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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
A dendritic magnetic particle with multiple branches, showing a complex pattern of swirling magnetisation.

Lead Supervisor: Richard Harrison, Earth Sciences

Co-Supervisor: Emilie Ringe, Earth Sciences

Brief summary: 
The whole of paleomagnetism is based on a lie – this project will reveal the truth about what REALLY carries paleomagnetic signals in rocks.
Importance of the area of research concerned: 
Paleomagnetism is one of the most important tools we have to study the history of the Earth, not only revealing the past movement of tectonic plates but the evolution of the Earth's magnetic field itself, with major implications for the thermochemical evolution of the planet. The physics of paleomagnetism is largely founded on the theory that sub 100 nm particles of magnetite act as uniformly magnetised 'single-domain' (SD) grains that acquire their magnetic memory at a single blocking temperature on cooling in the presence of a magnetic field. Paleomagnetic theories are almost entirely reliant on SD theory. However, we now know that the majority of magnetic remanence is carried by particles that are significantly larger than 100 nm and that these adopt highly non-uniform multi-vortex (MV) states that behave in much more complex ways than SD theory would have us believe. There is currently no practical theory that properly accounts for MV behaviour in rocks. However, the combination of recent developments in experimental and theoretical methods means we are finally in a position to tackle this problem head on.
Project summary : 
In this project we will develop a theoretical framework for MV states based on a combination of 3D tomography and micromagnetic simulations, leading to the development of a thermodynamic/kinetic model of the thermoremanent magnetisation acquisition process. The project will build upon proof-of-concept work performed within the nanopaleomagnetism group, which has established a roadmap towards a practical model of MV behaviour that can be applied to model and interpret real paleomagnetic data.
What will the student do?: 
A range of real particle geometries will be generated using high-resolution tomography methods (X-ray tomography, focussed ion-beam nanotomography, and scanning transmission electron tomography) . These geometries will be used as the input to micromagnetic simulations using existing code. These simulations will be used to explore the systematics of the MV state, focussing on the nature of the complex energy landscape created by the availability of multiple local energy minima and a range of energy barriers between them. These results will guide the development of a thermodynamic framework for calculating the equilibrium behaviour of ensembles of MV particles during cooling, combined with a kinetic model that enables the blocking/unblocking process to be calculated.
References - references should provide further reading about the project: 
Lascu, I., Einsle, J.F., Ball, M.R., Harrison, R.J., 2018. The Vortex State in Geologic Materials: A Micromagnetic Perspective. J. Geophys. Res. Solid Earth. doi:10.1029/2018JB015909
Roberts, A.P., Almeida, T.P., Church, N.S., Harrison, R.J., Heslop, D., Li, Y., Li, J., Muxworthy, A.R., Williams, W., Zhao, X., 2017. Resolving the Origin of Pseudo-Single Domain Magnetic Behavior. J. Geophys. Res. Solid Earth 122, 9534–9558. doi:10.1002/2017JB014860
Nagy, L., Williams, W., Muxworthy, A.R., Fabian, K., Almeida, T.P., Conbhuí, P.Ó., Shcherbakov, V.P., 2017. Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales. Proc. Natl. Acad. Sci. 114, 10356–10360. doi:10.1073/pnas.1708344114
You can find out about applying for this project on the Department of Earth Sciences page.
Dr Emilie Ringe
Prof Richard Harrison
Department of Earth Sciences Graduate Administrator