The polarization behavior of La-modified lead zirconate titanate relaxors has been investigated for various electrical and thermal histories. The field-cooled and zero-field-cooled behaviors were both studied. The magnitude of both polarizations were found to be equal above a critical temperature. A macroscopic polarization developed, when the zero-field-cooled state was warmed with a bias applied. The temperature of the maximum charging current decreased with increasing bias field. This decrease was modeled using the deAlmeida-Thouless relationship [J. deAlmeida and D. Thouless, J. Phys. A 11, 983 (1978)], which predicted an average moment size freezing of approximately 3×${10}^{\mathrm{-}27}$ C m. A glassy polarization mechanism was subsequently proposed with correlations between superparaelectric moments leading to the development of effective nonergodicity in a frozen state. Arguments are presented that this freezing process is dispersive due to a distribution of correlation strengths. The time dependence of the polarization was also investigated.

• Received 5 February 1992

DOI:https://doi.org/10.1103/PhysRevB.46.8013