In-situ control of magnetic multilayers studied with polarised neutron reflectometry

Polarised Neutron Reflectometry (PNR) is a powerful measurement technique used for studying structural and magnetic profiles. Here, we develop the ability to perform PNR measurements with external stimuli applied in situ. These in situ measurement techniques provide new avenues of research for functional spintronic devices. The work of this thesis is split into two main studies. In the first, PNR measurements with in situ annealing are performed on C60/CoB/C60 trilayers to monitor the migration of B ions from the CoB magnetic alloy into C60 molecular layers. This allows for control of magnetism by changing the B composition of CoB. Initial SQUID magnetometry measurements allowed for a comparison of the magnetisation before and after annealing as well as the monitoring of the magnetic moment during annealing. The C60/CoB/C60 sample magnetisation increased by approximately 50% due to annealing. Structural information was gathered using PNR measurements with in situ annealing. The results show a significant injection of B into the C60 layers due to annealing. For a 10 hour anneal at 300◦C the atomic percentage of the CoB layer reduces by (7 ± 4)% resulting in an increase in the magnetic moment of (80 ± 30)%. Nuclear Reaction Analysis (NRA) measurements show a broadening of the B profile as further evidence of diffusion.
In the second main study of this thesis, an in situ FMR-PNR measurement technique is developed. This technique aims to combine the depth dependent magnetisation measurement of PNR with the measurement of magnetisation dynamics from FMR to unlock the ability to study the depth profile of the cone angle and damping. This measurement was performed in two geometries, one where the sample magnetisation and neutron polarisation are aligned and the other where there is an angle between the magnetisation and neutron polarisation. The second of these allows for a measurement of the magnetisation angle as well as the magnetisation. PNR measurements were performed with the sample on and off resonance to attempt to observe the changes in magnetisation due to the FMR precession. It was found that the measurement sensitivity is much greater for the magnetisation angle than the magnetisation magnitude so the angular sensitivity is vital for this measurement. Simulations were used to determine the relationship between the measured angle between the magnetisation and neutron polarisation and the FMR cone angle. These demonstrated that the change in the measured magnetisation angle from PNR is proportional to the square of the FMR cone angle, ΔθM ∝ ϕ2. The experimental measurements in this geometry show a small change in magnetisation angle of (0.2 ± 0.3)◦ corresponding to a cone angle of (3 ± 2)◦.