Scientists baffled by magnetic material that freezes when heated

Physicists have found that certain magnetic material freezes when the temperature rises to a certain point. We have usually only seen this behavior when we cool magnetic materials, not when we heat them. As such, it has left physicists scratching their heads and baffled by the development.

These magnetic materials freeze when heated

changes observed in the magnetic material when it is heated
This figure shows how the magnetic spins stabilize and freeze when heated. Image source: Khajetoorians, AA (Alex) / Radboud University

Physicists Alexander Khajetoorians of Radboud University in the Netherlands say the freezing of the magnetic materials is the opposite of what we normally see. The result is “counter-intuitive, like water turning into an ice cube when heated up”, according to Khajetoorians

Normally, ferromagnetic materials such as iron have aligned spins. This means that the magnetic spins of the atoms all spin in the same direction. Essentially, the south and north magnetic poles are all aligned in the same direction. However, some alloys made of both iron and copper have randomized spins. Physicists call this condition spin glass.

Usually, physicists say that you have to combine materials to make spinning glass. Like the iron and copper mixture I mentioned in the previous section. However, neodymium is an element that experiences spinning glass without the need to mix additional materials with it. And when heated, this magnetic material freezes.

Unexpected behavior

example of metamaterials in physics laboratoryImage source: luchschenF/Adobe

The researchers published a study on the phenomenon in Nature physics† According to the paper, they heated the materials from -268 C to -265 C. By just heating it to those three degrees, the magnetic material froze into a fixed pattern, creating a magnet. When they cooled down again, the magnetic spins returned to their normal, random patterns.

As I noted above, this is intriguing because this freezing of the magnetic spins usually only occurs when the magnetic material is cooled and not heated. As such, the researchers had not expected to see this phenomenon. Typically, higher temperatures increase the energy in a solid, liquid, or gas. The same goes for magnetic material.

When you increase the temperature, the spins usually start to vibrate, which makes for more random spins. In this case, however, the magnetic material froze. These phenomena are not often encountered in nature. And if so, they completely turn everything we thought we knew on its head. Of course, this is not the only material that does not behave as expected.

The physicists say that the behavior exemplifies the concept of degeneracy. This is basically saying that many different states can have the same energy, causing the whole system to get frustrated. Then, when a certain temperature is reached, the system breaks, settling into one pattern. Freezing magnetic materials is a good example of this concept at work.

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