Scientists Discover a New Kind of Magnet

Its magnetic properties come from spin excitons, and could someday improve the speed and power consumption of data storage devices

In a normal magnetic material, dense magnetic moments try to align with their neighbors (left). By contrast, in a singlet-based material, unstable magnetic moments pop in and out of existence, and stick to one another in aligned clumps (right).
Illustration: Lin Miao/New York University
In a normal magnet, magnetic moments align with their neighbors (left). In a singlet-based magnet, unstable magnetic moments quickly appear and disappear, and stick together in clumps (right).
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A new kind of magnet, theorized for decades, may now have been experimentally proven to exist. And it could eventually lead to better data storage devices.

In a normal magnet, the magnetic moments of individual grains align with each other to generate a magnetic field. In contrast, in the new "singlet-based" magnet, magnetic moments are temporary in nature, popping in and out of existence.

Although a singlet-based magnet's field is unstable, the fact that such magnets can more easily transition between magnetic and non-magnetic states can make them well-suited for data storage application. Specifically, they could operate more quickly and with less power than conventional devices, says Andrew Wray, a materials physicist at New York University who led the research.

Now, Wray and his colleagues have discovered the first example of a singlet-based magnet that is robust—one made from uranium antimonide (USb2).

"Even though this looks like a magnet, it's profoundly different from other magnets on a microscopic scale," Wray says.

The concept for singlet-based magnets dates back to the 1960s. The temporary nature of their magnetic moments arises from a "spin exciton," which can occur when electrons collide with one another under the right circumstances. Excitons are quasiparticles made up of electrons bound to their positively-charged counterparts, known as holes. In normal excitons, the magnetic moments of the electrons and holes usually point in opposite directions and cancel each other out. In contrast, for spin excitons, the magnetic moments of the electrons and holes align the same way.

Although a single spin exciton is very unstable and disappears quickly, when many are together, they can in theory stabilize each other and catalyze the appearance of even more spin excitons in a kind of chain reaction, Wray says. However, until now, magnetism from spin excitons was only stable at extremely cold temperatures of less than 10 Kelvin (minus 263.15 degrees C).

The scientists discovered a robust singlet-based magnet as they were investigating uranium antimonide. Decades of research had found that magnetism and electricity behaved strangely within this material.

Using neutron-scattering and X-ray scattering scans, as well as computer simulations, the researchers discovered uranium antimonide's magnetism arose from spin excitons.

"It ends up taking very little energy to create spin excitons for uranium antimonide," Wray says. "This is essential for the singlet-based magnet, because if it took a lot of energy, then there wouldn't be enough spin excitons to condense, stabilize one another, and give you a magnet."

Uranium antimonide's magnetism emerged at relatively high temperatures of roughly 203 Kelvin (minus 70.15 degrees C). Wray notes "room temperature may be in shooting distance," if one doped the material with strongly magnetic components.

Uranium antimonide is fairly easy to synthesize, allowing scientists a way to uncover more about this new form of magnetism, Wray says. Future applications could find ways to create singlet-based magnets from other materials, such as "non-toxic heavy transition metal elements that would be more friendly for applications," he adds.

The scientists detailed their findings online on 7 February in Nature Communications.

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