  The physical properties of magnetic nanoparticles have been investigated with
focus on the influence of dipolar interparticle interaction. For weakly coupled
nanoparticles, thermodynamic perturbation theory is employed to derive
analytical expressions for the linear equilibrium susceptibility, the
zero-field specific heat and averages of the local dipolar fields. By
introducing the averages of the dipolar fields in an expression for the
relaxation rate of a single particle, a nontrivial dependence of the
superparamagnetic blocking on the damping coefficient is evidenced. This
damping dependence is interpreted in terms of the nonaxially symmetric
potential created by the transverse component of the dipolar field.
  Strongly interacting nanoparticle systems are investigated experimentally in
terms of spin glass behavior. Disorder and frustration arise in samples
consisting of frozen ferrofluids from the randomness in particle position and
anisotropy axis orientation. A strongly interacting FeC system is shown to
exhibit critical dynamics characteristic of a spin glass phase transition.
Aging, memory and rejuvenation phenomena similar to those of conventional spin
glasses are observed, albeit with much weaker rejuvenation effects than in both
a Ag(11 at% Mn) Heisenberg and an Fe_{0.5}Mn_{0.5}TiO_3 Ising spin glass.
Differences in the nonequilibrium dynamics of the strongly interacting
nanoparticle system and the two spin glass samples are discussed in terms of
anisotropy and different timescales, due to the much longer microscopic flip
time of a magnetic moment than of an atomic spin.
