Hard X-ray 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 thesis is 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. The fabrication of three dimensional magnetic structures of arbitrary design was addressed, which provides progress towards achieving three dimensional magnetic systems with controllable properties. In this thesis, a 6 µm diameter artificial buckyball structure, consisting of a polymer scaffold, was fabricated using two photon lithography and subsequently coated with a magnetic material. The coating was first achieved with sputtering, and a full structural and elemental characterisation was performed with hard X-ray resonant phase tomography with a spatial resolution of 25 nm, revealing shadowing of the deposition process, as well as providing information on the electronic state of the magnetic material. An optimised deposition of the magnetic material on the polymer scaffold, with which such shadowing effects could be avoided, was achieved with a combination of atomic layer deposition and electrodeposition. Preliminary measurements of the magnetic properties of the three dimensional magnetic structure gave an indication of the influence of the physical structure on the magnetic properties. For the determination of the magnetic configuration in a three dimensional system, an appropriate imaging technique was required. Up to now, high spatial resolution three dimensional magnetic imaging has been achieved with soft X-ray and electron techniques, which are suitable for the investigation of thin film systems of total thickness up to around 200 nm. However, it has not been possible to determine the internal magnetic nanostructure within larger systems. In this thesis, hard X-rays were chosen for magnetic tomography due to their high penetration depth, which comes at the price of a low magnetic signal. To achieve hard X-ray magnetic tomography, progress was first made in the experimental capabilities required for the imaging of tomographic projections. In particular, two dimensional hard X-ray magnetic imaging at the nanoscale was developed with dichroic ptychography. In a proof of principle measurement, the magnetic configuration of a 1 µm-thick FeGd i multilayer film was imaged with a spatial resolution of 45 nm, and the optimal imaging conditions determined. Next, a reconstruction algorithm was required to obtain the three dimensional magnetic vector field, for which an iterative gradient-based optimisation routine was developed. The validity of the reconstruction was demonstrated with numerical simulations for a variety of magnetic structures. Finally, the high spatial resolution magnetic imaging with hard X-rays was combined with the newly developed iterative reconstruction algorithm and a dual-axis tomographic setup to achieve hard X-ray magnetic tomography with nanoscale spatial resolution. This provides a technique to determine the three dimensional magnetic configuration at the nanoscale within micrometre-sized samples, without requiring prior knowledge of the magnetic properties of the sample. In a first demonstration of hard X-ray magnetic tomography, the complex internal three dimensional magnetic structure within the bulk of a micrometre-sized soft magnetic pillar was determined to a spatial resolution of 100 nm, revealing a complex magnetic configuration that consists of vortices and antivortices that form cross-tie and vortex walls along orthogonal planes. At the intersections of these magnetic structures, magnetic singularities - Bloch points - are found, and their surrounding magnetic configuration determined, representing the first direct observation of Bloch points since they were predicted in 1965. 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, including the study of fundamental magnetic structures to the development of hard and soft magnetic materials for technologically relevant applications