  Iron oxide nanoparticles have tremendous scientific and technological
potential in a broad range of technologies, from energy applications to
biomedicine. To improve their performance, single-crystalline and defect-free
nanoparticles have thus far been aspired. However, in several recent studies
defect-rich nanoparticles outperform their defect-free counterparts in magnetic
hyperthermia and magnetic particle imaging. Here, an overview on the
state-of-the-art of design and characterization of defects and resulting spin
disorder in magnetic nanoparticles is presented with a focus on iron oxide
nanoparticles. The beneficial impact of defects and disorder on intracellular
magnetic hyperthermia performance of magnetic nanoparticles for drug delivery
and cancer therapy is emphasized. Defect-engineering in iron oxide
nanoparticles emerges to become an alternative approach to tailor their
magnetic properties for biomedicine, as it is already common practice in
established systems such as semiconductors and emerging fields including
perovskite solar cells. Finally, perspectives and thoughts are given on how to
deliberately induce defects in iron oxide nanoparticles and their potential
implications for magnetic tracers to monitor cell therapy and immunotherapy by
magnetic particle imaging.
