Quantum-size-induced oscillations of the electron-spin motion in Cu films on Co(001)

We report on spin-polarized electron reflection experiments in which the electron-spin motion is studied in spin-dependent quantum well structures. Oscillations of the electron-spin motion due to quantum interference are observed in the model system Cu/Co(001) both as a function of electron energy and Cu overlayer thickness. The reflectivity as well as the spin-motion data can be well interpreted in terms of a Fabry-Pérot interferometer model. In particular, this opens the possibility of studying the spin-dependent reflection properties of the buried Cu/Co interface.     

Scattering of electrons on the Cu(001) surface by Co adatoms

Excited electrons at surfaces can be scattered by adsorbate atoms or defects, which changes the energy or momentum. Such scattering processes can be studied by energy, time and angle-resolved two-photon photoelectron spectroscopy. In this article the influence of statistically distributed Co adatoms on a Cu(001) surface on the dynamics of electrons in image-potential states is investigated. Different scattering mechanisms, such as interband, intraband, and bulk scattering are identified and analyzed quantitatively. Cobalt adatoms cause mainly quasielastic scattering of electrons in image-potential states. Inelastic processes are due to interactions with electrons in the substrate and are not significantly increased by Co adatoms. The results are compared to previous experimental and theoretical work on Cu adatoms. PACS 73.20.At; 68.49.Jk; 79.60.Ht     

Kerr-effect magnetometry of the single monolayer Co/Cu(001)

Experimental evidence is given for a four-fold in-plane magnetic anisotropy of the p(1×1) Co monolayer epitaxially grown on a Cu(001) substrate. Temperature dependent hysteresis curves show the magnetization remaining almost constant up to 400 K and the coercive field drastically increasing as the temperature is decreased.     

Vibrational spectroscopy of ordered oxygen adiayers on Ni/Cu(001) and Co/Cu(001) thin film systems

We report surface vibrations in c(2 × 2) oxygen adlayers on Ni and Co thin films on a Cu(001) substrate measured at gG by high resolution EELS. For the Ni thin film surface, one phonon peak is measured for varying film thicknesses from 1.3 ML (monolayer) to 6 ML with a constant energy of 221 cm⁻¹. For the Co thin film surface, three loss peaks are found, whose relative intensities change as the film thicknesses are varied. One loss peak at ˜520 cm⁻¹ is tentatively assigned to the Fuchs-Kliewer mode of cobalt oxide (CoO). The other two peaks at 317 and 376 cm⁻¹ are likely related to different bonding sites. Surface phonons on the p(2 × 2) Co thin film (389 cm⁻¹) and a bulk resonance mode (115 cm⁻¹) are also reported.     

Molecular dynamics simulation of Co thin films growth on Cu(001)

Results of MD simulations of the structure of ultrathin films of Co on Cu(001) are presented. Growth conditions corresponding to vacuum evaporation as well as laser ablation are considered. The dynamics of the growth process and the structure of the as-deposited films are investigated as a function of the kinetic energy of adatoms. The effect of fast interdiffusion due to a high impact energy is observed. Tight-binding potentials in the second-moment approximation are used. Co–Cu interaction parameters are determined from ab-initio electronic structure calculations.     

Ab initio study of microscopic processes in the growth of Co on Cu(001)

Using density-functional theory we investigate the energetics of various diffusion processes relevant for the heteroepitaxial growth of Co on Cu(001). We focus on how the barrier height depends on the local coordination, the interaction with the substrate and spin-polarization. We determine the temperature at which the different atomistic processes are activated and discuss their implications for the growth morphology. In particular, atomic exchange divides the temperature scale into two distinct regions: At lower temperatures growth proceeds similar to the homoepitaxial case, above the activation temperature of exchange a qualitatively different bimodal surface morphology evolves. PACS 68.55.-a; 68.36.Fx; 75.70.Rf; 68.47.De; 71.15.Mb     

SHG investigations of the magnetization of thin Ni and Co films on Cu(001)

Received: 22 November 1998     

Labyrinthine island growth during Pd/Ru(0001) heteroepitaxy

Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections.     

Embedded Co islands on Ge(001)

The adsorption of Cobalt (Co) on Germanium (Ge) (001) surfaces has been studied using scanning tunneling microscopy. Upon annealing at temperatures of 500–550 K well-ordered rectangular shaped embedded islands are formed. Based on our scanning tunneling microscopy data we propose that the elementary building block of these embedded islands consist of six Co atoms arranged in a hexagonal pattern. A statistical analysis reveals that the embedded Co islands exhibit an attractive interaction in a direction perpendicular to the substrate dimer rows and a repulsive interaction in a direction along the substrate dimer rows. The embedded Co islands eventually convert to perfectly straight and micrometers long nanowires upon annealing at temperatures that exceed 700–750 K.     

Uniaxial magnetic anisotropy in nanostructured Co/Cu(001): from surface ripples to nanowires

We have investigated the correlation between morphology and magnetic anisotropy in nanostructured Co films on Cu(001). The formation of nanoscale ripples by ion erosion is found to deeply affect the magnetic properties of the Co film. A surface-type uniaxial magnetic anisotropy with easy axis parallel to the ripples is observed. The origin of the magnetic anisotropy has been identified with the modification of thermodynamic-step distribution induced by ripple formation. At higher ion doses, when Co ripples detach and crystalline nanowires form, a strong enhancement of the magnetic anisotropy due to magnetostatic contributions is observed.     

Fermi surface nesting and structural transition on a metal surface: In /Cu(001)

Peierls-type instability and structural phase transition are shown to occur on the surface of a normal metal. An In overlayer on Cu(001) undergoes a reversible transition at approximately 350 K. Scanning tunneling microscopy of the low-temperature, reduced-symmetry phase indicates a strong periodic lattice distortion (PLD). Angle-resolved photoemission of the high-temperature phase reveals that the In-derived surface resonance constitutes a square-shaped, quasi-two-dimensional Fermi surface within the projected bulk Cu bands. The Fermi surface exhibits one-dimensional nesting upon the transition, which is in agreement with the PLD periodicity.     

Evidence for an anisotropy-induced non-collinear spin state in exchange-coupled Co/Cu(001)

The magnetization behaviour of a Co/Cu/Co(001) sandwich has been studied by magneto-optical Kerr effect measurements. The sample was grown by molecular beam epitaxy onto a sapphire (1 .2) substrate with a Cu/Cr/Nb(001) buffer system. The copper layer had the form of a wedge with the thickness range chosen to be around the second region of antiferromagnetic exchange coupling. The hysteresis loops in the regime of weak antiferromagnetic coupling show characteristic steps, which can be explained by an anisotropy-induced non-collinear spin state. Indication for a similar behaviour is also found in the regime of strong antiferromagnetic coupling. This behaviour is explained by taking into account the competition between anisotropy, interlayer exchange coupling and external field energy. The nature of this metastable non-collinear magnetization state is in marked contrast to the biquadratic (90°) exchange coupling which was discovered in Fe/Cr(001).