Controlling Magnets with Heat

By Michael Lucibella

Researchers from the University of California, San Diego, have found that in a narrow temperature range, some nanomaterials suddenly and dramatically become more resistant to changes in their magnetic orientation. The team, led by Ivan Schuller, discovered previously unseen spikes in this behavior in bilayers of nickel and vanadium oxide.

“It’s the control of magnetism without a magnetic field,” said Schuller, speaking at the APS March Meeting. “We can control it just by changing the temperature.”

Coercivity is the quantity that marks how resistant a ferromagnetic material is to having its magnetic orientation changed by a nearby magnetic field. In most materials, the hotter they are, the easier it is to change their magnetic directions, and the lower their coercivity.

Schuller’s team found a narrow temperature range where the coercivity of a 10 nm layer of nickel mated with a 100 nm layer of vanadium oxide unexpectedly jumps several hundred percent.

“It changes by a factor of five. It’s a huge effect,” Schuller said. “It’s significant because it provides a new control mechanism for the magnetism.”

The effect occurs at the temperature where the oxide goes from being an electrical insulator to a conducting state. The nickel-plus-V₂0₃ nanomaterial shows spikes between 160 and 180 kelvin, while nickel with VO₂ peaks around 340 kelvin.

Because the effect is so new, its potential applications are still unclear. Schuller speculated that this kind of manipulation could be used in the future to make longer-lasting magnetic memory, or build an electrical transformer that loses its conductivity when it starts to overheat.

“It’s an automatic fuse that doesn’t burn. Something that you wouldn’t have to go and replace,” Schuller said.

He added that he expected to see similar effects in other materials, and his group plans to find out if there is some way also to mimic the effect using different voltages and currents.

The team’s research was published in Applied Physics Letters.