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Structure, morphology, and magnetism of epitaxial Fe x Co 1~x alloy ultrathin "lms grown on Cu(1 0 0) in ultrahigh vacuum were investigated by a multi-technique approach over the whole composition range up to thicknesses of nine atomic monolayers (ML). Combining the results of the di!erent techniques it is found that the "lms grow at room temperature in a distorted FCC structure with random chemical order. The amount and sign of the distortion depend on thickness and composition. Below +60 to 70% Fe content the alloy "lms are smooth, and exhibit two di!erent vertical interlayer distances. At higher Fe concentrations several superstructures are observed, which are attributed to regular structural rearrangements. The structural relaxation above 4 ML thickness known from pure Fe "lms is not observed in the FeCo alloy "lms. Instead a gradual structural change towards a BCC(1 1 0) structure with increasing thickness occurs at higher Fe concentrations, and is still not complete in "lms as thick as 9 ML. The interrelation between morphology and structure in Fe x Co 1~x /Cu(1 0 0) is discussed in terms of composition-dependent lattice parameters and strain. An expansion of the "lms in the plane leads to a stabilization of the FCC structure, whereas a compression leads to corrugated "lms with a tendency towards structural transformations. The absence of the relaxed FCC structure found in pure Fe "lms is explained by Co atoms acting as defects in the FCC structure of the alloy "lms either in a geometrical sense and/or because of their di!erent magnetic moment with respect to the Fe atoms in both ferromagnetic and antiferromagnetic con"gurations of FCC Fe. ( 2000 Elsevier Science B.V. All rights reserved.

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).

The electronic structure of three monolayers (ML) Co on Cu(111), grown with Pb as a surfactant and capped with two ML Cu, was studied using magnetic circular dichroism in valence-band photoemission. The easy axis of magnetization in these films is perpendicular to the surface, allowing an experimental setup of extreme symmetry (photon spin, light incidence, electron emission, and magnetization all aligned and perpendicular to the surface). In such a geometry features observed in magnetic dichroism can be directly related to the symmetry character of the relativistic band structure. Experimental data in the photon energy range of 6--24 eV are presented. The observed magnetic dichroism and its dispersion with photon energy can be explained on the basis of direct transitions in a bulklike band structure of Co.

A spin-reorientation transition upon a variation of the Fe concentration $x$ is observed in ultrathin fcc ${\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ alloy films on Cu(001) for $x$g77%. For film thicknesses below 4 monolayers the magnetic easy axis changes from in-plane on the Co-rich side to out-of-plane on the Fe-rich side. Since no structural transitions accompany this spin-reorientation transition, varying the alloy composition can be interpreted as varying the number of $3d$ electrons. Upon changing the average $d$-band filling by as little as 0.1 electron strong changes of the anisotropy energy are observed.

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.

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.

The electronic properties of fcc Fe Co/Cu(100) were investigated by spin-resolved photoelectron spectroscopy. At a fixed concentration a dispersion of the majority and minority bands with Δ 5 -symmetry towards higher binding energy along the Δ-axis of the Brillouin zone could be followed upon increasing photon energy. Upon variation of the concentration the spectra for the alloy films reveal a gradual shift of the Δ 5 minority spin band through the Fermi level and a change of the sign of the spin polarization at the Fermi level in full agreement with the expected tendency from Co to Fe. Such a behavior agrees with both a simple mixture of the Fe and Co electronic properties in a randomly mixed alloy film, and a chemical separation (clustering) of both constituents.

Magnetic circular dichroism in valence-band photoemission from perpendicularly magnetized Ni thin films on Cu(001) has been studied both experimentally and theoretically. Over the photon energy range of 11--27 eV for normal emission and normal incidence, intensity asymmetries up to 20% upon light helicity or magnetization direction reversal were found. Fully relativistically calculated photoemission spectra agree well with experiment and reveal the interplay between spin-orbit coupling and exchange splitting, which is at the origin of magnetic dichroism. From comparison of theory and experiment the band dispersion is determined and points of hybridization along the \ensuremath{\Delta} axis are identified. \textcopyright{} 1996 The American Physical Society.

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.

The interpretation of measurements of magnetic dichroism in angle-resolved photoemission from the valence bands of 3d ferromagnetic metals is complicated by the simultaneous presence of spin-dependent surface transmission of the spin-polarized photoelectrons into the vacuum. As a result, two separate mechanisms contribute to the change in the photoemission intensity spectra when the sample magnetization is reversed. The magnitude of the surface transmission effect in the ``light'' 3d metals has been measured using Cu(001) as a sample, and an intensity asymmetry of \ifmmode\pm\else\textpm\fi{}3% has been found. This is smaller than, but approximately the same size as, the magnetic dichroism measured for iron and cobalt, and demonstrates experimentally that neither mechanism can be generally neglected in favor of the other.

The loss of spontaneous magnetization in ferromagnets with increasing temperature belongs to the class of thermodynamical second-order phase transitions. When the critical (Curie) temperature is approached thermal fluctuations of the spins overcompensate their aligning exchange interaction, and the ferromagnetic long-range order leading to the occurrence of the spontaneous magnetization

Deposition of Pb on Cu(111) prior to the growth of Co/Cu superlattices changes the magnetic properties of the latter. The use of a surfactant induces layer-by-layer growth of Co and delays the fcc to hcp structural transition. As a consequence, the easy axis of magnetization remains perpendicular to the plane for Co/Cu films grown with Pb. On the other hand, the elimination of twinning in the Cu spacers grown with surfactants allows the observation of complete antiferromagnetic coupling in Co/Cu superlattices for both in-plane and out-of-plane magnetization.

Magnetic dichroism measurements of the valence bands of films of fcc Co/Cu (001) have been performed using angle‐resolved UV photoemission in low‐symmetry, off‐normal emission geometries, and linearly polarized light. Asymmetries of magnitude 4% are observed upon magnetization reversal. Evidence of both spin‐dependent surface transmission and magnetic dichroism in the angular distribution of photoelectrons is seen. The asymmetry spectra indicate sensitivity to both magnetic exchange splitting and to spin–orbit splitting.

An experimental and theoretical study of magnetic circular dichroism in valence band photoemission from 15 monolayer thick fcc Ni films on Cu(001) is presented. A highly symmetric configuration (light incidence, electron emission, magnetization direction, photon helicity, and surface normal all parallel) allows the illustrative interpretation of the dichroism in terms of the relativistic band structure. Photoemission experiments in the photon energy range of 11–27 eV are compared to fully relativistic one‐step photoemission calculations. From this comparison, the dichroic features can be directly related to the double group symmetry of the initial states, which is demonstrated by two examples.

Ultrathin films of 3d‐transition metal alloys, and in particular FeCo alloys, currently receive considerable interest because of their potential technological application and the possibility to adjust magnetic properties via the variation of composition and structure. To study magnetic and structural properties of the otherwise unstable fcc phase of FeCo, this structural phase was stabilized by epitaxial growth on Cu(001). Ultrathin FexCo1−x films were deposited at room temperature by coevaporation from two separate Knudsen cells, operated under stabilized conditions. The film thickness was varied between 2 and 9 monolayers (ML) and the Fe concentration between x=0.2 and x=0.95. The growth process was monitored by medium energy electron diffraction (MEED). Auger electron spectroscopy and low energy electron diffraction (LEED) were employed to analyze the composition and structure of the films. A nearly perfect layer‐by‐layer growth up to at least 9 ML, as seen by MEED, is encountered for x≤0.7. For higher...