Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single Cobalt-Phathalocyanine molecules adsorbed on well characterized magnetic nanoleads. A spin-polarized electronic resonance is identified over the center of the molecule and exploited to spatially resolve stationary spin states. These states reflect two molecular spin orientations and, as established by density functional calculations, originate from a ferromagnetic molecule-lead superexchange interaction mediated by the organic ligands. 73.20.At,75.70.Rf,85.65.+h Conceptually new device structures accounting for sizable quantum effects will be needed if the downscaling of electronic and magnetic devices were to continue. One of these new concepts is the marriage of molecular electronics and spintronics, where functional molecules become active device components within a circuitry where information is carried by spins [1,2]. Progress toward this tantalizing goal rely on our understanding of spin transport and magnetism in reduced dimensions, where fundamental playgrounds extend to the extreme limit of single atoms and molecules. Studies include spin transport across a well chosen molecule sandwiched between two magnetic leads [3,4], or even atomic-size constrictions formed by bringing two leads into contact [5,6]. One of the limiting drawbacks is the variability of the resulting conductance, which comes from the incomplete knowledge we have of the molecule-lead interface. For instance, only a few experimental studies have focused on the interaction between a molecular spin and a magnetic substrate [7,8]. A better understanding would enable us to target the chemical engineering needed for building the desired spintronic functionalities into a molecule. In the past years, it became possible with spin-polarized (SP) scanning tunneling microscopy and spectroscopy (STM and STS) to directly observe the interplay between magnetism and surface structure with atomic resolution. SP-STM can also serve as a model tunneling magnetoresistance device since the junction includes two well defined magnetic leads -the tip and the sampleseparated by a vacuum barrier. A link can then be accurately established between spin transport and density of states. Recently, through SP-STM it was possible to evidence how the magnetization switching of a nanocluster is influenced by the spatial location of the spin injection [9]. Another example can be found in SP-STM of single atoms [10]. When the SP current flows across a magnetic atom adsorbed on a magnetic surface rather than directly into the surface, the tunneling spin transport is significantly affected, and some control can be exerted through the choice of the atom.Here we show how tunneling spin transport can be modified by "dressing" atoms with organic ligands. We combine low-temperature SP-STM and model calculations to study a model system consisting of individual Cobalt-Phthalocyanine (CoPc, Fig. 1a) molecules adsorbed on a magnetic substrate. The interaction between the m...

The tip of a low-temperature scanning tunneling microscope is brought into contact with individual cobalt atoms adsorbed on Cu(100). A smooth transition from the tunneling regime to contact occurs at a conductance of G approximately G0. Spectroscopy in the contact regime, i.e., at currents in a muA range, was achieved and indicated a significant change of the Kondo temperature TK. Calculations indicate that the proximity of the tip shifts the cobalt d band and thus affects TK.

The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump to contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the adatom and the surface atoms.

Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A lineshape analysis of the bound state indicates that native adatoms decrease the surface-state lifetime, while a cobalt adatom causes no significant change. PACS numbers: 73.20.Fz, 68.37.Ef, 72.15.Qm The unique ability of scanning tunneling microscopy and spectroscopy (STS) to access locally the density of states of single adsorbed atoms and clusters has been recently used to investigate magnetic adatoms which -owing to the Kondo effect -exhibit a sharp spectroscopic structure close to the Fermi energy E F [1,2,3,4,5,6]. Surprisingly, only few studies of metal surfaces have been reported for non-magnetic adatoms and for a wider energy range around the Fermi level [4,7,8,9,10]. There is a good reason to explore the physics beyond a narrow range around E F . For instance, it is known that a localized attractive perturbation of a two-dimensional electron gas should result in the appearance of a bound state, split off from the bottom of the continuum, and with a wave function localized around the perturbation [11]. Surface and image-potential states represent an opportunity to investigate this scenario with STS. In fact, it has been predicted that bound states should appear around single alkali atoms on metal surfaces as a consequence of their attractive perturbation on these twodimensional electron gases [12].In this Letter, we present a comparative lowtemperature STS study of a set of adsorbates on the Ag(111) and the Cu(111) surfaces. We show that the density of states (DOS) of single silver and cobalt atoms adsorbed on Ag(111), as well as single copper and cobalt atoms adsorbed on Cu(111), exhibit a resonance below the binding energy E 0 of the surface states of these substrates. Within the framework of a Newns-Anderson model extended to a two-band interaction, we assign this resonance to a bound state split-off from the bottom edge of the surface-state band. A lineshape analysis suggests that the scattering of the surface state at the adsorbate affects its lifetime, depending on the adatom nature. This appears to be a general property of atoms interacting with a two-dimensional electron gas. The measurements were performed in a homebuilt ultrahigh vacuum scanning tunneling microscope at a working temperature of T = 4.6 K. The Ag(111) and the Cu(111) surfaces were cleaned by Ar + sputter/anneal cycles. The single copper and silver adatoms were created by controlled tip-sample contact, whereas the single cobalt atoms were evaporated onto the cold substrates by heating a degassed cobalt wire wound around a pure W wire (> 99.95 %). The evaporation, through an opening of ...

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