By manipulating metal shavings with magnets, even children are able to give a bald guy some hair. Now, researchers have discovered that magnets can also be used to control heat and maybe even sound. Using a magnetic field roughly the size of a medical MRI, researchers slowed the movement of heat through a tuning fork. The study, published in Nature Materials this week, is the first to demonstrate that “acoustic phonons” have magnetic properties too.
Not to be confused with photons (or particles of light), phonons are particles that transmit heat and sound. These two are both essentially the vibration of atoms. “The hotter a material is, the faster the atoms vibrate,” Ohio State University’s Joseph Heremans says in a news release. “Sound is the vibration of atoms, too… It’s through vibrations that I talk to you, because my vocal chords compress the air and create vibrations that travel to you, and you pick them up in your ears as sound.”
With a powerful enough magnet, you should be able to control heat in materials that aren’t traditionally magnetic, like glass or plastic. (The effect goes unnoticed with metal since it transmits so much heat.) Heremans and colleagues used a 7-tesla magnet, the kind only found in hospitals and labs. They shaped a piece of indium antimonide semiconductor into a lopsided tuning fork: The wider arm is 4 millimeters wide, the narrower one is 1 millimeter wide. Heaters were planted at the base of the arms.
To slow the atoms in the material down enough for phonon movement to be detectable, the semiconductor was chilled to minus 268 degrees Celsius. Under those conditions, a larger sample can transfer heat faster than a smaller sample of the same material. “Imagine that the tuning fork is a track, and the phonons flowing up from the base are runners on the track. The runners who take the narrow side of the fork barely have enough room to squeeze through, and they keep bumping into the walls of the track, which slows them down,” Heremans explains. “The runners who take the wider track can run faster, because they have lots of room… The more collisions they undergo, the slower they go.”
The researchers measured the temperature change in both fork arms, then subtracted one from the other—with and without the magnetic field turned on. With no magnetic field, the larger arm transferred more heat than the smaller arm; with a magnetic field, however, heat flow through the larger arm slowed down by 12 percent. The magnetic field caused some of the phonons to vibrate out of sync, forcing them to bump into one another. So in the larger arm, the freedom of movement worked against the phonons, and more particles were knocked off course.
Their reaction indicates that phonons are sensitive to magnetism. “We’ve shown that we can steer heat magnetically,” Heremans adds. “We should be able to steer sound waves, too.” Next, they’re testing if sound waves can be deflected sideways with magnetic fields.
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