Defect structures on epitaxial Fe3O4(111) films

Epitaxial Fe3O4(111) films were grown onto a Pt(111) substrate by repeated cycles of iron deposition and subsequent oxidation in 10(-6) mbar oxygen. A previous low energy electron diffraction (LEED) intensity analysis reveale d the regular Fe3O4(111) surface to expose 1/4 monolayer Fe atoms over a cl ose-packed oxygen layer underneath. With scanning tunneling microscopy (STM ) a hexagonal lattice of protrusions with a 6 Angstrom periodicity is obser ved. The protrusions are assigned to the topmost layer Fe atoms, which agre es with the dominating Fe3d electron density of states near the Fermi level related to these surface atoms, as revealed by ab initio spin-density-func tional theory calculations. The most abundant type of point defects observe d by STM are attributed to iron vacancies in the topmost layer, which was c onfirmed by LEED intensity calculations where different types of vacancy de fects have been simulated. For oxidation temperatures around 870 K the regu lar Fe3O4(111) surface coexists with several different surface structures c overing about 5% of the films, which expose 3/4 ML iron atoms or close-pack ed iron and oxygen layers, resulting in surface domains that are FeO(111) a nd Fe3O4(111) in nature. These domains are arranged periodically on the sur face and farm ordered biphase superstructures. At 1000 K oxidation temperat ure they vanish and only the regular Fe3O4(111) surface remains. [S0163-182 9(99)07539-6].