The low-temperature dynamics of a noninteracting magnetic nanoparticle system ($\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$-alginate composite with an average nanoparticle size of $4\mathrm{nm}$) has been studied by superconducting quantum interference device magnetization measurements. Using different temperature and field protocols memory phenomena have been observed in the dc magnetization and magnetic relaxation of this nanoparticle system at temperatures below its blocking temperature $T_B=38\mathrm{K}$. Aging experiments show an absence of any waiting time dependence in the magnetization relaxation due to a field change after zero-field cooling and only a weak waiting time dependence after field cooling. This observation discriminates the dynamics of this system from the behavior of a classical spin glass, where frustration and disorder lead to an aging dependence of the system response. Moreover, the dynamics of this nanoparticle system is shown to be governed by a broad distribution of particle relaxation times arising from the distribution of particle sizes and sample inhomogeneities.

• Received 22 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.014445