Computational modelling for UK national research programme into magnetic skyrmions
Skyrmions are excitations of matter whose occurrence and properties are mysterious, and which hold promise for technological deployment as highly efficient memory elements. The discovery of skyrmions in magnetic materials and of their self-organisation into a skyrmion lattice has made skyrmion physics arguably the hottest topic in magnetism.
A consortium of UK universities is embarking on an EPSRC-funded National Research Programme on Skyrmions and Skyrmionic devices.
The project is led by Prof. Peter Hatton & Dr. Tom Lancaster (Durham), Prof. Paul Midgley FRS (Cambridge), Prof Hans Fangohr and Dr Ondrej Hovorka. (Southampton), Prof Thorsten Hesjedahl (Oxford), Prof. Geetha Balakrishnan & Prof. Don Paul (Warwick). We are launching a major research programme intended to achieve a step-change in our understanding of skyrmions in magnetic materials and engineer them towards application. Our EPSRC-funded programme is a six-year UK based project built around a consortium of five premier Universities, with input from international academic institutions and industrial partners.
We are recruiting five three-year Post Doctoral Research Associates to the project, one based in each University. We require expertise in (i) crystal growth and characterisation (ii) state-of-the-art experimentation (iii) thin film physics (iv) electron microscopy and (v) computational modelling. We will also shortly be recruiting PhD students at each institution. For enquiries about the project contact Prof. Peter D. Hatton (email@example.com).
Figure 1: a) Magnetic nanotechnology device, b) and c) isolated Skyrmions (see Figure 2), d) helical state, e) mixed helical and skyrmion state, f) array of skyrmions.
Ferromagnetic nanostructures underpin a wide range of technology, including hard disks, actuators and sensors, and provide scope for fundamental materials and physics research. The magnetisation vector field in such nanostructures is dominated by: (i) the strong and short-range exchange interaction that favours parallel alignment of the magnetisation, (ii) the weak and long-range demagnetisation interaction field that favours antiparallel alignment of domains of magnetisation, (iii) the local crystal anisotropies that favour particular directions in the crystal lattice and (iv) the locally acting external field. The theory often used to describe these effects at the length scale of micrometres and below is called Micromagnetics.
Recently, a new kind of interaction has been predicted and found to exist in magnetic materials: the (v) short-range and strong Dzyaloshinskii-Moriya Interaction (abbreviated as DM interaction or DMI) which favours 'curvature' in the magnetisation. The DM interaction is in direct competition with the exchange interaction: the exchange interactions wants to achieve a uniform vectorfield configuration and the DMI tries to twist the magnetisation: neighbouring magnetic moments would like to point in perpendicular directions. Just from the competition of the DMI and the exchange (which favours neighbouring magnetic moments to point in the same direction), together with an applied (uniform) external magnetic field, a number of new phases is observed (Figure 1 above), including regular arrangements of skyrmions (Figure 1f) and helical magnetisation patterns and mixed configurations (see Figure 1d) and 1e) above).
A particularly interesting and potentially very useful magnetisation vector field is that which looks a little bit like a vortex and which is known as a Skyrmion - see for example Nature Physics 7, 673-674, 2011 for an introductory summary on skyrmions in magnetic nanostructures.
Inspired by this recent experimental discovery of skyrmions in magnetic systems, further works have shown remarkable properties of skyrmions: due to the unexpected stability of the skyrmion lattice, they could potentially be used to record data at the nanoscale. It has also been found that a very low electric currents can drive the skyrmions through the film, which opens the door for a low-energy realisation of the currently much researched race-track memory device which has the potential to replace harddisks with a new technology that combines high capacity with low power requirements, and can sustain an unlimited number of read-write cycles.
Figure 1b) shows a single skyrmion in a small disk which as just large enough to accommodate that skyrmion (here the diametre is 120nm, and the material is FeGe), and a result from our work at Southampton .
A number of innovative suggestions have been made, how these skyrmions could create a step-change in mankind's data storage and processing capabilities. There is a multitude of interesting and technologically important studies possible, including static and dynamics of skyrmions, and their interaction with spatially confined structures (leading the path towards logic networks), and data storage device components and designs.
We will collaborate with academic and industrial partners, combining simulation work with analytical theory and experimental work, and progress with good understanding of industry design constraints.
In this project, we extend and apply the established micromagnetic framework with the interaction terms that allow to observe and study the skyrmion phase using computer simulation. We will also use Monte Carlo and Spin Dynamics methods.
The micromagnetic codes that will be used and extended include the successor of the open source tool Nmag, and uses a finite-element discretisation of space and employs the Fenics library, and the finite difference codes fidimag and OOMMF. Thermal behaviour of skyrmions and their self-organisation will be investigated using Monte-Carlo and stochastic Spin dynamics methods based on several in-house developed codes and also the atomistic spin model simulation code VAMPIRE (http://vampire.york.ac.uk). We use Python as the main tool to drive the simulations, combined with C/C++ code where necessary.
We believe in high standards of software engineering in Computational Science, and are looking for computational scientists sharing our passion. Commonly used tools include version control, automatic testing and continuous integration.
We are looking for a computational scientist, ideally with experience in condensed matter physics and simulation development, and experience in working in interdiscliplinary teams.
We appreciate that such a combination of skills is rare, and will provide training on the job to the successful candidate to achieve such a broad portfolio of skills.
Self organisation of skyrmions can be seen in figure 1f: The arrows show local magnetisation direction. The objects looking like vortices are skyrmions. Skyrmions act like particles and repel each other. In an infinite film, they would form a ordered hexagonal lattice to minimise their energy. In the example shown above (from a Metropolis Monte Carlo simulation), perfect hexagonal order can not be established due to the finite size and given shape of the simulated square. The sample is exposed to an external field applied into the screen plane which is necessary to stabilise this skyrmion state.
Skyrmions in confined geometries are shown in Figure 1b and 1c: A single Skyrmion confined in a disk (1b) and square-like (1c) structure. We have shown  that the boundaries of such small systems stabilise skyrmions, and that these can exist without applied magnetic fields. This is of great importance for data storage and processing applications, and will be explored further in this project.
 Scientific Reports 5, 17137 (2015) (summary)
Please apply at this URL: https://jobs.soton.ac.uk/Vacancy.aspx?ref=769516AK
Closing Date: Friday 09 September 2016
Interviews are likely to be held on Wednesday 28 September or Friday 30 September at Southampton.