Surface and near-surface diffusion

The topic of this project is diffusion studies in metallic films. Metallic films are currently a subject of extensive studies (potential applications in magnetic storage technologies, soft X-ray multilayer mirrors etc.). The general understanding of the kinetics of these systems which is responsible for their stability at increased temperatures is of great importance. Recently with tunneling and atomic-force microscopy diffusion on surfaces has been followed, however there remains the question about a possible perturbation of the diffusing system by the instrument. An alternative way of studying structure and dynamics of matter is by quasielastic scattering of quanta. Quasielastic scattering is by definition non-perturbing. Our idea is to combine grazing incidence reflectometry, a widely used scattering method for the characterisation of thin films and multilayers, with nuclear resonant scattering, and in this way to extend the field of synchrotron radiation investigations into diffusion in surfaces and near surface regions. The following tasks have to be accomplished. 1. Construction of a universal ultra high vacuum system to study diffusion in sensitive samples created by molecular beam epitaxy. A sophisticated design will allow to use the common synchrotron methods for investigation of layer-system in controlled vacuum conditions in the temperature range 80 K - 1300 K. A unique feature of the system will be the possibility to produce and to study in situ the epitaxial growth. This option will lead to a variety of experiments and will significantly increase the benefits of such a system. 2. We have proven the ability to measure bulk diffusion by nuclear resonant scattering.Up to now it was not possible to distinguish between bulk, near-surface and surface diffusion. Using a combination of grazing incidence reflectometry and the advantages of the resonant scattering will allow to study surface and near-surface diffusion in metallic films. Additionally a depth profile analysis can be obtained by an adjustment of the penetration depth simply by variation of the incidence angle. The experiments are meant as part of a cooperation with ab initio computational studies of the energy landscape at metallic surfaces.

Dynamical properties of condensed matter are key for the functionality of nanoscale devices. The role of the surface and interfaces between adjacent materials becomes increasingly relevant with decreasing size of the structural units. Moreover, new dynamical phenomena are expected in very small structures. Since the properties of surface structures are different from those of corresponding bulk materials, new methods have to be developed for their experimental characterization and modelling. An efficient way to achieve this goal is to use extremely brilliant X-rays from modern synchrotron radiation sources like the European Synchrotron Radiation Facility (ESRF) in order to study the dynamical properties under ultrahigh vacuum (UHV) conditions. A thorough understanding of these properties opens the way to tailor future functional nanoscale systems. The main goal of this project was the construction of a universal UHV system to study diffusion in sensitive samples made by evaporation of metallic elements in vacuum. A unique feature of the system is the possibility to produce, characterize and to study samples in a broad temperature range during their growth in the chamber system located at the ESRF. The sophisticated design of the UHV set-up is unique in the world. Thus, there is currently no possibility to perform investigations of this kind at any other synchrotron facility. In particular, studies of an iron monolayer on a tungsten substrate exploited entirely the unique possibilities of the apparatus and enabled the comparison with ab-initio calculations. Furthermore, that system is known to be ideal for studying a plenty of phenomena on the iron surface like dynamics, growth and magnetic phenomena. The very small amount of material - a single monolayer of atoms on a surface -represents a major challenge in the experiment. Somewhat surprisingly, the measured signal turned out to be much larger than expected. The exceptionally high sensitivity of the measuring technique even revealed diffusion of residual gas atoms on the iron surface. Other systems studied were intermetallic phases of iron with silicon and platinum. In both alloys, the mobility of iron atoms close to the surface was found to be different from the one in bulk materials of the same kind. The experimental facility developed within the project is permanently used and still improved and extended. These activities certainly strengthen the role of Austria in the field of nanoscience at large-scale research facilities.