Structural and phase transformation of cobalt films grown on amorphous carbon

- Cobalt layers have amorphous structure at thicknesses below ~ 2.0 nm.
- Nanoclusters are observed in the cobalt layers with a thickness of 3-4 nm.
- Cobalt layers are fully crystalline at thickness of > 4 nm.

Structural evolution of the ultra-thin cobalt layers grown on amorphous carbon by DC magnetron sputtering were studied in detail by transmission electron microscopy and low-angle X-ray diffraction for a range of the cobalt thickness from 1.5 nm to 4.6 nm. It was shown that atomic structure of cobalt layers was amorphous at the layer thicknesses below 2 nm, an amorphous matrix with embedded nuclei of the crystalline phase with in-plane size of 1-2 nm in the thickness range from 3 nm to 3.2 nm, and polycrystalline with the randomly oriented HCP cobalt grains at thicknesses over 4 nm. Increase of the cobalt thickness from 3.2 nm to 4.6 nm led to growth of the cobalt grains with in-plane average size up to ~ 70 nm by the normal grain coarsening process. Transition of the atomic structure of cobalt from the isotropic amorphous state to the anisotropic crystalline state in the thickness range of ~ 2-4 nm was accompanied by deterioration of the magnetization vector direction within the ferromagnetic domains due to high magnetic anisotropy of HCP lattice of cobalt.