  Ferroelectrics display spontaneous and switchable electrical polarization.
Until recently, ferroelectricity was believed to disappear at the nanoscale;
now, nano-ferroelectrics are being considered in numerous applications. This
renewed interest was partly fuelled by the observation of ferroelectric domains
in films of a few unit cells thickness, promising further size reduction of
ferroelectric devices. It turns out that at reduced scales and dimensionalities
the material's properties depend crucially on the intricacies of domain
formation, that is, the way the crystal splits into regions with polarization
oriented along the different energetically equivalent directions, typically at
180o and 90o from each other. Here we present a step forward in the
manipulation and control of ferroelectric domains by the growth of thin films
with regular self-patterned arrays of 90o domains only 7 nm wide. This is the
narrowest width for 90o domains in epitaxial ferroelectrics that preserves the
film lateral coherence, independently of the substrate.
