R. Koch

Magnetoelastic coupling of Fe at high stress investigated by means of epitaxial Fe(001) films

Abstract ME (magnetoelastic) effects frequently are held responsible for altered magnetic anisotropies of thin films. We show that at typical intrinsic stress values (> 0.1 GPa) the ME properties of thin films may deviate significantly from that described by the linear ME coupling constants of the bulk. Quantitative measurements on epitaxial Fe(001) films with a cantilever beam magnetometer provide a second order ME coupling constant of Fe, D 11 = 1.1 GJ / m 3 , yielding for the first time an experimental bulk value of a magnetic element.

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.

High-temperature STM investigation of Au(110), Pt(110) and Ag(110)

Abstract Au(110) and Pt(110), which both are (1×2) MR (missing-row) reconstructed at 300xa0K, are well known to undergo two phase transitions upon heating: an Ising transition, in which the surfaces deconstruct, and a 3D roughening transition. Our real-space investigation, with atomic-scale resolution by high-temperature scanning tunneling microscopy, reveals that Pt(110)(1×2) deconstructs via the formation of 2D island/hole units in accordance with theory. The MR configuration of Au(110) — in contrast to the present theoretical models — is stable close to the roughening temperature. The order–disorder transition seems to be due to the 2D roughening of already-existing step edges. In view of recent findings of tip-assisted movement of single atoms already at moderate tunneling conditions, e.g. on Ag(110), we discuss and eventually exclude the influence of tip/surface interactions on the relevant structural transformations of the two surfaces.

Can oxygen modify step arrangements? STM and LEED investigations on Ni(771)

Abstract Clean Ni(771), a vicinal Ni(110) surface, stabilizes in bulk truncated form with single terrace widths and regular (111) steps of single height parallel to the close-packed rows as revealed by scanning tunneling microscopy and low energy electron diffraction. Ni(771) therefore offers the interesting possibility to study the role of steps on the gradual formation of the oxygen-driven (2 × 1) reconstruction of the missing row (MR) type as a function of oxygen coverage. After oxygen exposures of 20 L the surface exhibits a perfect (2 × 1) MR configuration with the terraces and steps modified to double width and height, respectively. Thus the missing/added row mechanism for the formation of the MR-reconstruction is directly visualized. At low oxygen exposures isolated linear Ni/O chains are formed that already extend over twice the initial terrace width. At oxygen convrages of about 60% of that of the (2 × 1) phase a peculiar (6 × 1) superstructure is observed; its strucural units are separated by 3 a 1 ( a 1 = a /√2) and comprise of three linear metal chains with distances of 2 a 1 and a 1 , respectively.

Oxygen adsorption on Co (1010). The structure of p(2×1)2O

Abstract Three ordered oxygen superstructures form on Co(10 1 0), the hcp analog to fcc(110) with close-packed rows separated by the c-axis distance. The p(×1)2O phase, which develops from both the c(2×4) and the ‘double-layer missing-row’ p(2×1)O structure upon increasing the oxygen coverage from 0.5 to 1.0, is revealed to be the most stable one. As derived by scanning tunnelling microscopy and low-energy electron diffraction its structure consists of oxygen zigzag chains located in the troughs of the possibly slightly distorted Co(10 10 ) rows.

Magnetization, magnetostriction and intrinsic stress of polycrystalline Fe films measured by a UHV cantilever beam technique

Abstract The magnetization, magnetostriction and intrinsic stress of various polycrystalline Fe films have been investigated with an extended version of a UHV cantilever beam technique. Due to the high sensitivity of this technique absolute values of the respective properties can be determined of films approaching even monolayer thickness. The saturation magnetization of Fe glass is equal to the respective bulk value down to film thicknesses of 3 nm, where the films percolate; on mica and oxidized Si a decrease by about 30% is observed at mean film thicknesses lower than 10 nm, probably due to intermixing effects at the interface. On all three substrates a strong dependence of the saturation magnetostriction on film thickness is observed at mean film thicknesses lower than 10–20 nm.

Intrinsic Stress and Growth of Ag on p-Doped Si(001)(2 × 1): Influence of Dopant Concentration

Nucleation and growth of Ag on Si(001)(2 × 1) has been studied at tempertures of (300 ÷ 550) K, using intrinsic stress measurement together with several structural methods. Microstructure, growth mode and related intrinsic stress are strongly influenced by the Si(001) dopant concentration. On low p-doped Si(001)(2 × 1) film growth proceeds by Volmer-Weber mode resulting in polycrystalline films at (300 ÷ 400) K and high-quality epitaxial Ag(001) films at (450 ÷ 550) K. On highly doped substrates, on the other hand, epitaxial Ag(111) films grow via Stranski-Krastanov mode at 300 K, indicating the crucial effect of surface dopants on the initial stages of Ag nucleation.

Pyramidal growth on bcc(001) stabilises facets close to {012}: A Monte Carlo study

Abstract Epitaxial growth on bcc(001) has been studied by means of a realistic Monte Carlo algorithm. When Ehrlich–Schwoebel barriers suppress step-down diffusion, the surface may be corrugated on a mesoscopic scale by a pattern of pyramid structures that, in agreement with recent theoretical and experimental work, grow in time according to a power law of t 1/4 . Analysis of the surface current of different vicinal surfaces substantiates the preference to facets with slopes lying between that of {013} and {012} in a wide range of the growth parameters.

The magnetoelastic coupling constant B2 of epitaxial Fe(001) films

Magnetoelastic (ME) coupling, a property of major importance in heteroepitaxy, describes the dependence of the free energy of magnetic materials on strain/stress. Using our versatile ultrahigh vacuum cantilever beam magnetometer we have investigated the ME constant B 2 of epitaxial Fe(001) films in the thickness range of 2-100 nm, where the films are characterised by the magnetisation and the magnetic anisotropy of bulk Fe. Similar to B 1 both the magnitude and sign of B 2 depend on the film stress. B 2 decreases linearly with increasing stress and changes its sign at ca. 4 GPa. For stress-free Fe(001) films the bulk value of B 2 is obtained.

Carbidic and graphitic carbon on Ni(771) : aspects for catalysis

Abstract Vicinal surfaces are often used to model real catalysts, because they exhibit steps and kinks, which are believed to be catalytically active sites. On Ni(771) — a vacinal Ni(110) surface — major differences between carbidic and graphitic carbon were disclosed, using scanning tunneling microscopy, low-energy electron diffraction and Auger electron spectroscopy: The carbidic phase exhibits a (4×1) superstructure affecting only the inner regions of the Ni(110) terraces. Graphitic carbon, on the contrary, forms a macromolecular surface arrangement that preferentially decorates and thus inactivates the catalytically active step sites.