Physicists are startled by this magnetic material that “freezes” when heated.

When disordered magnetic materials are cooled to just the right temperature, something interesting happens. The spins of their atoms ‘freeze’ and lock into a static pattern, exhibiting cooperative behavior not normally exhibited.

Now, for the first time, physicists have seen the opposite. When fractionally heated, the naturally occurring magnetic element neodymium freezes, upending all our expectations.

“The magnetic behavior we observed in neodymium is actually the opposite of what ‘usually’ happens,” said physicist Alexander Khajetoorians of Radboud University in the Netherlands.

“It’s pretty counterintuitive, like water turning into an ice cube when heated.”

In a common ferromagnetic material like iron, the magnetic spins of the atoms all align in the same direction; that is, their north and south magnetic poles are oriented the same way in three-dimensional space.

But in some materials, like some alloys of copper and iron, the spins are quite random instead. This state is called spin glass.

You might be thinking, “but neodymium is known to make excellent magnets,” and you’re right…but it needs to be mixed with iron for the spins to align. Pure neodymium does not behave like other magnets; Just two years ago, physicists discovered that this material is actually best described as self-induced spin glass.

Well, it seems, neodymium is even stranger than we thought.

When you heat a material, the rise in temperature increases the energy in that material. For magnets, this increases the motion of the spins. But the opposite also occurs: If you cool down a magnet, the spins slow down.

For spin glasses, the freezing temperature is the point at which the spin glass behaves more like a traditional ferromagnet.

A team of scientists led by Radboud University physicist Benjamin Verlhac wanted to study how neodymium behaves with changing temperatures. Interestingly, they found that raising the temperature of neodymium from -268 degrees Celsius to -265 degrees Celsius (-450.4 to -445 Fahrenheit) produced the freezing state that normally occurs when a spin glass cools.

When the scientists cooled the neodymium down again, the spins became disordered again.

It’s unclear why this happens, as it’s very rare for a natural material to behave in the ‘wrong’ way, unlike all other materials of its kind. However, scientists believe it may be related to a phenomenon called frustration has to do.

This is the case when a material cannot achieve an ordered state, resulting in a disordered ground state such as we see in spin glasses.

It’s possible, the researchers said, that neodymium exhibits specific temperature-dependent correlations in its spin-glass state. Increasing the temperature weakens it and with it the frustration, which allows the spins to settle in one direction.

Further investigation could reveal the mechanism behind this strange behavior, where order emerges from disorder when energy is added; The researchers note that this has ramifications that go well beyond physics.

“This pattern ‘freezing’ does not typically occur in magnetic material,” explained Khajetoorians.

“If we can ultimately model how these materials behave, this could be extrapolated to the behavior of a variety of other materials as well.”

The research was published in natural physics.

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