Terahertz-Driven Interaction of Coherent Magnons in Solid Materials

A new, ultra-fast method for regulating magnetic materials could enable next-generation data processing technologies.

Terahertz-Driven Interaction of Coherent Magnons in Solid Materials

This image illustrates that a duo of robust THz laser pulses provokes spin waves in an antiferromagnetic substance, causing nonlinear emissions at sum- and difference-frequencies. Image Credit: University of Texas at Austin & MIT researchers

Researchers and engineers are exploring options for creating information processing systems that operate more rapidly as the request for computing resources continues to increase. Using spin waves – arrangements of electron spins – to transmit and process data much faster than conventional computers is one prospective solution. However, a significant hurdle has been the regulation of these extremely prompt spin waves for beneficial applications.

MIT and The University of Texas at Austin have made substantial advancements in specifically controlling these ultrafast spin waves using personalized light pulses. Their research, conducted 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 outlined in two reports provided in Nature Physics.

Magnetic data storage systems, fundamental to the internet, cloud computing, and mobile phones, are pivotal. The modification of magnetic spin states (up and down) in ferromagnetic materials, representing the binary bits “0” and “1”, is critical to this technology. The alignment of these spins, which are minute magnets, determines the magnetic characteristics of the material.

Researchers have found that when light is applied to a single collection of atoms within these materials, the atoms’ spins wobble in a way that resonates with adjoining atoms in a similar fashion to ripples on a pond following the splash of a stone. This is known as a spin wave.

Antiferromagnets, a distinctive group of magnetic materials, have spins aligned in opposing directions compared to traditional data storage materials. As the spin waves in these materials are generally much faster than those in ferromagnets, they might be used in next-generation high-speed data processing designs.

An orthoferrite, an antiferromagnet, was used in the researchers’ trials. This material typically has two distinct spin waves which do not interact with each other. By utilizing terahertz (THz) light, which is invisible to the human eye at extreme infrared frequencies, the researchers were able to effectively trigger interaction between these spin waves.

In one report, it was demonstrated that, analogous to the harmonic overtones produced when a guitar string is strummed, exciting a spin wave at one frequency using intense THz fields can initiate another spin wave at a higher frequency.

This really caught us off guard. It meant that we could nonlinearly regulate the energy flow within these magnetic systems.

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

In another report, it was found that the excitation of two distinct spin waves can lead to the creation of a new hybrid spin wave. This is particularly exciting because, as per Baldini, it may help propel spintronics technology into the burgeoning field of magnonics. While the information is carried by the spin of each individual electron in spintronics, the field of magnonics uses spin waves, or magnons, to convey information.

Here, rather than spintronics, you are leveraging these collective spin waves that involve numerous electron spins concurrently. This can transport information in an exceptionally efficient manner and operate at extremely fast timescales that are otherwise unattainable in spintronics.

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

The researchers established an advanced spectrometer for conducting this groundbreaking work, allowing them to recognize the mutual interaction between the various spin waves and to observe their basic symmetries.

Unlike visible light, which can be easily detected by the naked eye, THz light is challenging to detect. These experiments would be infeasible without the technique development, enabling us to measure THz signals with just a single light pulse.

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

This research was primarily funded 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. doi.org/10.1038/s41567-023-02350-7.

Source: https://cns.utexas.edu/

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