  Many dense magnetic nanoparticle systems exhibit slow dynamics which is
qualitatively indistinguishable from that observed in atomic spin glasses and
its origin is attributed to dipole interactions among particle moments (or
superspins). However, even in dilute nanoparticle systems where the dipole
interactions are vanishingly small, slow dynamics is observed and is attributed
solely to a broad distribution of relaxation times which in turn comes from
that of the anisotropy energy barriers. To clarify characteristic differences
between the two types of slow dynamics, we study a simple model of a
non-interacting nanoparticle system (a superparamagnet) analytically as well as
ferritin (a superparamagnet) and a dense Fe-N nanoparticle system (a superspin
glass) experimentally. It is found that superparamagnets in fact show aging (a
waiting time dependence) of the thermoremanent-magnetization as well as various
memory effects. We also find some dynamical phenomena peculiar only to
superspin glasses such as the flatness of the field-cooled magnetization below
the critical temperature and memory effects in the zero-field-cooled
magnetization. These dynamical phenomena are qualitatively reproduced by the
random energy model, and are well interpreted by the so-called droplet theory
in the field of the spin-glass study.
