Ultrathin Cobalt Films Research Articles

Magnetic domain walls in ultrathin fcc cobalt films.

The micromagnetic structure or 180 o -domain walls in ultrathin cobalt films, grown on Cu(100), has been investigated by means or SEMPA. The comparison or our experimental data with one-dimensional micromagnetic calculations demonstrates that the magnetostatic energy has a pronounced influence on the domain-wall structure, even in the monolayer thickness range. Discrepancies between experimental observations and the one-dimensional model may be explained by a two-dimensional inplane structure or the domain walls

Oxidation kinetics of thin and ultrathin cobalt films

We have prepared polycrystalline Co films on epitaxially oxidized Si(111) surfaces and studied their oxidation kinetics under atmospheric conditions using the fact that metallic cobalt is a ferromagnet but CoO is an antiferromagnet and therefore practically nonmagnetic. As a consequence, oxidation is associated with a loss in magnetism. Results show that all samples with an initial thickness of di ≳ 5 nm oxidize practically instantaneously, whereby a constant amount of 2.5 nm of metal is transformed into oxide. For di < 5 nm the time constant for oxidation increases considerably and follows an approximately linear dependence with decreasing film thickness, reaching an extrapolated value of τ=190 days for di → 0. This increasing time constant let all samples with di < 2.5 nm appear ferromagnetic within the course of this study due to a nonoxidized metallic rest. Auger electron spectroscopy analyses revealed that the main oxidation product is in fact CoO, especially near to the metal interface, but that other compounds are also likely to be formed summing up to a total thickness of 6 nm including adsorbates. A second point of interest was the unidirectional magnetic anisotropy which the oxide imposes to the metal due to an exchange interaction at the interface. This anisotropy shows a sharp onset at a CoO thickness as small as ∼0.25 nm from which an anisotropy constant of ∼4.5×106 J/m3 can be estimated for ultrathin CoO layers.

Symmetry-induced uniaxial anisotropy in ultrathin epitaxial cobalt films grown on Cu(1 1 13).

Uniaxial anisotropy has been found in ultrathin cobalt films grown on a Cu(1 1 13) surface. Our studies using scanning electron microscopy with polarization analysis clearly show that the easy axis of magnetization is parallel to the direction of the step edges of the Cu(1 1 13) substrate. In spite of the different anisotropy behavior, the domain structures in Co/Cu(001) and Co/Cu(1 1 13) are similar, which indicates that the domain pattern in ultrathin films is little affected by the anisotropy.

Cross section transmission electron microscopy observations of ultrathin films of cobalt in multilayers

We present results obtained by cross section transmission electron microscopy on ultrathin cobalt films, i.e. with a thickness going down to 0.5 nm, separated by gold layers. Observation of the profile views shows that our films are continuous, even for the smallest thickness, a fact which had not been proven until now.

Cobalt ultra-thin film epitaxy on Pt(100) surfaces

To test the possibility of obtaining good Co-Pt multilayers, we studied the initial stages of growth of Co on Pt(100) by surface physics techniques in order to determine the structural order of ultra-thin Co layers. The LEED-Auger data for room temperature deposition can be explained by a quasi layer-by-layer growth showing a large surface disorder. Photoemission experiments using the synchrotron radiation in the 40–130 eV range confirmed that the interface is not really abrupt and evidenced a limited interdiffusion process below the cobalt overlayers.Annealing these cobalt ultra-thin films (2–10 ML) in the 200–400 °C range gave rise to a progressive epitaxy of a cubic phase on platinum, clearly identified by LEED and RHEED. This showed the possibility of building in this temperature range an interface structurally better defined than at room temperature.

Cobalt ultra-thin film epitaxy on Pt(100) surfaces

Abstract To test the possibility of obtaining good Co-Pt multilayers, we studied the initial stages of growth of Co on Pt(100) by surface physics techniques in order to determine the structural order of ultra-thin Co layers. The LEED-Auger data for room temperature deposition can be explained by a quasi layer-by-layer growth showing a large surface disorder. Photoemission experiments using the synchrotron radiation in the 40–130 eV range confirmed that the interface is not really abrupt and evidenced a limited interdiffusion process below the cobalt overlayers. Annealing these cobalt ultra-thin films (2–10 ML) in the 200–400 °C range gave rise to a progressive epitaxy of a cubic phase on platinum, clearly identified by LEED and RHEED. This showed the possibility of building in this temperature range an interface structurally better defined than at room temperature.

NOVEL MANY-BODY EFFECTS IN PHOTOEMISSION AND INVERSE PHOTOEMISSION SPECTRA OF ULTRATHIN TRANSITION-METAL FILMS

We present an overview of some recent developments in the theoretical modeling of transition-metal systems, particularly the ultrathin-film structures, focusing on the effects of electron-electron interactions. We describe the progress in the understanding of how to model realistic strongly correlated electron systems using and going beyond the local-density-approximation single-particle electronic structures. Results of exact many-body calculations of photoemission and inverse photoemission spectra of ultrathin nickel, iron and cobalt films are shown to illustrate the application of our approach. Interesting new features induced by many-body effects are found and discussed. Comparison with available experimental results is presented and further work, both experimental and theoretical, is suggested.

Micromagnetic properties of ultrathin cobalt films

The properties of the magnetic domain structures of ultrathin fcc cobalt films epitaxially grown on Cu (001) have been examined using an ultrahigh vacuum surface magneto-optic Kerr effect instrument. The evolution of magnetic behavior is observed for film thicknesses ranging from 1.4 to 7.5 monolayers. The coercivity is sensitive to film growth temperature and thermal cycling history. The coercivity decreases with diminishing film thickness and falls to very low values for the thinnest layers. The results are discussed in terms of Néel domain-wall micromagnetics for ultrathin films.

Ultra-thin cobalt films on Mo(110): overlayer growth and chemisorption of CO and D 2

The overlayer growth of cobalt on Mo(110) and the adsorption of CO and deuterium on Co/Mo(110) have been studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD). Co grows layer by layer on Mo(110) at a sample temperature of 115 K. Annealing multilayers of Co on Mo(110) to > 400 K results in the formation of three-dimensional (3D) Co clusters. The TPD spectra of Co from Mo(110) reveal two desorption peaks, β 1 and β 2, with activation energies of 82 and 85 kcal/mol, corresponding to desorption from 3D Co clusters and the first uniform Co overlayer, respectively. Submonolayer Co( < 0.75 ML, where 1 ML corresponds to the atomic density of the Co(0001) surface) is believed to form a pseudomorphic layer on Mo(110) upon annealing to 400 K. Co coverages from 0.75 to multilayers, annealed to 500 K, exhibit the following LEED patterns: a 9 × 2 structure, a hexagonal structure, and a 2 × 2 structure. The 9 × 2 and hexagonal structures are interpreted to correspond to a distorted lattice of Co(0001) and to 3D Co clusters on Mo(110), respectively. The TPD spectra of CO or D 2 on Co overlayers show desorption features with lower activation energies than the analogous desorption features from bulk Co(0001).

Ultra-thin cobalt films on Mo(110): overlayer growth and chemisorption of CO and D 2: Jian-Wei He and D. Wayne Goodman Department of Chemistry, Texas A & M University, College Station, TX 77843-3255, USA

Ultra-thin cobalt films on Mo(110): overlayer growth and chemisorption of CO and D 2: Jian-Wei He and D. Wayne Goodman Department of Chemistry, Texas A & M University, College Station, TX 77843-3255, USA

Evidence of epitaxial growth of bcc Co on Cr(100)

We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 × 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.

Evidence of epitaxial growth of bcc Co on Cr(100)

We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 × 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.

Magnetic domain structure in ultrathin cobalt films

The magnetic domain structures in ultrathin fcc cobalt films, epitaxially grown on Cu(100), have been investigated by means of a scanning electron microscope with spin polarization analysis of the secondary electrons. In the ‘as-grown’ state the films show a single domain configuration with small oppositely magnetized edge domains. Demagnetization of the films creates multidomain structures. In all films, ranging from 3 to 19.5 monolayer thickness, qualitatively the same domain structure has been found with the {110} directions as the easy axes of magnetization. The domain boundaries exhibit an irregular shape which is explained by a decreasing magnetostatic energy contribution to the Néel wall energy. Large domains with lateral extensions of some hundred microns dominate. A coinciding second phase with considerably smaller domains has been found occasionally in the films.

Magnetic hysteresis of cobalt ultrathin films with perpendicular anisotropy

We have investigated the coercivity mechanism in cobalt ultrathin films with perpendicular anisotropy. Important time-dependent effects suggest that thermally activated phenomena play a dominant role in these materials. A simple model based on thermally activated wall motions accounts fairly well for the experimentally observed shape and time dependence of the hysteresis loops. Conversely, it provides useful information on the activation energy distribution.

Magnetization and Curie Temperature of Ferromagnetic Ultrathin Films: The Influence of Magnetic Anisotropy and Dipolar Interactions (invited)

AbstractTheoretical investigations of the magnetization and Curie temperature of ferromagnetic ultrathin films in the presence of magnetic anisotropy and long-range dipolar interactions are presented. The Curie temperature of fcc (001) cobalt ultrathin films is calculated and compared with experimental results on Co/Cu (001) films. The influence of an external magnetic field, and the surface gradient of the magnetization are also discussed.

Structure and growth mode of metastable fee cobalt ultrathin films on Cu(001) as determined by angle-resolved x-ray photoemission scattering

The structure and growth mode of metastable fcc Co(001) films on Cu(001) substrates were determined by using angle-resolved X-ray photoemission scattering (ARXPS). The results show that at room temperature on Cu(001), cobalt grows with a small but measureable deviation from ideal layer-by-layer mode, with the formation of two-layer thick islands for average coverages below two monolayers (ML). When the coverage exceeds two monolayers, cobalt grows to a well-ordered fcc-(001) film via a layer-by-layer growth mode. The combination of low-energy electron diffraction (LEED) and ARXPS as the structural probes shows that high quality fcc films of Co of up to 20 ML thickness can be stabilized at room-temperature by epitaxial growth. Upon heating to 400° C, an unusual structure is formed in which substrate Cu atoms diffuse through the cobalt film to form a flat layer on top of an ordered fcc cobalt film.

Very large magnetoresistance effects induced by antiparallel magnetization in two ultrathin cobalt films

Magnetoresistance (MR) and magnetization measurements by SQUID have been performed on Au/Co.Au/Co/Au multilayers in which the two ultrathin cobalt films have perpendicular magnetizations and different coercive fields Hc1 and Hc2 due to their different thicknesses. The two Co layers are magnetically uncoupled, as shown by their well separated square hysteresis loops. The MR exhibits a plateau for magnetic fields H such as Hc1&amp;lt;H&amp;lt;Hc2, giving antiparallel orientation of their magnetization. The δR/R value is 1.4% at 300 K and 6% at 4.2 K. These experiments confirm the mechanism of spin-dependent diffusion at the interfaces leading to an increase of the resistance when the two magnetic layers have antiparallel magnetizations.

Magneto-optical studies of Co/Au ultrathin metallic films

We report on static and dynamic magneto-optical measurements of the magnetization in ultrathin cobalt films deposited on Au (111). Their large Faraday rotation associated to the squareness of their magnetic hysteresis loop makes these samples very promising as magneto-optical storage media.

Direct experimental identification of the structure of ultrathin films of bcc iron and metastable bcc and fcc cobalt

X-ray-excited photoemission and Auger-electron angular distributions from epitaxial transition-metal films are shown to be characteristic of crystal structure but insensitive to elemental species. This feature is used to identify the structures of ultrathin films of iron and cobalt, grown at room temperature on single-crystal metal substrates. It is found that cobalt grows in the metastable bcc Co(001) structure on bcc Fe(001) thin films, and fcc Co(001) grows on fcc Cu(001). While bcc Fe(001) can be stabilized on fcc Ag(001) substrates, this is not the case for cobalt films. Epitaxy of cobalt on Ag(001) results in a complex structure that is neither fcc nor bcc but is consistent with a body-centered tetragonal structure. These results are used to determine an upper bound on the strain that bcc cobalt films can sustain in room-temperature growth.

Magnetic after-effects in ultrathin cobalt films with perpendicular anisotropy

Using an alternating gradient magnetometer (AGM) magnetic after-effects have been observed in ultrathin ferromagnetic films for the first time. The relaxation time of magnetic reversal in epitaxial cobalt films on Au(111) in a constant applied field varies over several orders of magnitude depending on field and film thickness. The extrapolated relaxation time in zero field increases from 14 days to 11 years if the film thickness is reduced from 9.5 to 5.4Å. This should be an adequate stability for most practical applications of such films.