We used a scanning tunneling microscope to probe the interactions between spins in individual atomic-scale magnetic structures. Linear chains of 1 to 10 manganese atoms were assembled one atom at a time on a thin insulating layer, and the spin excitation spectra of these structures were measured with inelastic electron tunneling spectroscopy. We observed excitations of the coupled atomic spins that can change both the total spin and its orientation. Comparison with a model spin-interaction Hamiltonian yielded the collective spin configuration and the strength of the coupling between the atomic spins.

SrTiO 3 . Whereas in oxygen-deficient SrTiO 3−x and Nb-doped SrTiO 3 films the Hall constant increases markedly below 100 K (29), it is less temperature dependent in LaAlO 3 /SrTiO 3 heterostructures. In addition, the upper critical field of the heterostructures is an order of magnitude smaller than that of Nb-SrTiO 3 with the same T c . Finally, our observation of both superconducting and insulating behavior on the same sample, depending on the precise LaAlO 3 layer thickness, is very hard to reconcile with a pure oxygen vacancy scenario. Magnetic anisotropy allows magnets to maintain their direction of magnetization over time. Using a scanning tunneling microscope to observe spin excitations, we determined the orientation and strength of the anisotropies of individual iron and manganese atoms on a thin layer of copper nitride. The relative intensities of the inelastic tunneling processes are consistent with dipolar interactions, as seen for inelastic neutron scattering. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network in the copper nitride. These structures, which provide atom-by-atom accessibility via local probes, have the potential for engineering anisotropies large enough to produce stable magnetization at low temperatures for a single atomic spin.

The synthesis of wafer-scale single crystal graphene remains a challenge toward the utilization of its intrinsic properties in electronics. Until now, the large-area chemical vapor deposition of graphene has yielded a polycrystalline material, where grain boundaries are detrimental to its electrical properties. Here, we study the physicochemical mechanisms underlying the nucleation and growth kinetics of graphene on copper, providing new insights necessary for the engineering synthesis of wafer-scale single crystals. Graphene arises from the crystallization of a supersaturated fraction of carbon-adatom species, and its nucleation density is the result of competition between the mobility of the carbon-adatom species and their desorption rate. As the energetics of these phenomena varies with temperature, the nucleation activation energies can span over a wide range (1-3 eV) leading to a rational prediction of the individual nuclei size and density distribution. The growth-limiting step was found to be the attachment of carbon-adatom species to the graphene edges, which was independent of the Cu crystalline orientation.

In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host 1 . Spin S = 1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin 2 . Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment's energy on the orientation of the spin relative to its surrounding atomic environment 3,4 . Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.A low density of magnetic impurities in a non-magnetic host metal can have dramatic effects on the magnetic, thermodynamic and electrical properties of the material owing to the Kondo effect, a many-body interaction between the metal's conduction electrons and the electron spin of the localized magnetic impurity 5 . This interaction gives rise to a narrow, pronounced peak in the density of states close to the Fermi energy 1 . The past decade has seen a surge of interest in the Kondo screening of individual atomic spins, as a result of experimental advances in probing individual magnetic atoms by using scanning tunnelling microscopes 6,7 and single-molecule transistors 8,9 . When magnetic atoms are placed on metal surfaces the Kondo interaction between the localized spin and the conduction electrons is very strong, leading to high Kondo temperatures T K , in the range from 40 to 200 K (ref. 10). In order to probe the Kondo physics it is desirable to reduce this interaction so that the Kondo screening competes on an equal footing with external influences such as magnetic fields. It was shown recently that this can be achieved by incorporating a decoupling layer between the atomic spin and the screening conduction electrons 11 .