NANOFERROELECTRICS · Nanosize ferroelectrics: ferroelectric nanotubes, self-patterning and size effect minimisation

Final Activity Report Summary - NANOFERROELECTRICS (Nanosize ferroelectrics: ferroelectric nanotubes, self-patterning and size effect minimisation)

The fellow has developed four interrelated lines of research: 1) Three-dimensional ferroelectrics: nanotubes and nanowires Manufacturing three-dimensional ferroelectric structures is essential in order to achieve higher storage density in the next generation of ferroelectric memories, and is identified a research priority by the International Roadmap for Reseach in Semiconductors. (i) Ferroelectric PZT was grown, for the first time, on carbon nanotubes. This is the first step towards the achievement of what could potentially be the smallest three-dimensional capacitors, consisting in concentric tubes of carbon, PZT and outer electrode. Work is now in progress in order to deposit the outer electrode and achieve a functional device. (ii) Control over domain size and polar orientation was achieved in ferroelectric nanowires, in collaboration with the Queen's University of Belfast. The author's article on the scaling relationships between domain size and lateral dimensions in ferroelectric nanowires has been selected as one of the top articles of 2007 in the Journal of Physics: Condensed Matter (http://herald.iop.org/JPCMTopPapers07EJnews/m1/jqr//deliver/Toppapers2007.pdf) (2) Self-organised nanostructures: nanodomains (i) In collaboration with scientists at the University of Groningen, the fellow has identified the existence of a minimum domain size for ferroelectric-ferroelastic twins in epitaxial thin films. This minimum domain size provides a route for the achievement of very fine-scale (7nm) self-organised regular stripes, which could in turn be used in the future as periodic templates for the fabrication of ferroelectric memory devices via selective etching. A thickness-induced transition to a new type of polar symmetry (not existent in bulk) was detected in nano-domains of epitaxial ultra-thin films of the archetypal ferroelectric PbTiO3. This new phase is expected to have better piezoelectric properties than the parent bulk ceramic. (ii) The fellow has also identified a new universal scaling law which relates domain size to film thickness and domain wall thickness. This scaling law was observed to be valid both for ferroelectrics and for ferromagnets, and it is expected to be also valid for multiferroics. (3) Magnetoelectrics and multiferroics This is a very hot area of research at the moment, because it might lead to a new generation of memory devices which can be written with a low voltage (low energy consumption) and read magnetically (non-destructive readout). The Fellow has made two contributions to this field: (i) A mechanism was identified whereby colossal magnetocapacitance can be achieved in materials which are not multiferroic. This was published as a single-author work in Applied Physics Letters, and as a corresponding author in Nature. (ii) Study of the ferroelectric domains of a room temperature multiferroic (BiFeO3) led to the identification of a potentially large magnetoelectric coupling in the domain walls and to the proposal of a new law which relates domain size to film thickness when the domains are fractal. This law is also "universal" in the sense that it should be valid for any ferroic material. (4) Deformation gradients and flexoelectricity Strain gradients appear naturally in very thin films (due to the relaxation of the strain generated by the substrate), and also in three-dimensional structures such as nanotubes, due to the radial stresses imposed by the concentric geometry. Understanding how strain gradients affect dielectric/ferroelectric properties is therefore of paramount importance. The Fellow and his PhD student (Pavel Zubko) have established unambiguously that strain gradients can induce polarisation even in materials that are not ferroelectric due to flexoelectricity. Importantly, they have also identified the existence of local polarisation in ferroelastic domain walls (i.e., domain walls in materials that are not polar). Furthermore, they have calculated that the local polarisation around defects in dielectrics such as SrTiO3 may exceed even the largest ferroelectric polarisations ever measured.

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