Magnetic interactions: A tool to modify the magnetic properties of materials based on nanoparticles

The magnetic behaviour of ensembles of magnetic entities depends on their intrinsic magnetic properties and many other factors, including short- and long-range magnetic interactions. This chapter focuses on the effect of interactions in assemblies of single-domain nanoparticles, each of them being considered as a 'superspin' with a magnetic moment related to the particle volume. Depending on the type and the strength of the interactions among the magnetic entities, the magnetic behaviour of an assembly of superspins evolves from ferromagnetic-like behaviour to paramagnetic-like behaviour, through spin glass-like behaviour. Due to the enhanced time and magnetization scale with respect to atomic systems, the suffix super has been used to define these magnetic states and the magnetism of nanoparticle assemblies has been often called supermagnetism. The aim of this chapter is to describe how the ability to synthesise suitable nanoparticles systems, makes interactions among magnetic entities a fundamental tool to modify magnetic properties of nanostructured-based materials. The last section of this chapter is dedicated to some examples of application, i.e. magnetic recording and permanent magnets, where the interactions among magnetic element plays a fundamental role in determining their performances. © 2014 Elsevier Ltd.