Year

2018

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials and School of Physics

Abstract

The work embodied in this thesis aims to investigate the occurrence of magnetic interface phenomena in low-dimensional thin-film systems which have conceivable utility in future condensed-matter technologies. Namely, the magnetic interface quality of an FePt3 nano-magnet formed via ion-induced chemical disorder will be critically analysed, in addition to a Co/Pd bilayer which features modifiable magnetic surface anisotropy upon exposure to hydrogen gas. The studies are enabled chiefly through advanced X-ray and neutron scattering techniques specifically chosen to probe interface structure as well as chemical and magnetic orders, and supplemented by traditional lab-based characterisation tools.

To begin, a much-anticipated experimental confirmation of the intrinsic sharpness of magnetic interfaces formed by locally driving magnetic phase transitions in materials using ion beams is presented. This is achieved through a unique experimental design whereby a room-temperature ferromagnetic nano-layer is encoded with depth-control onto a paramagnetic FePt3 film by inducing chemical disorder using energy-specific He+ ions. The magnetic transition is investigated through theoretical modelling, whereby the first density functional theory results for the entire suite of potential long-range magnetically ordered states of FePt3 are presented. In doing so, the energetically favourable ground-state spin structure is identified. By analysing several localised defect structures which may form in FePt3 under ion irradiation, the fundamental mechanism of the disorder-driven magnetic transition is revealed and shown to be caused by an intermixing of Fe and Pt atoms in anti-site defects above a threshold density.

In a second study, hydrogen-induced modifications to the layer-averaged static magnetisation and macroscopic magneto-dynamic behaviours of a Co/Pd heterostructure are investigated. The modifications are observed and examined in detail through simultaneously probing the magnetic anisotropy energy and studying the changing chemical and magnetic depth-profiles across the entire bilayer during primary hydrogengas absorption. It is revealed that the in-plane interfacial magnetisation of the Co/Pd bilayer irreversibly increases after primary hydrogen-gas absorption, indicating a weakening of the perpendicular magnetic anisotropy energy. To aid in conducting this analysis, an original experimental method is first developed which innovatively combines neutron scattering and microwave spectroscopy; equipment is then commissioned, and feasibility studies are performed.

This thesis is unavailable until Wednesday, December 04, 2019

Share

COinS