Aging of the field-induced asymmetry in a disordered ferroelectric

The isothermal aging of the asymmetry induced in the disordered dielectric crystal KTa1-xNbxO3 (x = 0.027) submitted to the biasing electric field E, is investigated. To this end, the response of the complex dielectric constant to infinitesimal field changes delta E, applied to the sample after a variable aging delay, has been measured for different magnitudes of E and after different aging delays. Two different experimental procedures have been used: in both cases the response strongly depends on the time spent under field. For short aging delays, the response has a strong contribution proportional to delta E and a weak quadratic contribution proportional to (delta E)(2). As time elapses, the linear and the quadratic contributions age in opposite ways: the former decreases whereas the latter increases. This paradoxical behaviour is analyzed in the framework of a model which attributes aging and the related effects (rejuvenation, memory) to the evolution of polarization domain walls: the decrease of the linear contribution is related to the decrease of the total wall area, while the increase of the quadratic term is attributed to wall reconformations.