Figure 1: A lattice of swirling magnetic vortices called skyrmions can be switched between two states using an electric field. (Image: Yoshinori Tokura, RIKEN Center for Emergent Matter Science)
Electrons act as tiny bar magnets. In some materials, these ‘magnets’ can tip and rotate at different angles, forming a swirling vortex—a skyrmion—just a few nanometers across. Many skyrmions can form an ordered, hexagonal lattice of magnetic vortex lines.
Yoshinori Tokura, who is director of the RIKEN Center for Emergent Matter Science, and colleagues from the University of Tokyo have studied skyrmion lattices in small crystals of the magnetic material copper selenium oxide (Cu2OSeO3). They found that these skyrmion lattices can exist for some time in states that are not energetically the most stable, known as metastable states. The team discovered that the metastable skyrmion lattices persist at very low temperatures and can be controlled by applying an electric field.
“Until our study, people believed that the skyrmion lattice should disappear when the temperature is lowered in this material,” says Yoshihiro Okamura, a PhD student in Tokura’s laboratory. “However, we found that the skyrmion lattice can persist even at very low temperatures under electric fields and that, once created, the lattice remained even after the electric field was switched off.”
The team showed that inverting the electric field could switch between the skyrmion lattice and a so-called conical state (Fig. 1). In particular, at 55.5 kelvin, the skyrmion lattice remained once created, which is important since it means that such systems could be used store data even when they are not being powered.
This kind of switching did not occur at 54 kelvin, while above 56.5 kelvin the system reverted to its original state when the electric field was turned off. That offers a narrow temperature window in which the two states are both stable and switchable.
“To realize the phase interchange across a wider temperature range, we would need larger electric fields,” says Okamura. “But since that could degrade the material used in the experiments, we instead intend to broaden the temperature window by using higher quality crystals.”
The team believes that other external factors such as mechanical pressure could also be used to control skyrmion lattices in similar ways. They also hope to demonstrate switching between stable states in a single skyrmion. Both advances would increase the versatility of skyrmions for applications.