Recent developments in high switching frequency wide-band gap semiconductors have resulted in increased performance of electronic circuits. The passive magnetic components (e.g. inductors), however, have not matched these advances and now account for disproportionate size and weight. Traditional magnetic materials (e.g. ferrites) can suffer from energy loss via heat generation by magnetic hysteresis and the formation of eddy currents. Therefore, a material is needed that possesses high magnetic saturation while eliminating these losses. Magnetite (Fe3O4) nanoparticles are a promising magnetic material, as they are air-stable and possess a high Msat of 92 Am2/kg. When reduced to a specific critical size, Fe3O4 nanoparticles display superparamagnetism: a phenomenon unique to magnetic nanoparticles in which the material has zero hysteresis. To maintain superparamagnetism at high switching frequencies, nanoparticle relaxation must occur faster than the switching magnetic field. This requires incredibly tight synthetic control as relaxation time is highly dependent on nanoparticle size. We recently showed that by synthesizing Wüstite (FeO) nanoparticles and then oxidizing to Fe3O4, we could achieve sub-nm control over nanoparticle diameter.1 However, some results indicate that internal defects can be introduced depending on oxidation temperature, which can reduce Msat up to 60%. Here, we use advanced electron microscopy techniques to observe and understand the emergence of structural defects in magnetic nanoparticles. We performed in-situ experiments using a Titan environmental TEM equipped with a Gatan K3-IS direct detection camera to identify synthetic conditions that lead to the reproducible formation of high quality, single crystal and compositionally pure Fe3O4 nanoparticles. As nanoparticle interactions can significantly influence relaxation kinetics we also used cryo-TEM to characterize nanoparticle assemblies as are present in solution.

In-situ electron microscopy to inform superior magnetic nanomaterials

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.