  This chapter provides an introduction to the fundamental physical ideas and
models relevant to the phenomenon of magnetic hysteresis in nanoparticle
assemblies. The concepts of single-domain particles and superparamagnetism are
discussed. The mechanisms of magnetization by coherent rotation and the role of
temperature in the gradual decay of magnetization are analyzed in the framework
of simple analytical models. Modern numerical techniques (Monte Carlo
simulations, Magnetization Dynamics) used to study dense nanoparticle
assemblies are presented. An overview of the most common experimental
techniques used to measure the magnetic hysteresis effect in nanoparticle
assemblies are presented and the underlying principles are exposed.
