Epitaxial growth and structure characterization of single-crystal Co(11{circle, and vertical bar}20) films on Cr(100) surfaces

Single-crystal Co films have been grown on two kinds of Cr(100) terminated surfaces on MgO(100) substrates by molecular beam epitaxy. The first was the growth of Co films on top of rough Cr(100) surfaces on MgO(100) substrate. hcp Co(11{circle, and vertical bar}20) films were prepared on such Cr(100) surfaces with about the same order of surface roughness as that of the Cr buffer layers. The second was the growth of high-quality and smooth Co(11{circle, and vertical bar}20) films on flat Cr(100) surfaces on Cr_xMo_1-x(100) graded buffer layers. We find that the use of Cr_xMo_1-x(100) graded buffer layers is critical to quickly prepare smooth and well-ordered Cr(100) surfaces. It is suggested that the symmetry breaking from four-fold Cr(100) to two-fold Co(11{circle, and vertical bar}20) may be due to the existence of surface steps under proper growth kinetics. The surface structure and crystal orientation of these films were characterized by reflection high-energy electron diffraction, atomic force microscopy and X-ray diffraction.