K. Baberschke

Magnetism in thin films

In the last decade enormous effort has been made in research and investment to study magnetic properties of thin films because of their obvious practical applications. Coincidentally, it happens that theory has made enormous progress. Ab initio calculations and microscopic theories allow us for the first time in the history of magnetism to study and manipulate the magnetism on an atomic scale. In contrast to bulk magnetic materials, ultrathin films allow us to manipulate magnetism via the thickness and, by use of artificial structure growth, to produce structures which do not appear in nature. Here we discuss the fundamental magnetic observables, i.e. magnetization, Curie temperature, magnetic moment per atom, susceptibility and magnetic anisotropy, for idealized prototype thin films like Fe, Co, Ni on metal substrates such as Cu, W, Re. Finally, we present studies on trilayers, i.e. magnetic thin films separated by a spacer, like Cu. These trilayers present prototypes of interlayer coupling relevant for practical use of multilayer structures.

The magnetism of nickel monolayers

Nickel allows to study the largest variety of phenomena in the magnetism of UHV ultrathin films. The low critical temperature of ≈ 630 K for the bulk favors experiments from 0 K to aboveTc and from one monolayer to infinite thick films. The thickness dependence ofTc (d) for the (001) and the (111) orientation is compared. Susceptibility measurements in UHV are presented, and from χmax the film geometry can be deduced. Ferromagnetic resonance measures the second- and fourth-order anisotropy constants. These give a clear understanding of when and how the reorientation transition from the in-plane to the perpendicular orientation occurs and its nature. Magnetic resonance and circular X-ray dichroism measure the spin and orbital parts of the magnetic moment µ, its anisotropy Δµ, and the 3d and 4sp contributions. Finally, we show how a 4 Monolayer (ML) Ni(001) film can be transformed into NiO by controlled oxygen dosage and thermal treatment.

Position of the σ-shape and π resonances of C2H2, C2H4 and C2H6 on Cu(100) at 60 K: A NEXAFS study

The position of the π and σ resonances in the C K edge spectra are determined for multilayer and submonolayer coverages of C2H2, C2H4 and C2H6 at 60 K, on a Cu(100) surface. The majority of the molecules are found lying flat on the surface in submonolayer coverages. We determine an elongation of the C-C bond length, for C2H2 and C2H4 on Cu(100) of 0.21(2) A and 0.10(2) A respectively, using the position of the σ resonance on a common energy scale with the gas phase. The results are compared with the ones of strong chemisorption on Pt(111). The σ-shape resonance of C2H6 chemisorbed on a metal substrate is reported for the first time. The molecule shows no elongation.

Nanostructuring of the Cu(001) surface by ion bombardment: a STM study

Abstract A detailed investigation of the effects of ion bombardment and thermal treatment on the topography of Cu(001) surfaces is presented. By means of 400 eV Ar + bombardment (flux = 6 × 10 12 ions/cm 2 s) at 300 K rectangularly shaped vacancy islands are created. These wells are on average 5 atomic layers deep, typically 20 nm wide at the top and regularly spaced about 27 nm apart. The dependence of this periodic well structure on the ion beam parameters and thermal treatment is discussed. Annealing the crystal to 370 K completely flattens this structure and the smooth topography of the clean unbombarded Cu(001) surface is recovered. We also present evidence that pinning of step flow at crystal defects or contamination on the order of 10 −4 leads to much larger terrace sizes than would be expected from the miscut of the surface.

In situ ferromagnetic resonance: an ultimate tool to investigate the coupling in ultrathin magnetic films

The benefit of using in situ ultrahigh vacuum ferromagnetic resonance (FMR) to study exchange coupled magnetic films is demonstrated. Structurally well defined trilayer systems consisting of two ultrathin magnetic films (Ni or Co) separated by a non-magnetic Cu spacer layer are examined. The stepwise construction of the sample reveals how the single uniform resonance mode of the bottom magnetic layer is influenced upon depositing the second layer on top. The coupling leads to an acoustical and an optical mode. The positions, intensities and linewidths of these modes are compared to theory which uses a continuum approach in the framework of the Landau–Lifshitz equation of motion. We discuss how to determine the coupling strength in absolute units which for our trilayers ranges from a few to about 100μeV/atom and then systematically investigate the parameters that influence this coupling. The effect of the spacer thickness and its roughness is studied in detail with the help of scanning tunnelling microscopy to obtain a realistic and quantitative picture of the spacer morphology. In a next step we investigate the influence of a non-magnetic overlayer usually used as a protection layer. For small coupling strengths such a capping layer can change the sign of the coupling. The temperature dependence of the coupling in the framework of existing theoretical models is discussed. Moreover, the resonance method is used to address via the linewidth the dynamical response of the magnetic layers upon being coupled. It is demonstrated that 2D spin fluctuations originating from one layer influence the damping properties within the other. To support the results obtained from the FMR, the measurements are complemented by magneto-optic Kerr effect experiments on the same systems.

Pseudomorphic growth of Ni films on Cu(001): a quantitative LEED analysis

Abstract We present LEED structure determinations of ultra-thin epitaxial Ni films on Cu(001) for coverages of 3, 5 and 11 ML. From full dynamical intensity analyses, a tetragonal distortion of all films can be deduced, in good accordance with FMR results. The structural parameters of the 5 and 11 ML films are practically identical. So, obviously there are no structural changes at about 7 ML, where the orientation of the magnetization is reported to switch from in-plane to out-of-plane. The film growth is pseudomorphic with an in-plane lattice parameter a p = 2.53 A at coverages of 3 and 5 ML. This value, which is slightly reduced with respect to the copper bulk, was recently also found to produce the best fit for a similarly prepared clean Cu(100) surface. With further increasing coverage, a p tends to approach the value of the nickel bulk (2.49 A) but does not fully reach it even at 11 ML ( a p = 2.51 A ). The films at 5 and 11 ML (and probably also in the regime between) show considerable tetragonal distortions. The top Ni layer shows an outward relaxation of 8% relative to the distances between deeper layers, which, due to the tetragonal distortion, are reduced compared to the ideal value of the nickel bulk.

Ferromagnetic Resonance in coupled ultrathin films.

Ferromagnetic resonance (FMR) is known to be one of the most informative techniques to measure basic physical quantities such as magnetic anisotropy energies, the g tensor in solids or the interlayer exchange coupling Jinter. We investigate prototype Cu/Ni/Cu/Ni/Cu(001) and Ni/Cu/Co/Cu(001) trilayers as well as Fen/Vm superlattices. We show for the case of trilayers how in situ ultrahigh vacuum FMR can be used to determine Jinter in absolute energy units in a straightforward way: we first prepare and measure the bottom magnetic layer together with the Cu spacer in situ and then evaporate the second magnetic film on top. Thus, it is possible to investigate the FMR signal before and after the two magnetic films become coupled. We discuss results, showing that the temperature dependence of Jinter follows a T3/2 law over a wide temperature range. This indicates that thermally excited spin waves at the interface of the ferromagnetic layers dominate the temperature dependence of Jinter. The second part focuses on the measurement of the g value. From the g value, the ratio of orbital to spin magnetic moment can be obtained via the relation μL /μS = (g − 2)/2. We show for Fen/Vm superlattices how μL /μS increases with decreasing Fe-layer thickness.

An angle-dependent magnetic circular X-ray dichroism study of Co/Cu(100): experiment versus theory

Values for the magnetic moments of 3d transition metals have been calculated from magnetic circular X-ray dichroism measurements using sum rules. In this work we reveal that these calculations are sensitive to a damping effect resulting from a decrease in photon penetration at the L3.2 peaks. Known as saturation effects, we present a simple correction to compensate for this phenomenon. We also show that the magnetic moments, as determined using sum rules, are relatively insensitive to the step function fitted to the absorption spectra. The magnetic moments derived from our data differ from the known ground-state moments. The corresponding collective factors appear to be transferable between cobalt and nickel.

Chloride perovskite layer compounds of [NH3-(CH2)n-NH3]MnCl4 formula

Abstract [NH3-(CH2)n-NH3]MnCl4 compounds with n = 2, …, 5 are chloride perovskite layer structures. The room temperature phase of members with odd numbers of carbon atoms is orthorhombic, whereas even numbers lead to monoclinic structures. Structural phase transitions were found in all compounds with n > 2. The magnetic behaviour is similar to the (CnH2n+1NH3)2MnCl4-family.

High resolution x‐ray absorption spectroscopy of linear hydrocarbons adsorbed on noble metal surfaces

Recent progress in the performance of soft x‐ray monochromators has revealed new features in soft x‐ray molecular photoabsorption spectra. We present here a comprehensive high resolution x‐ray absorption study on the linear hydrocarbons C2H2n and C2D2n, adsorbed on Ag(100) and Cu(100) at 50 K and in the solid and the gas phase. A line shape analysis is given, for the bound as well as for the continuum resonances and the absorption thresholds, by means of models easily accessible to the experimentalist, leading to a quantitative understanding of the observed spectra, including the vibrational envelopes. Changes in intramolecular bond lengths upon core‐hole excitation can be determined and tracked as a function of deuteration and chemisorption. The effects of the substrate bonding on the vibrational envelopes are analyzed.