Cap-layer-dependent oxidation of ultrathin cobalt films and its effect on the magnetic contrast in scanning electron microscopy with polarization analysis

We investigate the influence of a non-magnetic cap layer on the magnetic contrast of a Co film by scanning electron microscope with polarization analysis (SEMPA). Ex-situ sputter-deposited [Ir\Co\Pt]2 samples are terminated with wedge-shaped Pt layers with thicknesses ranging from 0 to 2 nm. The magnetic contrast almost vanishes at low capping, as well as towards the maximum Pt thickness. The highest contrast is found around 1 nm Pt cap layer, where about 25% of the contrast of a clean uncapped Co film is obtained. This contrast is sufficient to record high-quality SEMPA images. Energy-dispersive X-ray spectroscopy (EDX) reveals complete protection against oxidation from 1.1 nm onwards. Analysis of the contrast reduction towards higher coverages yields a spin-decay length of 0.7 nm in Pt for electrons with energies of roughly 6–10 eV above Fermi level. Quantitative EDX analysis indicates the formation of cobalt(II)hydroxide Co(OH)2 at a lower Pt coverage. The oxidation causes a roughening of the surface (...