Magnetism is slightly weird. Iron becomes magnetic because lots of electrons agree to align their individual magnetic moments. If you hit a thin layer of iron with a strong, very short pulse of light, however, the strength of its magnetic field will drop almost immediately.

How does this happen? Well, until now, we weren’t really sure. Thanks to the magic of X-ray lasers we now know why: sound waves carry angular momentum of electrons away.

Disappearing magnetism induces a headache

The problem with disappearing magnetism is related to something called the conservation of angular momentum. Think of the Earth, a gigantic spheroid spinning in space: its spin is angular momentum, which is always conserved. If the Earth loses angular momentum, something else must gain angular momentum to compensate.

Angular momentum isn’t just about the rate of spin; the orientation of the spin is also critical. Imagine that the Earth’s poles were aligned vertically. To rotate the Earth’s pole so that the spin was oriented some other way means that the Earth has lost angular momentum in the vertical direction and gained it in a horizontal direction. For that to happen, something else has to gain angular momentum along the vertical direction and lose it in the horizontal direction.

Electrons are little magnets, but they also have angular momentum. The direction of their angular momentum aligned with the magnetic north pole of the electron. To make iron magnetic, lots of electrons have to align their magnetic poles, which also means that their angular momentum lines up.

When a light pulse hits and the magnetic field drops, some of the electrons must have had to flip their pole, which also means their angular momentum must change. But angular momentum is conserved, so where does all that angular momentum go?

Finding spare angular momentum under the couch

Researchers had suggested that the angular momentum might be carried away as vibrations in the structure of the magnet. Effectively, as the electron flips, it jiggles a nearby atom, changing its angular momentum. These vibrations are similar to sound waves, but not quite like the sound waves in air, where the air molecules move back and forth (a longitudinal wave). Instead, we should observe that the atoms also move side to side (called a transverse or shear wave). Overall, you might think of the atoms making a kind of twisting motion.

To search for the missing angular momentum, the researchers used something called a pump-probe experiment. A very short and intense pulse of light hits the thin layer of iron (this is called the pump). As a result, lots of electrons flip their spin at nearly the same time, resulting in the iron atoms all vibrating together and a sound wave propagating away from the point where the light hit the sample.

To search for the sound waves, the researchers hit the sample with a very short pulse of X-rays just after the optical pulse. The X-ray pulse probes the structure of the iron crystal, providing a snapshot of the positions of the atoms.

In this case, the atoms are all in motion due to the sound wave. This blurs the X-ray picture of the crystal structure. By examining how this blurring varies with the time interval between the optical pump pulse and the X-ray probe pulse, we can see the atoms moving in response to the electrons flipping their orientation.

Where is the angular momentum?

The researchers found that sound waves with the right atomic motion were traveling away from where the light hit the iron crystal. They estimated that the electrons were able to transfer their angular momentum to the iron crystal in about 200 femtoseconds (one femtosecond is 10-15 second). The sound wave that they observed accounts for about 80 percent of the loss of magnetism.

For the last decade or so, researchers have been working on increasing and decreasing the magnetism of materials optically. These extremely fast processes hold promise in creating data storage that operates on femtosecond time scales. The big problem is not just making the process fast but also making it efficient. At present, optically switching magnetic materials still takes too much energy, but research like this may help find more efficient mechanisms. After all, no one wants a 50kW power supply attached to their PC, unless their basement is really cold.

Nature, 2019, DOI: 10.1038/s41586-018-0822-7 (About DOIs)