Physicists magnetize a material with light

In a study, it will appear natureresearchers report using a terahertz laser, a light source that oscillates more than a trillion times per second, to directly stimulate atoms in antiferromagnetic materials. The laser’s oscillations are tuned to the natural vibrations between the material’s atoms, shifting the balance of the atomic spins into a new magnetic state.

The findings provide a new way to control and switch antiferromagnetic materials, which have attracted interest for their potential in advancing information processing and storage chip technology.

In an ordinary magnet (called a ferromagnet), the atomic spins point in the same direction, so the ensemble is easily affected and pulled by the direction of any external magnetic field. In contrast, antiferromagnets are composed of atoms with alternating spins, each pointing in the opposite direction to its neighbor. This sequence of up, down, up, down essentially cancels out the spin, giving the antiferromagnet a net-zero magnetization unaffected by any magnetic forces.

If memory chips could be made of antiferromagnetic materials, data could be “written” into microscopic regions of the material, called magnetic domains. A certain configuration of spin directions in a given domain (e.g., up-down) will represent a classical bit “0”, while a different configuration (down-up) will represent a “1.” Data written on such a chip can resist the influence of external magnetic fields.

For this and other reasons, scientists believe antiferromagnetic materials may be a more powerful alternative to existing magnet-based storage technologies. However, a major obstacle is how to control antiferromagnets in a way that reliably switches the material from one magnetic state to another.

“Antiferromagnetic materials are very strong and are not affected by unwanted stray magnetic fields,” said Nuh Gedik, Donner Professor of Physics at MIT. “However, this robustness is a double-edged sword; their insensitivity to weak magnetic fields makes these materials difficult to control.”

Using carefully tuned terahertz light, the MIT team was able to controllably switch an antiferromagnet into a new magnetic state. Due to the stability of their magnetic domains, antiferromagnets could be integrated into future memory chips to store and process more data while using less energy and taking up a fraction of the space of existing devices.

“Generally speaking, such antiferromagnetic materials are not easy to control,” Gidik said. “Now we have some knobs to adjust them.”

Gedik is the senior author of the new study, which also includes co-authors Batyr Ilyas, Tianchuang Luo, Alexander von Hoegen, Zhuquan Zhuquan and Keith Nelson from MIT and the Max Planck Institute for the Structure and Dynamics of Matter in Germany The institute’s collaborators are the University of the Basque Country in Spain, Seoul National University, and the Flatiron Institute in New York.

lose balance

Gedik’s team at MIT has developed techniques to manipulate quantum materials, where interactions between atoms can produce exotic phenomena.

“Generally, we excite materials with light to learn more about why they fundamentally hold together,” Gadik said. “For example, why is this material an antiferromagnet, and is there a way to disrupt the microscopic interactions so that it becomes a ferromagnet?”

In their new study, the team collaborated with FePS₃ —A material that transitions to its antiferromagnetic phase at a critical temperature around 118 Kelvin (-247 degrees Fahrenheit).

The team suspected they could control the material’s transformation by tuning atomic vibrations.

“In any solid, you can think of it as a periodic arrangement of different atoms with tiny springs between them,” von Hogan explains. “If you pull an atom, it vibrates at a characteristic frequency that typically occurs in the terahertz range.”

The way atoms vibrate is also related to how their spins interact with each other. The team reasoned that if they could stimulate the atoms with a terahertz source that oscillated at the same frequency as the atoms’ collective vibrations, called phonons, the effect could also shift the atoms’ spins away from their perfectly balanced Magnetic alternating arrangement. Once out of equilibrium, atoms should spin more in one direction than in the other, creating a preferred direction that transforms an inherently non-magnetized material into a new magnetic state with finite magnetization.

“The idea is that you kill two birds with one stone: You excite terahertz vibrations of the atoms, which are also coupled to the spin,” Gedik said.

Shake to write

To test this idea, the team used FePS samples₃ This was synthesized by colleagues at Seoul National University. They placed the sample in a vacuum chamber and cooled it to temperatures of 118 K and below. They then shined terahertz light at the sample.

“We use terahertz pulses to change the sample,” Luo said. “It’s like ‘writing’ a new state in the sample.”

To confirm that the pulses triggered changes in the material’s magnetism, the team also aimed two near-infrared laser beams at the sample, each with opposite circular polarization. If the terahertz pulses had no effect, the researchers shouldn’t see any difference in the intensity of the transmitted infrared laser.

“Just seeing the difference tells us that this material is no longer the original antiferromagnet and that we are inducing a new magnetic state by essentially shaking the atoms with terahertz light,” Ilyas said.

Through repeated experiments, the team observed that the terahertz pulses successfully transformed the previously antiferromagnetic material into a new magnetic state. This transition lasted for a surprisingly long time, more than a few milliseconds, even after the laser was turned off. The same is true.

“People have seen these light-induced phase transitions before in other systems, but typically their lifetimes are very short, on the order of picoseconds, or trillionths of a second,” Gedik said.

In just a few milliseconds, scientists may now have a decent window of time during which they can probe the properties of the temporary new state before it reverts to its intrinsic antiferromagnetic properties. Then, they might be able to find new knobs to tune antiferromagnets and optimize their use in next-generation memory storage technologies.

This research was supported in part by the U.S. Department of Energy’s Materials Science and Engineering Division, Office of Basic Energy Sciences, and the Gordon and Betty Moore Foundation.