Research challenge 1: skyrmion materials

Research challenge 1: Skyrmion materials

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 βMn 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.

A 2-mirror optical furnace used for crystal growth at Warwick

Polycrystalline materials synthesis and single-crystal growth

Much of the synthesis of polycrystalline samples and single-crystals of skymion 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 vapor transport.

Polycrystalline materials; Polycrystalline samples of new and interesting materials are prepared using a suite of box and annealing furnaces.

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.

Flux technique: Suitable flux materials for the materials under study are chosen from an analysis of the phase diagrams.

Bridgman and vapor transport methods For some materials, the Bridgman or vapor 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 vapor transport (CVT) technique can be used. This is the most popular technique to obtain several skyrmion materials. Here, the material is mixed with a transporting agent (such as iodine or chlorine) and placed in an evacuated sealed quartz tube. A furnace with several temperature zones is used to transport the material to a colder zone 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.

Schematic of the 2-zone furnace used for the chemical vapour transport technique.

Studies of skyrmion materials in the laboratory

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.

In-depth experimental studies are carried out at all the Skymion 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.

AC susceptibility measurement of Cu2OSeO3 showing the skyrmion phase.

The materials we have prepared are used for further studies by members of the Skymion Project under Research Challenges 2 and 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. Single crystals have also been shared with collaborators both here in the UK and overseas for studies complementary to those of the Skyrmion Project team.