ANOMALOUS HYSTERESIS LOOPS IN KNBO3 SINGLE-CRYSTALS

It is found that KNbO3 single crystals containing cooperatively ordered impurity dipoles exhibit anomalous hysteresis loop behaviour. At frequencies of 50 Hz and lower, large sudden changes in polarization and electric field are observed. The orientation, shape and size of the loop change strongly with frequency, and the loop traces are broad, particularly at low frequencies of the order of 1 Hz. There is no well defined coercive field or remnant polarization. Earlier work on switching in these crystals has shown that domains in the crystal have a characteristic response time and can participate in the switching process only if the tracing frequency is of the order of this time. Against this background, it is shown that the sudden changes in polarization are due to nucleation or evaporation of a domain in the crystal. The origin of the anomalous nature lies in the fact that the polarization is not only a function of field E but also of time t. Experiments conducted with a d.c. bias confirm the role of impurity dipoles in the nucleation process, and hence in switching. The loops observed at various temperatures showed that the domains have stability on account of impurity dipoles associated with them. The importance of this work lies in the fact that cooperative ordering of the dipoles has to be avoided to prevent anomalous switching effects coming into play.