Abstract: The present invention relates to methods for producing oxide ferrimagnetics with spinel structure and iron oxide nanoparticles by soft mechanochemical synthesis using inorganic salt hydrates, oxide ferrimagnetics with spinel structure and iron oxide nanoparticles of ultra-small size and high specific surface area obtainable by the methods, biocompatible aqueous colloidal systems comprising oxide ferrimagnetics with spinel structure and iron oxide nanoparticles, carriers comprising oxide ferrimagnetics with spinel structure and iron oxide nanoparticles, and uses thereof in medicine.