  Magnetic single-domain nanoparticles constitute an important model system in
magnetism. In particular ensembles of superparamagnetic nanoparticles can
exhibit a rich variety of different behaviors depending on the inter-particle
interactions. Starting from isolated single-domain ferro- or ferrimagnetic
nanoparticles the magnetization behavior of both non-interacting and
interacting particle-ensembles is reviewed. A particular focus is drawn onto
the relaxation time of the system. In case of interacting nanoparticles the
usual Neel-Brown relaxation law becomes modified. With increasing interactions
modified superparamagnetism, spin glass behavior and superferromagnetism is
encountered.
