X-ray Imaging Studies of Topological Magnetism

Abstract

This thesis applies resonant soft x-ray scattering techniques to the study of single crystal lamellae hosting topological magnetic states. In particular, imaging methods are used to examine exotic magnetisation textures whose properties are affected by their topology. These include investigating magnetic skyrmions, anti- skyrmions, reports of biskyrmions, and the identification of a surface spiral state, previously theorised to exist in chiral helimagnets.

Tilted x-ray holography, in conjunction with micromagnetic simulations and Lorentz transmission electron microscopy measurements, was performed on the uniaxial ferromagnet (Mn1−xNix)0.65Ga0.35 (x = 0.5) to reveal that previous reports of biskyrmions, a bound pair of opposite chirality skyrmions, likely misidentified topologically trivial magnetic bubbles. We show that pseudo-tomographic imaging methods can map the topology of the magnetic bubbles formed, and our simple model structure can be used to account for previous experimental observations without the need to invoke an exotic bound-state texture. Further x-ray holography measurements on lamellae of the archetypal chiral helimagnet FeGe show that, in a thickness regime inaccessible to many other common magnetic imaging techniques, a previously theorised stacked spiral spin state can be stabilised.

Small angle x-ray scattering measurements confirm the existence of this stacked spiral spin state in lamellae of Cu2OSeO3 and its presence was used to inform skyrmion-skyrmion interaction potentials in the context of melting transitions undergone by the skyrmion lattice. We find that skyrmion lattice melting is mediated by the proliferation of topological defects through the lattice, in a two step transition. The skyrmions behave as a condensed ensemble of particles and we present evidence consistent with a grain boundary theory of melting. This is in contrast to a previous study, which reports the skyrmion lattice melts according to KTHNY theory. We theorise that differing concentrations of skyrmions in the systems can reconcile this difference, in line with recent Monte Carlo simulations, and propose a future experiment to conclusively test the melting phase transition. Magnetic skyrmions confined in lamellae are shown to provide a rich environment for the study of two dimensional phase transitions, with a field-tunable interaction potential.

Finally, scanning transmission x-ray microscopy and x-ray ptychography are used to investigate lamellae of the inverse Heusler system Mn1.4PtSn, a system with the appropriate symmetry to host antiskyrmions. By utilising high-resolution x-ray ptychography, we find previous reports of helimagnetism in the system have misidentified nanoscale ferromagnetic stripe domains, leading to inconsistent estimations of the material parameters. In addition to antiskyrmion bubbles, we identify the coexistence of topologically trivial bubbles and elliptical skyrmions. Micromagnetic simulations, performed with empirically calculated material parameters support these observations.