Richard L. Greene Dissertation Award Talk: Resonant Ptychographic Tomography of Three Dimensional Magnetic Structures

Three dimensional magnetic systems promise significant opportunities for applications, for example providing higher density data storage devices and new functionality associated with complex topology and greater degrees of freedom. The aim of this work was to address two main challenges concerning the experimental investigation of three dimensional magnetic structures, namely the fabrication of three dimensional tailored architectures, and the characterisation of three dimensional magnetisation vector fields. In particular an artificial magnetic buckyball was fabricated, and a detailed characterisation of the structural and elemental properties performed [1]. For the determination of the magnetic configuration in a three dimensional system, hard X-ray magnetic imaging at the nanoscale was developed [2], and combined with a new iterative reconstruction algorithm to achieve hard X-ray magnetic tomography [3]. In a first demonstration, the three dimensional magnetic nanostructure within the bulk of a soft magnetic micropillar was determined, revealing a complex magnetic configuration consisting of vortices and antivortices that form cross-tie and vortex walls. At the intersections of these magnetic structures, magnetic singularites - Bloch points - are found and their surrounding magnetic configuration determined, allowing for different Bloch point types to be identified [3]. With its high penetration depth, hard X-ray magnetic tomography opens the possibility for the elucidation of complex three dimensional magnetic structures for a range of extended magnetic systems with high spatial resolution.
The nanofabrication and characterisation techniques developed in this thesis provide a basis for future nanoscale investigations of three dimensional magnetic structures in a wide variety of systems.
[1] Donnelly et al, PRL 114, 115501 (2015)
[2] Donnelly et al, PRB 94, 064421 (2016)
[3] Donnelly et al, Nature 547, 328–331 (2017)