Multiscale investigation of the structure and morphology of the Co / Fe 2 O 3 ( 0001 ) interface

We report a detailed structural characterization of Co films grown on $\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3}(0001)$ for thicknesses up to 25 nm. Epitaxial and single-crystalline 20 nm thick $\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3}(0001)$ layers deposited on $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}(0001)$ and Pt(111) single crystals were used as substrate. The $\text{Co}/\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3}(0001)$ interface is a prototypical magnetic exchange coupled system for which the magnetocrystalline parameters are crucial to address. We evidence medium range order requiring a multiscale approach to reach a reliable description of the crystalline structure. Surface extended x-ray absorption fine structure (EXAFS) and surface x-ray diffraction (SXRD) have been combined to describe the structure in an extended thickness range. Additional grazing incidence small angle x-ray scattering (GISAXS) measurements show that the growth is three dimensional up to $\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\text{nm}$ and follows a nucleation\char22{}growth\char22{}coalescence scheme. For all thicknesses cobalt is found to grow with a lattice parameter close to its bulk lattice parameter. In the early stages of growth a disordered 2\char21{}3 monolayer thick interface exhibiting oxidized cobalt and metallic iron is evidenced. Long range order sets in for thickness above 4 nm showing the coexistence of fcc, twinned fcc, and hcp stacking within direct and in-surface-plane $30\ifmmode^\circ\else\textdegree\fi{}$ rotated epitaxial relationships.