The logarithmic relaxation rate of the thermoremanent magnetic moment $m\left(t\right)$ of interacting magnetic nanoparticles in discontinuous ${\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{Al}}_2{\mathrm{O}}_3$ 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 $t\to \infty$ 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 $T⪅T_c$ violates $T\ln (t∕{\tau }_0)$ scaling and seems to indicate a crossover from random-field domain state to SFM behavior.

• Received 20 July 2004
• Corrected 6 December 2004

DOI:https://doi.org/10.1103/PhysRevB.70.172411