Breakthroughs in material science are often little things created by unusual manipulations of atomic properties. By magnetising a material only with light, a team of physicists at the Massachusetts Institute of Technology (MIT) has reached a major milestone, a development that could transform data storage and memory technologies.
Now in Nature, a study demonstrates, for the first time, that atomic spins in an antiferromagnetic material, FePS3, can be influenced via a terahertz laser—millions of times every second. In addition to this, it produced a new magnetic state, one that is also persistently present for an unusually long time, potentially leading the way to new applications.
The Unique Nature of Antiferromagnets
Unlike ordinary ferromagnets, whose atomic spins line up in a single direction to create a net magnetisation, antifermagnets consist of alternating spins that cancel each other. Their resistance to external magnetic fields, which is important for robust memory storage, is a result of this one property. But this same resilience has historically been an obstacle to antiferromagnetic materials control.
“Antiferromagnetic materials are robust and not influenced by unwanted stray magnetic fields,”
explained Nuh Gedik, the senior author and Donner Professor of Physics at MIT.
“However, this robustness makes them difficult to control.”
The MIT team targeted these materials with terahertz light tuned to their natural atomic vibrations, or phonons, and brought their atomic spins into a magnetised state. Using this method, the authors demonstrate a new path to ‘writing’ information into these materials – a critical step to the integration of these materials into next generation memory chips.
From Light to Magnetism
In the centre of this research lies the interaction between light and matter. To guide resonance with the atomic vibrations of FePS3, the terahertz laser is designed. The spins of the neighbouring atoms, like tiny oscillations in a spring like lattice of atoms, are influenced by these vibrations. The researchers stimulated these vibrations, nudging the spins into preferred orientations and hence creating a finite magnetisation.
“The idea is to excite the atoms’ terahertz vibrations, which couple to the spins,” Gedik said. “This dual effect enables us to create a new magnetic state.”
To do this the team cooled FePS3 to its antiferromagnetic phase transition temperature, below 118 kelvins (or -247 degrees Fahrenheit), and exposed it to a precisely calibrated terahertz pulse. Next, using near infrared lasers, they signalled that the material had transitioned to a magnetic state by seeing differences in the polarisation of transmitted light.
A Long-Lasting Transition
One surprise with the induced magnetic state is that it remained after the crystal is heated to 40 Kelvin. The new state, however, did not disappear for several milliseconds following the end of the laser pulse, unlike previous phase transitions induced by light, which typically last just for picoseconds. This time scale is sufficiently long to offer a practical window to further explore for potential application.
“We now have a decent window of time during which we can probe the properties of this temporary state,”
said co-author Batyr Ilyas.
“This could help identify new ways to optimize antiferromagnets for technology.”
A Path Toward Energy-Efficient Memory
This research has far reaching implications beyond academic curiosity. Then, ultra dense and energy efficient memory storage could be based on antiferromagnetic materials with inherent stability. The material could be made to ‘write’ data using light, with spin configurations being read out as the ‘binary’ state of the material. Such chips could resist interference from external magnetic fields with less energy and less physical space than today’s technology.
But the MIT team’s work is a major step forward in tackling the challenges of scaling and refining this technique. They’ve data the methods by which light can be used to control magnetic states in antiferromagnetic materials, keying new avenues for fundamental research and practical application in materials science.
As Gedik aptly summarized,
“We now have some knobs to be able to tune and tweak these materials.”
With this newfound control, the future of memory technology might just hinge on the flicker of a laser.
