High-sensitivity SQUID magnetometric techniques are currently used to study the magnetisation process in metallic, core-shell and dispersed magnetic nanoparticles in polymeric and nanoporous matrices used for biomedical and environmental applications. Magnetic interactions vary with composition, morphology and aggregation and are studied and analysed using analytical models. The figure shows the magnetisation curves of iron oxides, with a diameter of approximately 6 nanometres, acquired after cooling in a magnetic field and in the absence of a field.