A unique new phenomenon in neodymium, a magnetic material, has been found when it is heated up. In a behavior that normally happens when the temperature decreases, physicists at Radboud University observed magnetic spins ‘freeze’ into a static pattern as the temperature rose.
The researchers characterized the material neodymium several years ago as a self-induced spin glass.
Typically, spin glasses are alloys in which elements, iron atoms, for example, 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 different directions.
Dissimilar to ordinary spin glasses – which feature random combining of magnetic materials – neodymium is an element. Barring significant amounts of any other material, it shows glassy behavior in its crystalline form.
Therefore the neodymium spins form patterns that rotate like a helix. This rotation is random and constantly changes.
Frozen Magnetic Helixes In Neodynium
Researchers found in this study that when neodymium was heated up from -268 C to – 265 C, the spins ‘freeze’ into a solid pattern forming a type of magnet, at the higher temperature. When the cooled down the material, the random whirling helix patterns resumed.
“This ‘freezing’ of the pattern does not normally occur in magnetic material”,
said Radboud’s Alexander Khajetoorians, professor of scanning probe microscopy.
Credit: Radboud University
In a solid, liquid, or gas, as temperature increases, so does its energy. This is also true for a magnet – with more temperature, spins start to shake.
“The magnetic behaviour in neodymium that we observed is actually the opposite of what ‘normally’ happens. It’s quite counterintuitive, like water that becomes an ice cube when its heated up,”
said Khajetoorians.
This type of phenomena is not often found in nature. In fact, there are very few materials known that behave in the wrong way.
Another well-known exception is that of Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, where charges build up and form an ordered pattern at a higher temperature, whereas at a lower temperature they are randomly distributed.
Modeling Spin Glasses
Why is it called a spin glass? The word “glass” refers to an analogy between the magnetic disorder in spin glass and the positional disorder of conventional, chemical glass, such as window glass.
In window glass, the atomic bond structure is highly irregular; in contrast, a crystal has a uniform pattern of atomic bonds. In magnetic solids, magnetic spins all align in the same direction; this is analogous to a crystal’s lattice-based structure.
The complex theoretical description of spin glasses was the subject of the Nobel Prize in Physics awarded to Italian theoretical physicist Giorgio Parisi in 2021. Determining how these spin glasses work also has relevance in 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,”
explained Khajetoorians.
The unusual behavior is associated with the concept of degeneracy, where many different states have the same energy, and the system becomes frustrated.
The effect of temperature is to break this predicament. Certain states survive, allowing the system to clearly settle into one pattern. We may also be able to harness this behavior toward new types of information storage or computational concepts, such as brain-like computing.
Reference: Thermally induced magnetic order from glassiness in elemental neodymium, Nature Physics, 4-Jul-2022
