Using a terahertz (THz) laser, researchers at MIT initiated a long-lasting magnetic state of more than 2.5 milliseconds, in an antiferromagnetic material. Typically, light-induced phases of matter return to equilibrium on ultrafast timescales after the light is removed. A timescale of milliseconds could provide enough time to probe the properties of the new phase state before the material settles back into antiferromagnetism, and offer insight into how to optimize the material. Antiferromagnetic materials have the potential to store and process more data than existing magnetic-based storage technologies, while using less energy. However, a controllable way to switch material from one magnetic state to another has yet to be discovered. MIT physicists have created a new, long-lasting magnetic state in a material using light. (From left) Tianchuang Luo, professor Nuh Gedik, and researcher Alexander von Hoegen. Courtesy of Adam Glanzman. Antiferromagnets are composed of atoms with alternating spins, with each spin pointing in the opposite direction from its neighbor. The alternating spins essentially cancel each other out, leaving the antiferromagnetic material with a net zero magnetization that resists any magnetic pull. “Antiferromagnetic materials are robust and not influenced by unwanted, stray magnetic fields,” professor Nuh Gedik said. “However, this robustness is a double-edged sword. Their insensitivity to weak magnetic fields makes these materials difficult to control.” The researchers worked with the van der Waals antiferromagnet iron phosphorus trisulfide (FePS3). FePS3 transitions to an antiferromagnetic phase at a critical temperature of around 118 K, or -247º F. The metastable state of the material grows more robust as the temperature approaches the antiferromagnetic transition point. The researchers hypothesized that the material’s transition to antiferromagnetism could be controlled by tuning its atomic vibrations. They used THz light to directly stimulate the atoms in the FePS3, shifting the balance of atomic spins in the material toward a magnetic state. “In any solid, you can picture it as different atoms that are periodically arranged, and between atoms are tiny springs,” researcher Alexander von Hoegen said. “If you were to pull one atom, it would vibrate at a characteristic frequency which typically occurs in the terahertz range.” The team further hypothesized that if the atoms in the FePS3 sample could be stimulated with a THz source that was the same frequency as the collective vibrations (phonons) of the atoms in the material, the effect could be to knock the atoms’ spins out of their balanced, magnetically alternating alignment. Once out of balance, the spins would be larger in one direction than the other, the team reasoned, and would create a preferred orientation that would shift the FePS3, an inherently nonmagnetized material, into a new state with finite magnetization. “The idea is that you can kill two birds with one stone,” Gedik said. “You excite the atoms’ THz vibrations, which also couples to the spins.” To test their hypothesis, the researchers placed a sample of FePS3 in a vacuum chamber and cooled it to below 118 K. They generated a THz pulse by aiming a beam of NIR light through an organic crystal, causing the light to be converted to THz frequencies, and shone the light on the sample. “This THz pulse is what we use to create a change in the sample,” researcher Tianchuang Luo said. “It’s like ‘writing’ a new state into the sample.” Using carefully tuned THz light, an MIT team was able to controllably switch an antiferromagnet to a new magnetic state. The transition persisted over several milliseconds, even after the laser was turned off. Courtesy of Adam Glanzman. To confirm that the pulse did indeed trigger a change in the material’s magnetism, the researchers aimed two NIR lasers at the sample, each with an opposite circular polarization. If the THz pulse had no effect, there should be no observable difference in the intensity of the light transmitted by the NIR lasers, but the team did see a difference. “Just seeing a difference tells us the material is no longer the original antiferromagnet, and that we are inducing a new magnetic state, by essentially using THz light to shake the atoms,” researcher Batyr Ilyas said. The researchers repeated the experiments and observed each time that the THz pulse switched the antiferromagnetic material to a new magnetic state. The magnetic state persisted over several milliseconds, even after the laser was turned off. “People have seen these light-induced phase transitions before in other systems, but typically they live for very short times — on the order of a picosecond, which is a trillionth of a second,” Gedik said. By combining calculations with classical Monte Carlo and spin dynamics simulations, the researchers were able to determine that the displacement of a specific phonon mode modulated the exchange couplings in a way that favored a ground state with finite magnetization near the Néel temperature. They also gained clarity on how the critical fluctuations of the dominant antiferromagnetic order can amplify both the magnitude and the lifetime of the new magnetic state. The results provide a new way to control and switch antiferromagnetic materials, which have the potential to advance information processing and memory chip technology. Antiferromagnets could be integrated into future memory chips, and due to the stability of magnetic domains, these chips would be able to store and process more data while using less energy and less space than existing devices. The research was published in Nature (www.doi.org/10.1038/s41586-024-08226-x).