An antiferromagnetic skyrmion may sound like the content of science fiction, but they could be the future of data storage. By characterising the material BiFeO3 using neutron diffraction alongside other techniques, and observing both magnetic and electric chirality, a group of researchers has opened the door to the possibility of creating spintronic systems using antiferromagnetic materials.
In most materials, spins of neighbouring atoms tend to align in the same direction as each other, or in opposite directions resulting in ferromagnets or antiferromagnets, respectively. In some materials, known as chiral magnets, there are unusual physical interactions between the spins because of a peculiar crystalline or multilayer structure of the material. In these cases, the atom's spins can align in a more intricate fashion, creating what is known as a topological spin texture.
One of these spin textures is a magnetic skyrmion. In a skyrmion, the orientation of spins rotates progressively from the up direction at the edge of the circular texture, to the down direction at the centre, or vice versa. As there are two possible directions for the winding, skyrmions can be left- or right-handed; a property called chirality.
Skyrmions have very specific stability, dynamics and scalability properties that make them well suited for possible applications in quantum computing. As described in a previous ISIS Science Highlight, antiferromagnetic materials are less susceptible to interference with an external magnetic field. Therefore, combining these two properties to create an antiferromagnetic skyrmion is very appealing.
A group of materials that may offer this property are multiferroics. These are materials which display magnetic order such as antiferromagnetism, as well as ferroelectricity; they have a spontaneous electric polarization that can be reversed by the application of an external electric field. One of the original multiferroics is BiFeO3; it is one of very few materials that exhibit both spin and dipole ordered phases above room temperature.
The areas of particular interest in materials such as these are the domain walls: the boundaries between the regions in the material that have different spin and electric orientations. A group of researchers from France studied BiFeO3 on the WISH diffractometer at ISIS alongside investigations using X-ray scattering and other techniques to characterise the material and its domain structure in both real and reciprocal spaces.
In particular, the work done on WISH allowed the magnetic structure to be determined for each ferroelectric variant throughout the whole sample. The high resolution and high flux of the diffractometer was essential to detect subtle splitting and be able to measure such thin samples. Combining this technique with resonant X-ray scattering, they observed, for the first time, both antiferromagnetic and electric chiral textures at domain walls in BiFeO3 at room temperature.
The appearance of both magnetic and electric chirality opens a new avenue (or paradigm) for the creation and control of skyrmion systems. Future research on the interplay of these unusual spin textures with spin currents and light will enable further development of these materials for spintronic applications.
Further information
The full paper can be found at: https://www.nature.com/articles/s41563-019-0516-z
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