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 , 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-->infinity 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 Tless than or similar toT(c) violates T ln(t/tau(0)) scaling and seems to indicate a crossover from random-field domain state to SFM behavior.