  Ultra-dense domain walls are increasingly important for many devices but
their microscopic properties are so far not fully understood. In this study we
combine atomistic and coarse-grained molecular dynamic simulations to study the
domain wall stability in the prototypical ferroelectric BaTiO3. We transfer the
discussion of the field-driven nucleation and motion of domain walls to
thermally induced modifications of the wall without an external driving force.
Our simulations show that domain wall dynamics and stability depend crucially
on microscopic thermal fluctuations. Enhanced fluctuations at domain walls may
result in the formation of critical nuclei for the permanent shift of the
domain wall. If two domain walls are close - put in other words, when domains
are small - thermal fluctuations can be sufficient to bring domain walls into
contact and lead to the annihilation of small domains. This is even true well
below the Curie temperature and when domain walls are initially as far apart as
6 unit cells. Such small domains are, thus, not stable and limit the maximum
achievable domain wall density in nanoelectronic devices.
