Time decay of the remanent magnetization in spin glasses (invited)

We begin with a brief history of magnetoviscosity measurements in materials. We then show that the relaxation of the thermoremanent magnetization in spin glasses is accurately characterized, over several decades in time, by the stretched exponential: σ_TRM=σ₀ exp[-C(ωt)^1-n/(1-n)]. The time-stretch exponent (n) is given as a function of temperature for both 2.6% AgMn+0.46% Sb and 1.0% CuMn; dissimilarities in the temperature dependence of n may be due to the different anisotropy energy of the samples. We emphasize the experimental fact that a field-cooled sample is not in equilibrium and show that the time development of the field-cooled state may be characterized as an exponential decrease of the relaxation frequency with increasing wait time: ω=ω₀ exp(-t_w/t₀). We briefly outline three general theories which have the stretched-exponential time dependence, but no theory can yet explain all of the observed behavior in spin glasses. We conclude by showing that the remanent magnetization in a ferromagnet may also have the stretched-exponential time dependence.