C. M. Schneider

Growth, morphology, and crystalline structure of ultrathin Fe films on Cu3Au(100)

The growth mode, morphology, and crystalline structure of Fe films on Cu 3 Au(100) are studied for diVerent growth temperatures (300 and 160 K ), using in situ scanning tunneling microscopy and low energy electron diVraction. Multilayer growth is found to be predominant for both growth temperatures. Only in films of 3‐4 monolayers (ML) grown at 300 K is a mixed mode of layer-bylayer growth and island growth observed. An fcc-to-bcc structural transformation, accompanied by a distinct change in the surface topography, starts at about 3.5 ML and 5.5 ML for the growth temperatures of 300 and 160 K, respectively. For both growth temperatures bcc-like Fe in Fe/Cu 3 Au(100) assumes, most likely through a Bain path, a surface plane with the (100) rather than the (110) orientation found in the Fe/Cu(100) system. Both the surface morphology and the onset thickness of the fcc‐bcc structural transformation are shown to be strongly aVected by the growth temperature.

Interplay between structure and magnetism in Fe/Cu(1 0 0) upon temperature variation

Abstract Interleaved low-energy electron diffraction (LEED) and magneto-optical Kerr effect (MOKE) measurements were carried out for room-temperature epitaxially grown FCC-like Fe/Cu(1 0 0) in the temperature range of 120–400 K. The structure of the Fe film was found to be not only dependent on thickness, as was believed previously, but also to be influenced by the temperature. Temperature-driven structural transitions were observed in the 4 and 4.5 ML films, this effect being more pronounced at 4 ML thickness. Whereas the whole 4 ML film assumes an FCC-like structure with a strong tetragonal expansion (FCT-like) at temperatures below 313 K, the bulk of the film relaxes into the ‘isotropic’ FCC-like structure and only the top layers remain expanded at temperatures above 333 K. Because only the FCT-like (expanded) Fe possesses ferromagnetic properties, the film becomes paramagnetic after heating above 333 K. This finding represents a new type of magnetic order-disorder transition and explains the lower value of the Curie temperature in the 4 ML Fe film as compared to 3 ML. In the 4.5 ML Fe film the similar correlation between the temperature-driven structural transition and an occurrence of the ferromagnetic long-range order was observed. Additionally, a pronounced difference in the energy positions of the characteristic maxima in the LEED I ( E ) curves for the (0 0) beam as well as a kinematic analysis of these curves imply a difference in the value of the tetragonal expansion for the entirely expanded FCC-like Fe film and the film expanded only in the topmost layers.

Epitaxial fcc FeCo alloy films on Cu(001)

Abstract Fcc FeCo alloy films (1.5–9 monolayers) were prepared on Cu(001) by epitaxial deposition at room temperature. Growth, structural and magnetic properties of these films were investigated by medium- and low-energy electron diffraction, Auger electron spectroscopy, as well as magneto-optical Kerr effect measurements. Addition of Fe in the fcc Co films results in an uneven vertical expansion of a part of the alloy films, its extent depending on composition. For more than 70 at.% Fe, essentially the whole film is vertically expanded, which is accompanied by a change of the growth mode and the appearance of some uniaxial structural irregularity in the plane of the films. The easy axis of the magnetization was found to lie within the film plane over the whole range of compositions for the investigated thicknesses. A linear increase of the Kerr signal at saturation magnetization with increasing thickness indicates that the whole alloy film is magnetic. A constancy of the Fe and Co local magnetic moments can be expected below 70–80 at.% of Fe.

Annealing effect on morphology and magnetism of ultrathin films of Fe and Ni on Cu(100)

Abstract Scanning tunneling microscopy (STM) and the magneto-optical Kerr effect (MOKE) have been used to investigate the effects of annealing on the morphology and magnetism of Fe and Ni films on Cu(100). Relatively mild annealing (490 K) created pinholes in the Fe/Cu(100) films, opening channels for the surface diffusion of copper to the top of the film surface along the walls of the pinholes. The magnetization of the Fe films is strongly affected by annealing. Annealing smooths the surface of Ni/Cu(100) films significantly, but no interdiffusion was observed upon annealing below 500 K. For Ni films thicker than 7 ML, the magnetization remains perpendicular after annealing, with only a slight reduction in its amplitude.

Element specific imaging of magnetic domains in multicomponent thin film systems

Abstract We employ a new form of photoemission microscopy to image magnetic domains with both element specificity and surface sensitivity. Its contrast mechanisms are based on magneto-dichroic effects in photo-induced Auger electron emission. The method is especially beneficial to the investigation of magnetic domains in multicomponent thin film structures. We demonstrate the capabilities of this technique for the case of epitaxial Cr/Fe(100) thin film systems. The magnetic domain structures of a monatomic Cr layer on Fe, a Cr wedge on Fe, and a wedge shaped Fe/Cr/Fe sandwich structure are investigated for both elements. An antiferromagnetic alignment of the Cr monolayer to the Fe substrate is directly revealed in the images. Exploiting the angular dependence of the magnetic dichroism, regions of bilinear and biquadratic exchange coupling in the Fe/Cr/Fe sandwich are unequivocally identified.

Direct evidence for complete antiferromagnetic coupling between Co films epitaxially grown on Cu(1 1 1) using Pb as surfactant

Abstract Co/Cu films were grown epitaxially on Cu(1 1 1), using Pb as a surfactant. The exchange coupling between Co layers separated by Cu is studied with magnetic circular dichroism in valence band photoemission. Direct evidence for complete antiferromagnetic coupling is observed at 4 monolayer Cu thickness for Co films both with perpendicular and in-plane remanent magnetization from the sign reversal of the dichroic asymmetry.

Perspectives in element-specific magnetic domain imaging

The discovery of magnetic dichroism in high-energy spectroscopies has opened up new opportunities for the spatially resolved investigation of magnetic microstructures. Involving characteristic electronic levels of the solid, soft X-ray magneto-dichroic effects combine magnetic sensitivity with elemental selectivity and thus permit element-specific analyses of magnetic phenomena. Of particular interest is their use as a contrast mechanism in magnetic domain imaging.

Magnetic dichroism in Co films on Cu(001) using unpolarized light: A thickness and temperature dependent study

Abstract The thickness and temperature dependence of the angular distribution of the magnetic dichroism in photoemission from Co 2p core levels of Co/Cu(001) ultra-thin films has been investigated, using unpolarized Mg Kα radiation. Photoelectron spectra were collected for emission angles in a range of ±9° around the surface normal in the [001] azimuth. All asymmetry spectra were found to exhibit an identical spectral shape within the experimental error bar, but different scaling. The angular distribution of the Co 2p dichroic signal shows that at emission directions off the sample normal strong variations are present, in contrast to predictions from simple atomic theory. These crystallinity-related modulations were studied for Co films of different thicknesses [5, 3, 2, and 1.5 atomic monolayers (ML)] and different temperatures (100 and 300 K). They are higher for thicker films and lower temperatures. This experimental finding proves a diffraction picture, in which this modulation around the surface normal is explained by diffraction effects in forward scattering along the sample normal. As diffraction effects in magnetic dichroism combine both diffraction and magnetic dichroism, its behavior is influenced by both, structural and magnetic properties of the film.

Tailoring epitaxial growth of low-dimensional magnetic structures by using surfactants

Abstract Ideal artificial materials such as magnetic thin films and superlattices are expected to possess unique properties owing to their reduced symmetry and dimensionality. In real systems, however, the actual behavior is extremely sensitive to the morphological features of the films and interfaces. {Co/Cu} heterostructures on Cu(111) are a prototypical example: their growth is complicated by several difficulties, such as the lack of interlayer diffusion or the appearance of stacking faults. We have been able to overcome these problems by using Pb as a surfactant during growth. In this way, ultrathin Co films and {Co/Cu} superlattices can be grown with custom-chosen properties: we can independently select to have either in-plane or out-of-plane magnetization, and ferro- or antiferro magnetic exchange coupling through the Cu spacer. The surfactant also prevents intermixing at the atomic steps, thus allowing us to grow one-dimensional structures (quantum wires).

Comparison of magnetism and morphology of ultrathin Fe films on Cu(100) and Cu3Au(100)

In order to obtain a deeper insight into the inter-relation of magnetism and morphology we compare the properties of Fe films on Cu(100) and Cu 3 Au(100) grown at different temperatures, using the magneto-optical Kerr effect and scanning tunnelling microscopy. In Fe films on Cu 3 Au(100) for both room- and low-temperature growth (RT and LT growth) neither an antiferromagnetic phase nor a good layer-by-layer growth, as found for RT growth in Fe/Cu(100), was observed. The critical thickness at which the spin reorientation transition from perpendicular to in-plane easy axis starts, is found to be 3.5 ML for RT-grown Fe/Cu 3 Au(100) as compared with 5.5 ML for the LT-grown films on both substrates.