This research programme

A consortium of UK universities is advancing Skyrmion research through the 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 (Southampton and European XFEL) and Dr Ondrej Hovorka. (Southampton), Prof Thorsten Hesjedahl (Oxford), Prof. Geetha Balakrishnan & Dr. Martin Lees (Warwick). Our ambition is to achieve a step-change in our understanding of skyrmions in magnetic materials and support engineering 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.

Approaching the second half of project, we are recruiting a three-year Post Doctoral Research Associates to the project (starting as soon as possible, with employment to the end of the project in July 2022).

This particular post is to be hosted at the University of Southampton under the supervision of Hans Fangohr and Ondrej Hovorka.

The project partners communicate between the sites of Oxford, Cambridge, Durham, Warwick, and Southampton daily using Slack (chat client), and regularly using video conferences. Multiple meetings per year bring together academics, post-docs and associated PhD students from the partner universities to facilitate real collaboration and exploitation of complementary skills and experience.

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.

Background and Context

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.

Somewhat 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. Other application may include neuromorphic computing.

Research Questions

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 [1].

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.

Methods

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 Finmag 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, OOMMF, JOOMMF/Ubermag and Mumax. 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.

Desired profile of skills and experience

We are looking for a computational scientist, ideally with experience in condensed matter physics and simulation development, and experience in working in interdiscliplinary teams. Alternatively, a physicist with experience in ferromagnetics at the nanoscale and skyrmion physics, with aptitude and interest in developing skills in computational physics. Programming skills and experience are essential.

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.

The role of the successful candidate is to support the experimental work at partner sites in the Skyrmion programme grant through theory and simulation. This includes development of new computational models - both in terms of equations and of implementation, use and maintenance of existing simulation tool software, and very close collaboration with staff and students at Southampton and other UK and international sites. Contributions of ideas and suggestions for research directions within the programme grant are encouraged.

More on the research questions

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.

Our results so far include the findings that the boundaries of such small systems stabilise skyrmions, and that these can exist without applied magnetic fields [1]. This is of great importance for data storage and processing applications, and will be explored further in this project. We have found that for finite size geometries, skyrmion nucleation and destruction via the boundary is a lower energy path circumventing topological protection [2]. We predict a new stable and manipulable Bloch point configuration ("a blochion") in chiral materials across chirality interfaces [3], and show evidence for Biskyrmions being magnetic bubbles [4]. To support better science through better software, we have defined a series of standard problem for DMI simulations, and solved them using different numerical tools. [5]

[1] Scientific Reports 5, 17137 (2015) (summary, online article)

[2] Scientific Reports 7, 4060 (2017) (summary>, online article)

[3] Scientific Reports 9, 7059 (2019) (online article)

[4] Advanced Materials March 2019, 1806598 (2019) (online article)

[5] New Journal of Physics 20, 113015 (2018), (online article)

Figure 2: A 3d skyrmionic system. While there is some understanding starting to emerge for the magnetic skyrmion system in thin films, there are even more questions arising for 3d systems as shown here.

How to apply

Please apply at this URL: https://jobs.soton.ac.uk/Vacancy.aspx?ref=1158119DA

Application deadline: Sunday 04 August 2019

Interview Date: Friday 30 August 2019 (in Southampton, UK)

Contact

If you wish to discuss any details of the post informally, please contact Hans Fangohr, Email: hans.fangohr@mpsd.mpg.de or Ondrej Hovorka, Email: o.hovorka@soton.ac.uk