Scientists from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) and <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

” data-gt-translate-attributes=”[{"attribute":"data-cmtooltip", "format":"html"}]” tabindex=”0″ role=”link”>MIT have achieved a groundbreaking feat — creating a stable, long-lasting magnetic state in an antiferromagnetic material using only light. This discovery could lead to major advancements in memory chip technology and information processing.

By using a <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

” data-gt-translate-attributes=”[{"attribute":"data-cmtooltip", "format":"html"}]” tabindex=”0″ role=”link”>terahertz laser, which oscillates more than a trillion times per second, the researchers were able to directly excite the material’s atoms. They carefully tuned the laser’s frequency to match the natural vibrations of the atoms, triggering an ultrafast shift in the atomic structure and pushing the material into a new magnetic state. Their findings, recently published in Nature, highlight the potential of light to control magnetism in innovative ways.

Breakthrough in Magnetic Control

In everyday magnets, like the ones on your fridge, the atoms inside align their magnetic moments in the same direction, creating a strong overall magnetic field. These materials, known as ferromagnets, are effective but can easily be influenced by external magnetic fields.

In contrast, antiferromagnets have a different structure — their atomic spins alternate in an up-down pattern, canceling each other out and resulting in no overall magnetization. This makes them highly resistant to outside magnetic interference, which could be useful for building more stable and interference-resistant memory chips. However, a major challenge has been finding a reliable way to switch their magnetic states to make them practical for real-world applications.

In a recent study published in the journal Nature, researchers at the Max Planck Institute for the Structure and Dynamics of Matter and MIT used terahertz light to control and switch an antiferromagnet into a new magnetic state. This breakthrough demonstrates the potential of antiferromagnetic materials for future memory chips that could store and process more data, use less energy, and take up less space.

“Generally, such antiferromagnetic materials are not easy to control, but now we’ve found some knobs to tune and tweak them,” said Angel Rubio, Director of the Theory Department at the MPSD, and Nuh Gedik, Donner Professor of Physics at MIT, who co-led the study.

Tuning Magnetic States with Light

The team worked with FePS₃, a material that transitions to an antiferro magnetic phase at around 118 Kelvin (-115°C). They hypothesized that its magnetic state could be controlled by tuning into its atomic vibrations, known as phonons. “You can picture any solid as a set of atoms periodically arranged, connected by tiny springs,” explains Alexander von Hoegen, a postdoctoral researcher in Gedik’s group. “If you pull one <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

” data-gt-translate-attributes=”[{"attribute":"data-cmtooltip", "format":"html"}]” tabindex=”0″ role=”link”>atom, it vibrates at a characteristic frequency, typically in the terahertz range.”

The team reasoned that by exciting these phonons with a terahertz laser tuned to their natural frequency, they could nudge the atoms’ spins out of their perfectly balanced alignment. This imbalance would create a preferred orientation, shifting the material into a new state with finite magnetization.

“The idea is that you excite the atoms’ terahertz vibrations, which also couple to the spins,” says Emil Viñas Boström, a postdoctoral researcher in Rubio’s group.

“Seeing a difference in the material’s optical properties tells us that it is no longer the original antiferromagnet, and that we are inducing a new magnetic state, essentially by using terahertz light to shake the atoms,” adds Batyr Ilyas, a graduate student in Gedik’s group.

Longevity of the New Magnetic State

Repeated experiments showed that a terahertz pulse could successfully switch the antiferromagnet into this new magnetic state. This state persisted for several milliseconds after the laser was turned off.  To understand the mechanism behind this long-lived magnetization, the researchers developed a model describing the interaction between spins and phonons. They identified a specific phonon mode—a pattern of oscillations within the crystal lattice—that mediated a coupling between the material’s antiferromagnetic and ferromagnetic states.

“This is a highly unusual situation where the change in magnetic fluctuations leads to a new type of magnetic order,” says Rubio. “Typically, fluctuations destroy magnetic order, but here they have a constructive effect.”

Simulations revealed that the lifetime of the induced magnetization, near the transition temperature, was determined by the slow dynamics of the antiferromagnetic order, a phenomenon known as critical slowing down. “Close to the ordering temperature, it’s like time slows down within the antiferromagnet, and the spins begin to move very slowly,” says Viñas Boström. The phonons act as a “glue,” coupling the magnetization to the antiferromagnetic fluctuations and slowing down the magnetization’s relaxation.

This extended lifetime provides a window for scientists to study the temporary magnetic state before it reverts to antiferromagnetism. Understanding these dynamics could open new pathways for controlling antiferromagnets and optimizing their use in next-generation memory storage technologies.

For more on this research, see Scientists Just Made a Material Magnetic Using Light.

Reference: “Terahertz field-induced metastable magnetization near criticality in FePS₃” by Batyr Ilyas, Tianchuang Luo, Alexander von Hoegen, Emil Viñas Boström, Zhuquan Zhang, Jaena Park, Junghyun Kim, Je-Geun Park, Keith A. Nelson, Angel Rubio and Nuh Gedik, 18 December 2024, Nature.
DOI: 10.1038/s41586-024-08226-x