Magnetic contrast layers with functional SiO2 coatings for soft matter studies with polarised neutron reflectometry

This study introduces silicon substrates with a switchable magnetic contrast layer (MCL) for polarised neutron reflectometry experiments (PNR) at solid/liquid interface to study soft matter surface layers. The advantage with neutron reflectometry (NR) data is that structural and compositional information can be enhanced by using different isotopic contrast on the same sample. This approach is normally referred to as contrast matching, which can be achieved by using solvents with different isotopic contrast, e. g. different H2O/D2O ratio, and/or by selective deuteration of the molecules. However, some soft matter system might be perturbed by this approach, or it might not be possible, particularly for biological samples. In this scenario, solid substrates with a MCL are an appealing alternative, as the magnetic contrast with the substrate can be used to partially recover the information on the sample structure. More specifically, in this study, a magnetically soft Fe layer coated with SiO2 was produced by ion-beam sputter deposition on silicon substrates of different sizes. The structure was evaluated using XRR, AFM, VSM, and PNR. The collected data showed high quality and repeatability of the MCL parameters, regardless of the substrate size and thickness of the capping SiO2 layers. As compared to other kinds of substrates with MCL layer previously proposed, which used Au capping layer, the SiO2 capping layer allows to reproduce the typical surface of standard silicon substrate used for NR experiment and compatible with a large variety of soft matter samples. We demonstrated such application, by using ready-to-measure 50*50*10 mm3 substrates in PNR experiments for the characterisation of a lipid bilayer in a single solvent contrast. Overall, the article highlights the potential of PNR with MCL for investigation of soft matter samples.