UBIQUITOUS NONEXPONENTIAL DECAY - THE EFFECT OF LONG-RANGE COUPLINGS (INVITED)

Many physical systems exhibit a dynamic response referred to either as slow relaxation, a quasilogarithmic time dependence, or a stretched e xponential response. Historically this time dependence has been attrib uted to the presence of disorder which creates a distribution of relax ation times. In two papers [D. K. Lottis, E. Dan Dahlberg, J. Christne r, J. I. Lee, R. Peterson, and R. White, J. Appl. Phys. 63, 2920 (1988 ); D. K. Lottis, R. M. White, and E. Dan Dahlberg, Phys. Rev. Lett. 67 , 362 (1991)], we have shown that this time dependence can alternative ly be explained to be a consequence of interactions or couplings. In t he model, the interactions between relaxing spins, the dipole-dipole c ouplings, drive the system from an initial state towards equilibrium. As the system relaxes, the dipolar energy is reduced and the driving f orce diminishes. This process gives rise to the observed slow relaxati on-time dependence in a very natural manner. To guarantee the absence of disorder, the model considers the dipolar coupling or interaction b etween relaxing spins with a mean-field approximation, the demagnetiza tion field. Another feature observed in physical systems which the mod el explains is the nonmonotonic temperature dependence of the logarith mic decay slope. In addition to a description of the model, measuremen ts to determine the presence of interactions in some of the systems wi ll be discussed.