Influence of Self-trapping, Clamping and Confinement on Hydrogen Absorption

The dissociation of hydrogen molecules at surfaces is the first step in the absorption process. If the absorbing material is covered by an oxide, this layer will determine the effective uptake rate of an underlying absorbing material. This effect is illustrated when determining the rate of transport of hydrogen through amorphous aluminium oxide layers. The transport rate was determined to be strongly thickness dependent. Hydrogen absorbed in a transition metal causes a volume expansion generated by a strain field around the absorbed hydrogen. This strain field causes a self-trapping of the hydrogen and a temperature dependent distribution in the atomic distances. The local strain field generated by the self-trapping process is found to be crucial for understanding both the hydrogen induced volume expansion as well as the diffusion of hydrogen. Ab-initio molecular dynamics simulations were used to reveal the temperature dependence of the unbinding of the hydrogen and the local strain field and its influence on the diffusion rate. The symmetry of the local strain field is also important for phase formation in metallic films and superlattices which are clamped to a substrate. As the thicknesses reduced from 50 to 10 nm thick vanadium films, substantial finite size effects become apparent in the phase diagrams. The volume change associated with the strain field cannot be accurately measured using x-ray diffraction because of its sensitivity to local arrangements of atoms. X-ray and neutron reflectivity were found to be more reliable probes of global effects of the sumof the local strainfields. Finite size effects in extremely thin V layers were also explored in metallic superlattices composed of iron and vanadium. The co-existence region, composed of a hydrogen gas and a solid-like phase, was found to be suppressed by at least 100 K to below 300 K. The hydrogen-hydrogen interaction can also be influenced by the electronic states in the non hydrogen absorbing layers, as demonstrated when comparing hydrogen absorption in Fe/V and Cr/V superlattices.  Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 72

Core code of the software Överlåtaren

The core code of the software Överlåtaren, as is presented in the article PO5077 ''Överlåtaren' a fast way to transfer and orthogonalize 2D offspecular reflectivity data' in the Journal of Applied Crystallograph

Discrete Layer-by-Layer Magnetic Switching in Fe/MgO(001) Superlattices

We report on a discrete layer-by-layer magnetic switching in Fe/MgO superlattices driven by an antiferromagnetic interlayer exchange coupling. The strong interlayer coupling is mediated by tunneling through MgO layers with thicknesses up to at least 1.8 nm, and the coupling strength varies with MgO thickness. Furthermore, the competition between the interlayer coupling and magnetocrystalline anisotropy stabilizes both 90 degrees and 180 degrees periodic alignment of adjacent layers throughout the entire superlattice. The tunable layer-by-layer switching, coupled with the giant tunneling magnetoresistance of Fe/MgO/Fe junctions, is an appealing combination for three-dimensional spintronic memories and logic devices