Physicists have successfully developed a new, durable magnetic state in a material using just light. This breakthrough offers a novel method to manipulate and flip antiferromagnetic materials, which hold promise for enhancing information processing and memory chip technologies.
MIT physicists have successfully developed a new, durable magnetic state in a material using just light.
In a research paper set to be published in Nature, the scientists share their findings on how they employed a terahertz laser—an intense light source oscillating over a trillion times per second—to directly stimulate the atoms in an antiferromagnetic material. The laser’s oscillations were precisely matched to the natural vibrational frequencies of the atoms within the material, altering the balance of atomic spins and converting them to a new magnetic state.
This discovery provides a fresh approach to manipulate and switch antiferromagnetic materials, which have potential advantages for the future of information processing and memory chip innovation.
In typical magnets, known as ferromagnets, all atomic spins point in the same direction, allowing the overall magnetization to easily respond to external magnetic fields. Conversely, in antiferromagnets, the spins of neighboring atoms alternate—one points up while the next points down. This alternating arrangement results in a net zero magnetization, rendering antiferromagnets immune to external magnetic influences.
Imagine a memory chip made from antiferromagnetic materials where data can be stored in tiny regions known as domains. A specific spin orientation configuration (like up-down) within a domain could signify the classical bit “0,” while a different orientation (down-up) would represent “1.” Such data storage systems would be highly resistant to external magnetic disturbances.
Due to these properties, scientists believe that antiferromagnetic materials could serve as a more durable alternative to current magnetic storage technologies. However, a major challenge has been controlling these materials to reliably transition between different magnetic states.
“Antiferromagnetic materials are robust and not easily affected by stray magnetic fields,” explains Nuh Gedik, the Donner Professor of Physics at MIT. “Yet, this robustness poses a challenge; their insensitivity to weak magnetic fields makes it hard to exert control over them.”
By employing precisely tuned terahertz light, the team at MIT was able to switch an antiferromagnet to a new magnetic state in a controlled manner. Antiferromagnets may be integrated into future memory chips capable of storing and processing a larger volume of data while requiring less energy and occupying less space compared to traditional devices, thanks to their stable magnetic domains.
“Traditionally, controlling such antiferromagnetic materials has been quite complex,” notes Gedik. “Now, we have the means to adjust and refine them more effectively.”
Gedik leads the research, which also features contributions from MIT team members Batyr Ilyas, Tianchuang Luo, Alexander von Hoegen, Zhuquan Zhang, and Keith Nelson. Collaborators include researchers from the Max Planck Institute for the Structure and Dynamics of Matter in Germany, the University of the Basque Country in Spain, Seoul National University, and the Flatiron Institute in New York.
Off balance
Gedik’s team focuses on developing methods to manipulate quantum materials whose atomic interactions lead to unique phenomena.
“We generally excite materials with light to better understand their fundamental properties,” Gedik explains. “For example, we’re exploring what makes this material an antiferromagnet and whether we can tweak the microscopic interactions to convert it to a ferromagnet.”
In their latest study, the team examined FePS3, which shifts into an antiferromagnetic phase around 118 Kelvin (-247 degrees Fahrenheit).
The researchers hypothesized that by tuning into the atomic vibrations of the material, they could control its transition.
“You can think of a solid as being made up of different atoms arranged periodically, with tiny springs in between,” von Hoegen explains. “Pulling an atom would cause it to vibrate at a unique frequency, usually within the terahertz range.”
The way atoms vibrate is also connected to how their spins interact. The team theorized that by stimulating the atoms using a terahertz source that matched their collective vibrational frequency (called phonons), they could slightly disrupt the balance of atomic spins. This would create a scenario where the spins lean in one direction, thus changing the material from a non-magnetized state to a new magnetic state with a defined magnetization.
“Our goal is to achieve two objectives at once: excite the atomic vibrations while also influencing the spins,” Gedik remarks.
Shake and write
To validate this concept, the team worked with a sample of FePS3 provided by their colleagues at Seoul National University. They placed the sample in a vacuum chamber and cooled it to the critical temperatures below 118 K. Using a near-infrared light source, they generated a terahertz pulse by directing it through an organic crystal, thus producing terahertz frequencies. They then aimed this terahertz light at the sample.
“This terahertz pulse is effectively used to induce a change in the sample,” Luo explains. “It’s akin to ‘writing’ a new state into the material.”
To verify that the pulse caused a change in the material’s magnetic properties, the team directed two near-infrared lasers with opposing circular polarizations at the sample. If the terahertz pulse had no effect, there would be no change in the intensity of the transmitted infrared lasers.
“Detecting any difference confirms that the material is no longer in its original antiferromagnetic state, thus indicating the induction of a new magnetic state through the action of terahertz light,” says Ilyas.
Throughout numerous experiments, the team found that the terahertz pulse successfully transformed the antiferromagnetic material into a new magnetic state—one that endured for several milliseconds, even after the laser was turned off.
“While light-induced phase transitions have been observed in other systems, those typically last for only very brief moments, almost on the order of a picosecond, or trillionth of a second,” Gedik explains.
With the ability to maintain this new state for a few milliseconds, researchers now find themselves with a usable time frame to explore the characteristics of this temporary state before it reverts to its original antiferromagnetic state. This knowledge may lead to new methods for optimizing antiferromagnets for advanced memory storage solutions.
This research received partial support from the U.S. Department of Energy’s Materials Science and Engineering Division, Office of Basic Energy Sciences, and the Gordon and Betty Moore Foundation.
