Surface studies of magnetite (100)

Abstract

The (100) surface of magnetite (Fe3 O4 ) has been studied using scanning tunneling microscopy (STM) in ultra high vacuum (UHV) with a view to establishing a test surface for spin polarised STM experiments. Two structured surface arrangements were found. The first was found to form after annealing in UHV at 970 ± 15 K for about 4 hours. This consists of cubic terraces with edges aligned along [110]. The terraces are separated by 2.0 ± 0.2 A in the z-direction. This is twice the interplanar distance for magnetite (100) and implies that the observed terraces consist of a combination of octahedral and tetrahedral planes. However, this surface state is only metastable. Further annealing for a total of approximately 22 hours causes the surface to transform into another state. This state consists o f rows running along [110] directions which rotate by 90° every 2.0 ± 0.3 A in the z-direction. They have a width of 18 to 42 A and are found to become generally narrower with increasing anneal time. The corrugation perpendicular to the direction of the rows is approximately 2.0 A leading us to invoke the term nanoterrace to describe this structure. Once formed it persists until removed by mechanical polishing.