Switching in KNbO3 single crystals containing cooperatively ordered impurity dipoles.

Abstract

In ${\mathrm{KNbO}}_{3}$ single crystals having an impurity dipole content of about ${10}^{16}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ it is found that switching is strongly affected by the presence of these dipoles, and the hysteresis loop shape can be correlated to the domain structure that itself is determined by the distribution of the impurity dipoles that have cooperative ordering among them. A theoretical model for the shape of the loop has been developed considering nucleation of microdomains by the impurity dipoles and their subsequent evaporation under the reversed electric field. The model is found to describe well the observed hysteresis loops traced at various frequencies. The effect of clamping produced by external compressive forces or dc bias fields can also be understood in terms of the ideas used in the development of the model. The measurements carried out at various temperatures show that the effect of the dipoles in the loops is shown more prominently with higher temperatures. The nature of the switching transients using square-wave pulses confirms that the nucleation and growth mechanism suggested by Merz is only partially operative, as the process becomes strongly dependent on the role of the impurity dipoles.