  Atomic force microscopy was used to investigate ferroelectric switching and
nanoscale domain dynamics in epitaxial PbZr0.2Ti0.8O3 thin films. Measurements
of the writing time dependence of domain size reveal a two-step process in
which nucleation is followed by radial domain growth. During this growth, the
domain wall velocity exhibits a v ~ exp[-(1/E)^mu] dependence on the electric
field, characteristic of a creep process. The domain wall motion was analyzed
both in the context of stochastic nucleation in a periodic potential as well as
the canonical creep motion of an elastic manifold in a disorder potential. The
dimensionality of the films suggests that disorder is at the origin of the
observed domain wall creep. To investigate the effects of changing the disorder
in the films, defects were introduced during crystal growth (a-axis inclusions)
or by heavy ion irradiation, producing films with planar and columnar defects,
respectively. The presence of these defects was found to significantly decrease
the creep exponent mu, from 0.62 - 0.69 to 0.38 - 0.5 in the irradiated films
and 0.19 - 0.31 in the films containing a-axis inclusions.
