J.J.P. Peters, G. Apachitei, R. Beanland, A. Sanchez, M. Alexe
1 Department of Physics, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, UK
It has been theoretically shown that in very thin and/or small systems the formation of ferroelectric domain walls is not energetically favourable. Instead vortex-type structure, involving polarization curling at unit cell, will form.  This has been recently shown at all scales, from macroscopic to medium range super-lattices and atomic scale.[3-5] Arguably, all reports on atomic scale curling of polarisation and vortex structures are on systems without electrodes. Whilst this favours vortex structures through enhancement of depolarising fields, manipulation and real use of these vortices necessitates metal electrodes to apply electric fields.
The screening effect of free carriers in the electrodes will apply a different set of constraints to those found in previously studied vortex structures. This raises the fundamental question of the stability of these vortex structures in metal-ferroelectric-metal, i.e. capacitor, devices.
Here we discuss occurrence and thickness dependence of topological features in multiferroic tunnel junction devices with ultra-thin, respectively 3, 6 and 9 unit cells (u.c.), PbTiO 3 films sandwiched between Co and La 0.7 Sr 0.3 MnO 3 (LSMO) electrodes. The structural data acquired using aberration-free scanning transmission electron microscopy (STEM) will be presented and discussed in relation with electrical characteristics of the multiferroic tunnel junctions.
As pointed recently,  the present study might have implications in defining a new state of matter in polar materials. The presence of these chiral topological states in bare capacitors, i.e. metal-ferroelectric-metal heterostructures, opens avenues in electrical control including local switching of chirality.
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