Surfaces and interfaces are some of the most important topics in condensed matter research, chemistry, materials research and life sciences: e.g. all catalytic reactions occur at surfaces, thin film nucleation and growth start at a given surface and the resulting microstructure and morphology of the deposited layers can be correlated to surface or interfacial effects. Further, effects like adhesion, adsorption, wetting, strain, corrosion, flow patterns or lubrication are equally influenced by the interfaces. The universality of interfacial influences is further evident in the energy transfer processes in solar cell devices, the charge transfer processes in batteries and essentially all electronic devices. Heterostructures involving magnetic materials are widely used for information technology and as magnetic sensors in industrial applications such as the automotive industry. Here, the performance of the magnetic devices relies on the magnetic and elastic properties of the individual layers and on the coupling between them which are both influenced by the interfacial quality.
Today, a large variety of complementary analytical techniques are routinely employed to determine and analyze surface structures and interfaces in situ. Electron microscopy, AFM/MFM, STM, LEED, photo electron spectroscopy, ellipsometry as well as classical x-ray and neutron diffraction/reflectometry and grazing incidence surface scattering give a wealth of information from the Ångström to the micrometer length scale. However, classically, electron or ion based surface science techniques are limited to the vacuum environment. To determine structural or magnetic parameters of surfaces in ambient environmental conditions, like a surface submerged in a liquid, surface x-ray diffraction (SXRD) is used. Likewise, to access buried interfacial or magnetic properties within a heterostructure of layers, reflectometry with polarized neutrons has proven to be a very powerful tool, which allows a non-destructive depth profile vector magnetometry, to be carried out.
Structural in situ characterization of heteroepitaxial growth with x-rays can be used to track details and quantify parameters generally inaccessible to other non-destructive methods applied during growth. A variety of instrumental developments allow for optimum growth conditions during x-ray scattering experiments and for an in-depth analysis of the complete epitaxial process showing the different growth phases and a quantification of a variety of parameters involved in such processes. Coherent x-ray diffraction is capable to provide not only the structure but also dynamic information on atomic monolayer deposition on surfaces.
More recent developments allow the magnetic characterization during thin film growth to be carried out using magnetic x-ray scattering techniques. Further, currently the experimental capabilities for polarized neutron reflectometry during thin film growth are being developed to allow the understanding of the evolution of magnetic domains and the movement of domain walls in the nanometer regime.
In the past, neutrons have been applied to study surface and interface structures mainly using specular neutron reflectometry, which is an established method today and available at all major neutron sources world-wide. Within the last decade also off-specular neutron reflectometry and grazing incidence small angle neutron scattering (GISANS), either monochromatic or using time-of-flight, have moved into the focus of interest with significant developments in measurement techniques and data analysis. These methods are hence very powerful for the determination of structured surfaces in the Ångström to micrometer length scale (see the ESS Workshop on Off-Specular neutron Scattering, Brussels 2012).
Nevertheless, grazing incidence surface diffraction with neutrons is still often unknown to the potential user base, mainly due to the general understanding that due to the low available neutron flux and the large sample sizes required, these experiments would be rendered impossible. Some unique properties of neutron scattering like the isotopic contrast variation to label specific molecular structures or atoms in e.g. soft matter systems are hence unexploited.
Undoubtedly, the current trend in research with neutrons is to decrease the sample sizes. Therefore, it is very important to adapt the experimental techniques to cope with small scattering cross sections and little sample material. Higher flux neutron instruments and focusing on small sample areas are, hence, the pre-requisite to enter this field of research. The option for more intense neutron beams at new spallation neutron sources (SNS, J-PARC, ESS) and the development of novel neutron optical concepts open completely new and challenging possibilities for the structural analysis of surfaces and interfaces with the vision to perform structural and magnetic surface reconstruction by neutron scattering.