with the facilities of the Electron Microscopes Unit within the Centre of Nanostructured Media from Queen's University where I am a PDRA since 2004. As a result of this work, I have published 13 papers on this subject in international journals over the last 5 years (25 articles published at present). Three more articles from the last year's results are awaiting publication this year.The prospect of controlling the ferroelectric polarization at the nanoscale by creating completely new dipole configurations is particularly exciting. I consider this the basis for a new generation of ferroelectric memory devices.