Magnets can control heat and sound

US researchers have discovered that heat, and possibly sound, can be controlled with a strong magnetic field.
Researchers at The Ohio State University used a magnetic field roughly the size of a medical Magnetic resonance imaging (MRI) to reduce the amount of heat flowing through a semiconductor by 12 per cent.
The study is the first to prove that acoustic phonons - the elemental particles that transmit both heat and sound - have magnetic properties.
"This adds a new dimension to our understanding of acoustic waves," said Joseph Heremans, professor of mechanical engineering at Ohio State.
"We've shown that we can steer heat magnetically. With a strong enough magnetic field, we should be able to steer sound waves, too," Heremans said.

Researchers, however, cautioned that there won't be any practical applications of this discovery any time soon: 7-tesla magnets like the one used in the study don't exist outside of hospitals and laboratories, and the semiconductor had to be chilled to minus 268 degrees Celsius - very close to absolute zero - to make the atoms in the material slow down enough for the phonons' movements to be detectable.

Taking a thermal measurement at such a low temperature was tricky, said Hyungyu Jin, Ohio State postdoctoral researcher and lead author of the study.
Jin took a piece of the semiconductor indium antimonide and shaped it into a lopsided tuning fork.

One arm of the fork was 4 mm wide and the other 1 mm wide. He planted heaters at the base of the arms.
The design worked because of a quirk in the behaviour of the semiconductor at low temperatures. Normally, a material's ability to transfer heat would depend solely on the kind of atoms of which it is made.
But at very low temperatures, such as the ones used in this experiment, another factor comes into play: the size of the sample being tested.
Under those conditions, a larger sample can transfer heat faster than a smaller sample of the same material. That means that the larger arm of the tuning fork could transfer more heat than the smaller arm.

In the experiment, Jin measured the temperature change in both arms of the tuning fork and subtracted one from the other, both with and without a 7-tesla magnetic field turned on.
In the absence of the magnetic field, the larger arm on the tuning fork transferred more heat than the smaller arm, just as the researchers expected.
But in the presence of the magnetic field, heat flow through the larger arm slowed down by 12 per cent.
Heremans said that the magnetic field caused some of the phonons passing through the material to vibrate out of sync so that they bumped into one another.

In the larger arm, the freedom of movement worked against the phonons - they experienced more collisions. More phonons were knocked off course, and fewer - 12 per cent fewer - passed through the material unscathed.