Abstract: The invention relates to nanoparticles of noble metals, having a controlled microstructure which leads to the appearance of ferromagnetic behaviour in said nanoparticles, thereby enabling the use of very small magnets (<5 nm) in a range in which standard ferromagnetic metals behave as superparamagnetic entitles (disappearance of hysteresis cycle). The inventive nanoparticles can be used, for example, to reduce the dimensions in magnetic recordings, as well as in biomedicine as tools for biomolecule recognition, nuclear magnetic resonance imaging, drug-release control or hypothermia treatments.