Tracing Invisible Spin Currents

The Science   

Newswise — In “spintronics”, researchers use the spin of electrons (or spin waves) to carry information. This approach is in contrast to current electrical devices that use electric charges to carry information. In spin waves, angular momentum (a measure of electrons’ “twist”) is what moves. These “spin currents” can operate faster and with less heat than ordinary currents of charge. But detecting a pure spin wave current is extremely hard. Researchers have designed most experiments to sense electric charge, not angular momentum. In this work, researchers showed that Resonant Inelastic X-ray Scattering (RIXS) can overcome this challenge. RIXS is a method that is sensitive to tiny changes in the momentum and energy of spin waves. (The quantum description of a spin wave is also known as a magnon.) By applying a temperature gradient across a magnetic insulator, this study drove a magnon spin current. The scientists then used subtle shifts in the RIXS signal to observe the magnon spin current. This test demonstrated that RIXS can “see” spin currents.

The Impact

This discovery marks a major step toward controlling spin currents, which are a key ingredient for next-generation electronics. By watching how spin moves through materials, researchers can better understand and design systems that use spin instead of charge to carry information. This could lead to smaller, faster, and more energy-efficient devices. This technology could be useful in a variety of applications, from advanced computer memory to new logic components. In the long term, precise control of spin flow may also open pathways to quantum technologies. These technologies would exploit the quantum nature of spin for information processing.

Summary

Magnon-based transport in insulators is fostering the next generation of ultra-fast, low-power, miniaturized electronics. Scientists’ understanding of the magnon spin current has been hampered by how difficult it is to detect the flowing electron angular momenta. For example, detecting this characteristic and determining its transport properties can require scientists convert it to charge current. Because there are many complex mechanisms happening during the magnon/charge conversion, this process may hinder the extraction of fundamental, microscopic transport parameters. In this project, researchers built a device that produces spin currents through a temperature gradient. They then integrated it into a specialized experimental setup at the National Synchrotron Light Source II (NSLS-II, a Department of Energy Office of Science User Facility) designed for RIXS experiments. They created a new tool to directly visualize the energy and momentum of magnon currents. They also used a mathematical model to calculate how long the magnons lasted as a function of their momentum. This is called the momentum-resolved magnon lifetime and knowing it is essential for developing magnon-based spintronic devices in the future. 

Funding

This work was supported by the DOE Office of Science, Basic Energy Sciences, Early Career Research Program and the Programmable Quantum Materials Energy Frontier Research Center. Research was conducted at the National Synchrotron Light Source II and the Center for Functional Nanomaterials, both DOE Office of Science User Facilities.