Aging, Memory and Oxygen Vacancies in PLZT

Aging and memory are phenomena that can be found in various disordered systems. Memory of multiple aging stages has been found in spin glasses, below the so called freezing temperature Tf and in the relaxor ferroelectric PLZT also above this temperature, but it is not sure whether memory at these temperature is intrinsic of the glassy relaxor system or is rather due to migration of mobile defects. We measured anelastic and dielectric susceptibility in two different PLZT compositions: relaxor (PLZT 9/65/35) and partially ferroelectric (PLZTN 7/60/40) with 0.02 oxygen vacancies. We introduced O vacancies, H defects and mobile charges resulting from their ionization in the relaxor and we found that in this sample, memory, in elastic compliance and dielectric susceptibility, is not enhanced but depressed by an increase of O vacancies, while in the ferroelectric, there is no memory unless aging is protracted for long times. This is considered as evidence that in the relaxor case memory is intrinsic while in the non relaxor case it is due to slow migration of defects.