Deterministic control of antiferromagnetic domain walls by circular phonons

Magnetic materials have long been the backbone of how we store information, from the hard drives in our computers to the credit cards in our wallets. For decades, scientists have focused on ferromagnets, the materials that make up ordinary magnets, because their magnetic fields are easy to measure and control. However, a different class of materials called antiferromagnets holds the promise of much faster and denser data storage. In these materials, the tiny magnetic spins of atoms point in opposite directions, canceling each other out so that the material has no overall magnetic field. This makes them invisible to standard magnetic sensors and incredibly difficult to manipulate. The challenge lies in controlling the boundaries between different regions, or domains, within these materials. If scientists could learn to move these boundaries with precision, they could create a new generation of memory devices that are both ultrafast and incredibly stable.

A team of researchers has now demonstrated a way to steer these invisible boundaries using the vibrations of the crystal lattice itself. Working with a layered mineral called manganese phosphorus trisulfide, which has been lightly doped with nickel to make its internal structure easier to see, the scientists used pulses of mid-infrared light to shake the atoms in a specific, circular pattern. By directing this light with a specific twist, known as circular polarization, they were able to push the boundaries between magnetic domains in one direction or the other. The result is a method to deterministically switch the magnetic state of the material, turning it from one stable configuration into another and back again, simply by changing the handedness of the light used to excite it.

The material they studied, manganese phosphorus trisulfide, is a van der Waals antiferromagnet, meaning it is made of thin sheets held together by weak forces, much like a deck of cards. Inside each sheet, the magnetic atoms are arranged in a honeycomb pattern. Below a certain temperature, these atoms align in two opposing groups, creating a state where the magnetic forces balance perfectly. This balance creates two possible states for the material, which are mirror images of each other in time. In a perfect crystal, these states would be equally likely, but in reality, the material is filled with tiny defects and imperfections. These imperfections act like anchors, pinning the boundaries between the different magnetic regions in place. This pinning is usually a problem, but the researchers found that it is also the key to making the switching work.

To see what was happening inside the crystal, the team used a special imaging technique that relies on how light bounces off the material. Because the magnetic arrangement breaks certain symmetries in the crystal, it produces a unique signal when hit with a laser, allowing the researchers to map out the maze-like pattern of magnetic domains. They observed that the boundaries between these regions were not static; they could be moved. The researchers then turned to a free-electron laser, a powerful source of light capable of producing very specific wavelengths. They tuned this laser to match the natural vibration frequency of a specific group of atoms within the crystal, the phosphorus and sulfur complex. When they fired pulses of light at this frequency, the atoms began to vibrate.

The crucial step was how they shaped the light. Instead of just shaking the atoms back and forth, they used circularly polarized light to make the atoms move in a circle. This created a chiral motion, a microscopic spinning of the crystal lattice that has a specific direction, either clockwise or counter-clockwise. This spinning motion acts like a magnetic field, but one that is generated by the movement of the atoms themselves rather than by electric currents or external magnets. The researchers found that when they used light spinning in one direction, the magnetic boundaries moved to expand one type of domain. When they switched the light to spin in the opposite direction, the boundaries moved back, shrinking that domain and expanding the other.

This process is not instantaneous. A single pulse of light is not strong enough to move the boundaries all the way across the sample. Instead, the researchers found that the effect is cumulative. They fired a sequence of light pulses, and with each pulse, the boundary moved a tiny bit further, overcoming the resistance of the crystal defects that were holding it in place. It is similar to pushing a heavy object up a hill with small, repeated nudges; each push gets it a little higher until it crests the top. In their experiments, it took about four pulses to build up a significant change in the domain shape, creating a protrusion that extended into the neighboring region. However, reversing the direction of the light was much more efficient. A single pulse of light with the opposite spin was enough to collapse the protrusion and return the boundary to its original position.

The researchers confirmed that this effect was not just a random fluctuation but a direct result of the light's interaction with the crystal's vibrations. They tested different wavelengths of light and found that the effect was strongest only when the light matched the specific frequency of the circular atomic vibrations. If they used a different frequency, the boundaries did not move. They also tested the system at different temperatures, finding that the control worked reliably from very cold temperatures up to about 45 degrees above absolute zero. Beyond that point, the thermal energy became too strong, and the light could no longer steer the boundaries with the same precision.

Through computer simulations, the team modeled how the boundaries moved across the landscape of crystal defects. The models showed that the circular motion of the atoms creates a force that helps the boundary climb over the energy barriers created by the defects. Once the boundary moves past a defect, it gets stuck again at the next one, stabilizing the new position. This step-by-step movement explains why the changes accumulate with each pulse. The simulations also reproduced the asymmetry seen in the experiment, showing that moving the boundary against the tension of the stretched domain wall requires many small pushes, while letting it snap back releases the tension in a single, swift motion.

This work demonstrates that the magnetic order in these materials can be controlled not by magnetic fields, which are weak in antiferromagnets, but by the dynamic shape of the crystal lattice itself. The researchers showed that by resonantly exciting specific vibrations with circularly polarized light, they could generate an effective field that acts directly on the magnetic order. This field is strong enough to overcome the pinning forces of the crystal defects and move the domain walls in a predictable, reversible way. The findings suggest that the key to controlling these materials lies not just in the strength of the light, but in the interplay between the light-induced motion and the natural imperfections of the crystal. By understanding how these defects stabilize the movement, scientists can potentially engineer materials with tailored responses, opening the door to new ways of storing and processing information using the hidden magnetic states of antiferromagnets.