  An evidence that the dielectric ageing in the polydomain Pb(Zr0.45Ti0.55)O3
thin films is controlled by progressive pinning of 180 domain walls is
presented. To provide such a conclusion, we use a general method, which is
based on the study of the time evolution of the nonlinear, but anhysteretic,
dielectric response of the ferroelectric to a weak electric field. A
thermodynamic model of the ferroelectric system where the dielectric response
is controlled by bending movements of pinned 180 domain walls is developed.
Within this model, the nonlinear permittivity of the ferroelectric is expressed
as a function of the microstructural parameters of the domain pattern. It is
shown that using the analysis of the time evolution of the nonlinear
permittivity, it is possible to estimate changes in the concentration of the
pinning centers that block the movements of the 180 domain walls during aging
in polydomain perovskite ferroelectrics.
