Aging and memory in a superspin glass

Abstract

Magnetization relaxation measurements show that aging occurs in a discontinuous metal-insulator multilayer (DMIM) $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(0.9\mathrm{nm})/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\mathrm{nm}){]}_{10}$ below the spin-glass transition temperature, ${T}_{\mathrm{g}}\ensuremath{\approx}44\mathrm{K}.$ Furthermore, the DMIM system memorizes the structure of a quasiequilibrium state reached after an intermittent stop-and-wait protocol during cooling. These results unambiguously corroborate the collective nature of the low-temperature spin dynamics. The correlation length achieved at ${T=0.95T}_{\mathrm{g}}$ after a wait time of ${10}^{4}\mathrm{s}$ is estimated to extend to ${10}^{5}$ superspins, which seems to imply a crossover from three- to two-dimensional growth of the correlation length.