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Additional file 1. SPIRIT Checklist.

The depth-resolved chemical structure and magnetic moment of Co2FeAl0.5Si0.5 thin films grown on Si(111) have been determined using x-ray and polarized neutron reflectometry. Bulk-like magnetization is retained across the majority of the film, but reduced moments are observed within 45 Å of the surface and in a 25 Å substrate interface region. The reduced moment is related to with compositional changes due to oxidation and diffusion, which are further quantified by elemental profiling using electron microscopy with electron energy loss spectroscopy. The accuracy of structural and magnetic depth-profiles obtained from simultaneous modeling is discussed using different approaches with different degree of constraints on the parameters. Our approach illustrates the challenges in fitting reflectometry data from these multi-component quaternary Heusler alloy thin films.

Halfmetal-semiconductor interfaces are crucial for hybrid spintronic devices. Atomically sharp interfaces with high spin polarisation are required for efficient spin injection. In this work we show that thin film of half-metallic full Heusler alloy Co2FeSi0.5Al0.5 with uniform thickness and B2 ordering can form structurally abrupt interface with Ge(111). Atomic resolution energy dispersive X-ray spectroscopy reveals that there is a small outdiffusion of Ge into specific atomic planes of the Co2FeSi0.5Al0.5 film, limited to a very narrow 1 nm interface region. First-principles calculations show that this selective outdiffusion along the Fe-Si/Al atomic planes does not change the magnetic moment of the film up to the very interface. Polarized neutron reflectivity, x-ray reflectivity and aberration-corrected electron microscopy confirm that this interface is both magnetically and structurally abrupt. Finally, using first-principles calculations we show that this experimentally realised interface structure, terminated by Co-Ge bonds, preserves the high spin polarization at the Co2FeSi0.5Al0.5/Ge interface, hence can be used as a model to study spin injection from half-metals into semiconductors.

We show that Co2FeAl0.5Si0.5 film deposited on Si(111) has a single crystal structure and twin related epitaxial relationship with the substrate. Sub-nanometer electron energy loss spectroscopy shows that in a narrow interface region there is a mutual inter-diffusion dominated by Si and Co. Atomic resolution aberration-corrected scanning transmission electron microscopy reveals that the film has B2 ordering. The film lattice structure is unaltered even at the interface due to the substitu- tional nature of the intermixing. First-principles calculations performed using structural models based on the aberration corrected electron microscopy show that the increased Si incorporation in the film leads to a gradual decrease of the magnetic moment as well as significant spin-polarization reduction. These effects can have significant detrimental role on the spin injection from the Co2FeAl0.5Si0.5 film into the Si substrate, besides the structural integrity of this junction.

Half-metallic (HM) materials are promising candidates for spintronic devices due to their 100% spin polarisation at the Fermi level and their high TC, making them robust to thermal fluctuations. For HM to be used in devices, they need to be in thin film form and retain both SP and magnetisation through the film and at the substrate/interface. High quality thin films of both Co2FeAl0.5Si0.5 and Fe3O4 described in this thesis have been grown by collaborators, and are theoretically predicted to be HM. HM in a film can be indicated by measurements of the magnetic moment. This thesis shows that polarised neutron reflectivity (PNR) data from these HMs can be fitted to obtain the magnetisation profiles perpendicular to the surface.\ud \ud PNR measurements are fitted simultaneously with X-ray reflectivity. Complementary techniques such as microscopy, magnetometry and X-ray diffraction are used in order to limit the fit, give confidence in the extracted structural and magnetic profiles and to help explain the underlying reasons for the magnetic properties. One chapter is dedicated to describing the set up and methodology of reflectivity fitting.\ud \ud Co2FeAl0.5Si0.5 grown on Si(111) has both extended surface and interface regions, the latter due to Si diffusion as shown by microscopy. The structural and magnetic parameters in this region are not well defined and become strongly coupled creating non-unique solutions to the fit of the reflectivity data. One model uses electron energy loss spectroscopy data to constrain the composition across the interface, which limits the fit. This sample is found to have a magnetic dead layer at the interface. Co2FeAl0.5Si0.5 grown on Ge(111) on the other hand, has a small interface region due to lack of Ge diffusion from the substrate. As the fit parameters are better defined, a unique solution to the reflectivity data is obtained. A magnetic moment corresponding to HM is obtained for both Ge grown films with the decrease in magnetisation towards the substrate. Three samples of Fe3O4 grown on MgO(111) with different post-annealing temperatures were also studied. Structural and magnetic experimental techniques are combined to understand the affect post-annealing temperature has on the film properties. An unusual substrate/film epitaxial relationship is found out of plane: MgO(111)||Fe3O4(100). Further temperature dependent studies investigate the change in magnetism below the Verwey transition TV ~ 120 K.