Contents

• Motivation and introduction
• X-ray scattering from thin films
  • Surface scattering of X-rays
    • Accessible reciprocal space
    • The used geometry - X-Ray Reflectivity (XRR)
  • Snell´s law and Fresnel equations
    • The refractive index and Snell's law
    • The Fresnel equations and the penetration depth of X-rays
  • Reflectivity from a homogeneous layer
  • Reflection from a multilayer
  • Reflection from rough surfaces and interfaces
• Mössbauer effect and Nuclear Resonant Scattering
  • Mössbauer effect
    • Lamb-Mössbauer factor
  • Nuclear resonant scattering (NRS)
    • The refractive index including nuclear resonance
    • Quantum beats
    • Polarisation aspects
  • Fresnel equations for resonant media
  • Diffusion effects
    • Rate equations
    • Bravais lattice - the simplest case
    • Diffusion coefficient
• Bibliography
• About this document ...

Abstract







The Nuclear Resonant technique combines brilliant intensity and coherence properties of third generation synchrotron sources and the excellent characteristics of Quasielastic Mössbauer Spectroscopy. Its ability to resolve the atomic jump diffusion mechanism in bulk materials was shown in many experiments. The aim of this work is the application of the Nuclear Resonant Scattering (NRS) method for studies of diffusion and dynamics in thin films. To enhance the surface sensitivity the geometry of grazing incidence is used. In this way thin films produced by Molecular Beam Epitaxy can be investigated. By varying preparation conditions (deposition temperature, coverage, substrate, substrate miscut, annealing temperature ) model systems with unique properties, that can be investigated by nuclear resonant scattering, are produced. For the first experiments two systems, an iron film on MgO(001) and an iron submonolayer on W(110), were chosen.

The Fe film on a MgO substrate is a model for NRS system due to the high $^{57}$Fe content, the very well known structure and good growth properties. It offers the possibility to compare directly the diffusion mechanism in the bulk part of the film with the enhanced diffusion in the near-surface region using the NRS method.

Fe on a W(110) substrate offers the opportunity to investigate the relationship between structure, hyperfine interactions and dynamics. The well known structure and high temperature stability predestines the system for diffusion investigations. NRS with its ability to investigate hyperfine parameters and dynamics is a powerful tool in this case. Another unique property of the method, the isotopic selectivity, allows to investigate surface diffusion even in one monolayer thin films. Investigations of hyperfine-relaxation phenomena are rare in bulk materials. The reason is a relatively low concentration of defects, especially vacancies, and high diffusion barriers. Surfaces offer the unique opportunity to study atoms in the vicinity of a vacancy defect--the vacuum. The vacuum induces an intrinsic electric field gradient (EFG) with the main axis perpendicular to the surface in the case of an atom embedded in the uppermost layer. The direction orientation strongly depends on the nearest neighbour coordination of an atom. A perturbation by an additional defect, e.g. an embedded vacancy or an ad-atom moving on the layer, will result in a reorientation of the EFG main axis. Since the diffusion barriers on surfaces are lower than in bulk fast moving vacancies or ad-atoms can lead to a fast fluctuating hyperfine field and hence to hyperfine relaxation effects. The relaxation manifests in a decrease of the quadrupole interaction effectively felt by the atom.

A large part of this thesis was devoted to the development and construction of a universal UHV system for synchrotron investigations in grazing incidence geometry. The system became the core of a 6th EU framework programme ``DYNASYNC'' and one of the most important tools of the ``Materials Dynamics network''. It was extended to a unique system for sample preparation (MBE), analysis (LEED, AES) and X-ray scattering experiments (XRD, XRR, GISAXS, XPCS, ...). It joins the interests of institutes and universities from Austria, Belgium, Czech republic, France, Germany, Hungary, Poland and Slovakia in the field of dynamics and magnetism studies in nanostructures.