We investigate the fine structure of graphene on iridium, which is a model for graphene weakly interacting with a transition metal substrate. Even the highest quality epitaxial graphene displays tiny imperfections, i.e. small biaxial strains, ca. 0.3
%, rotations, ca. 0.5^{circ}, and shears over distances of ca. 100 nm, and is found incommensurate, as revealed by X-ray diffraction and scanning tunneling microscopy. These structural variations are mostly induced by the increase of the lattice parameter mismatch when cooling down the sample from the graphene preparation temperature to the measurement temperature. Although graphene weakly interacts with iridium, its thermal expansion is found positive, contrary to free-standing graphene. The structure of graphene and its variations are very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pining of graphene at steps.
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.
Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of de
fects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e. much higher than on metal thin films used so far.
Graphene is attractive for spintronics due to its long spin life time and high mobility. So far only thick and polycrystalline slabs have been used as ferromagnetic electrodes. We report the growth of flat, epitaxial ultrathin Co films on graphene. T
hese display perpendicular magnetic anisotropy in the thickness range 0.5-1nm, which is confirmed by theory. PMA, epitaxy and ultrathin thickness bring new perspectives for graphene-based spintronic devices such as the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface effects such as Rashba and electric field control of magnetism.
