Aging of the asymmetry induced by the biasing electric field in a disordered ferroelectric

Abstract

Isothermal aging of the disordered dielectric crystal KTa1−xNbxO3 (x = 0.027), the symmetry of which is broken by the biasing electric field E, is studied by means of AC dielectric-constant measurements. The asymmetry of the crystal induced by the field is put in evidence by the response of the dielectric constant to the small field change δE. This response contains an anisotropic contribution proportional to δE and an isotropic positive contribution proportional to (δE) 2 . The former slowly decreases whereas the latter increases as time elapses. Our results are well interpreted in the framework of a model which attributes the time-dependent part of the dielectric constant to the evolution of the polarization domain walls: the slow decrease (aging) of the linear contribution is related to the decrease of the total wall area, which is a consequence of the domain growth, while the fast increase (rejuvenation) of the quadratic contribution is the signature of wall reconformations.Introduction. -In many materials, some properties vary with time in a manner which depends on the thermal history of the sample. This phenomenon is called aging [1]. It is generally observed in materials which contain some type of disorder [2], such as structural glasses [3], spin-glasses [4] and disordered dielectrics [5].Recently, experiments performed in potassium niobo-tantalate (a disordered dielectric material -KTN in short) showed that a static electric field of amplitude E seems to act as temperature [6,7]: small field changes produce variations of the complex dielectric constant ε(t) opposite to the aging evolution (i.e. rejuvenation), while large back and forth field variations keep partial memory of aging. However, in these experiments the electric field was only considered as an additional parameter, not essentially different from temperature. This means that it was not taken into account that the electric field breaks the symmetry of the total system (sample + applied parameter) while the temperature does not. This difference may be understood as follows: below the transition temperature T tr , the KTN sample enters a disordered ferroelectric phase; it contains a large number of ferroelectric domains randomly oriented along the eight possible [111] directions and distributed in such a way that the total polarization is equal to zero in the absence of electric field; consequently, the system presents