Vortex Domains in Ferroelectric Nanostructures

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

Abstract

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.

Publications

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Evans DM (2014) Switching ferroelectric domain configurations using both electric and magnetic fields in Pb(Zr,Ti)O3-Pb(Fe,Ta)O3 single-crystal lamellae. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

 
Description In this project experimental work has sought to study complex arrangements of dipoles such as flux-closure, vortex, vertex or skyrmion patterns (predicted by theory) in ferroic materials. While there is undoubtedly still considerable work to be done, enough insight has been gained. A key aspect was determining the differences and the interchangeable features between vortex and vertex domains. Vertex domains imply simple crossing of three or more domain walls. Vortex domains require a curl of polarisation.

One extremely important finding is that vertex domains in nano-ferroics of different geometries are controlled by kinetics and not by thermal equilibrium. Direct imaging using scanning transmission electron microscopy (TEM) of BaTiO3 platelets machined from bulk single crystal revealed that domains form into sets of four quadrants (each quadrant composed of a number of 90° stripe domains). The spatial arrangement of these quadrants, in particular the position of the point at which they met (the quadrant core), was found to be highly sensitive to the platelet shape. In detail, the quadrant core was found to progressively migrate away from the center of the platelet as the aspect ratio of the platelet increased.

Other key finding was the influence of notches (machined along bars of BaTiO3 single crystal) onto the static and dynamic behaviour of ferroelectric domains. While static domain configurations suggested a notch-domain wall interaction that should inhibit domain wall motion during axial switching, direct functional measurement found switching to be accelerated in the presence of notches. Modelling suggested that this counterintuitive observation is related to the unusual field focusing effect that the notch geometry creates.

Furthermore a very important finding was that close to the ferroelectric-to-paraelectric phase transition the equilibrium domain periodicity is dependent on the critical temperature and gives rise to domain annihilation events when thermal gradients exist. Comparison between the observations made under this thermal gradient and those made in thin BaTiO3 platelets where a thickness gradient exists, demonstrate that changing domain period through domain annihilation is a general feature when gradient vectors are not perpendicular to the domain walls with which they interact.

Another significant finding is that electron beam induced radial electric fields stabilize quadrant patterns which have been observed in TEM imaging of ferroelectric domains in nanodots. This might serve as a warning that the domain configuration under electron beam observation may be a nonequilibrium property. The electron densities required to form the quadrant patterns are significantly higher than typical defect densities in ferroelectrics, implying that external charging may be involved. This points to the fact that an electron beam may be intentionally used to ''write'' such patterns and suggests a possible avenue toward new classes of ferroelectric devices that utilize such electron beam engineered domain states.

Probably the most essential finding of this work is that the flux-closure core singularities that are required as pre-condition of vortex formation are avoided in BaTiO3. Detailed examination by piezoelectric force microscopy (PFM) of boundaries between bundles of 90° domains investigated as possible sites for the existence of naturally occurring flux closure states in thin single crystal BaTiO3 lamellae were found to consist of a series of 180° domain walls arranged ideally into zigzag patterns.

The discovery of a new room temperature multiferroic material - Pb(Zr,Ti)O3-Pb(Fe,Ta)O3- is definitely a major finding of this project. This was driven by a strong desire to demonstrate new electronic devices. While some applications rely on distinct ferroelectric and magnetic states which can be poled separately, other applications are based on a strong coupled magnetoelectric response (applied electric field causes changes in magnetisation and a magnetic field causes changes in electrical polarisation). The technological possibilities that could arise from magnetoelectric multiferroics are considerable and a range of functional devices has already been envisioned. Realising these devices, however, requires coupling effects to be significant and to occur at room temperature. Switching ferroelectric domain configurations using both electric and magnetic fields in Pb(Zr,Ti)O3-Pb(Fe,Ta)O3 single crystal lamellae was achieved by PFM and TEM measurements (published in the Nature Communications journal). An order of magnitude estimate of the effective coupling coefficient suggested a value of approximately 1x 10-7 sm-1.

The last key finding of this project was an alternative origin for the Forsbergh pattern reported in 1949 (spontaneous spatial ordering in the birefringence patterns seen in flux-grown BaTiO3 crystals, under the transmission polarized light microscope), in which sheets of orthogonally oriented ferroelastic stripe domains simply overlay one another. A critical value of periodicity of the stripe domains just below the Curie temperature is required for the Forsbergh birefringence pattern to occur.

All key findings are reflected in publications (18 articles, 2 book chapters, and conference presentations and proceedings) in high impact international journals (i.e., Nature Communications, Nano Letters, and Advanced Materials etc.) over the last 6 years.

Book chapters:
Mapping topological defects in ferroelectrics by PFM - J.M. Gregg, A. Schilling, M.P.D. Campbell, R. G. P. McQuaid, A. Kumar (2014).

Ferroelectric Vortices and Related Configurations from Experiment - Prosandeev, S., Naumov, I., Fu, H., Bellaiche, L., , M.P.D. Campbell, R. G. P. McQuaid, L-W. Chang, A. Schilling, L.J. McGilly, A. Kumar, J.M. Gregg, Nanoscale Ferroelectrics and Multiferroics. John Wiley & Sons, Ltd, p. 700-728 (2016).

Refereed Journal Papers:
1. 1. A. Schilling, A. Kumar, R. G. P. McQuaid, A. M. Glazer, P. A. Thomas, J. M. Gregg, Reconsidering the Origins of Forsbergh Birefringence Patterns, Physical Review B (Condensed Matter) 94, 2, 024109. , 024109 (2016).

2. Chapman, Jacob; Gregg, J.; Schilling, Alina; Kimmel, Anna; Duffy, Dorothy, Novel Ferroelectric Nanobubble Domains in Strained Tetragonal Prototypical Perovskite Films, NanoLett.submitted (2016).

3. D. Evans, M. Alexe, R. McQuaid, A. Schilling, A. Kumar, D. Sanchez, N. Ortega, R. Katiyar, J. F. Scott, J.M. Gregg, The Nature of Room-Temperature Magnetoelectric Coupling in Pb(Zr,Ti)O3-Pb(Fe,Ta)O3, Advanced Materials, 10.1002/adma.201501749 (2015)

4. J. Scott, A. Schilling, S. Rowley, J. M. Gregg, Some Current Problems in Perovskite Nano-Ferroelectrics and Multiferroics: Kinetically-limited Systems of Finite Lateral Size, Sci. Technol. Adv. Mater. (2015)

5. A. Schilling, B. Barton, J.R. Jinschek, L. Mele, P. Dona, J Einsle, J. Ringnalda, M. Arredondo, J.M. Gregg, Live Imaging of Reversible Domain Evolution in BaTiO3 on the Nanometer Scale Using In Situ STEM and TEM, Microscopy and Microanalysis journal, 1560-1561 (2014).

6. Schiemer J, Carpenter M A, Evans D M, Gregg J M, Schilling A, Arredondo M, Alexe M, Sanchez D, Ortega N, Katiyar R S, Echizen M, Colliver E, Dutton S and Scott J F, Studies of the Room-Temperature Multiferroic Pb(Fe0.5Ta0.5)0.4 (Zr0.53Ti0.47)0.6O3: Resonant Ultrasound Spectroscopy, Dielectric, and Magnetic Phenomena, Adv. Functional Materials 24, DOI: 10.1002/ adfm.201303492 (2014) - Citations: 0.

7. D.M. Evans, A. Schilling, A. Kumar, D. Sanchez, N. Ortega, R. S. Katiyar, J. F. Scott, J. M. Gregg, Switching ferroelectric domain configurations using both electric and magnetic fields in Pb(Zr,Ti)O3-Pb(Fe,Ta)O3 single-crystal lamellae, Phil. Trans, R. Soc. A 372, 20120450 (2014) - Citations: 0.

8. D.M. Evans, A. Schilling, A. Kumar, D. Sanchez, N. Ortega, M. Arredondo, R.S. Katiyar, J.M. Gregg, J.F. Scott, Magnetic Switching of ferroelectric Domains at Room Temperature in a New Multiferroic, Nature Comms 4, 1534 (2013) - Citations: 16.

9. D. Sanchez, N. Ortega, A. Kumar, G. Sreenivasulu, R. S. Katiyar, J. F. Scott, D. M. Evans, M. Arredondo, A. Schilling, J. M. Gregg, Room-temperature single phase multiferroic magnetoelectrics: Pb(Fe, M)x(Zr,Ti)(1-x)O3 [M=Ta, Nb], J. Appl. Phys. 113, 074105 (2013) - Citations: 6.

10. R. Ahluwalia, N. Ng, D. Srolovitz, A. Schilling, R. McQuaid, D. M. Evans, J. F. Scott, J. M. Gregg, Manipulating ferroelectric domains in nanostructures under electron beams, Phys. Rev. Lett. 111 (16), 165702 (2013) - Citations: 1.

11. L. J. McGilly, T.L. Burnett, A. Schilling, M.G. Cain, J.M. Gregg, Domain annihilation due to temperature and thickness gradients in single-crystal BaTiO3, Phys. Rev. B 85, 054113 (2012) - Citations: 2.

12. A. Schilling, S. Prosandeev, L. Belaiche, J. F. Scott, J. M. Gregg, Shape-induced phase transition of domain patterns in ferroelectric platelets, Phys. Rev. B 84, 064110 (2011) - Citations: 21.

13. L.J. McGilly, A. Schilling, J. M. Gregg, Domain Bundle Boundaries in Single Crystal BaTiO3 Lamellae: Searching for Naturally Forming Dipole Flux-Closure/Quadrupole Chains, Nano Letters 10, 4200 (2010) - Citations: 31.

14. M.McMillen, R. McQuaid, S. C. Haire, C. D. McLaughlin, L. W. Chang, A. Schilling, J. M. Gregg, The influence of Noches on Domain Dynamics in Ferroelectric Nanowires, Appl. Phys. Lett. 96, 042904 (2010) - Citations: 5.

15. A. Schilling, D. Byrne, G. Catalan, K.G. Webber, Y.A. Genenko, G.S. Wu, J. F. Scott, J. M. Gregg, Domains in Ferroelectric Nanodots, Nano Letters 9, 3359 (2009) - Citations: 75.

16. G. Catalan, I. A. Luk'yanchuk, A. Schilling, J. M. Gregg, J. F. Scott, Effect of wall thickness on the ferroelastic domain size in BaTiO3, J. Mater. Sci. 44, 5308 (2009) - Citations: 3.

17. L. McGilly, D. Byrne, C. Harnagea, A. Schilling, J. M. Gregg, Imaging Domains in BaTiO3 Single Crystal Nanostructures: Comparing Information from Transmission Electron Microscopy and Piezo-Force Microscopy, J. Mater. Sci 44, 5197 (2009) - Citations: 10.

18. I. A. Luk'yanchuk, A. Schilling, J. M. Gregg, G. Catalan, J. F. Scott, Origin of Ferroelastic Domains in Free-Standing Single Crystal Ferroelectric Films, Phys. Rev. B 79, 144111 (2009) - Citations: 24.

Unfortunately no more outcomes, apart from the results submitted last year, have resulted from this work as my employment within Queen's University Belfast ended at the end of this grant 31 July 2015.
Exploitation Route My findings are important in area of experimental research and to applications in electronic devices. For example the discovery of a new multiferroic material might contribute to creating new and powerful electronic devices.
Sectors Education,Electronics

 
Description Ferroelectric Playground 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact This 3 page article was published by the Research Media (www.researchmedia.eu) in a special issue entitled International Innovation which helps disseminating science, research and technology.
Year(s) Of Engagement Activity 2013