Acoustic emission from domain wall motion in ferroelectric lead germanate

Abstract

Simultaneous visual observation and monitoring of the ultrasonic signals show that acoustic emission (AE) produced as Pb 5Ge 3O 11 c-plate crystals undergo the ferroelectric hysteresis cycle results from domain wall nucleation and collapse. AE activity takes place predominantly at applied electric fields large enough to bring a crystal substantially towards its saturation polarisation. A notable feature observed in Pb 5Ge 3O 11 and Pb 5− x Ba x Ge 3O 11 alloys is the existence of an abrupt threshold electric field, denoted by E ∞ in the high gain limit, for production of AE: as the electric field is increased beyond the threshold value, the amount of AE observed increases by several orders of magnitude. E ∞ depends linearly on the inverse of the c-plate sample thickness (1/ d) and also upon sample temperature—falling to a minimum at about 70°C for Pb 5Ge 3O 11 and then rising steeply as the Curie temperature (178°C) is approached. E ∞ is also dependent on the rate of change of the applied electric field and as this is increased extrapolates to the high frequency limit of the coercive field. Measurements of crystal polarisation indicate a one-to-one correspondence between AE and the electrical Barkhausen pulses which occur during polarisation reversal—further evidence for domain nucleation and collapse as the source of AE. Optical and AE studies made simultaneously on gadolinium molybdate add confirmation that these particular processes are responsible for the AE produced by ferroelectric crystals.