The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2
times 2)$ or a $(sqrt{3} times sqrt{3})$R$30^{circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(sqrt{3} times sqrt{3})$R$30^{circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.
The structure and magnetic properties of Co clusters, comprising from 26 to 2700 atoms, self-organized or not on the graphene/Ir(111) moire, were studied in situ with the help of scanning tunneling microscopy and X-ray magnetic circular dichroism. Su
rprisingly the small clusters have almost no magnetic anisotropy. We find indication for a magnetic coupling between the clusters. Experiments have to be performed carefully so as to avoid cluster damage by the soft X-rays.
