Resonant Soft X-ray Scattering from Complex Magnetic Structures

Abstract

This thesis lays out a number of investigations into different magnetic systems governed by short- and long-range magnetic interactions. We take advantage of the sensitivity of soft x-rays to magnetism and the polarisation dependence of magnetic x-ray scattering to investigate these systems. Full polarisation analysis is a highly sensitive technique which can be utilised to refine small deviations in magnetic structures. We present work on improving the efficiency of these measurements, finding efficiencies of up to 75\% in the measurement method by rotating the incident light angle with the polarisation analyser fixed at the peak of intensity. Full polarisation analysis has been applied to the study of magnetism in the heavy fermion system CeRu$_2$Al$_{10}$. Although this system has a relatively simple magnetic structure it is highly unstable to doping and has an anomalously high transition temperature. Magnetic scattering was observed at the cerium $M$-edges and the polarisation dependence of the scattering is only consistent with a non-collinear magnetic structure. Least squares fitting of the non-collinear structure revealed a small spin canting of the magnetic structure towards the $a$-axis. This canting requires a symmetry lowering of the space group from $Cmcm$ to the subgroup $Pmnm$ to allow for the Dzyaloshinskii-Moriya interaction. Full polarisation analysis as well as neutron powder and single crystal measurements have been undertaken on the multiferroic skyrmion system Cu$_2$OSeO$_3$. Measurements were taken in the helical, conical, skyrmion, and ferrimagnetic regions. The (1, 0, 0) reflection at the Cu $L$-edge was found to be due to the anisotropy of the tensor of x-ray susceptibility (ATS). The satellite reflections around the ATS peak were observed not to display the polarisation dependence expected for a helix. The polarisation dependence observed is not consistent with a non-collinear structure and can be fitted with a long spin density wave with the moment pointing along the scattering vector. This structure would be expected to produce stepped features in the magnetisation measurements which are not observed. This long spin density wave structure is also not consistent with previous Lorentz transmission electron microscopy measurements. Neutron scattering measurements confirm the three up one down magnetic structure in the ferrimagnetic phase. However, the magnetic intensity at the $(1, 0, 0)$ position in the helical phase indicates a change in the magnetic structure Rietveld refinement fitting with a reduced ferrimagnetic structure where one of the $4a$ site copper moments is coupled antiferromagnetically to the $12b$ and remaining $4a$ site moments. Finally, the magnetic properties of thin films have been investigated using absorption techniques and x-ray magnetic circular dichroism. Manganese oxide monolayers and bilayers have been grown in different charge environments provided by heterostructuring with SrTiO$_3$ and LaAlO$_3$, with the aim of controlling the magnetic properties of the manganese \textit{via} charge transfer. It was found that the magnetic properties of the manganese is strongly linked to the capping layer used. This is due to different effects introduced by the growth of the different capping layers; LaAlO$_3$ introducing strong ion intermixing, and SrTiO$_3$ introducing oxygen vacancies. Both capping layers displayed superparamagnetic temperature and field dependent properties with small clusters (around 4) of strongly interacting manganese ions. The strongly interacting manganese ions are separated by either the ion intermixing or oxygen vacancies. Both capping layers displayed superparamagnetic temperature and field dependent properties. Superparamagnetism is usually only observed in nanoparticles. The elimination of the magnetic interactions required for nanoparticle-like properties to be possible are due to a combination of ion intermixing and oxygen vacancies