Tailoring the Magnetism of Co Atoms on Graphene through Substrate Hybridization

We determine the magnetic properties of individual Co atoms adsorbed on graphene (G) with x-ray absorption spectroscopy and magnetic circular dichroism. The magnetic ground state of Co adatoms strongly depends on the choice of the metal substrate on which graphene is grown. Cobalt atoms on G/Ru(0001) feature exceptionally large orbital and spin moments, as well as an out-of-plane easy axis with large magnetic anisotropy. Conversely, the magnetic moments are strongly reduced for Co/G/Ir(111), and the magnetization is of the easy-plane type. We demonstrate how the Co magnetic properties, which ultimately depend on the degree of hybridization between the Co $3d$ orbitals and graphene $\pi$ bands, can be tailored through the strength of the graphene-substrate coupling.

Figure 1

(a) Normalized XAS spectra of Co atoms on G/Ru(0001) taken at normal and grazing incidence ($\mathrm{\Theta }=0.01\mathrm{ML}$, $B=6.8\mathrm{T}$, $T=2.5\mathrm{K}$). The spectra are offset for clarity. (b) STM image of 0.008 ML Co on G/Ru(0001) (mean cluster size $1.07\pm 0.04$ atoms, $V_{\mathit{t}}=-50\mathrm{mV}$, $I_{\mathit{t}}=100\mathrm{pA}$, $T=4.7\mathrm{K}$).

Figure 2

(a) and (b) Background subtracted XAS and XMCD of Co/G/Ru(0001), together with normal incidence spectra from the multiplet calculations [$\mathrm{\Theta }=0.01\mathrm{ML}$, $B=6.8\mathrm{T}$, $T=2.5\mathrm{K}$, average over 10 (a), respectively, 5 (b) spectra]. (c) Splitting of $d$ orbitals induced by the uniaxial crystal field according to multiplet calculations.

Figure 3

Magnetization curve of Co/G/Ru(0001) ($\mathrm{\Theta }=0.01\mathrm{ML}$, $T=2.5\mathrm{K}$). The points correspond to the maximum of the XMCD obtained from a single pair of ${\mu }_{+}$ and ${\mu }_{-}$ spectra. Since each of them is measured on a different surface spot, we accounted for intensity variations by dividing by the maximum of the background subtracted XAS at the Co $L_3$ edge. The XMCD maxima are proportional to the total moments of a Co atom, but are less accurate than the full sum rule analysis taking into account integrals over both $L_{2,3}$ edges. Therefore, we scaled the normal and grazing curves such that their ratio at 6.8 T is equal to the ratio of the corresponding total moments from sum rules, see Table 1. The solid lines show the fits with Eq. (1).

Figure 4

(a) and (b) Background subtracted XAS and XMCD of Co/G/Ir(111), together with normal incidence spectra simulated with multiplet calculations ($\mathrm{\Theta }=0.01\mathrm{ML}$, $B=6.8\mathrm{T}$, $T=2.5\mathrm{K}$, (a) and (b) average over six spectra). (c) Adsorption site deduced from STM image of a Co atom on G/Ir(111) with atomically resolved graphene. Black lines mark the sixfold graphene hollow sites imaged as protrusions ($3.2\times 3.2{\mathrm{nm}}^2$, $V_{\mathit{t}}=-50\mathrm{mV}$, $I_{\mathit{t}}=200\mathrm{pA}$, $T=4.7\mathrm{K}$). (d) Splitting of $d$ orbitals induced by the uniaxial crystal field according to multiplet calculations.