Atomic-scale observation of vacancy ordering in magnetite nanoparticles

Magnetic nanoparticles have been extensively investigated for use in various advanced technologies. Among them, magnetite nanoparticles garner significant attention owing to their biocompatibility and superior magnetic properties. However, their performance can be affected by the presence of discrepancies from the ideal bulk structure. In this study, we carried out an atomistic microscopy observation of magnetite nanoparticles by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), which demonstrates the formation of vacancies in their structure reflected in changes in the atomic column contrast. In addition, the HAADF-STEM images, acquired along the [111] orientation, demonstrate the tendency of symmetric ordering of the observed vacancies in opposite atomic columns around the central brightest atomic columns. Atomic crystal structure optimization simulations confirm the lowest energies for such ordering configurations, in a good agreement with the experimental observations.