UK Skyrmion ProjectSkyrmions are excitations of matter which hold promise for technological deployment as highly efficient memory elements. We wish to increase our understanding of these mysterious particles and engineer them towards application.
The skyrmion is a topological defect much like the magnetic domain wall and the superconducting vortex. If skyrmions can be made energetically stable they should occur within many systems exhibiting broken symmetry and show a characteristic set of excitations.
The UK Skyrmion Project (UKSP)In July 2016 the EPSRC funded a five year Programme Grant - Skyrmionics: From Magnetic Excitations to Functioning Low-Energy Devices (EP/N032128/1Link opens in a new window). The project involving 5 UK universities as well as a number of national and international partners, set out to understand the physics of skyrmions and develop skyrmionic materials towards the point of deployment in device applications. The project was extended due to the COVID 19 pandemic and formally ended in January 2023. The following pages outline the work carried out under the three Research Challenges set by the project team, introduce the team members, and list the publications that resulted from this work. If you have any questions about this project, please do not hesitate to contact the Research Challenge team leaders.
Meet the Team
In order to better understand the science behind skyrmions we have built a multidisciplinary team with a variety of expertise, based at the Universities of Cambridge, Durham, Oxford, Southampton and Warwick.
The project also involved a number of national and international partners based at other academic institutions and central facilities, as well as industrial organisations.
A full list of team members can be found on the Project Team page.
Details of the papers published as a result of work carried out by the UK Skyrmion Project team in Research Challenges 1, 2 and 3, including a link to each paper in the journal can be found on the Publications section of these web pages.
Some of the early papers are the result of pilot studies carried out my members of the UKSP team before the project was funded by the EPSRCLink opens in a new window.
Skyrmion materials - Team leader Professor Geetha Balakrishnan
Research Challenge 1 concerns the synthesis and characterization of skyrmionic materials. Polycrystalline and single-crystals of skyrmion materials are produced using a range of techniques.
In the study of skyrmion lattices, one class of materials has occupied a prominent place in the literature: crystals that adopt the P213 space group (or B20 phase). The first systems identified were a group of transition metal compounds, all of which lack space-inversion symmetry and host the Dzyaloshinskii-Moriya (DM) interaction. Examples of this class are FeGe/Si, Fe1-xCoxSi and MnSi. An interesting example of the B20 type materials that is not metallic is the magneto-electric Cu2OSeO3, also exhibiting helimagnetic order. It has a much more complex unit cell, with the Cu building blocks of which offer opportunities to tune the structure-property relationship observed in the variety of B20 structures.
In our programme of materials preparation we have grown examples of B20 materials. In addition to the B20's, there are other related structures such as the β-Mn types that may host exotic magnetic objects and allow the study of skyrmion lattices and related phenomena. The search for new materials that exhibit skyrmion lattices has been a significant part of our materials research in the programme. We have investigated compounds that exhibit chiral/helical magnetic ordering or alternatively, a ferromagnetic phase that can be switched to a helimagnetic phase.
Polycrystalline materials synthesis and single-crystal growth
Much of the synthesis of polycrystalline samples and single-crystals of skyrmion hosting materials is carried out in the Physics Department at the University of Warwick, whose Superconductivity and Magnetism Group has excellent facilities for research into skyrmionic materials. Warwick is well-known for the growth of high-quality single-crystals of oxides, intermetallics and related materials, with over 50 national and international partners engaged with this work. It is equipped with three optical mirror furnaces (including one with Xe arc lamps which uniquely allows us to reach temperatures of up to 2800 °C), a tetra-arc furnace, as well as facilities for crystal growth by the Bridgman method, the flux method, and chemical vapour transport.
Polycrystalline samples of new and interesting materials are prepared using a suite of box and annealing furnaces.
Suitable flux materials for the materials under study are chosen from an analysis of the phase diagrams.
Optical mirror furnaces
We employ either the Floating Zone (FZ) or Solvent Floating Zone (TSFZ) techniques to grow single crystals. The different optical furnaces provide a range of preparation conditions allowing us to vary, for example, the temperature definition in the molten zone and the growth rates. A wide variety of systems, which have both congruent and incongruent melting points can be prepared using these methods. Different growth atmospheres and pressures can be used. Materials that have been prepared include oxides as well as intermetallic materials.
A 2-mirror optical furnace used for single crystal growth at Warwick.
Bridgman and vapour transport methods
For some materials, the Bridgman or vapour transport growth techniques are more suitable. In the Bridgman method the material is contained in a crucible, e.g. a sealed quartz ampoule or a boron nitride crucible, and cooled from the molten state through a temperature gradient.
For crystals that cannot be obtained from molten material, the chemical vapour transport (CVT) technique can be used. This is the most popular technique to obtain several skyrmion materials.
Here, the precursor material is mixed with a transporting agent (such as iodine or chlorine) and placed in an evacuated sealed quartz tube. A furnace with a hot zone at temperature T2 heats a precursor, that is then transported to a colder zone T1 where it is deposited as crystals. These growths typically last for several weeks for each growth, and the growth periods and conditions have to be optimized for each material.
CVT grown single crystals of Cu2OSeO3
Single crystals of several pure and doped systems hosting skyrmions have been prepared using CVT.
Single crystals of Cu2OSeO3 were grown from pre-reacted polycrystalline powder using chemical vapour transport with TeCl4 as a transporting agent. The skyrmion lattices was studied using x-rays
(a) Schematic diagram of the experimental setup.
(b) Small-angle x-ray scattering pattern of the helical state of Cu2OSeO3.
(c) Energy scan of x-ray transmission (blue) and magnetic scattering (red) for a 200-nm-thick Cu2OSeO3 lamella at 20 K and zero magnetic field, aligned with the incoming beam ∥ . Representative transmitted intensity was determined by summing inside the blue circle on (b), and the scattered intensity was taken by summing the red circles and subtracting the green circles as background [18 - 2020].
Characterization of skyrmion materials
The materials that have been prepared are characterized in the laboratories of the Skyrmion Project partners. Structural studies include x-ray diffraction and electron microscopy. Samples are also analyzed using thermogravimetric and differential thermal analysis. A real-time x-ray Laue back reflection system at Warwick allows fast, direct examination of the quality of the as-grown single crystals and the alignment and cutting of samples.
Comparative study of the structural and magnetic properties of single crystals of Mn1/3NbS2 and Cr1/3NbS2 (a) Unit cell of Mn1/3NbS2 or Cr1/3NbS2 viewed down the  direction. (b) Simulated electron diffraction patterns along the  direction for both the noncentrosymmetric P6322 structure and the centrosymmetric P63/mmc structure, which differ from each other in the relative intensities of key spots (200) and (030), assuming kinematic scattering. An electron micrograph (c) of a crystal of Mn1/3NbS2 and an electron diffraction pattern from the same area are shown and match well with the simulation for the P6322 space group, with the especially faint (200) spot marked with an arrow. (d) The unit cell viewed along  direction. (e) A simulation of the corresponding electron diffraction pattern. (f) Bright-field images and electron diffraction patterns from Cr1/3NbS2 and (g) Mn1/3NbS2. While the Cr1/3NbS2 pattern matches well with the simulation, Mn1/3NbS2 displays extra reflections indicating a superlattice with a period of 3c in the c direction and 7d010 in the b∗ direction [8 - 2022].
In-depth experimental studies are carried out at all the Skyrmion Project host institutions using a range of probes including AC and DC magnetic measurements, specific heat studies, transport measurements (resistivity, Hall effect, thermopower), as a function of temperature, magnetic field, and pressure. We have facilities for magnetometry, transport, and thermodynamic measurements from 50 mK to 800 K in magnetic fields up to 17 T. Transport (30 kbar) and magnetization (10 kbar) studies can also be carried out as a function of pressure.
The figure shows the real component of AC susceptibility in constant field Bext for GaV4S8-ySey where (a) y = 0 (b) y = 0.1, (c) y = 8, and (d) y = 7.9. Dashed black lines indicate fields where μSR measurements were performed with white highlighting proposed skyrmion lattice regions. [11 - 2020].
Skyrmion materials for Research Challenges 2 and 3
The materials we have prepared are used for further studies by members of the Skyrmion Project under Research Challenge 2 and Research Challenge 3. At Cambridge University advanced microcopy techniques are used to investigate materials right down the level of an individual skyrmion. Samples are used for experiments at central facilities, both here in the UK at ISIS and Diamond, as well at international facilities such as PSI, ILL, ESRF and SOLEIL.
Small-angle neutron scattering
Pressure stability of the skyrmion lattice in Cu2OSeO3 in the μ0H parallel ki geometry. Small angle neutron scattering (SANS) patterns of the skyrmion lattice at T = 56 K, μ0H = 25 mT, and (a) ambient pressure or (b) P = 5 kbar. (c) and (d) show SANS patterns obtained at ambient pressure and P = 5 kbar, respectively, each with fixed T = 58.5 K, μ0H = 21 mT [9 - 2020].
Crystals for other projects
Single crystals have also been shared with collaborators both here in the UK and overseas for studies complementary to those of the UK Skyrmion Project team.