Single layer graphene is grown on 10 nm-thick Ir(111) thin films grown on sapphire substrates. These samples have high structural...

Graphene on Ir(111) is studied using scanning tunneling microscopy (STM) and x-ray standing waves (XSW). The graphene layer has a corrugated shape due to the moire pattern formed as a result of a spatially varying interaction strength between graphene and its substrate. The coherent fraction F-(111) determined in XSW allows us to determine the amplitude of this intrinsic corrugation when the incoherent contributions of wrinkles, step edges, graphene edges, and contaminations are taken into account. The corrugation is found to depend on the stress state of the carbon sheet. Using density functional theory (DFT) calculations, we show that graphene can compensate stress by increasing its corrugation rather than by a reduction of the C-C bond length.

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 x 2) or a (root 3 x root 3) R30 degrees 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 x 2) Eu structure displays plain paramagnetic behavior, whereas for the (root 3 x root 3) R30 degrees 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.

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 of similar to 0.3%, rotations of similar to 0.5 degrees, and shears over distances of similar to 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 the sample from the graphene preparation temperature to the measurement temperature. Although graphene weakly interacts with iridium, its thermal expansion is found to be positive, contrary to free-standing graphene. The structure of graphene and its variations is very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pinning of graphene at steps. DOI: 10.1103/PhysRevB.86.235439

Magnetic properties of nanometer-thick Co films intercalated at the graphene/Ir(111) interface are investigated using spin-polarized low-energy electron microscopy and Auger electron spectroscopy. We show that the graphene top layer promotes perpendicular magnetic anisotropy in the Co film underneath, even for relatively thick intercalated deposits. The magnetic anisotropy energy is significantly larger for the graphene/Co interface than for the free Co surface. Hybridization of the graphene and Co electron orbitals is believed to be at the origin of the observed perpendicular magnetic anisotropy.

The extended carbon-metal contact in graphene-metal hybrids opens new avenues for manipulating the properties of both constituents of the hybrid and for combining the functionalities of each of them. We developed a two-step ultrahigh vacuum route to fabricate high-quality nanometer-thick metal films having abrupt interfaces, sandwiched between a protective graphene layer and its substrate, using chemical vapor deposition and metal intercalation made effective at mild temperatures. We demonstrate functional hybrid systems with ferromagnetic metal films whose topmost graphene interface allows us to manipulate the direction of the magnetization of the film to a large extent. We obtain prominently perpendicular magnetization for a large range of Co thickness. The preparation and properties of the graphene/ferromagnet hybrid are analyzed using a set of surface-sensitive in situ and ex situ techniques together with first-principles calculations, altogether providing extensive topographic, chemical, magnetic, and vibrational characterization.

The structure and magnetic properties of Co clusters, comprising 26–2700 atoms, self-organized or not on the graphene/Ir(111) moiré, were studied in situ with the help of scanning tunneling microscopy and x-ray magnetic circular dichroism. Surprisingly, the small clusters have almost no magnetic anisotropy. We find indication for a magnetic coupling between the clusters.

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 defects, 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 an attractive candidate in spintronics for a number of reasons, among which are its electric-field-controlled conductivity, its expected long spin lifetime and its two-dimensional nature. A number of recent proposals call for the development of high-quality ferromagnetic thin films in contact with graphene, whereas only thick polycrystalline or three-dimensional (nanoclusters) morphologies have been demonstrated so far. We report on the growth of flat, epitaxial ultrathin Co films on graphene using pulsed laser deposition. These display perpendicular magnetic anisotropy (PMA) in the thickness range 0.5-1 nm, in agreement with our first-principles calculations. PMA, epitaxy and ultra-small thickness bring new perspectives on graphene-based spintronic devices making use of the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface phenomena such as the Rashba effect and electric field control of magnetism.

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. These 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.