Ch. Rath

A structural analysis of the Co(0001) surface and the early stages of the epitaxial growth of Cu on it

The epitaxial growth of copper on Co(0001) was investigated by combined quantitative LEED and STM, including the clean surfaces Cu(111) and Co(0001). The mode of growth is a modified Stranski‐Krastanov type, i.e. pseudomorphic and approximately layer-like up to four monolayers of copper; later on, three dimensional islands develop. The stacking of the copper adlayers is exclusively fcc from the very beginning, whereby the ABAB stacking on a given terrace of the substrate is continued as ABABca. Since there are also terraces with an ABA stacking, fcc-twins (e.g. ABAcb) develop in the Cu films. Whilst in the submonolayer coverage regime of copper the cobalt substrate remains hcp stacked, films with more than about two copper layers are found to have induced a stacking fault in the substrate corresponding to a registry shift of the uppermost cobalt layer. As indicated by the analysis of a film slightly above one monolayer coverage, the registry shift seems to be triggered when the terraces of the substrate are fully covered by copper whereby the rearrangement of atoms probably starts at the step edges.

Surfactant-induced surface restructuring: (4×4)-Pb/Cu(111)

The atomic structure of a monolayer of Pb on Cu(111) was investigated by an in situ combination of low-energy electron diffraction, scanning tunnelling microscopy and Auger electron spectroscopy. The pronounced (4 × 4) superstructure observed is found to be due to a (in surface projection) hexagonally close-packed but vertically buckled Pb layer which induces a substantial complex restructuring of the first three Cu layers. In this interface to the copper bulk no intermixing of Pb and Cu atoms is observed. Surprisingly, the overall buckling amplitude of the first Cu layer is even larger than that of the lead layer and the positional height of Pb atoms residing on top of Cu atoms is lower than for all other atoms. The structural results agree qualitatively with recent effective-medium-theory calculations. The reconstruction induced by the lead adatoms shows that the substrate reacts in a structurally flexible way to their presence. It is proposed that this might be related to the surface-active (‘surfactant’) role that Pb plays in homogeneous and heterogeneous epitaxial growth on Cu(111).

ORDERED AND DISORDERED RIPPLING IN THE CoAl(110)-(1×1) SURFACE

A quantitative structural analysis of CoAl(110)-(1×1) by low energy electron diffraction shows that the topmost layer of the surface is rippled by an amplitude of 0.18 A, with aluminum atoms pulled out of the surface and the average distance to the second layer only slightly expanded. In contrast to the perfect stoichiometry found for the top layer and the chemical order in the bulk, about 20% of the second layer Al atoms are substituted by Co. This is probably due to preferential sputtering during the course of sample preparation and incomplete subsequent reordering. A new version of Tensor LEED allows the detection of some disordered rippling induced by the substitutional disorder in this layer, with substitutional cobalt being 0.05 A above the ideal aluminum position. The quality of the theory–experiment fit is mirrored by a Pendry R factor value of R=0.10.

Face-dependent segregation and relaxation in Mo0.75Re0.25 random alloy surfaces

Abstract The structure and composition of the three low index surfaces (100), (110) and (111) of the substitutionally disordered alloy Mo 0.75 Re 0.25 was investigated by full dynamical LEED intensity analyses. Strong face-dependencies are found for the composition of the surface layers as well as for the relaxation of their spacings. The (110) orientation turns out to be almost bulk-like terminated geometrically as expected for a quasi-close packed surface as well as chemically. The more open (100) and (111) surfaces, however, exhibit heavily oscillating layer stoichiometries and strong relaxations of interlayer distances both extending deep below the surface. The face-dependent segregation behaviour can be consistently explained by a surface induced tendency to stabilize an ordered intermetallic compound with the order parameter rapidly decaying with depth.

Surfactant-induced structures in the heteroepitaxial growth of Co on Cu(111)

The growth of Co on Cu(111) with Pb as a surfactant is found to be accompanied by a considerable surface reconstruction of (4 × 4) symmetry induced by the surfactant. Its crystallography was investigated by quantitative low-energy electron diffraction (LEED) for an initial and later stage of growth with deposition of 1.3 monolayer (ML) and 7 ML Co, respectively. In the low coverage case the surface reconstruction is rather similar to that of Pb/Cu investigated earlier. It extends deep into the substrate with simultaneous minimization of the Pb layer buckling. The structure seems to be controlled by dominating fcc-stacking characteristic of this early stage of growth. With further cobalt deposition the type of reconstruction changes. For the 7 ML Co film it is restricted to the Pb and top Co layer only with the buckling of the surfactant layer considerably increased. We discuss that this may be attributed to the slightly different lattice parameters of Cu and Co, though the influences of the different stacking involved cannot be ruled out. The top film layer is always reconstructed during the whole growth which might be responsible for the easy exchange processes which take place during the growth and are essential for the layer-by-layer growth as found earlier.

Structure and morphology of epitaxial Cu/Co bilayers grown on Cu(111) with Pb as a surfactant

The use of Pb as a surfactant has been shown to improve the magnetic properties of ultrathin Co/Cu heterostructures grown on Cu(111): the thickness range of Co films displaying perpendicular magnetic anisotropy is extended and a complete antiferromagnetic coupling between them is made possible. In this work, we apply low energy electron diVraction (LEED) and scanning tunneling microscopy (STM ) to study the growth of epitaxial Cu/Co bilayers on the Cu(111) surface precovered by one monolayer of Pb forming a (4◊4) superstructure, aiming to understand the origin of their magnetic properties. A quantitative comparison of experimental LEED spectra shows that the Cu layers grow in the twinned fcc structure, the amount of twinning depending on the number of stacking faults in the Co films. STM images of the capping Cu layers show not more than two atomic levels simultaneously exposed. This morphology is similar to that of Co films, which grow in the layer-by-layer mode in the presence of Pb. The result is the formation of sharp interfaces that favour the interface contribution to the magnetic anisotropy and allow the growth of layers with well defined thicknesses.

Phase transition and atomic structure of an Fe3Si(100) single crystal surface

The surface of Fe 3 Si(100) was investigated by low-energy electron diffraction (LEED) and Auger electron spectroscopy. Three different phases were observed depending on the sample annealing temperature. Two (1 x 1) structures with the full 2D symmetry of the DO3-type crystal surface can be prepared at temperatures T≤400°C and T≥ 680°C. The two phases exhibit different LEED spot intensities, indicating that their structures are different. In the temperature regime between 525 and 625°C only a subset of diffraction spots is visible, corresponding to a c(1 x 1) periodicity. In this phase obviously the chemical order of the DO3 structure is lost in the surface region. For the low temperature (1 x 1) phase the LEED structure analysis favours a model consisting of a bulk-like layer stacking sequence with an additional silicon layer on top of a mixed Fe-Si layer.

Reduced coercivity in ferromagnetic Co–Cu coevaporated epitaxial films on Cu(111)

Epitaxial films grown by coevaporation of Co and Cu on Cu(111) were investigated by low-energy electron diffraction and surface magneto-optical Kerr-effect measurements. The films are dominantly face-centered-cubic stacked up to high Co concentrations and exhibit ferromagnetism. Their coercivity is significantly reduced compared to pure Co films produced by thermal Co deposition on Cu(111) independent of the use of Pb as a surfactant.

Cosegregation-induced epitaxial growth of two- and three-dimensional compounds on multicomponent alloy surfaces

In this contribution the cosegregation-induced epitaxial growth of two- and threedimensional chromium nitrides on ferritic Fe-15%Cr-N(lOO) (CN = 30 wt-ppm) single crystal surfaces will be discussed. The two-dimensional CrN surface compound is stable between 600 and 720°C. From the (1 x 1) LEED pattern it is inferred that the surface compound is epitaxial to the bcc(100) alloy surface. XPD and LEED-IV investigations have revealed that this surface compound consists of a single CrN compound layer plus an additional subsurface chromium layer with a huge interlayer expansion between both layers. The CrN surface precipitate formed at temperatures T < 600°C is also epitaxially arranged on the bcc(100) substrate surface. Its structure corresponds to the rocksalt structure, i.e. the structure of the well-known bulk CrN. Starting from a sputter cleaned alloy surface the growth of the epitaxial CrN surface precipitate proceeds via the two-dimensional CrN surface nitride. After completion of this two-dimensional CrN layer the nucleation and growth of the three-dimensional CrN surface precipitate takes place.