The dynamics of a magnetic particle system consisting of ultrafine Fe-C particles of monodisperse nature has been investigated in a large time window, ${10}^{-9\mathrm{}}–{10}^4\mathrm{s}$, using Mössbauer spectroscopy, ac susceptibility, and zero field cooled magnetic relaxation measurements. By studying two samples from the same dilution series, with concentrations of 5 and $6\times {10}^{-3\mathrm{}}\mathrm{vol}\%$, respectively, it has been found that dipole-dipole interaction increases the characteristic relaxation time of the particle system at all temperatures investigated. The results for the most concentrated particle assembly are indicative of collective magnetic dynamics and critical slowing down at a finite temperature, $T_g\approx 40\mathrm{K}$. Close to and below the transition temperature, an aging phenomenon is observed, another manifestation of collective magnetic dynamics.

• Received 15 August 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.5154