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Terahertz-Induced Coupling Between Coherent Magnons in Solids

Next-generation information processing technologies may be made possible by a new, ultrafast technique for controlling magnetic materials.

Terahertz-Induced Coupling Between Coherent Magnons in Solids

This illustration shows that a pair of intense THz laser pulses drives spin waves in an antiferromagnetic material, which radiates nonlinear emissions at the sum- and difference-frequencies. Image Credit: University of Texas at Austin & MIT researchers

Scientists and engineers are searching for ways to create information processing systems that operate more quickly as the demand for computing resources keeps rising. Utilizing spin waves - patterns of electron spins - to transfer and process information far more quickly than in traditional computers is one potential remedy. Until now, a significant obstacle has been controlling these extremely fast spin waves for practical purposes.

Researchers at MIT and The University of Texas at Austin have made great progress in precisely manipulating these ultrafast spin waves with customized light pulses. Their research, led by MIT Graduate Student Zhuquan Zhang, University of Texas at Austin Postdoctoral Researcher Frank Gao, MIT Professor of Chemistry Keith Nelson, and UT Austin Assistant Professor of Physics Edoardo Baldini, is described in two studies published in Nature Physics.

Magnetic data recording technology, which stores and retrieves enormous amounts of data, is a fundamental part of the internet, cloud computing, and smartphones. The manipulation of magnetic spin states (up and down) in ferromagnetic materials - which stand for the binary bits “0” and “1” - is the key to this technology. The magnetic properties of the material are determined by the alignment of these spins, which are tiny magnets.

Researchers have discovered that when light is applied to a single group of atoms in these materials, the atoms’ spins wobble in a way that spreads to nearby atoms like ripples on a pond after a stone falls in. This is a spin wave.

A unique class of magnetic materials known as antiferromagnets has spins aligned in opposite directions from these conventional data storage materials. Since the spin waves in these materials are generally much faster than those in ferromagnets, they could be used in future high-speed information processing architectures.

An orthoferrite, an antiferromagnet, was used in the researchers’ experiments. Two unique spin waves are present in this material, which typically do not interact with one another. The researchers were able to effectively induce interaction between these spin waves by utilizing terahertz (THz) light, which is undetectable to the human eye at extreme infrared frequencies.

In one paper, they demonstrated that, similar to the harmonic overtones that naturally occur when a guitar string is plucked, using strong THz fields to excite a spin wave at one frequency can start another spin wave at a higher frequency.

This really surprised us. It meant that we could nonlinearly control the energy flow within these magnetic systems.

Zhuquan Zhang, Graduate Student, Department of Chemistry, Massachusetts Institute of Technology

It was discovered in the other paper that a new hybrid spin wave can be produced by the excitation of two distinct spin waves. This is especially exciting, according to Baldini, because it may help advance spintronics technology into the emerging field of magnonics. Information is carried by each individual electron’s spin in spintronics. Information is transported by spin waves, or magnons, in the field of magnonics.

Here, unlike with spintronics, you are using these collective type of spin waves that are involving many, many electron spins simultaneously. That can lead you to extremely fast timescales that are not reachable in spintronics and also move information in a more efficient way.

Edoardo Baldini, Assistant Professor, Department of Physics, The University of Texas at Austin

The researchers created an advanced spectrometer to perform this ground-breaking work, which allowed them to identify the mutual coupling between different spin waves and to see their underlying symmetries.

Unlike visible light that can be easily seen by the eye, THz light is challenging to detect. These experiments would be otherwise impossible without the technique development, which allowed us to measure THz signals with only a single light pulse.

Frank Y. Gao, Postdoctoral Researcher, Department of Physics, The University of Texas at Austin

This work was primarily supported by the US Department of Energy’s Office of Basic Energy Sciences, the Robert A. Welch Foundation, and the US Army Research Office.

Journal Reference

Zhang, Z., et al., (2024) Terahertz-field-driven magnon upconversion in an antiferromagnet. Nature Physics.


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