Abstract: The present invention relates to magnetic particle dispersions comprising coated monocrystalline and/or polycrystalline single nanoparticles of iron oxides and nano-particulate 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 treatment of iron deficiency anemia and for applications in therapeutic hyperthermia, particularly passive partial-body hyperthermia or cell tracking and magnetic resonance imaging (MRI).