Characterisation of static and dynamic magnetisation in a Heusler-based exchange coupled trilayer: the role of biquadratic coupling

Thesis

This thesis presents a comprehensive characterisation of static and dynamic magnetisation in epitaxial Co2MnSi/Cr/Co2MnSi exchange coupled trilayers, with a focus on understanding the role that biquadratic coupling plays in determining the magnetic properties of this system.

Chapter 1 outlines the motivation for studying exchange coupled trilayers within the context of Spintronic and Magnonic applications, highlighting the unique properties of Heusler compounds that make them the ideal candidate for such technologies. Chapter 2 provides the framework for interpreting both static and dynamic magnetisation and outlines the theoretical background of the three techniques used to characterise the magnetic properties of the Co2MnSi/Cr/Co2MnSi exchange coupled trilayers.

Chapter 3 describes the fabrication of the epitaxial Co2MnSi/Cr/Co2MnSi samples, using molecular beam epitaxy, and presents the results of the structural analysis carried out using transmission electron microscopy and X-ray reflectivity.

Chapter 4 investigates the static magnetisation using magnetometry, confirming the presence of dominant biquadratic coupling, which varies as a function of the Cr interlayer thickness. The biquadratic coupling is found to influence the magnetisation behaviour, leading to highly rounded magnetisation curves and increased saturation fields. Chapter 5 presents the magnetoresistance measurements, which provide further confirmation that biquadratic coupling is the dominant coupling mechanism over a wide range of Cr interlayer thicknesses.

Chapter 6 presents the ferromagnetic resonance measurements used to probe the dynamic magnetisation. Acoustic and optic resonance modes are identified, and their coupling amplitudes are shown to depend on the biquadratic coupling strength and the applied field direction.

Finally, in Chapter 7 concluding marks are given on the role that biquadratic coupling plays in the static and dynamic magnetisation properties of epitaxial Co2MnSi/Cr/Co2MnSi exchange coupled trilayers. The results demonstrate the potential for engineering interlayer coupling, enabling tunable magnonic devices.

Authors: Lord, SJ

• DOI: 10.5287/ora-bpe762dqb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Thin films; Exchange coupled trilayers; Magnetism; Biquadratic exchange coupling; Heusler alloys; Multilayers