  Mixing of atoms at the interface was studied for Mn/Fe magnetic
hetero-epitaxial layers on Cu(001) by scanning tunneling
microscopy/spectroscopy. The formation of a surface alloy was observed when the
Mn layer was thinner than 3 atomic layers. From the fourth layer, Fe
segregation is suppressed, and a pure Mn surface appears. Accordingly,
spectroscopic measurements revealed the electronic difference between the
surface alloy and Mn layers. The surface electronic structure of the fourth Mn
layer is slightly different from that of the fifth layers, which is attributed
to the hybridization of the fourth layer with the underneath Fe-Mn alloy.
