Time-resolved imaging of magnetisation dynamics using x-ray holography with extended reference and autocorrelation by linear differential operator (HERALDO)

Abstract

Within this thesis the theory and practicality of the holography with extended reference autocorrelation by linear differential operator (HERALDO) technique will be presented and discussed in detail along with the results of a series of experiments performed on nano-scale magnetic elements using this technique. The aim was to investigate the static and dynamic time-dependent behaviour of thin-film square magnetic elements. The magnetic elements were predominantly Permalloy (Py) squares however some contained topological structures whilst others were formed from multiple square elements, creating a larger conjoined structure. Magnetic dynamics were studied by exciting the ground state of the magnetic elements with a pulsed Oersted field produced by applying an electronic pulse through the central signal line of an integrated co-planar waveguide (CPW) which supported the elements. These dynamics were then observed by utilising circularly polarised x-rays from synchrotron sources and exploiting the x-ray magnetic circular dichroism (XMCD) effect. The experimental set-up, data collection and analytical process for each experiment will be discussed in detail with a particular focus on the HERALDO technique and any remarkable results obtained on each of the samples. These results have been supplemented with micromagnetic simulations in order to gain a deeper understanding of what is causing the effects seen. The results of the experiments show that for thicker Py elements (>50 nm) there is a complex three-dimensional structure to the magnetic states that exist within, specifically that out-of-plane magnetic moments are more prominent. Interpretation of the experimental results with micromagnetic simulations showed that the square elements contained out-of-plane magnetic singularities at each corner of the square which could be switched in polarity by magnetic bullet-modes which propagate along the domain walls when the vortex core is excited to gyrate. In addition, when the element is exposed to a moderate strength magnetic field that remains below its saturation magnetisation strength the vortex core distorts into an extended domain wall-like region of both in-plane and out-of-plane magnetisation. Concerning simulations of an element with an off-centre topological structure (defect), when the vortex core gyrates, a series of events occur that cause the polarity of the core to switch. Regarding a structure formed of multiple elements, these samples are able to exhibit one of two ground states: an antivortex (AV) state or a continuous state. The continuous state was observed to result in a more regular vortex core gyration pattern. This result opens up potential for gyration synchronisation of multiple vortex cores which could overcome the limitations of current methods by allowing direct interaction between vortices via the exchange interaction, as opposed to the weaker stray field effects or complex nano-contact spin-torque driven devices.