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The in-plane orientation of the magnetization in the center of domain walls is measured in Co/Ir (111) as a function of Co thickness via scanning electron microscopy with polarization analysis. Uncapped, thermally evaporated cobalt on an Ir(111) single-crystal surface is imaged in situ in ultra-high vacuum. The initial pseudomorphic growth with an atomically flat interface of cobalt on iridium ensures comparability to theoretical calculations and provides a study of an interface that is as ideal as possible. Below a cobalt thickness of 8.8 monolayers, the magnetic domain walls are purely Néel oriented and show a clockwise sense of rotation. For larger thicknesses the plane of rotation changes and the domain walls show a significant Bloch-like contribution, allowing to calculate the strength of the Dzyaloshinskii-Moriya interaction (DMI) from energy minimization. From the angle between the plane of rotation and the domain-wall normal an interfacial DMI parameter s = −(1.07 ± 0.05) pJ m ⁄ is determined, which corresponds to a DMI energy per bond between two Co atoms at the interface of tot = −(1.04 ± 0.05) meV.

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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 (4.5 Å RMS), while the surface of the closed Pt film is smooth (1.2 Å RMS), as proven by atomic force microscopy. Island growth of the Pt film on Co is observed, yielding a laterally inhomogeneous oxidation of uncovered patches up to an effective Pt coverage of about 0.9 nm.

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