(Left) At cooler temperatures, the spins in neodymium form random patterns, where each pattern whirls like a helix with a particular twist. (Right) But when neodymium is heated, the spins adopt one particular helix pattern, a phenomenon that normally occurs in magnetic materials when the temperature decreases. Image: Radboud University.
(Left) At cooler temperatures, the spins in neodymium form random patterns, where each pattern whirls like a helix with a particular twist. (Right) But when neodymium is heated, the spins adopt one particular helix pattern, a phenomenon that normally occurs in magnetic materials when the temperature decreases. Image: Radboud University.

Researchers have observed a strange new type of behavior in a magnetic material when it’s heated up. The magnetic spins in this material ‘freeze’ into a static pattern when the temperature rises, a phenomenon that normally occurs when the temperature decreases. The researchers report their findings in a paper in Nature Physics.

The researchers discovered this phenomenon in the material neodymium, an element that they described several years ago as a ‘self-induced spin glass’. Spin glasses are typically metal alloys, where, for example, iron atoms are randomly mixed into a grid of copper atoms. Each iron atom behaves like a small magnet, or a spin. These randomly placed spins point in all kinds of directions.

Unlike conventional spin glasses, produced by the random mixing of magnetic materials, neodymium is an element. Without significant amounts of any other material, it shows glassy behavior in its crystalline form. The spins form patterns that whirl like a helix, and this whirling is random and constantly changes.

In this new study, the researchers discovered that when they heated neodymium up from -268°C to -265°C, its spins ‘freeze’ into a solid pattern, forming a type of magnet, at the higher temperature. When cooling down the material, the random whirling helix patterns came back.

“This ‘freezing’ of the pattern does not normally occur in magnetic material,” says Alexander Khajetoorians, professor of scanning probe microscopy at Radboud University in Nijmegen, the Netherlands.

Higher temperatures increase the energy in a solid, liquid or gas. The same holds true for a magnet: at higher temperatures, spins usually start to shake.

“The magnetic behavior in neodymium that we observed is actually the opposite of what ‘normally’ happens,” says Khajetoorians. “It’s quite counterintuitive, like water that becomes an ice cube when it’s heated up.”

These kinds of counterintuitive phenomena are not often found in nature – very few materials are known to behave in the wrong way. One other well-known example is the Rochelle salt: its charges build up and form an ordered pattern at higher temperatures, but are randomly distributed at lower temperatures.

The complex theoretical description of spin glasses was the subject of the Nobel Prize in Physics in 2021. Figuring out how these spin glasses work also has importance for other scientific fields.

“If we ultimately can model how these materials behave, this could also be extrapolated to the behavior of a wide range of other materials,” says Khajetoorians.

The underlying odd behavior is linked to the concept of degeneracy: where many different states have the same energy, and the system becomes frustrated. Temperature can alter this situation: only certain states survive, allowing the system to clearly settle into one pattern.

This odd behavior could potentially be harnessed for new types of information storage or computational concepts, such as brain-like computing.

This story is adapted from material from Radboud University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.