Studies in polarization switching characteristics of ferro electrics

Abstract

Ferroelectrics are characterized by a macroscopic spontaneous polarisation whose direction can be reoriented by the application of a suitable electric field. This reorientation of the ferroelectric polarisation through the action of an external electric field is called polarisation switching. During the switching process, a transient switching current flows in the ferroelectric. The characteristic parameters, such as the switching current and the switching time, can be evaluated from the switching current transient. These parameters depend on the nature of the substance and the external physical conditions such as electric field, temperature, pressure etc. The process of polarisation switching takes place through a three?stage process. The first stage consists of the nucleation of field?reoriented domains which takes place as soon as the field is applied. The subsequent stages consist of the sideways and the forward growth of the domains nucleated in the first stage. The process is completed when the overall polarisation is reoriented into the field direction. The mechanism of the switching process can be known from the nature of the applied electric?field dependences of the switching current and the switching time. Information about the switching process can also be obtained by the comparison of parameters such as the activation field (which can be calculated from the experimental values of the switching time or the switching current) with the applied field. Large fluctuations of polarisation occur near the Curie point in a ferroelectric sample which exhibits a second?order phase transition. The polarisation fluctuations generate random noise voltages which can be related to the sample impedance by the application of the Nyquist theorem. Thus an experimental method based on the measurement of the noise voltages can be used to evaluate the sample resistance, capacitance and the dielectric constant. In contrast to the usual bridge techniques which are used to measure these parameters, no measuring voltage need be applied in the experimental technique based on the measurement of the noise voltage. The experimental technique based on the measurement of the noise to determine the sample parameters can be thought of as a zero?field technique. Since the magnitude of the noise voltage depends on the sample temperature, it is called the thermal noise voltage. This thesis consists of six chapters. The first five chapters concern themselves with the polarisation?switching process in ferroelectrics. The sixth and final chapter deals with the study of the phase transition in radiation?damaged ferroelectrics by the measurement of the thermal noise (generated by the polarisation fluctuations). The polarisation switching was studied in irradiated triglycine sulphate crystal, irradiated sodium nitrite crystal, dicalcium strontium propionate crystal and azoxybenzene (liquid film). The phase transition in ??irradiated triglycine sulphate crystal was examined by the method based on the measurement of the thermal noise. The first chapter introduces the concept of polarisation switching. Ferroelectrics are usually divided into domains. The domain structure plays an important role in polarisation switching. The essential features of the domain structure have been described. The difference between polarisation reorientation and polarisation reversal has been pointed out. In the second chapter, a brief survey of the methods used to study the polarisation switching and domains in ferroelectrics has been presented. A review of the relevant literature alone has been given and the survey is therefore not exhaustive. The reasons for the choice of the Merz method for use in the present studies have been mentioned. The third chapter provides a brief review of some of the theories which have been proposed to explain polarisation switching in ferroelectrics. The Miller–Weinreich model, the Hayashi model and the Pulvari–Kuebler theory lay down criteria which help decide the relative importance of the various stages of the switching process. The Pulvari–Kuebler theory provides expressions for the applied electric?field dependences of the switching time and the switching current. These expressions have been used in the present studies. The Patuzzo theory establishes the fact that the domain?wall motion and the switching?current transient studies yield similar information about the polarisation?switching process. The fourth chapter describes the experimental methods used in the present study. Circuits to measure the spontaneous polarisation and the switching current transient have been described. The spontaneous polarisation was measured by the Sawyer–Tower circuit and the Schubring, Nolte and Dork grounded?sample circuit. The switching current transient was studied by the use of the Merz method. A variable?temperature sample chamber and a temperature?control circuit to be used in association with the sample chamber for varying and controlling the sample temperature have been described. All the circuits and the sample chamber were fabricated with indigenously available components and materials. The final section of the chapter describes the methods of sample preparation. The results of the switching?characteristics studies have been presented in Chapter Five. The theory and the method used for analysis of the experimental results have been described. The switching time depends exponentially on the applied electric field in the case of irradiated triglycine sulphate and irradiated sodium nitrite. In dicalcium strontium propionate and azoxybenzene, the switching time depends linearly on the applied electric field. The effect of irradiation is to increase the importance of nucleation and sideways motion of the domain walls in the polarisation?switching process. The switching processes in dicalcium strontium propionate and azoxybenzene are linear processes. The effect of the radiation damage on the switching processes in triglycine sulphate and sodium nitrite has been discussed. The sixth and final chapter deals with the phase transition in gamma?irradiated triglycine sulphate. The thermal noise voltage from the sample was measured by a method based on that developed by Brophy and Webb. From the thermal?noise voltage, the sample resistance, capacitance and dielectric constant were determined. The variations with temperature of the sample resistance, capacitance and dielectric constant have been given. The phase transition has been discussed in the light of the damage due to irradiation.