J.J. de Miguel

Influence of the growth conditions on the magnetic properties of fcc cobalt films: from monolayers to superlattices

Abstract The growth and magnetic properties of films of fcc cobalt on Cu(100) substrates has been characterized by a multitechnique approach. The films are ferromagnetically ordered in-plane at temperatures below Tc. The Curie temperature of the films displays a linear dependence with the coverage reaching bulk-like behaviour at coverages of 5–6 monolayers. Spin-polarized photoemission shows that the band structure is already close to that of the bulk at 5 ML. Crystalline Co/Cu sandwiches and superlattices have been grown on Cu(100) substrates. The magnetic ordering of Co slabs across Cu layers of varying thicknesses, as explored by SMOKE, changes from ferromagnetic to antiferromagnetic and back to ferromagnetic. The antiferromagnetic ordering has been confirmed by polarized neutron diffraction. As a function of the external magnetic field, the magnetic ordering changes to ferromagnetic with a complex intermediate behaviour.

The surface morphology of a growing crystal studied by thermal energy atom scattering (TEAS)

The growth of a Cu(100) surface from its vapour has been investigated by TEAS for different surface temperatures. The specularly reflected intensity of the growing surface shows temporal oscillations for in-phase and out-of-phase scattering conditions. In-phase data have been analyzed in terms of the, diffuse scattering from the surface steps whereas out-of-phase oscillations in terms of a kinematical and purely elastic description of the scattering from a random distribution of terraces. In both cases the temporal evolution of the step concentration of the growing crystal was obtained. Also, the surface diffusion coefficient of Cu adatoms was directly estimated from the experiments resulting in a preexponential of 1.4×10 −4 cm 2 s −1 and an activation energy of 0.40 eV.

QUANTITATIVE EVALUATION OF THE PERFECTION OF AN EPITAXIAL FILM GROWN BY VAPOR DEPOSITION AS DETERMINED BY THERMAL ENERGY ATOM SCATTERING

Abstract The growth of a Cu(100) surface from its own vapor has been investigated by monitoring the temporal variations of the intensity of the specularly reflected beam of thermal He atoms. The specular intensity shows oscillations with a periodicity corresponding to the growth of an atomic layer. These oscillations have been investigated for in-phase an out-phase scattering conditions and have been analyzed in terms of diffuse and purely elastic scattering respectively. Both analyses have led to the same quantitative results in terms of the surface concentration of steps of the growing crystal at a variety of surface temperatures. Also, the surface diffusion coefficient of Cu adatoms on Cu(100) has been estimated from the experimental data.

Spin- and angle-resolved photoemission from single crystals and epitaxial films using circularly polarized synchrotron radiation

Abstract We describe some novel results on the experimental spin-resolved electronic structure of a layered system, consisting of ultrathin films of fcc-Cobalt epitaxially grown onto a Cu(100) single crystal surface. Films more than 2 mono-atomic layers (ML) thick are found to be ferromagnetic at room temperature with an in-plane remanent magnetization. The electronic structure of 5 ML fcc-Co films turns out to be already bulk-like. On the other hand, the excitation with circularly polarized light leads to strong spin-polarization effects from the clean Cu(100) surface. This proves a significant influence of the spin-orbit interaction even in copper, resulting in strongly hybridized electronic energy bands.

Influence of surfactants on atomic diffusion

Abstract We have used Monte Carlo simulations with realistic interatomic potentials, combined with experimental results obtained by He diffraction (thermal energy atom scattering) and STM to investigate the effect of a surfactant agent such as Pb on the mechanisms of atomic diffusion involved in epitaxial metal growth. We find that the main role of the surfactant is to hinder fast diffusion by hopping over the surface, which is the dominant mechanism on a compact face such as Cu(111), and to promote exchange. As a side effect, this facilitates interlayer diffusion and hence layer-by-layer growth, because islands are smaller and have rougher borders; adatoms reaching an edge have more opportunities to cross them by exchange with a step atom.

Thickness-dependent coercivity of ultrathin Co films grown on Cu(111)

By using the magneto-optic Kerr effect (MOKE) we have investigated the coercivity of ultrathin epitaxial Co films grown by molecular beam epitaxy with the aid of Pb as the surfactant on Cu(111). We find two different regimes: below ~6 ML (ML≡monolayer), the coercive field Hc increases continuously, whereas for thicker layers it falls at a rate that is inversely proportional to the film thickness. While this latter behaviour is typical for bulk systems, we show that the initial one is an effect of reduced dimensionality, reflecting the variation of the Curie temperature of the magnetic films in the same thickness range. This phenomenon could have important implications for atomic-scale engineering of magnetic materials.

Temperature dependence of the step structure of vicinal Si(001) surfaces

Abstract Si(001) surfaces miscut slightly towards the [110] direction consist of alternating domains of (1 × 2) and (2 × 1) reconstruction, separated by inequivalent, single-atomic-height steps; at higher miscut angles, the surface is mainly monodomain, with terraces separated by double-atomic-height steps. High-resolution low-energy electron diffraction has been used to determine the step structure of vicinal Si(001) as a function of both temperature and miscut angle. The concentration of double-atomic-height steps continuously increases with miscut angle and, for vicinalities greater than ~ 2°, decreases with increasing temperature. From a comparison of the experimental results with the predictions of a one-dimensional model treating the problem in terms of chemical equilibrium in a two-component system we obtain information on the energetics of the structure transformation.

Epitaxy and magnetic properties of fcc cobalt films on Cu(100)

The epitaxial growth of fcc cobalt films on top of a Cu(100) substrate has been investigated by a variety of methods including Auger Electron Spectroscopy (AES), Medium Energy Electron Diffraction (MEED) and Thermal Energy Atom Scattering (TEAS). In a simultaneous AES/MEED experiment the growth mode was determined to be of the Frank-v.d. Merwe type. The magnetic properties of these films have been studied in situ by means of the Surface Magneto Optical Kerr Effect (SMOKE). We observed a strong thickness dependence of the Curie temperature leading to T c ≈ 300 K for a cobalt coverage of 2 atomic layers. The orientation of the magnetic moments, which was always found to be within the film plane, shows a distinctive in-plane magnetocrystalline anisotropy. In fcc cobalt films with a thickness of more than 3 monolayers we identified the 〈110〉 directions as the easy axes of magnetization.

Phase diagram of sulphur on Mo(110)

Abstract A preliminary study of the S/Mo(110) system indicates that it may be a rich testing ground for a variety of 2D transitions. Three ordered structures exist below θs = 0.5, namely p(2 × 2), c(2 × 2) and ( 2 1 2 1 ) . The p(2 × 2) ordered phase seems to disorder into the 2D gas phase through a continuous phase transition which should belong to the universality class of the XY model with cubic anisotropy. The corresponding order-disorder transition for the c(2 × 2) phase is predicted to belong to the Ising model. The ( 2 1 2 1 ) ⇄ c (2 × 2) phase transition is an example of a less-common order-order transition. The complete 2D phase diagram has been obtained and compared with MC simulations yielding the sulphur-sulphur lateral interactions.

Structural characterisation and homoepitaxial growth on Cu(111)

Abstract A comprehensive study of the homoepitaxial MBE growth of Cu on Cu(111) is presented. This system displays a wealth of features and a large accumulation of morphological and structural defects. It is demonstrated that all of them can be ascribed to two basic characteristics of fcc-(111) faces: the presence of two threefold adsorption sites at the surface, which allows the formation of stacking faults, and the existence of high Ehrlich–Schwoebel barriers at steps, hindering interlayer diffusion. This behaviour, therefore, must be common during growth on compact metallic faces, and could have important implications for the preparation of low-dimensional heterostructures.