AbstractSTM tips made from antiferromagnetic MnNi have been used to investigate the atomic structure of the (001) and (111) surfaces of Fe3O4. The clean (001) surface displays a (√2 × √2)R45° superlattice, which is attributed to charge‐ordering on the surface, where Fe2+‐Fe2+ and Fe3+‐Fe3+ dimers can be discriminated. The oxygen‐terminated (111) surface is characterized by a hexagonal superlattice with a periodicity of 42 Å. Oxygen vacancies are observed in atomically resolved images of this superlattice. In the presence of an external magnetic field of 60 mT, a significant change in the atomic corrugation in the topmost oxygen layer around each of these defects is observed. The results on both (001) and (111) surfaces are discussed in terms of possible spin‐polarized effects. Microsc. Res. Tech. 66:85–92, 2005. © 2005 Wiley‐Liss, Inc.