Exotic structures on magnetic multilayers

To characterize the possible magnetic structures created on magnetic multilayers a model has been formulated and studied. The interlayer inhomogeneous structures found indicate either (i) a regular periodic, (ii) a quasiperiodic change in the magnetization or (iii) spatially chaotic glass states. The magnetic structures created depend mainly on the ratio of the magnetic anisotropy constant to the exchange constant. With the increase of this ratio the periodic structures first transform into the quasiperiodic and then into the chaotic glass states. The same tendency arises with the depolarization of the magnetic moments of the first layer deposited on the substrate.Comment: 4 pages, 1 figur

X-ray absorption fine structure of adsorbates on metal surfaces

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THE ADSORPTION SITE OF FORMATE (HCOO) ON COPPER SURFACES

Les études SEXAFS précédentes de la nature de formiate sur Cu(100) et Cu(110) ont conduit de différentes conclusions. En particulier la distance Cu-O sur Cu(100) est trouvée très longue (~2.3Å) et est crue comme évidence pour une bond de surface anomale. En réanalysant les deux spectres par l'utilisation de la simulation de multicoquille atomique, les résultats montrent que la structure est, en fait, très similaire pour les deux surfaces avec une distance voisine Cu-O d'environ 1.97Å, qui est similaire que celle trouvée dans le formiate de cuivre anhydre. L'assignation précédente résulte de l'échec de la filtration de Fourier de la "contribution du premier voisin'' pour séparer les effets de deux liaison similaires pour cette coquille atomique. Le problème se pose par le déplacement de la distance O-O dans le formiate et de la distance Cu-Cu à la surface qui force les atomes d'oxygène à adopter les sites d'adsorption de faible symétrie.Previous SMAFS studies of the structure of formate on Cu(100) and Cu(110) have led to very different conclusions. In particular the Cu-O distance on Cu(100) was found to be very long (~2.3Å) and believed to be evidence for an 'anomalous' surface bond. Reanalysis of both sets of data using a multishell simulation shows that the structure is, in fact, very similar on both surfaces with a Cu-O nearest neighbour distance of about 1.97Å, similiar to that in anhydrous copper formate. The previous assignment arises from the failure of the Fourier filtering of the 'nearest neighbour contribution' to resolve the effects of two similar bondlengths to this shell. The problem arises because of the mismatch of the O-O distance in formate and the Cu-Cu distance on the surface which forces the oxygen atoms to adopt low symmetry adsorption sites

The effect of energetic ion bombardment during growth on the interface structure of Co/Cu multilayers

A modified unbalanced magnetron sputtering technique was applied to the growth of multilayer Co/Cu films in order to systematically study the interface structure evolved under ion bombardment. X-ray reflectivity measurements revealed an interface smoothing effect in samples deposited under ∼200 eV ion bombardment. A reduction in the Bragg peak intensity for samples with a greater number of bilayer repeats was also observed. This was attributed to roughening of the final surface in the thicker films

Microstructural modification in Co/Cu multilayers prepared by low energy ion-assisted deposition

The microstructure of Co/Cu multilayers deposited by low energy ion-assisted deposition was investigated. The samples studied were grown by unbalanced magnetron sputtering with and without an applied d.c. substrate bias of −50 V. Specular and off-specular X-ray reflectivity measurements were performed on the samples and revealed the presence of roughness at the interfaces that was partially correlated throughout the film. The effect of applying a −50 V bias was to suppress the correlation of the lower frequency roughness and to slightly reduce the bilayer period of the multilayer. The differences between the samples are discussed in terms of possible ion bombardment induced smoothing of the layers and densification of the microstructure

THE STRUCTURE OF THE Cu(111) (√3 x √3) R30°-Cl SURFACE : A COMBINED SEXAFS AND PHOTOELECTRON DIFFRACTION STUDY

L'étude de la surface EXAFS de Cu(√3 x √3)R30°-Cl, en enregistrant le spectre du rendement total électronique au delà du seuil d'absorption - K de Cl en utilisant SRS Daresbury, montre que la distance interatomique CuCl dans cette phase est de 0.05 ± 0.02Å plus longue que celle de CuCl massif. Ce résultat est donné a la fois par l'analyse de Fourier en filtrant une seule coquille atomique at par simulation de multicoquille atomique. Les deux analyses révèlent aussi l'adsorption dans les sites creux de coordination - 3, cependant aucune de ces deux méthodes permet de faire une distinction entre deux sites, creux inéquivalents, de coordination - 3 à la surface (111) du cubic face centrée (cfc). Ces deux sites montrent que la contribution pour l'EXAFS des atomes les plus proches est identique dans toutes les directions de polarisation et diffère seulement à la troisième (et plus) coquille atomique la plus proche. Par contrast les spectres obtenus par diffraction photoélectronique pour la photoémission 1s de Cl, le long des directions [111] et [110], fournissent de claires distinctions entre ces sites en montrant que les atomes de Cl sont adsorbés dans les mêmes sites du "cfc" que la prochaine couche atomique de Cu pourrait occuper, sur les sites creux de coordination - 3 dans la couche atomique supérieure (ou extérieure) et la second du métal. La nature complémentaire de ces deux techniques dans l'enregistrement du spectre et de son analyse est clairement démontré par cette étude.A Surface EXAFS study of the Cu(√3 x √3)R30°-Cl structure, collecting data above the Cl K-edge in total electron yield on the SRS at Daresbury, indicates that the Cu-Cl distance in this phase is 0.05 ± 0.02Å longer than in bulk CuCl. This result is given by both single shell Fourier filtering analysis and by a multishell simulation treatment. Both analyses also reveal that adsorption is in a 3-fold coordinated hollow site, although neither method is able to distinguish between the two inequivalent 3-fold hollow sites present on this fcc (111) surface. These two sites show identical nearest neighbour contributions to the EXAFS at all polarisation directions, and differ only in the third (and higher) nearest neighbour shell structure. By contrast photoelectron diffraction data for the Cl 1s photoemission, collected along the [111] and [110] directions, do provide a clear distinction between these sites, showing that the Cl atoms are adsorbed in the same "fcc" sites as the next Cu atom layer would occupy, above 3-fold hollow sites in both the top and second metal atom layers. The complementary nature of these two techniques in the collection and analysis of data is highlighted by this study

Photoemission studies of the Pt 3 Cr(110) surface

Ultraviolet photoemission spectroscopy measurements have been made of the electronic structure of the (110) surface of the ferrimagnetic alloy Pt3Cr in the photon energy range 40 to 180 eV. The measurements indicate that the Pt and Cr d bands completely overlap, but that the strongest contribution from the Pt d band is to be found at binding energies of 2–7 eV. The strongest contribution from the Cr d band is to be found at the Fermi level but the electrons at the Fermi level are not completely associated with this element