No abstract

By using Auger electron spectroscopy and secondary-electron imaging, we show that, as a result of bombardment with 1.5-keV Ar ϩ ions, Al is preferentially sputtered and the surface structure of a decagonal Al 70 Co 15 Ni 15 quasicrystal transforms to body-centered cubic. Annealing the sample at 700 K restores the Al concentration at the surface and, thereby, the quasicrystalline structure. Real-time observations establish the orientational relationship between the quasicrystalline and cubic surface structures, which we use to propose an atomic model of their interface.

Secondary-electron imaging is presented as a practical method which allows investigation of the near-surface structure in real time. It is based on the observation that electrons backscattered from surfaces in the keV range show a strong enhancement of intensity along directions defined by atomic rows. The spatial imaging of such electrons reveals the symmetry of near-surface regions in real space. Three-dimensional views of the solid are readily obtained, which makes this method ideally suited for the study of unknown structures and any material system where there is a change of symmetry.

The atomic structure of Co films deposited on Cu(111) at room temperature is studied by imaging backscattered secondary electrons. The patterns show in-registry growth for the first two atomic layers.The next two monolayers show features of the fcc as well as the hcp structure. The hexagonal structure characteristic for bulk Co is found for subsequent layers.Thin films of magnetic metals epitaxially grown on nonmagnetic substrates primarily offer the possibility of studying magnetic phenomena in two dimensions. It is believed that the magnetic properties, like anisotropy, are intimately connected to the very atomic structure of these films. So it is not surprising that recently much effort has been dedicated to study the growth of a model system like Co films on Cu(111), as well as epitaxial Co/Cu superlattices. A further point of interest is that Co films on Cu may serve to stabilize the metastable fcc phase of Co. A natural question arises up to what thickness the fcc phase remains really stable.Cobalt metal crystalhzes in both the face-centeredcubic (fcc) and the hexagonal-closed-packed (hcp) structures with the transition temperature of 706+4 K for the hcp -+fcc transition and 691+4 K for the fcc -+hcp transition. ' The crystallographic data for bulk metals of Co and Cu are also very similar. These involve a negligible

Local real-space symmetry properties of the decagonal quasicrystal Al 70 Co 15 Ni 15 are determined using the secondary-electron imaging technique. A basic symmetry unit consisting of atoms on the vertices of tubelike stacked alternating pentagons and center-decorated decagons accounts for the observations.