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


When disordered magnetic materials are cooled to the right temperature, something interesting happens. The spins of their atoms “freeze” and lock into place in a static pattern, exhibiting cooperative behavior that is not usually displayed.

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

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

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

In a conventional ferromagnetic material, such as iron, the magnetic spins of the atoms all line up in the same direction; that is, their north and south magnetic poles are oriented the same in three-dimensional space.

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

You might be thinking “but neodymium is well known for making great magnets” and you’d be right…but it needs to be mixed with iron for the spins to line up. Pure neodymium does not behave like other magnets; it wasn’t until two years ago that physicists determined that this material is, in fact, best described as self-induced spin glass.

Now, it looks like neodymium is even weirder than we thought.

When you heat a material, the increase in temperature increases the energy of that material. In the case of magnets, this increases the motion of spins. But the reverse also happens: when you cool a magnet, the rotations slow down.

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

Led by physicist Benjamin Verlhac of Radboud University, a team of scientists set out to probe the behavior of neodymium under changing temperatures. Interestingly, they found that increasing the temperature of neodymium from -268 degrees Celsius to -265 degrees Celsius (-450.4 to -445 Fahrenheit) caused the frozen state typically seen when cooling a glass spin.

When the scientists cooled the neodymium, the spins again fell into disarray.

It is not known why this happens, as it is very rare for a natural material to behave in the “wrong” way, unlike how all other materials of this type behave. However, scientists believe it may be linked to a phenomenon called frustration.

It is when a material is unable to reach an ordered state, resulting in a disordered ground state, as seen in spin glasses.

It is possible, the researchers say, that neodymium has certain correlations in its spin glass state that are temperature dependent. The increase in temperature weakens them, and therefore also the frustration, allowing the spins to settle into an alignment.

Further investigation might reveal the mechanism behind this strange behavior in which order emerges from disorder with the addition of energy; the researchers note that this has implications far beyond physics.

“This ‘freezing’ of the pattern doesn’t normally occur in magnetic materials,” the Khajetorians explained.

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

The research has been published in Natural Physics.