RIKEN physicists have shown that tiny amounts of heat can be used to control the movement of magnetic hot tubs called skyrmions1. This ability could help develop energy-efficient forms of computers that use waste heat.
Skyrmions are tiny eddies that form when the magnetic flux of a group of atoms organizes into swirling patterns. Skyrmions can move within a material and, under certain conditions, group themselves into a regular arrangement known as a skyrmion lattice (upper part of Fig. 1).
Skyrmions are promising information carriers in next-generation computer chips that have very low energy requirements. Researchers can control skyrmions by applying electric currents and magnetic fields, but instead try to manipulate them with the help of heat currents. “This is an exciting prospect as it would open up the possibility of using waste heat to move skyrmions,” says Xiuzhen Yu of the RIKEN Center for Emergent Matter Science.
Yu and her colleagues have now shown how a temperature gradient can be used to drive skyrmions in an electrically insulating magnetic material.
The team built a device that consisted of a plate of this material, a miniature heating element, and two electrical thermometers. They then created skyrmions with a width of around 60 nanometers in the plate by cooling it to around -253 degrees Celsius and applying a magnetic field. These skyrmions gathered into a stable honeycomb structure known as the hexagonal skyrmion lattice.
Yu’s team then raised the temperature slightly at one end of the plate and used a transmission electron microscope to see how this affected the skyrmions. A temperature gradient of 100ths of a degree per millimeter of plate was sufficient to set the skyrmions in motion. Above this threshold, the edge of the honeycomb lattice migrated from the cooler to the warmer end of the plate and migrated in the opposite direction to the heat flow (lower part of Fig. 1). This required a very low heating power of only 10 microwatts, which is a hundred or a thousand times less than the power required to move skyrmions with an electric current or magnetic field. With a slightly higher power, individual skyrmions could be driven through the plate by the temperature gradient.
The researchers say this is the first time heat-driven skyrmion motion has been observed in an insulating magnet. “This finding should inspire researchers to develop energy-efficient devices using skyrmions,” says Yu.
The team is now studying the heat-induced dynamics of skyrmions, including their conversion to their anti-particle anti-skyrmions in metallic systems at room temperature.