Skyrmions are small, whirling magnetic spin structures that occur in many materials, including manganese silicide thin films (in which they were first discovered) and cobalt-iron-silicon. In the present work, the researchers studied thin multilayers of platinum, cobalt and magnesium [Pt(3.2 nm)/CoFeB(0.7 nm)/MgO(1.4 nm)]. They can be imagined as 2D knots in which the magnetic moments rotate about 360° within a plane.

Skyrmions could form the basis of future magnetic data-storage technologies. Today's disk drives use magnetic domains (in which all the magnetic spins are aligned in the same direction) to store information, but there are fundamental limits to how tiny such domains can become and how tightly they can be packed together. Skyrmions, on the other hand, might be more stable when they are small and thus be used to create storage devices with higher density. What is even more important, such a memory would retain information even when the power is switched off.

'Racetrack' memories

Another good thing about skyrmions is that, thanks to their smaller sizes and shorter spacing between them, they could be moved rapidly and efficiently along nanowires or other nanostructures in future “racetrack” memories. Such memories would not require any moving parts either – unlike the read-write heads in traditional hard disk drives – making these future devices much more robust. They are also less affected by defects, such as edge roughness, than conventional domain walls so they can move more smoothly and stably along the tracks.

Skyrmions have been predicted to exhibit an effect that is similar to the Hall effect for electrons – and which is thus called the skyrmion Hall effect – when a current is applied to them. As a result, the current should not only make the skyrmions move in a longitudinal direction but in a transversal direction too. Such motion needs to be considered in applications such as logic gates, and the effect plays an important role in the behaviour of skyrmions since it can change the transversal position of individual quasi-particles within a racetrack and therefore pose additional challenges for memory applications.

Nanoscale pump-probe X-ray microscopy

Thanks to nanoscale pump-probe X-ray microscopy, researchers led by Mathias Kläui of Johannes Gutenberg University Mainz and Geoffrey Beach of the Massachusetts Institute of Technology have now succeeded in imaging individual skyrmions as they move in real time along a magnetic racetrack made of [Pt(3.2 nm)/CoFeB(0.7 nm)/MgO(1.4 nm)]. The technique can be used to observe how skyrmions move with sub-100 picosecond time resolution on spatial scales of less than 15 nm.

The researchers found that the skyrmions move at a well-defined angle that can exceed 30° with respect to the angle at which a current is applied. This angle increases with the speed that the skyrmions are travelling (up to at least 100 m/s).

'Skyrmion Hall angle'

“This ‘skyrmion Hall angle’ has previously been predicted to depend on the static skyrmion properties, such as the diameter, but was predicted not to vary dynamically with speed,” explains Kläui. “Here we find that this angle strongly depends on the velocity – behaviour that cannot be explained using conventional models. We have thus developed a new model that takes into account dynamic changes of the skyrmion spin structure that can qualitatively explain the observations.

“We also show that we can thus tailor this angle and that we need to take into account the dynamic changes of the skyrmion trajectories when designing future devices,” he tells nanotechweb.org.

The researchers obtained their results by studying a film consisting of layers of platinum, cobalt and magnesium that they fabricated specially to contain a minimum of defects. In this material, so-called additive Dzyaloshinskii–Moriya (DM) interactions make adjacent magnetic spins in the material line up perpendicularly to the direction of applied current. This leads to helical spin structures (the skyrmions) that have well-defined handedness, or chirality. The DM interaction is part of the spin-orbital interaction, which couples electronic orbital and intrinsic spin magnetism and bestows skyrmions with additional stability and a smaller size, which allows them to travel more smoothly in nanostructures.

Kläui and colleagues detail their research in Nature Physics doi:10.1038/nphys4000.