Magnetic Compton scattering studies of novel phases

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

Micro-Hydropower in Nepal:Analysing the Project Process to Understand Drivers that Strengthen and Weaken Sustainability

Evaluating the sustainable operation of community-owned and community-operated renewable energy projects is complex. The development of a project often depends on the actions of diverse stakeholders, including the government, industry and communities. Throughout the project cycle, these interrelated actions impact the sustainability of the project. In this paper, the typical project cycle of a micro-hydropower plant in Nepal is used to demonstrate that key events throughout the project cycle affect a plant’s ability to operate sustainably. Through a critical analysis of the available literature, policy and project documentation and interviews with manufacturers, drivers that affect the sustainability of plants are found. Examples include weak specification of civil components during tendering, quality control issues during manufacture, poor quality of construction and trained operators leaving their position. Opportunities to minimise both the occurrence and the severity of threats to sustainability are identified. For the micro-hydropower industry in Nepal, recommendations are made for specific actions by the relevant stakeholders at appropriate moments in the project cycle. More broadly, the findings demonstrate that the complex nature of developing community energy projects requires a holistic consideration of the complete project process

Learning off the job : engineers and professional education.

This thesis identifies a framework of critical occasions documented by engineering students taking residential modules during the taught element of an MSc. It develops a categorisation of critical events on which future research could be built and will be of interest to practitioners, learners and academics. Building on this nomenclature the effect of important episodes on respondents' cognition and professional development is examined by applying fuzzy logic. Using a reflective interview based case study students were questioned about their background, attitudes and landmark events to investigate the classification. A focus group provided another perspective and validated early findings. The choice of a case study and use of interviews are discussed within the methodology. Previous literature on critical incidents, professional development and cognition was considered to illuminate the framework. The resulting data was analysed and patterns identified in the fieldwork chapters to catalogue the critical happenings. The developing professional identity of respondents is another area examined providing an insight into how and why such professional development occurs. Findings include: that milestones volunteered were representative of critical episodes found in previous writing and that attitudinal changes revealed within the subsets of Apprentices and Graduates appeared to converge as the course progressed. Practitioners may facilitate critical incidents and so enhance their own professional development. Learners will be interested to know that discussions with respondents highlighted occasions that resulted in increased cognition, improved confidence and presented opportunities for networking leading to professional formation. For academics the research presents ways in which students learn using critical thinking, highlighting a continuum of critical happenings on which future research could be encouraged. While offering no strong claim to generalisability the taxonomy identifies areas for further examination, which could lead to generalisability in the future

Design for Localisation:a case study in the development and implementation of a low head propeller turbine in Nepal

This paper proposes a methodology for “design for localisation” that addresses the challenge of designing a product for local manufacture and use, whilst considering the production process availability and the context of the local geographic area. The methodology is derived from a case study of the development of a propeller turbine in Nepal. In the case study, the initial challenge was the absence of a low head turbine that could be manufactured, used and repaired in Nepal. A potential solution from previous academic work was identified however its intended operating environment differed considerably. Through identification of the specific local requirements, the design priorities for individual sub-systems in the new context were developed. Using three examples, design changes driven by improving the ease of manufacture and applicability to the local context are explained. Multiple phases of field and laboratory-based testing were used to validate, adapt and improve the design and its method for implementation. The experiences of the case study lead to three rules for design for localisation using an identified potential solution for a local problem: firstly, to derive local product requirements; secondly to develop solutions appropriate for local manufacturing; and finally, to conduct field-testing phases to ensure the product is suitable for its intended application

Understanding sustainable operation of micro-hydropower:a field study in Nepal

Off-grid renewable energy technologies are important in improving electricity access for rural communities. However, methods for ensuring their sustainable operation are often poorly understood. In this article, existing approaches for the assessment of off-grid projects are examined. Reliability of the technology, financial viability and community engagement are identified as the 3 key areas governing the sustainability of projects. Focusing on these areas, a methodology is proposed to understand the sustainability of micro-hydropower plants. A mixed-methods approach including a maintenance assessment and interviews with managers, operators and consumers is used to evaluate 24 sites in Nepal. Technically, the results of the study showed that trained operators delivered a higher standard of maintenance, however, technical issues were identified that arise during the design, manufacture and installation phases. The financial viability of plants was aided by charging consumers based on consumption, whilst plants with a higher rated capacity tended to benefit from a larger number of productive end uses. Community engagement was fostered through the local identity of the plant however this was threatened by societal changes. Inherent features of the site, such as rated power and the population density, internal resilience to short-term shock events (failure of components, insufficient collection of tariffs and departure of trained operators) and long-term external development (increased incomes, increased energy consumptions and growth in rural settlements) were found to affect the sustainability of plants

Determination of spin and orbital magnetization in the ferromagnetic superconductor UCoGe

International audienceThe magnetism in the ferromagnetic superconductor UCoGe has been studied using a combination of magnetic Compton scattering, bulk magnetization, X-ray magnetic circular dichroism and electronic structure calculations, in order to determine the spin and orbital moments. The experimentally observed total spin moment, Ms, was found to be-0.24 ± 0.05 µB at 5 T. By comparison with the total moment of 0.16 ± 0.01 µB, the orbital moment, M l , was determined to be 0.40 ± 0.05 µB. The U and Co spin moments were determined to be antiparallel. We find that the U 5f electrons carry a spin moment of Us ≈-0.30 µB and that there is a Co spin moment of Cos ≈ 0.06 µB induced via hybridization. The ratio U l /Us, of −1.3 ± 0.3, shows the U moment to be itinerant. In order to ensure an accurate description of the properties of 5f systems, and to provide a critical test of the theoretical approaches, it is clearly necessary to obtain experimental data for both the spin and orbital moments, rather than just the total magnetic moment. This can be achieved simply by measuring the spin moment with magnetic Compton scattering and comparing this to the total moment from bulk magnetizatio