Defect-induced charge freezing on epitaxial Fe3O4(001) film surfaces studied by spin-polarized scanning tunneling microscopy

We report the role of atom defects on the charge freezing of Fe3O4(00 1) Surfaces studied by spin-polarized scanning tunneling microscopy (SP-STM) using a Ni tip. Epitaxially grown Fe3O4(001) films on a MgO(001) substrate were used as samples. Atomically flat surfaces are obtained by annealing in an ultrahigh vacuum and in oxygen. The surfaces exhibit a (root 2 x root 2)R45 degrees reconstruction as revealed by STM with a W tip. STM images indicate surface termination at B-sites. An atomic structure with a 0.3 urn periodicity is observed within the cation rows that are aligned along the [110] direction. SP-STM images show a pronouncedly different periodicity of 1.2 nm on areas having surface defects such as cation vacancies. This corrugation with a 1.2 nm periodicity can be attributed to a charge localization of Fe3+ and Fe2+ ions that are trapped by cation vacancies and then isolated from the electron hopping process. The results also indicate the important role of oxygen vacancies in modifying the 1.2 nm periodicity.