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 fall into place in a static pattern, exhibiting cooperative behaviors not usually displayed.

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

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

“It’s kind of counterintuitive, like water turning into an ice cube when it’s heated up.”

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

But in some materials, such as some alloys of copper and iron, the spins are quite random. This condition is what is known as a spinning glass.

You may be thinking “but neodymium is known for making excellent magnets” and you are right… but it needs to be mixed with iron to align the spins. Pure neodymium does not behave like other magnets; it was only two years ago that physicists determined that this material is, in fact, best described as a self-induced spinning glass

Now it seems that neodymium is even stranger than we thought.

When you heat a material, the increase in temperature increases the energy in that material. In the case of magnets, this increases the movement of the spins. But the opposite also happens: when you cool a magnet, the spin slows down.

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

Led by physicist Benjamin Verlhac of Radboud University, a team of scientists wanted to investigate how neodymium behaves 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 freezing condition commonly seen when cooling a spinning glass.

When the scientists cooled the neodymium again, the spins got confused again.

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

This is when a material is unable to reach an ordered state, resulting in a disordered ground state, as we see in spin glasses.

It’s possible, the researchers said, that neodymium has certain correlations in its spin glass state that depend on temperature. Increasing the temperature weakens it, and thus the frustration, which can cause the spins to align.

Further investigation could 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 the physics.

“This ‘freezing’ of the pattern does not normally occur in magnetic material,” Khajetorians explained

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

The research was published in Nature physics

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