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Title: Spintronics physics of thin-film interfaces
Author: Aboljadayel, Razan Omar M.
ISNI:       0000 0004 7961 7942
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Magnetic heterostructures may exhibit new features at the interface while leaving the bulk properties of the films unaltered, which make them ideal for technological applications. Here, I explore the interfaces of EuO(001)/LaAlO₃(001), EuO/NiO(111) and graphene/metal heterostructures. Published DFT calculations postulated the formation of a spin polarised two-dimensional electron gas (2DEG) in the EuO film at ~1.5 nm away from the EuO(001)/LaAlO₃(001) interface, due to the polar catastrophe which would also enhance the Tc of the EuO. Magnetisation depth profiles of the interface with the LaO⁺- and mixed-terminated LaAlO₃(001) substrate are investigated using low energy muon spin relaxation (μSR) and polarised neutron reflectivity (PNR). The behaviour of our EuO(001) film is unlike that reported in the literature for stoichiometric EuO and EuO₁₋ₓ films, and no increase in the Tc was detected by the μSR. Instead, a reduction of ~2.6 K in the Tc is measured by the SQUID which is attributed to the lattice mismatch between the EuO(001) and the LaAlO₃(001) substrate. However, contradictory results were obtained by the PNR for the LaO⁺-terminated sample, where a magnetic moment of 3.59 μBEu atom, which is consistent with the value reported by Barbagallo et al. for a 9% oxygen-deficient EuO, was detected at 20 nm from the EuO(001)/LaAlO₃(001) interface. Therefore, I question the accuracy of the DFT calculations. Exchange bias and the magnetisation depth profile of the polar interface of the antiferromagnet NiO(111) with EuO are investigated using PNR, and the results are compared with those of an uncoupled EuO grown on a MgO(001) substrate. A small double shifted hysteresis loop with enhanced coercivity, which arise as a result of the competition of the random NiO(111) S-domains, is measured by the SQUID magnetometer for the polar interface. However, a larger spin-split in the PNR is observed for the EuO(001)/MgO(001) sample in contrast to the EuO/NiO(111). The small spin-split in the latter is attributed to the pinning of the EuO magnetisation by the randomised NiO(111) S-domains which restrict it from aligning fully with the applied field. This postulation is verified by further PNR measurements on a field-cooled EuO/NiO(111) sample, which showed equal probability for the presence of compensated and uncompensated NiO(111) spins at the interface. A magnetic moment in graphene can be induced in the presence of a ferromagnetic layer as a result of the proximity effect. PNR measurements show that a magnetic moment of ~0.57 μB is induced in the graphene when it is grown on Ni(111) and on Co(111) films, which is about 5x higher than that reported in the literature. The origin of the induced magnetic moment is found to be due to the opening of the graphene's Dirac cones as a result of the strong Cp-3d hybridisation which is confirmed by additional measurements using a non-magnetic Ni₉Mo₁(111) substrate. The results were also validated by the Bayesian uncertainty analysis, and proved by the X-ray magnetic circular dichroism measurements.
Supervisor: Barnes, Crispin H. W. Sponsor: Government of Saudi Arabia
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Spintronics ; Thin film ; Magnetism ; nanotechnology