Numerical Modelling of Spin Ice Dynamics

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Why can't you pull the north pole off a magnet? If you try to cut a magnet in half you get two smaller magnets, each with a north and south pole. Repeat this process and you ultimately end up with a single atom with a magnetic moment, which again has both poles. Essentially, the answer is that while there are particles with electric charge (electric 'monopoles', such as electrons) there are no magnetically charged particles -- magnetic monopoles -- in our universe.However, there is growing evidence that this is not the whole story [1]. In 1997 a mysterious new type of magnet was discovered, called a spin ice. Our leading theories suggest that the magnetic fields of individual ions in these crystals orient themselves so as to create local sources and sinks of magnetization, which bear a striking resemblance to magnetic monopoles. Recent numerical [2] and experimental work [3] indicates that magnetic monopoles produce unique signatures in the fluctuations of the magnetic field around spin ice crystals, signatures known as 'pink noise'.This project is aimed at understanding how monopoles generate pink noise; how that understanding might be used to definitively prove the monopoles' existence; and how we might put this understanding to use in developing technological applications. The work will combine numerical approaches (kinetic Monte Carlo) with analytical modelling. Topics will include noise spectra, fractal percolation clusters, loop erased random walks, out-of-equilibrium dynamics, and glassy behaviour. While the work is theoretical in nature, there is scope to engage with experimental colleagues, and the project will be co-supervised by two experimentalists. The models developed for spin ices will be applicable to the artificial spin ice structures, arrays of nanometre-scale magnetic domains which capture the key features of spin ices, developed by the group of Dr S. Ladak. These same structures are being made superconducting by the group of Dr G. Klemencic, offering the possibility to include quantum effects into the work.The project includes substantial external funding for personal development, including public speaking and media training. There is also funding available for 3 months' salary to undertake an industrial placement if desirable. Both are part of the supervisory team's commitment to Responsible Innovation.