Atomic scale observation of domain boundaries on c(2 x 2) Fe(001) thin film surfaces

Scanning tunneling microscopy (STM) measurements reveal that Fe thin films epitaxially grown on a MgO(001) substrate under appropriate conditions have an atomic step-terrace surface with a c(2 x 2) surface reconstruction, and that this surface is often accompanied by line-like patterns along the Fe(110) direction. We have investigated the electronic properties of the patterns in detail by atomically resolved STM. We found that the pattern is a domain phase boundary of the c(2 x 2) domains, and that the domain boundaries consist of atoms exhibiting a (1 x 1) structure. The STM image shows remarkable bias-voltage dependency as indicated by the contrast inversion between the patterns and c(2 x 2) domains as the polarity of the applied bias-voltage is changed, and the contrast achieved its maximum value at +0.2 V. This bias-voltage dependency of the patterns can be explained by the existence of surface states which are observed at around 0.2 eV above the Fermi level on the Fe(001) (1 x 1) surface and 0.4 eV on the c (2 x 2) domains.