In-depth structural characterisation of the bct-hcp phase transition in Co epitaxial films

In-depth structural evolution of Co layers grown on Fe(001) has been investigated by modulated electron emission measurements during both film growth, by thermal evaporation, and erosion by mild ion sputtering. Co growth results in an epitaxial body-centered tetragonal (bct) phase which progressively turns to hexagonal close packed (hcp) in the 10-35 monolayers (ML) range. Erosion of a 40 ML thick film, performed under sputtering conditions such as to preserve structural order, shows that the bct/hcp interface is located significantly deeper than expected from measurements during growth. It is concluded that the transition to the hcp phase in the growing film begins on top of the bet layer and afterwards extends both upward and downward, reducing the thickness of the underlying bet layer.