J. P. Bucher

Nanoscale Magnetic Domains in Mesoscopic Magnets

The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.

Visualizing the spin of individual cobalt-phthalocyanine molecules.

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.

The Quest for Nanoscale Magnets: The example of [Mn12] Single Molecule Magnets

Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects.

Size-Dependent Surface States of Strained Cobalt Nanoislands on Cu(111)

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.

Transition from One- to Two-Dimensional Growth of Cu on Pd(110) Promoted by Cross-Exchange Migration

A transition from one- to two-dimensional structures is observed in the growth of Cu islands on Pd(110). When Cu is deposited at substrate temperatures below 300 K, the adatoms move along the easy [10] direction forming 1D Cu chains. At higher temperatures transverse diffusion of adatoms is activated and 2D islands start to grow. Migration barriers are found to be (0.51 ± 0.05) eV and (0.75 ± 0.07) eV along and across the [10] directions, respectively. In the two-component driven diffusion regime, the shape of the 2D Cu islands is determined by the energy difference of the two orthogonal diffusion barriers.

Thermally induced disorder and conformational defects of alkane monolayers on graphite

Abstract The two-dimensional molecular lamellar arrangement of long chain alkanes adsorbed on the basal plane of graphite and its thermally induced disordering has been studied by variable temperature scanning tunneling microscopy. Atomic scale resolution of the disordered phase is achieved by studying the quenched high temperature phase. Overall as well as conformational disorder, in particular molecular kink defects, have been observed. High temperature snapshots show that the thermal evolution towards molecular disorder proceeds via small longitudinal fluctuations of molecules or blocks of molecules around their average positions. For increasing temperatures, molecular excursions along the long axis of the molecules become larger until eventually the lamellae lose their individuality, accompanied by substantial loss of lateral order.

Nanometric cartography of tunnel current in metal–oxide junctions

Recently, ferromagnet/insulator/ferromagnet trilayer junctions were shown to exhibit large magnetoresistance effects. However, these effects proved to be poorly reproducible from sample to sample. To get a nanoscopic insight on the origin of these fluctuations, we have used a combined atomic force microscope/scanning tunneling microscope setup to map the tunnel current that flows through metal/oxide stacks. The current histogram, which extends over 1–2 orders of magnitude, is found to be well described by a simple model which takes into account tiny spatial fluctuations of tunnel barrier thickness (typically, 0.1 nm). Moreover, our analysis shows that the total conductance of imperfect metal/oxide junctions tends to be dominated by very few sites. This result allows us to relate the sample-to-sample conductance fluctuations to slight local variations of the tunnel barrier parameters.

Electronic properties and local densities of states in clean and hydrogen covered Pt particles

Abstract 195 Pt pulsed nuclear magnetic resonance has been applied to the study of electronic properties of small supported platinum particles, having a mean size between 16 and 27 A. Special attention has been given to the effect of hydrogen adsorption. The relevant quantities for the analysis are the Knight shift related NMR intensities and spin lattice relaxation times. This information together with information from the size distributions obtained by electron microscopy allow a determination of the local density of states (LDOS) at the Fermi energy inside and at the surface of the particles. The LDOS at E F the clean surface shows a decrease of 39% with respect to the bulk which is mostly due to d-like electrons. The LDOS at E F of the hydrogen covered surface is further diminished and we have to go three Pt layers deep to recover an almost bulk LDOS.

Selective imaging of self-assembled monolayers by tunneling microscopy

The properties of thin organic films offer many challenging opportunities for science and technology. A crucial requirement for the advancement of molecular film technology is the selective characterization and modification on an atomic level. Local proximal probes like Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) bear certainly the potential for this purpose. So far, however, mainly adsorbed organic molecules lying flat on a smooth substrate have been imaged with near atomic resolution. Here, we demonstrate the ability of STM to selectively image self-assembled monolayers of long-chain molecules (hexanethiol) oriented upright on the substrate Au(111) with molecular resolution. Upon proper choice of the tunneling parameters we can image the molecular head-group anchored at the substrate and/or the molecular tail group.

Stability and lattice contraction of small platinum particles

Abstract The structural stability of Pt clusters of cubooctahedral and icosahedral shapes has been analytically studied by taking an approximation based on the second moment of the density of states to calculate the cohesive energy. We show that the predictions of this model are in agreement with the phenomenological ideas of Inos model and the known experimental results for the transition from cubooctahedral to icosahedral geometrical structure. The agreement between the calculated values for the contraction of the interatomic distance as a function of decreasing size, and very recent experimental results is excellent.