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Title: Characterisation of half-metallic thin films using polarised neutron reflectometry
Author: Glover, Stephanie E.
ISNI:       0000 0004 7431 7107
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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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. 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. 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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics