Physicists at MIT have directly stimulated atoms in an antiferromagnetic material using a terahertz laser, a light source that oscillates more than a trillion times per second. Their results provide a new way to control and switch antiferromagnetic materials, which are of interest for their potential to advance information processing and memory chip technology.
Ilyas et al. demonstrated the efficient manipulation of the magnetic ground state in layered magnets through non-thermal pathways using terahertz light and established regions near critical points with enhanced order parameter fluctuations as promising areas to search for metastable hidden quantum states. Image credit: Adam Glanzman.
In common magnets, known as ferromagnets, the spins of atoms point in the same direction, in a way that the whole can be easily influenced and pulled in the direction of any external magnetic field.
In contrast, antiferromagnets are composed of atoms with alternating spins, each pointing in the opposite direction from its neighbor.
This up, down, up, down order essentially cancels the spins out, giving antiferromagnets a net zero magnetization that is impervious to any magnetic pull.
If a memory chip could be made from antiferromagnetic material, data could be ‘written’ into microscopic regions of the material, called domains.
A certain configuration of spin orientations (for example, up-down) in a given domain would represent the classical bit ‘0,’ and a different configuration (down-up) would mean ‘1.’ Data written on such a chip would be robust against outside magnetic influence.
For this and other reasons, scientists believe antiferromagnetic materials could be a more robust alternative to existing magnetic-based storage technologies.
A major hurdle, however, has been in how to control antiferromagnets in a way that reliably switches the material from one magnetic state to another.
Using carefully tuned terahertz light, MIT Professor Nuh Gedik and colleagues were able to controllably switch an antiferromagnet to a new magnetic state.
“Antiferromagnetic materials are robust and not influenced by unwanted stray magnetic fields,” Professor 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 FePS3, a material that transitions to an antiferromagnetic phase at a critical temperature of around 118 K.
They suspected they might control the material’s transition by tuning into its atomic vibrations.
“In any solid, you can picture it as different atoms that are periodically arranged, and between atoms are tiny springs,” said MIT’s Dr. Alexander von Hoegen.
“If you were to pull one atom, it would vibrate at a characteristic frequency which typically occurs in the terahertz range.”
The way in which atoms vibrate also relates to how their spins interact with each other.
The scientists reasoned that if they could stimulate the atoms with a terahertz source that oscillates at the same frequency as the atoms’ collective vibrations, called phonons, the effect could also nudge the atoms’ spins out of their perfectly balanced, magnetically alternating alignment.
Once knocked out of balance, atoms should have larger spins in one direction than the other, creating a preferred orientation that would shift the inherently non-magnetized material into a new magnetic state with finite magnetization.
“The idea is that you can kill two birds with one stone: You excite the atoms’ terahertz vibrations, which also couples to the spins,” Professor Gedik said.
To test this idea, they placed a sample of FePS3 in a vacuum chamber and cooled it down to temperatures at and below 118 K.
They then generated a terahertz pulse by aiming a beam of near-infrared light through an organic crystal, which transformed the light into the terahertz frequencies.
They then directed this terahertz light toward the sample.
“This terahertz pulse is what we use to create a change in the sample,” said MIT’s Dr. Tianchuang Luo.
“It’s like ‘writing’ a new state into the sample.”
To confirm that the pulse triggered a change in the material’s magnetism, the authors also aimed two near-infrared lasers at the sample, each with an opposite circular polarization.
If the terahertz pulse had no effect, they should see no difference in the intensity of the transmitted infrared lasers.
“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 terahertz light to shake the atoms,” said MIT’s Dr. Batyr Ilyas.
Over repeated experiments, the team observed that a terahertz pulse successfully switched the previously antiferromagnetic material to a new magnetic state — a transition that persisted for a surprisingly long time, 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,” Professor Gedik said.
The study was published in the journal Nature.
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B. Ilyas et al. 2024. Terahertz field-induced metastable magnetization near criticality in FePS3. Nature 636, 609-614; doi: 10.1038/s41586-024-08226-x
This article is a version of a press-release provided by MIT.
Source : Breaking Science News