Specific-heat measurements on amorphous ${\mathrm{Fe}}_{\mathrm{x}}$${\mathrm{Zr}}_{100\mathrm{-}\mathrm{x}}$ (x=90 and 92) by a thermal relaxation method reveal a temperature decay of the form T(t)=${\mathrm{T}}_0$+ΔT exp[-(t/τ${)}^{\mathrm{\beta }}$] below 20 K, where the alloys are asperomagnetically ordered. β≃0.5 when ${\mathrm{T}}_0$→0. The stretched exponential is a manifestation of nonergodic behavior, because of the inability of the spin system to explore the available random magnetic configurations on the 100-ms times scale of the experiment.

• Received 2 October 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.385