Discovery of a novel magnetic skyrmion surface state

The search for improved information carriers remains a major task in data storage technology research. The ideal candidate should be stable against external disturbance such as temperature or field fluctuations, it should allow for low-power manipulation and easy readout, combined with a small footprint allowing for high packing densities. In principle, magnetic skyrmions fit the requirements well and are therefore undergoing a data storage health check. Their key advantage arises from their topological properties: its particular magnetic moment configuration, i.e., the self-protected vortex shape, is robust against certain distortions. Their topology also induces novel magnetoelectrical effects, allowing for efficient manipulation and detection.

In principle, the most straightforward way of making use of skyrmions for memory devices is to assign a logic 0 or 1 to the absence or presence of a skyrmion in a magnetic racetrack memory scheme. Many of the components needed for an experimental realization of this technology have recently been reported in the literature. Nevertheless, two key issues remain challenging. First, skyrmions have to be tamed to keep their distance when being moved along the racetrack, i.e., the encoded information has to be preserved. Second, the storage density in this conventional racetrack design is governed by the skyrmion diameter, as shown in Fig. 1(a). In analogy to the introduction of perpendicular memory, an improved version of a racetrack can be achieved by erecting the bits as shown in Fig. 1(b) – a `perpendicular skyrmion array’. In this case, the storage density is merely governed by the skyrmion depth itself, as the thickness of a skyrmion is theoretically vanishing due to its two-dimensional nature!

When carrying out their REXS study in the in-plane geometry on the well-known skyrmion crystal Cu₂OSeO₃, the research team was surprised by the emergence of an exotic surface skyrmion lattice state. First, the skyrmion array survives at very low temperatures, much different from the known, tiny skyrmion pocket in out-of-plane fields. In fact, at low temperatures, skyrmions only ‘float’ at the very surface of the bulk sample, above a sea of conical spirals. The thickness of this surface state is estimated to be only 120 nanometers. Second, by applying an oblique magnetic field, the team showed that the surface skyrmion array is strongly ‘pinned’ to the surface, resisting the field tilting.

Lead author Prof Shilei Zhang clarifies:

For more information on the subject matter, please contact either Professor Shilei Zhang (ShanghaiTech University): zhangshl1@shanghaitech.edu.cn, Professor Gerrit van der Laan (Diamond): gerrit.vanderlaan@diamond.ac.uk, or Professor Thorsten Hesjedal (Oxford Physics): Thorsten.Hesjedal@physics.ox.ac.uk.

To find out more about the I10 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Paul Steadman: paul.steadman@diamond.ac.uk.