Spatially averaging techniques revealed that Co nanostructures on Pt(111) have an extraordinary high magnetic anisotropy due to a combination of unquenched orbital moments and a strong spin-orbit coupling induced by the platinum substrate [1]. The anisotropy of monolayer islands is determined to a large extend by the atoms at the perimeter of the islands [2]. Furthermore, the structure of the interior part of monolayer islands is strongly heterogeneous and changes on the scale of only a few atoms due to a dislocation network. These facts suggest that local inhomogeneities might be of importance for their magnetic properties. A study by spin-resolved scanning tunneling spectroscopy (SP-STS) is thus highly desirable.
We used SP-STS to study the electronic and magnetic properties of Co nanostructures on the Pt(111) surface. Within Co monolayer islands the d-like surface resonance is very sensitive to the local Co-Pt stacking resulting in a heterogeneous electronic structure [3]. A similar stacking dependence has been observed on single Co atoms where spectroscopic differences are related to the two possible adsorption sites of the atoms on the Pt(111) substrate.
Despite this electronic heterogeneity, the magnetic domains in the monolayer are clearly observed by SP-STS. New insights into the anisotropy of monolayer islands were obtained by analyzing the width of the domain walls. Based on our analysis we propose an out-of-plane anisotropy between +0.08 meV/atom and +0.17 meV/atom for atoms within the islands. Although some triangular islands have a base length up to fifteen times the domain wall width, they do not show a reorientation from out-of-plane to in-plane magnetization. Such a reorientation would be expected from the decreasing importance of perimeter atoms [2].
[1] P. Gambardella et al., Science 300, 1130 (2003).
[2] S. Rusponi et al., Nat. Mater. 2, 546 (2003).
[3] F. Meier et al., Phys. Rev. B 74, 195411 (2006).
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