Magnets could cool future refrigerators, laptops

Magnets can act as wireless cooling agents and may one day be used to cool refrigerators and laptops, MIT scientists say.
A new theory formulated by researchers from Massachusetts Institute of Technology describes the motion of magnons - quasi-particles in magnets that are collective rotations of magnetic moments, or 'spins'.
In addition to the magnetic moments, magnons also conduct heat. The researchers found that when exposed to a magnetic field gradient, magnons may be driven to move from one end of a magnet to another, carrying heat with them and producing a cooling effect.
"You can pump heat from one side to the other, so you can essentially use a magnet as a refrigerator," said Bolin Liao, a graduate student in MIT's Department of Mechanical Engineering.

"You can envision wireless cooling where you apply a magnetic field to a magnet one or two meters away to, say, cool your laptop," Liao said.
In theory, Liao said, a magnetically driven refrigerator would require no moving parts, unlike conventional iceboxes that pump fluid through a set of pipes to keep things cool.

Liao, graduate student Jiawei Zhou and Department of Mechanical Engineering head Gang Chen, have published a paper detailing the magnon cooling theory in Physical Review Letters.
In many ways, magnons are similar to electrons, which can simultaneously carry electrical charge and conduct heat. Electrons move in response to either an electric field or a temperature gradient - a phenomenon known as the thermoelectric effect.

In recent years, scientists have investigated this effect for applications such as thermoelectric generators, which can be used to convert heat directly into electricity, or to deliver cooling without any moving parts.
Liao and his colleagues recognised a similar 'coupled' phenomenon in magnons, which move in response to two forces: a temperature gradient or a magnetic field.
Because magnons behave much like electrons in this aspect, the researchers developed a theory of magnon transport based on a widely established equation for electron transport in thermoelectrics, called the Boltzmann transport equation.
From their derivations, Liao, Zhou, and Chen came up with two new equations to describe magnon transport.

With these equations, they predicted a new magnon cooling effect, similar to the thermoelectric cooling effect, in which magnons, when exposed to a magnetic field gradient, may carry heat from one end of a magnet to the other.
Liao used the properties of a common magnetic insulator to model how this magnon cooling effect may work in existing magnetic materials.
He found that while the effect was small, the material was able to generate a cooling effect in response to a moderate magnetic field gradient. The effect was more pronounced at cryogenic temperatures.