  Deterministic polarization reversal in ferroelectric and multiferroic films
is critical for their exploitation in nanoelectronic devices. While
ferroelectricity has been studied for nearly a century, major discrepancies in
the reported values of coercive fields and saturation polarization persist in
literature for many materials. This raises questions about the atomic-scale
mechanisms behind polarization reversal. Unconventional ferroelectric switching
in $\epsilon$-Fe2O3 films, a material that combines ferrimagnetism and
ferroelectricity at room temperature, is reported here. High-resolution in-situ
scanning transmission electron microscopy (STEM) experiments and
first-principles calculations demonstrate that polarization reversal in
$\epsilon$-Fe2O3 occurs around pre-existing domain walls only, triggering local
domain wall motion in moderate electric fields of 250 - 500 kV/cm. Calculations
indicate that the activation barrier for switching at domain walls is nearly a
quarter of that corresponding to the most likely transition paths inside
$\epsilon$-Fe2O3 domains. Moreover, domain walls provide symmetry lowering,
which is shown to be necessary for ferroelectric switching. Local polarization
reversal in $\epsilon$-Fe2O3 limits the macroscopic ferroelectric response and
offers important hints on how to tailor ferroelectric properties by domain
structure design in other relevant ferroelectric materials.
