Growth and Surface Structure of Thin Co Films on Au(001) Studied by Scanning Tunneling Microscopy

We have studied the room-temperature growth of Co films on Au(001) substrate in the thickness range up to 3.4 monolayers (ML) and also studied the effects of annealing at 500 K. The surface morphology and structure at various Co coverages ($\theta_{\text{Co}}$) were investigated in scanning tunneling microscopy (STM) and low-energy electron diffraction studies. As-grown Co films up to 3.4 ML thick show bcc structures with Co islands. Annealing at 500 K activates several diffusion mechanisms, inducing surface segregation of Au and phase separation between Au and Co. These mechanisms change the structure and surface morphology of the films significantly. Consequently, regular nanostructures aligned in the $\langle 100\rangle$ direction and consisting of buried Co islands are formed and characteristic reconstruction of the Au(001) surface were observed on annealed Co films. From the observed STM height profile data we have inferred detailed surface and interface structures due to the large difference between the monolayer heights of the bcc Co(001) and fcc Au(001). In samples with $\theta_{\text{Co}}=3.4$ ML we observed a regular network structure with the ($2 \times 1$) reconstruction. The mechanism by which the buried Co islands form is discussed based on thermodynamic considerations.