Crystallography of magnetic nanostructures - structural complexity versus accuracy

It is well known that there are close correlations between structural and magnetic properties of materials and certainly this holds also for nanostructured epitaxial films. So, the knowledge of the crystallography of a certain structure - i.e. the knowledge of the precise coordinates of the atoms involved - is essential for a quantitative understanding of a system's magnetic properties. Unfortunately, real space methods such as scanning tunneling microscopy provide crystallographic data of only rather limited accuracy and - even worse - only of the top layer. The full structure of a film can only be resolved by techniques applying surface penetrating probes, for example, x-rays or electrons (or both). The present talk illuminates the power and limitations of using electrons or, more precisely, low-energy electron diffraction (LEED) in its quantitative version (applying tensor LEED for the intensity analysis). The talk concentrates on metallic epitaxial films of nickel, cobalt and iron on low index copper surfaces as well as iron on a reconstructed iridium surface. It will be shown that in favourable, i.e. structurally simple cases, quantitative LEED can resolve atomic positions with an accuracy of the order of 0.01A, as well as the chemical nature of the atom under consideration. However, the accuracy reduces with increasing structural complexity. This is because of both correlations between parameters and, quite often, some lack of the scientist's imagination in considering all relevant parameters. Complexity can hold even in simple cases when different structural domains exist, with each of them to be analysed, but only the sum of the intensities is available for the fit. On the other hand, the method can be extremely sensitive to a parameter (for example, vertical layer spacings) or to a certain structural arrangement. An example is the stacking of layers during growth, so that different sequences of, for example, fcc and hcp stacking can be clearly differentiated. Complex crystallographic structures are unavoidable when there is competition between pseudomorphic growth and a film's tendency to assume its native structure. Then, substantial distortions can develop within the film as, for example, in the case of iron on reconstructed iridium, where the substrate's structure influences the film growth in an unusual way. The power of LEED is demonstrated for these scenarios also. abstract only