Abstract
The electronic configurations of Fe, Co, Ni and Cu adatoms on graphene and graphite have been studied by x-ray magnetic circular dichroism and charge transfer multiplet theory. A delicate interplay between long-range interactions and local chemical bonding is found to influence the adatom equilibrium distance and magnetic moment. The results for Fe and Co are consistent with purely physisorbed species having, however, different 3d-shell occupations on graphene and graphite ( and , respectively). On the other hand, for the late 3d metals Ni and Cu a trend towards chemisorption is found, which strongly quenches the magnetic moment on both substrates.
Highlights
Graphene is recognized as an excellent candidate for spintronics applications
We suggest that on highly oriented pyrolitic graphite (HOPG), for both Co and Fe atoms, the activation barrier ∆ph from high- to low-spin configurations is higher than on FLG, so that impinging Co and Fe atoms are trapped in the first potential minimum [31]
Our suggested substrate dependent activation barriers for Transition metal (TM) physisorption raises the question on the detailed mechanism distinguishing the seemingly similar multilayer graphene samples HOPG and FLG
Summary
Measurements were performed at the beamline ID08 of the European Synchrotron Radiation Facility using circularly polarized light tuned to the 3d element L2,3 absorption edges. Magnetic fields of B = 5T were applied along the x-ray beam, both making an angle Θ relative to the surface normal. The XAS (XMCD) signal is defined as the average (difference) between positive and negative circularly polarized absorption spectra, which were measured at T = 8K in the total electron yield mode for polar (Θ = 0◦) and in-plane (Θ = 70◦) geometries. All spectra shown in this work correspond to a maximum total x-ray exposure time of 2 minutes, which minimizes observed beam-induced time effects. In our simulations we model the effect of electron charges at the surrounding carbon sites of the honeycomb network of graphene by a trigonal crystal field (CF) with symmetry C6v for hollow sites and C3v for top sites (see SI for a definition of the relevant CF parameters)
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