Long-lived magnetic state induced by intense ultrashort laser pulses
Researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) and MIT have successfully created a new, long-lasting magnetic state in an antiferromagnetic material using only light. This breakthrough holds significant promise for advancing information processing and memory chip technology. The team reports the direct stimulation of atoms in an antiferromagnetic material with a terahertz laser—a light source oscillating more than a trillion times per second. By tuning the laser’s frequency to match the natural vibrations of the material’s atoms, they induced an ultrafast change in the atomic structure, ultimately driving the system into a novel magnetic state. Their work has been published in Nature.
In common magnets like the ones on your fridge, called ferromagnets, the magnetic moments of all the atoms align, like a lattice of miniature compasses pointing in the same direction. This alignment produces a finite magnetization but also makes the material sensitive to external magnetic fields. Antiferromagnets, on the other hand, have alternating atomic spins—up, down, up, down—such that the magnetic contributions cancel out, resulting in zero net magnetization. This unique property makes antiferromagnetic materials immune to magnetic perturbations, which could make them ideal for robust, interference-resistant memory chips. However, a key challenge has been finding a reliable way to switch these materials between magnetic states.
In the recent study published in 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”, say Angel Rubio, Director of the Theory Department at the MPSD, and Nuh Gedik, Donner Professor of Physics at MIT, who co-led the study.
The team worked with FePS3, a material that transitions to an antiferromagnetic 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 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.
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.
Text based on MIT’s press release about this work.
