Ultrathin Cobalt Films Research Articles

Rotated incommensurate domains of Co ultrathin films on Pt(111)

Low energy electron diffraction (LEED) and Auger electron spectroscopy were used to study the initial growth of cobalt ultrathin films on a Pt(111) surface at room and low temperatures. The films show incoherent epitaxy at room temperature. Unrotated and rotated incommensurate Co domains with two equivalent angles of rotation, +4.9° and −4.9°, with respect to the aligned substrate, are observed by LEED for 2 monolayers of Co Pt(111) after applying an annealing treatment. From a calculation of the mismatch for the corrugated surface, we confirm this reorientation angle. The evolution of the LEED satellite pattern for the deposition at 140 K is the same as for the deposition at room temperature, but a faint (2 × 2) LEED pattern was observed for higher coverages. UV photoelectron spectroscopy was used to monitor the evolution of the density of electronic states during deposition.

Magnetic domains of cobalt ultrathin films observed with a scanning tunneling microscope using optically pumped GaAs tips

We performed scanning tunneling microscope experiments with GaAs tips under illumination by circularly polarized light in ultrahigh vacuum at room temperature. Ferromagnetic ultrathin cobalt layers with perpendicular magnetization were employed as test samples. The spin-polarized tunneling current was shown to exhibit a magnetic sensitive component, changing its sign from one place to another on the sample surface. Reproducible images exhibiting magnetic domains of a 300–500 nm characteristic size were obtained. These results agree with magnetic force microscope measurements performed on a typical sample.

Surface x-ray diffraction fromCo/Pt(111)ultrathin films and alloys: Structure and magnetism

Surface x-ray-diffraction measurements have been done in cobalt ultrathin films (2--12 atomic layers) grown on Pt(111). Cobalt grows, at room temperature, in an imperfect layer-by-layer mode with its own lateral lattice spacing. The packing of the atomic planes is predominantly fcc but it changes to hcp upon annealing to $\ensuremath{\sim}450\mathrm{K}.$ Further annealing to $\ensuremath{\sim}550\mathrm{K}$ results in alloying. Several alloys have been identified with stoichiometries close to the bulk stable phases. Resonant magnetic diffraction experiments in the ${L}_{\mathrm{III}}$ absorption edge of Pt allow one to determine the magnetism of the Pt atoms at the interface. By measuring magnetic crystal truncation rods, it has been found that only the Pt atoms in contact with the Co overlayer are magnetized $(\ensuremath{\sim}0.2{\ensuremath{\mu}}_{B})$ being the magnetization of the second Pt layer about ten times smaller. The induced magnetic moment of the Pt interface has been found to depend rather linearly on the thickness of the Co film, similarly to the magnetization of the overlayer. The transformation fcc to hcp of the cobalt stacking results in a \ensuremath{\sim}25% enhancement of the magnetization at the interface.

Perpendicular magnetization in ultrathin electrodeposited cobalt films

Electrodeposited ultrathin cobalt films on Au(111) covered with a protective copper film can show perpendicular magnetization exactly like their ultrahigh-vacuum-grown counterparts. At high deposition rates of cobalt, out-of-plane magnetization is stabilized in the thickness range from 2 to 8 atomic layers while low deposition rates favor in-plane magnetization at any thickness. The cobalt films possess hcp structure with the $c$ axis perpendicular to the Au(111) plane. The technique is versatile and leads to quality standards comparable to those obtained by molecular-beam epitaxy, but at a much lower cost.

Magnetization reversal in ultrathin ferromagnetic films with perpendicular anisotropy

This paper presents a detailed study of the magnetization reversal dynamics in ultrathin cobalt films ( t Co = 6–12 A ̊ ) sandwiched by gold (1 1 1) layers, their magnetic anisotropy being perpendicular to the film surface. The domain wall (DW) velocity and nucleation rate are determined from direct time-resolved domain structure imaging. Measurements as a function of the applied magnetic field and temperature enable us to estimate the magnetic parameters, such as activation volumes, coercive fields, Gilbert damping parameters, etc., controlling the magnetization reversal. Depending upon the applied magnetic field value three different regimes are evidenced and studied. At low fields the DW pinning by structural inhomogeneities controls the thermally activated magnetization reversal dynamics. Above the propagation field value the dynamics is due to viscous DW motion. In higher fields a DW velocity breakdown is observed. The DW jaggedness and its change with the applied field and temperature is examined through the DW fractal dimension. A simple DW motion simulation taking into account an activation volume distribution explains qualitatively the data.

Investigation of the domain contrast in magnetic force microscopy

Domains were imaged by magnetic force microscopy (MFM) on materials where the domain size exceeds the sample thickness by three orders of magnitude. Selected samples are a magnetooptical medium and ultrathin cobalt films, all with perpendicular magnetization. A strong domain contrast is observed in both cases. This fact is confronted to the usual theory of MFM image formation, in which the stray field from one body (tip or sample) is sensed by the other, without altering of the magnetization distributions. It is shown that the domain contrast in such extreme conditions cannot be explained with that theory. On the contrary, Abraham and McDonald’s model, which considers the response of the sample to the tip field, is quantitatively compared to experiment. It is shown to provide a good qualitative description of the results, but not a quantitative one, because of oversimplification.

The growth mechanism and structure of ultrathin cobalt films deposited on the Pd(111) surface

Ultrathin films of cobalt were deposited by thermal evaporation on the Pd(111) surface at room temperature. The combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) shows that cobalt grows forming multi-atomic layer islands. X-ray photoelectron diffraction (XPD) and low energy electron diffraction (LEED) indicate that for the first 1–2 atomic layers the cobalt islands adopt an fcc structure with the same in-plane parameter as the substrate. For thicker films, cobalt maintains the fcc stacking sequence with a gradual relaxation to the lattice parameter of bulk Co.

Magnetotransport in microstripes patterned in ultrathin cobalt films

The magnetotransport properties are an attractive alternative to standard observation techniques for the characterization of the magnetization reversal in structures with reduced lateral dimensions (stripes). In particular, magnetic films exhibit the so-called anomalous Hall effect which is very sensitive to the local magnetic domain structures. We began to work with the model system Co/Au(111) grown on silicon dioxide. We present here a new method for microstructuring magnetic multilayers, that avoid the use of technological processes on the multilayers. Preliminary magnetotransport results in 50 μm wide stripes are presented and discussed.

Ultrathin films of cobalt on Fe{001} and the effect of oxygen.

Deposition of Co on a clean Fe{001} surface in ultrahigh vacuum yields initially films with 1\ifmmode\times\else\texttimes\fi{}1 low-energy electron-diffraction (LEED) patterns up to thicknesses of about 15 \AA{}, then films with increasingly high-background c(2\ifmmode\times\else\texttimes\fi{}2) LEED patterns for thicknesses up to about 30 \AA{}, and finally disordered films for larger thicknesses. Quantitative LEED analysis finds the 1\ifmmode\times\else\texttimes\fi{}1 films to have a body-centered-tetragonal structure with in-plane lattice constant a=2.87 \AA{} (pseudomorphic with the Fe{001} substrate) and c=2.792 \AA{}; the first interlayer spacing is contracted by about 11%. Strain analysis of the bulk identifies this structure as a tetragonal distortion of metastable bcc Co. The c(2\ifmmode\times\else\texttimes\fi{}2) films are found to be pseudomorphic distortions of hcp Co(112\ifmmode\bar\else\textasciimacron\fi{}0), which has a rectangular unit mesh that requires a 6.6% compression in one and a 0.4% compression in the other direction in the surface plane; the film has a bulk interlayer spacing of 1.29 \AA{} (expanded 2.9% over the equilibrium value 1.2535 \AA{}) and about 10% contraction of the first interlayer spacing. It was found that a monolayer of oxygen chemisorbed on the Fe{001} surface before deposition of Co extends the stability range of the strained bcc phase considerably. The oxygen ``floats'' on the surface of the Co film and a 30-\AA{} film still has the 1\ifmmode\times\else\texttimes\fi{}1 distorted bcc structure, but with a monolayer of oxygen on top. Thicker Co films no longer produce measurable LEED patterns, so that the onset of the c(2\ifmmode\times\else\texttimes\fi{}2)-distorted hcp (112\ifmmode\bar\else\textasciimacron\fi{}0) structure cannot be observed. \textcopyright{} 1996 The American Physical Society.

Growth mechanism and epitaxy of cobalt on the Pt(110) surface

Ultrathin cobalt films were deposited by thermal evaporation on the Pt(110) surface at room temperature. The growth mechanism was studied by a combination of low energy He + scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). The results indicate that during the initial stages of growth cobalt forms a layer of single atomic thickness. X-ray photoelectron diffraction (XPD) shows that the multi-atomic layer cobalt film is ordered and has a well defined epitaxial relationship with the substrate.

Visibility of cobalt ultrathin films in standard bright field electron microscopy: Possibilities of qualitativeobservations

Abstract We present here a study of the possibilities of standard bright field microscopy for studying cobalt layers 4 atomic planes thick: this microscopy is often used when qualitative information on many multilayer samples is needed. However, it is important to ascertain what conclusions can be drawn from the bright field images of such small objects without having to resort to a difficult quantitative analysis. We have compared experimental images obtained from ultrathin cobalt films to images calculated with or without defects, including the influence of thickness, defocus and tilt on the images.

Magnetization reversal processes in ultrathin cobalt films starting from demagnetized states

Remagnetization processes were investigated in ultrathin cobalt films with perpendicular anisotropy, after interrupting the magnetization reversal of a saturated sample and applying a large in-plane magnetic field. This study was performed by magnetooptical magnetometry and microscopy. Drastic differences were observed in magnetic properties depending upon the magnetic sample history.

Effect of submonolayer coverage on magnetic anisotropy of ultrathin cobalt films M/Co/Au(111) with M=Au, Cu, Pd

Using in situ polar magneto-optical Kerr-effect measurements, the variation of the Kerr rotation and magnetic anisotropy of a (0001)Co ultrathin film on a (111)Au substrate has been precisely recorded, during the first stages of the growth of an overlayer of metal M=Au, Cu, and Pd. As reported earlier, a drastic increase of the magnetic anisotropy was observed, with a peak around 1 monolayer of overlayer thickness. From a careful study of the variation of the remanent Kerr rotation, it could be shown that only the interface contribution to the anisotropy changes with the overlayer thickness, while the bulk contribution remains mostly constant. The overall behavior can be interpreted in terms of electronic effects in the metal overlayer, acting on the interface anisotropy via band hybridization at the interface.

Room temperature instability of Co/Cu(111).

The compositional and structural stability of UHV-grown ultrathin cobalt films on Cu(111) are investigated by low-energy ion scattering. Even at room temperature migration of copper substrate atoms onto the cobalt film by surface diffusion occurs during and after the deposition process. The resulting film and interface structure therefore depends critically on deposition rate and temperature as well as on time of measurement. Our findings provide an explanation for the controversial results on structural and magnetic properties of Co/Cu films and multilayers.

Magneto-optical Kerr effect study of ac susceptibilities in ultrathin cobalt films

The ac susceptibility of ultrathin Co/Cu(100) films has been investigated in situ under ultrahigh vacuum conditions using the magneto-optical Kerr effect. A conventional Kerr effect setup was combined with an ac-field modulation technique to improve the experimental detection limit. A sensitivity in the range of 10−8 rad for the Kerr ellipticity has been achieved in the present study. The ac susceptibility χac of the investigated cobalt films is mainly characterized by a pronounced and sharp maximum as a function of the temperature T. This χac(T) dependency corresponds to the ferromagnetic-paramagnetic phase transition occurring at the Curie temperature Tc of the cobalt films, which is also confirmed by experimental observation of the hysteresis behavior. Besides the measurement of Curie temperatures, the experimental technique presented here allows the determination of magnetic properties for ultrathin films even in the paramagnetic regime far above Tc.

Electronic structure of ultrathin cobalt films on W(110).

Ultrathin films of cobalt on W(110) are investigated by angle-resolved photoelectron spectroscopy excited with unpolarized and linearly polarized He i radiation. The epitaxial growth of the films is studied by reflection high-energy electron diffraction, x-ray-excited photoelectron diffraction, and Auger electron spectroscopy. The influence of symmetry, packing density, and film thickness on the electronic structure is discussed. The band structure of the quasi-two-dimensional systems is mapped out and compared with full-potential linearized augmented-plane-wave calculations for the pseudomorphic monolayer of Co on W(110) and for the unsupported close-packed monolayer.

Magnetic domain walls in ultrathin fcc-cobalt films

We present an experimental investigation of the domain wall fine structure in ultrathin cobalt films which we have studied by means of SEMPA. Our results demonstrate the influence and importance of the magnetostatic field energy for the structure of domain walls even in monolayer films.

Magnetic interface anisotropy in ultrathin cobalt films Au/Co/M/Au, with M = Au, Cu, Pd or Pt

We have measured the bulk and interface contributions to magnetic anisotropy on mainly hcp (0001) cobalt ultrathin films grown on polycrystalline textured Au(111) substrate, and covered with different metals. We have studied magnetic anisotropies versus nature of coverage metal M = Au, Cu, Pd or Pt. Large interface anisotropy constants K s were found for induced magnetic moment per interface atom was found in Co/Pd interface.

Expansion of ultrathin cobalt films in (Au/Co) multilayers measured by diffraction methods

In this paper, we present the results obtained from diffraction techniques, which give information on the structure of (00.1) h.c.p. cobalt thin films embedded in (111) f.c.c. gold layers. Electron and X-ray experiments were performed on gold-cobalt multilayers with smooth interfaces; they both show an expansion of the cobalt cell in the direction perpendicular to the interfaces; a smaller isotropic expansion in the interface plane is also shown, leading to a volume expansion and to a distortion of the cobalt cell. This expansion is found to be linked with the cobalt thickness and the nature of the spacer material.

Spin-polarized low-energy electron microscopy study of the magnetic microstructure of ultra-thin epitaxial cobalt films on W(110)

Magnetic domain formation in ultra-thin Co films on W(110) was studied with spin-polarized low-energy electron microscopy (SPLEEM). Films with the expected hexagonal structure were grown on the clean tungsten surface, but also fcc structured films with a (100) plane parallel to the surface could be prepared on a stabilizing tungsten carbide layer. Generally, fcc Co films showed very irregular in-plane magnetization between 3 and 11 ML thickness. We were able to record subtle changes in the magnetic domain structure with increasing film thickness. For films with hexagonal structure, first in-plane contrast was observed at 3 ML. Rotation of the substrate crystal established the presence of a uniaxial in-plane anisotropy with the easy axis along the W[1 1 0] direction for a 6 ML film.