Ubiquitous nonexponential decay: The effect of long-range couplings? (invited)

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Abstract
Many physical systems exhibit a dynamic response referred to either as slow relaxation, a quasilogarithmic time dependence, or a stretched exponential response. Historically this time dependence has been attributed to the presence of disorder which creates a distribution of relaxation times. In two papers [D. K. Lottis, E. Dan Dahlberg, J. Christner, 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 alternatively be explained to be a consequence of interactions or couplings. In the model, the interactions between relaxing spins, the dipole–dipole couplings, drive the system from an initial state towards equilibrium. As the system relaxes, the dipolar energy is reduced and the driving force diminishes. This process gives rise to the observed slow relaxation-time dependence in a very natural manner. To guarantee the absence of disorder, the model considers the dipolar coupling or interaction between relaxing spins with a mean-field approximation, the demagnetization field. Another feature observed in physical systems which the model explains is the nonmonotonic temperature dependence of the logarithmic decay slope. In addition to a description of the model, measurements to determine the presence of interactions in some of the systems will be discussed.