Low-temperature growth favours hcp structure, flatness and perpendicular magnetic anisotropy of thin (1-5 nm) Co films on Pt(111)

The structural and magnetic properties of thin Co films grown on Pt(111) were investigated in situ under UHV using simultaneous surface x-ray diffraction, resonant magnetic surface x-ray diffraction at the Pt LIII absorption edge and the surface magneto-optic Kerr effect. We focus on the difference between low-temperature growth and growth at 300 K. Thin (1-5 nm) Co films grown at 120 K (low-T films) have the hcp structure and a small concentration of stacking faults compared to films of the same thickness grown at 300 K (RT films), which contain many stacking faults and have a predominant fcc structure. In addition, the low-T films are flatter. The RT films exhibit a clear reorientation transition of the magnetization, from perpendicular to parallel, at a thickness of 1 nm. In contrast, the low-T films display intense polar Kerr signals up to the largest investigated thickness (5 nm). In these films, the reorientation transition takes place over a large thickness range, through a canted phase. Heating up a low-T film to 300 K induces a rotation of the average easy axis towards the film plane, still with a high polar Kerr signal at 300 K. This behaviour with temperature is mostly reversible. These results are discussed on the basis of the temperature dependence of the anisotropy constants and magnetocrystalline anisotropy differences between the fcc and hcp phases