The present invention relates to magnetic particle dispersions comprising coated individual monocrystalline and/or polycrystalline nanoparticles of iron oxides and nanoparticulate aggregates (multi-core particles) thereof with improved nonlinear magnetization behavior and improved heating properties in alternating magnetic fields. When measured in a magnetic particle spectrometer (MPS) the particle dispersions show a pronounced overtone structure, especially in the higher harmonics, which surpasses all previously known particle systems many times over. Therefore, the dispersions are especially useful for applications such as MPI (magnetic particle imaging). In addition, the new particle dispersions are suitable for applications in therapeutic hyperthermia, particularly passive partial-body hyperthermia or cell tracking and magnetic resonance imaging (MRI). Hence, the diagnostic and therapeutic use of the dispersions as well as pharmaceutical compositions of diagnostic or therapeutic interest comprising these dispersions are also objects of the present invention.
