Universal and scaled relaxation of interacting magnetic nanoparticles

The logarithmic relaxation rate of the thermoremanent magnetic moment $m(t)$ of interacting magnetic nanoparticles in discontinuous Co₈₀Fe₂₀∕Al₂O₃ multilayers follows a universal power law, whose exponent n increases with increasing particle concentration as predicted by recent simulations [Ulrich et al., Phys. Rev. B 67, 024416 (2003)]. While $n<1$ characterizes the stretched exponential decay of the dilute superspin glass (SSG) regime, $n>1$ refers to algebraic decay with finite remanence for t→∞ as observed in more concentrated superferromagnets (SFM). In the crossover regime from SSG to SFM, an increase from $n<1$ at low temperature to $n>1$ at T⪅Tc violates T0.2em0exln(t∕τ₀) scaling and seems to indicate a crossover from random-field domain state to SFM behavior.