By Amit Malewar 21 Dec, 2024
Collected at: https://www.techexplorist.com/magnetizing-material-light/94981/
Common magnets, also known as ferromagnets, have all spins pointing in the same direction, making the aggregate quite susceptible and thereby influencing it to align in the direction of the applied magnetic field. On the contrary, antiferromagnets are composed of atoms whose spins are aligned in opposite directions with respect to their neighboring atoms; these spins alternatively point upwards and downwards, giving an average magnetic moment of zero.
Thus, it derives the characteristic wherein the spins are canceled out or net zero magnetization and remain more or less indifferent to magnetic pull.
This would allow a memory chip to be composed of antiferromagnetic material and write information into microscopic regions of the material, which are called domains. A certain configuration of spin orientations would represent the classical bit “0”, and others would represent “1”; all written data would be impervious to magnetic influence.
Overall, there is a wide field of envisagement where antiferromagnetic materials are heralded as possibly a much more robust alternative to existing magnetic-based storage technologies. However, the reliability of materials in switching from one magnetic state to another remains a huge hurdle.
Controlling the functional properties of quantum materials by light has emerged as a front of condensed matter physics. This has led to discovering various light-induced phases of matter, such as superconductivity, ferroelectricity, magnetism, and charge density waves. However, in most cases, the photoinduced phases return to equilibrium on ultrafast timescales after light is turned off, thus restricting their practical applications.
MIT physicists have created a new and lasting magnetic state in a material using only light. Using a terahertz laser—a light source that oscillates more than a trillion times per second—the researchers directly stimulated atoms in an antiferromagnetic material. The laser’s oscillations are tuned to the natural vibrations among the material’s atoms to shift the balance of atomic spins to a new magnetic state.
The work offers a new way to control and switch antiferromagnetic materials, of interest for their potential in advanced information processing and memory chip technology.
With resonant terahertz radiation, the MIT team could controllably switch an antiferromagnet to a new magnetic state. Because of the stability of magnetic domains, antiferromagnets could eventually be incorporated into future memory chips that store and process more data while consuming less energy and occupying a fraction of the space occupied by their counterparts.
Revealing their latest study, the team sought to exploit FePS3-which becomes antiferromagnetic below a critical temperature of about 118 kelvins, -247 degrees Fahrenheit.
The team suspected that they could control the transition of the material by tuning into its atomic vibrations.
The vibrating behavior of the atoms was also related to how their spins influenced each other. They conceived that if they could wiggle the atoms from their perfectly balanced state to magnetically alternating alignment with a terahertz laser that oscillates at the same frequency as the atomic collective vibrations (phonons), the spins of the atoms would be similarly nudged out of their aligned states.
Once out of balance, the pendulum swung for them fairly quickly, allowing the dominant spin orientation to become so. That led to the preferred orientation, which could nudge the previously nonmagnetic material into a new magnetic phase with net magnetization.
Nuh Gedik, the Donner Professor of Physics at MIT, said, “The idea is that you can kill two birds with one stone: You excite the atoms’ terahertz vibrations, which also couples to the spins.”
Seoul National University provided a test sample of FePS3 for this mission. After placing the sample in a vacuum chamber, they cooled it to temperatures around and below 118 K. They generated a terahertz pulse when a beam of near-infrared light passed through an organic crystal, transforming light into terahertz frequencies. They also directed this terahertz light into the sample.
“This terahertz pulse is what we use to create a change in the crystal,” says Tianchuang Luo, study co-author. “It is similar to ‘writing’ a new state into the sample.”
To confirm that the pulse affected the samples’ magnetism, the team also bombarded the sample twin near-infrared lasers, each with circularly polarized light. If the terahertz pulse had no effect, there would be no difference in the intensity of the transmitted infrared lasers.
Batyr Ilyas, co-author of the study, said, “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.”
Through repeated experiments, the team trained their lenses on a terahertz pulse that captured an NH cool switch of previously antiferromagnetic material into a new magnetic state. This process lasted surprisingly long, for a few milliseconds after the laser shut off.
“People have seen these light-induced phase transitions before in other systems, but they typically live only for very short times on the order of picoseconds-that is, a trillionth of a second,” Gedik says.
In a few milliseconds, scientists now might have, after that laser excitation has switched the antiferromagnetic material into a new magnetic state, quite a decent timeframe in which they may probe the properties of that temporary new state before it settles back into its immanent antiferromagnetic state. Then, they may be able to spot new knobs to tweak antiferromagnets to be optimally used in next-generation memory storage technologies.
Journal Reference:
- Ilyas, B., Luo, T., von Hoegen, A. et al. Terahertz field-induced metastable magnetization near criticality in FePS3. Nature 636, 609–614 (2024). DOI: 10.1038/s41586-024-08226-x
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