Magnetic Compton scattering studies of novel phases

Abstract

Magnetic Compton scattering is already a well established technique for the study of ferromagnetic and ferrimagnetic systems. The inelastic collision of a photon with an electron is sensitive to the electron momentum density. Advances in X-ray beam technology with the advent of dedicated synchrotron radiation facilities have allowed the tunability of various properties of the X-ray beam, permitting the weak coupling between it and the spin, S, of a target electron to be exploited. Recent developments in sample conditions (field, temperature, pressure) have enabled previously forbidden regions of the phase diagram of a sample to be explored. Other improvements in recent years of the technique combined with novel analytic interpretation via ab initio band structure calculations allow for a deeper understanding of the ground state to be gained from magnetic Compton scattering experiments. In this thesis, the unique properties of magnetic Compton scattering are applied to novel materials where high magnetic fields are essential and in conjunction with modelling methods the underlying electronic structure is elucidated. In the first study, investigations into the spin density of two complex oxides, Ca3Co2O6 and Sr3Ru2O7, are detailed. Ca3Co2O6 is a frustrated, metamagnetic spin chain compound which has been the subject of much investigation over recent years. From magnetic Compton scattering experiments, direct measurements of the bulk spin moment (1.78 ± 0.05 μB for 2 T and 3.93 ± 0.05 μB at 5 T and 7 T) confirms the existence of a large unquenched Co orbital moment (1.3 ± 0.1 μB at 7 T) together with an oxygen spin moment of ≈ 0.9 μB. Calculations from which theoretical magnetic Compton profiles have been extracted are shown to be in good agreement with the experimental data and unexpectedly reveal the existence of a Fermi surface in this system. With regards to the orbital occupation, molecular orbital calculations on the active [CoO6]9− cluster are discussed and from which the Co 3d orbitals responsible for the observed electronic and magnetic behaviour are determined. It is suggested that it is the double occupation of the dx2−y2,xy orbital that gives rise to the large unquenched orbital moment. The second magnetic oxide, Sr3Ru2O7, is a model system displaying a metamagnetic quantum critical point (MMQCP) reached via field tuning. Magnetic Compton profiles were measured in the metamagnetic phase along three crystallographic directions. LSDA band structure calculations and molecular orbital simulations were performed to reveal the extent of Ru 4d - oxygen 2p hybridisation, and also determine the occupation numbers of the t2g and the eg orbitals in the metamagnetic phase. The oxygen spin density is estimated to be approximately 30 - 31 % of the total spin density in agreement with NMR results. Furthermore, a spin moment of 0.70 ± 0.03 μB in the ab-plane reveals a negligible orbital moment. In the second study, the magnetic properties of the uranium superconductors UCoGe and UGe2 are presented. For UCoGe, the spin and orbital moments have been measured using a combination of magnetic Compton scattering, X-ray magnetic circular dichroism and density functional calculations to reveal the magnetic structure of this ferromagnetic superconductor. The compound is found to be a weak ferromagnet with a small total moment of 0.16 ± 0.01 μB. The uranium spin and orbital contributions nearly cancel, it is suggested that the uranium 5f electrons carry a spin moment of ≈ -0.30 μB and an opposing orbital moment of ≈ 0.38 μB, these values imply a strongly delocalised system. This is in contradiction with recent ab initio calculations which over-estimate the moments and also at odds with recent polarised neutron diffraction data. In addition to the uranium magnetism, there exists a cobalt spin moment of 0.06 μB anti-aligned to the uranium spin moment induced via significant 3d-5f hybridisation which could be responsible for delocalising the uranium electrons. The magnetic structure is verified by X-ray magnetic circular dichroism measurements at the cobalt L2,3 edges which confirm that the uranium and cobalt moments are anti-parallel. For the compound UGe2, temperature dependent magnetic Compton scattering experiments were conducted to investigate the shape of the momentum density across the T∗ phase transition. It is this transition which is thought to play a vital part in the observed superconducting phase. A change in the orbital occupation is inferred from a small shape change in the magnetic electron distribution with increasing temperature, lending evidence to a Fermi surface driven phase transition. For both compounds attention has been given to the degree of delocalisation of the uranium 5f electrons. In the final study, results from high temperature magnetic Compton scattering experiments on the Invar alloys Fe1−xNix and pure nickel are presented. The aim of which is to investigate any change in the shape of the spin-polarised momentum distribution. The spin density of Fe1−xNix (x = 0.20, 0.35 and 0.60) was first studied over a temperature range 50 - 450 K to investigate a possible change in band structure associated with the mechanism behind the Invar effect. The magnetic Compton profiles for the non-Invar compositions (x = 0.20 and 0.60) show no significant change in the electron momentum distribution in accordance with a similar study on Fe3Pt. However, the Invar composition (x = 0.35) shows a distinct change between the high temperature and low temperature momentum density. We associate this with a change in the electron momentum density occupation. In addition, the size of the occupation change is shown to be temperature dependent. The study into the spin density of pure nickel within the ferromagnetic phase and at its Curie temperature revealed no change in the momentum distribution in the two temperature regimes along the [100] crystal direction. This observation is in accordance with a Stoner-like reduction in the exchange-splitting with no enhancement or reduction of the sp-hybridisation, but at odds with other experimental work