Magnetisation Dynamics in Multilayer Spintronic Materials

Abstract

The search for new advanced materials to enable technology to operate at ever faster speeds still is one of the key challenges as the information society develops. This has led to an interest in exploiting the high frequency dynamics observable in ultrathin multilayer magnetic films. The functionality of these multilayers is dependent on the many aspects of the structure such as interface quality, interlayer couplings and magnetism energies. Thus, the study of the magnetisation dynamics of these systems through techniques such as ferromagnetic resonance spectroscopy is essential for the adoption of such materials into technology and is the focus of the work reported in this thesis. An investigation concerning the dynamics of the Synthetic Antiferromagnets (SAFs) CoFeB/Ru/CoFeB is detailed with the highest resonant frequency at zero applied field observed in a SAF to date reported. This work also demonstrates the importance of interface quality to achieving high frequency 10s GHz dynamics at zero field. The results have been successfully modelled using an analytical approach allowing static and dynamic magnetic parameters to be extracted. Studies concerning the high frequency dynamics of synthetic ferromagnets are also reported. Through experiments and numerical simulations, it is shown that the dynamic response of the two ferromagnetic layers has an orthogonal dependence on the difference in layer magnetization and interlayer coupling allowing both parameters to be determined accurately. Subsequently, the phase relations of the resonant modes can be determined, hitherto a challenging experimental measurement. Magnetisation dynamics depend on the exchange interaction which is parameterised by the exchange constant. Perpendicular Stationary Spin Waves (PSSWs) are increasingly used to determine this parameter where the value of the exchange constant is extracted from first order PSSWs and analysed assuming a rigid surface pinning model. The work presented in this thesis demonstrates through a systematic study of multiple PSSW modes in NiFe films, where both thickness and cap layer material are varied, that this simple analysis does not adequately describe the experimental data. Specifically, a single value of the exchange constant cannot be used to fit all the data measured, as is expected from a consideration of the physics of the material. However, adopting a dynamic pinning model where the magnetic properties close to the surface are different to those of the bulk of the film does allow a complete description of the data. This approach is verified through a comprehensive set of micromagnetic simulations which are able to reproduce the key features of the experimental data. This work demonstrates that the current practice of using first order mode PSSWs to determine the exchange constant through a rigid surface pinning model must be treated with caution.

Date of Award	31 Jan 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Thomson (Supervisor) & Ivan Vera Marun (Supervisor)