Electronic structure studies of exotic phenomena using magnetic Compton scattering

• Main Author: Kersh, David Alexander
• Published: University of Warwick 2016
• Subjects: 621.34; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.687178

The presented thesis investigates the spin densities of three exotic magnetic materials: Co2MnSi, a proposed half-metallic material which has gathered much interest in recent years due to the very high spin polarisation measured in thin films. CeB6, a Kondo material with a very complex phase diagram where the nature of the origin of the magnetism remains controversial and finally Ca3Co2O6, a complicated low-dimensional system where the electronic structure of the magnetic sites is poorly studied and controversial. These studies would not be possible were it not for the improved statistical quality and greatly enhanced experimental capacity, achieved by upgrading the experimental setup on the BL08W beamline at the SPring-8 synchrotron. The primary technique used to investigate the magnetic properties of these materials was Magnetic Compton Scattering. This technique directly probes the spin-dependent electron momentum density of a material, enabling an isolated measurement of its spin moment. In addition, through comparison with theoretical models, the technique can be used to gain insight into the electronic structure of the material, and determine which bands contribute to the magnetism in the system. These theoretical models were calculated using ab initio methods such as Density Functional Theory (DFT). In addition to magnetic Compton scattering, a range of complementary experimental techniques have been used to provide further useful analysis for these materials. This includes techniques such as SQuID magnetometry, Energy Dispersive X-ray Spectroscopy, Powder X-ray Diffraction and Laue Diffraction. The upgrade to the BL08W beamline at the SPring-8 synchrotron in Hyogo, Japan was prompted by technical issues which were present at the ID15 beamline at ESRF in Grenoble, France. Modelling the stray field effects proved the presence of such a powerful magnet would not impact the electron beam during operation, ensuring the magnet could be used on the beamline without affecting other experiments. Upgrading the 3 T cryostat at SPring-8 to the 9 T Oxford Instruments Spectromag magnet improved the experimental capacity - the range of physical phenomenon which can be measured, considerably. The study of Co2MnSi marks one of the first bulk investigations of the potential half-metal. A single crystal was measured along the [100], [110] and [111] directions using magnetic Compton scattering. Modelling of the system was performed using the ELK DFT code which calculated the system to be a half-metal with a 5uB spin moment. Comparing the experimental and theoretical profiles yielded good agreement, with the theoretical profiles very accurately modelling the broadness and tails of the MCPs but with deviations at low momentum. Comparing the anisotropies found very good agreement between the experimental and theoretical profiles. Characterisation of the material was performed using SQuID magnetometry, Energy Dispersive X-ray Spectroscopy and Powder X-ray Diffraction. The SQuID work found the sample to saturate at 5uB, in very good agreement with the theoretical calculation and literature. Studying the stoichiometry of the system, the EDX suggested a small Co-Si disorder, with the excess Si occupying the Co sites. Finally, Powder XRD using a Co and Cu source was used to probe the disorder in the sample by comparing the differences in the anomalous scattering from the two sources. Due to uorescence and a poor background:noise ratio, this study remained inconclusive. However, the good agreement between the DFT work and the measured magnetic Compton Profiles, the anisotropies and the SQuID work contribute strong evidence for the half-metallicity of Co2MnSi. A sample of CeB6 was measured along the [100] and [110] directions using magnetic Compton scattering. A small anisotropy between the two directions was found which prompted a further investigation. DFT calculations were capable of reproducing an anistropy but were inadequate in describing the shapes of the MCPs. The 4f profiles of the Ce ion were calculated using the GAMESS code. None of the calculated orbitals were found to be in good agreement with the experimental data. A calculation which combined the Ce 4fx(z2-y2) orbital with the calculated 2p orbitals of the B6 octahedra found excellent agreement with the experimental data. A fixed-spin moment calculation performed using DFT where a small moment was allocated to the B sites did not improve agreement with the experimental data. This result gives strong indication that the spin-density of CeB6 requires both a Ce and B moment to be described adequately. The final experimental chapter measured a sample of Ca3Co2O6 along the c axis using magnetic Compton scattering. Modelling the CoO6 trigonal site in GAMESS gave nonphysical results for describing the data. This lack of physicality was reproduced in the Co3O12 and Co5O18 calculations. Removing the O atoms from the calculation improved agreement with the experimental data considerably, finding the orbital contributions to be physical. The results of this work suggest Ca3Co2O6's electron momentum density originates entirely from the Co trigonal site in the 3+ state, with no spin-density originating from the O sites.