# Skyrmion-skyrmion interactions in skyrmion racetrack memory

Summary of our recent paper on Skyrmion dynamics in race track memory devices:

Xichao Zhang, G. P. Zhao, Hans Fangohr, J. Ping Liu, W. X. Xia, J. Xia & F. J. Morvan
Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory in
Scientific Reports 5, 7643 (2015)
(Online: journal arXiv.org)

## Context: Skyrmion racetrack

### Racetrack memory

The racetrack memory is a proposed data storage technology in which bits are encoded as magnetic domains on a wire. Using an electric current, the domains can be moved along the wire, and made to pass one or multiple read/write points (which are static.)

Figure A: In the racetrack memory design, magnetic domains are moved along a wire using an electric current. As there are no mechanical parts moving, the system is more robust that the hard disk technology. It has the potential to deliver high data density as the track can be stored in a 3d volume (in contrast to the 2d-area of platters in hard disks). (Source of animated gif)

### Skyrmion racetrack memory

Recent work from 2013 shows that a similar design can potentially be used where the domain walls are replaced with skyrmions. Skyrmions are particular patterns in the magnetic field, and each skyrmion can be regarded as a particle (within some limits). Instead of encoding 1 with a magnetic domain pointing up, and 0 with a domain pointing down, one could encode 1 with the presence of a skyrmion and zero with its absence in such a skyrmion track memory.

Figure B: The skyrmion racetrack memory design is based on the racetrack memory idea (as shown in Figure A), but uses moving skyrmions instead of moving magnetic domains to store the data. A spin-current, in this design flowing from bottom to top perpendicular to the layer of the track, is used to move the skyrmions along the track.

## This study

In this study we study some of the details that are important to realise this design, including the repulsion behaviour of skyrmions, the optimum spacing of skyrmions in a racetrack, and a mechanism to allow skyrmions to escape at the end of the track.

### Skyrmion repulsion

For $$d_i=60$$ nm, as shown in subplots c and d, both skyrmionic bits move $$13.8$$ nm with a steady velocity of $$46$$ m/s. We see that an initial spacing of 30nm is too small, and 60nm is sufficient.

In the manuscript, we study how the skyrmion repulsion varies as a function of skyrmion distance, and other parameters, such as the applied field, the track width and anisotropy.

### Skyrmion clogging

When skyrmions are moved along a wire, it is an important part of the design to have some mechanism that allows the skyrmions to self-destruct at the end of the wire.

Figure D: Subplots a and b show how skyrmions fail to exit at the right-hand side end of the track. They are squeezed together and their size is reduced, but given the strength of the current pushing them along the track, they cannot escape. This is undesirable.

Subplots c to l show the successful escaping of skyrmions from the track for the same driving current, due to a modified end of track geometry. The mechanism of skyrmion destruction can be seen from the number of snapshots.

## Summary

• Study skyrmion motion in interfacial DMI based skyrmion nanotrack memory
• Study skyrmion-skyrmion repulsion and
• Skyrmion-edge repulsion in the nano tracks
• Using OOMMF simulation package
• Propose a end of track geometry to support efficient escaping of skyrmions from the track

## Future work

Skyrmionics is an active field of research - if you are interested to pursue a PhD in computational modelling of such systems, please get in touch.

## Full text access and paper details

Xichao Zhang, G. P. Zhao, Hans Fangohr, J. Ping Liu, W. X. Xia, J. Xia & F. J. Morvan, Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory, in Nature Scientific Reports 5, 7643 (2015)

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