Understanding the Behaviour of Magnetic Damping in Ferromagnetic Thin-Film Multilayers

Abstract

Magnetisation processes including rotation of magnetisation, and dynamic propagation of domains walls in magnetic materials are regulated by the precessional damped motion of magnetic moments about an effective field. In thin-film systems, when a ferromagnetic (FM) layer is in contact with non-magnetic (NM), and excited with a microwave, spin-current is transmitted across the interface. This process is known as spin pumping and results in the transmission of energy and angular momentum. The structure of both the FM and NM layers greatly affects spin pumping, with the change of crystalline structure affecting both spin-pumping and two-magnon scattering. In Ferromagnetic Resonance (FMR) measurements, we can assess spin pumping by observing changes in the FMR linewidth when a non-magnetic layer is introduced. These alterations serve as exhibits of the transfer of spin angular momentum from the ferromagnetic material to the non-magnetic layer. Two-magnon scattering is the energy transfer from uniform-to-nonuniform precessional modes.
This thesis focuses on understanding variation in damping parameters between two identical 10 nm thick CoFe magnetic layers separated by Ag. The precessional dynamics were measured using FMR. The thickness of the Ag interlayer has a considerable impact on the damping parameter. When certain Ag layer thickness corresponds to an antiferromagnetic coupling peak that might lead to enhancement of the damping in our samples. The interaction of the Ruderman-Kittel-Kasuya-Yosida (RKKY) across the Ag layer could results in the formation of stationary spin-wave modes that are out of phase within the effective field. This could be a reason of the additional bump term to the frequency-dependence of the resonance linewidth over a certain frequency.
In the polarised neutron reflectivity and polarisation analysis (PNR/PA) investigation, when we relaxed the magnetization from hard-axis saturation, we expected the layers to align antiparallel along/against the beam direction as the field is reduced. The sample was rotated about the film's normal axis and measured twice, at 0 and 45 degrees. The results indicate that there was a reduced structural scattering length density (SLD) at the interface between Ag and CoFe, while there was an increase in the magnetic SLD during a 45-degree scan. These changes may be recognised as variations in composition and the presence of magnetic properties that exhibit anisotropic behaviour. In a further magnetic anisotropy study, we made an unusual sample different from typical samples where the sample stage remained static during the growth of the first and third NiFe layers. The Ag layer was formed in the usual way with the sample stage rotating. Then, FMR measurements were carried out at various angles. The findings reveal that the frequency behaviour of both single and double resonance data remains linear and is not influenced by variations in measurement angles. This suggests that the anomalous enhanced linewidth seen in other samples with Ag spacer layers is not purely the result of misaligned magnetic anisotropies.