“The magnetic orientation of a material can be determined much faster with light than with current pulses,” explains Dr. Jan-Christoph Deinert from the Institute of Radiation Physics at the HZDR. The physicist and his team used a special type of light that is invisible to the human eye – so-called terahertz radiation. With a wavelength of just under one millimeter, this light lies between thermal and microwave radiation in the electromagnetic spectrum. They used the ELBE radiation source at the HZDR as a light source. Here, scientists generate extremely short and intense terahertz pulses, among other things. These proved to be ideal for analysing the magnetization of wafer-thin material samples.
The samples consisted of at least two extremely thin layers stacked on top of each other. For the bottom layer, the researchers chose a magnetic material, for example from the element cobalt or from an iron-nickel alloy. The upper layer consisted of metals such as platinum, tantalum or tungsten. None of these metallic layers was thicker than three nanometers. “Only when the layers are this thin can the material be penetrated by part of the terahertz radiation,” explains Deinert. This partial transparency is a key prerequisite for being able to read the magnetization of the lower layer with light at all.
Simple material, complex mechanism
“In our experiments, the terahertz flashes generate a variety of interactions between light and matter,” explains Dr Ruslan Salikhov from the Institute of Ion Beam Physics and Materials Research at the HZDR, who was responsible for producing the samples. In combination with other optical short-pulse lasers, the team was able to visualize and decipher the very fast relativistic quantum effects in the wafer-thin layers. First of all, the terahertz pulses with their electric field generate extremely short-lived electric currents in the upper metal layer. Remarkably, the electrons sort themselves according to the orientation of their intrinsic angular momentum, the spin, and a spin current is generated perpendicular to the layers. At the interface between the layers, an accumulation of electrons with a very specific spin orientation is formed in direct succession. And depending on the alignment between these spins and the magnetization direction of the lower layer, the electrical resistance of the interface changes. The researchers call this effect unidirectional spin Hall magnetoresistance – USMR for short.
The USMR effect was discovered a few years ago by researchers at ETH Zurich. But the HZDR team has now gone a big step further. Thanks to this effect, the researchers can read out the magnetization direction extremely quickly by using the extremely short terahertz pulses. These ensure that the spin current changes direction around one trillion times per second. The electrical resistance of the boundary layer is also varied ultra-fast thanks to the USMR effect. And the quantum effect thus provides feedback to the terahertz radiation itself: “Depending on the orientation of the magnetization, we generate a rapid fluctuation in the transparency of the sample,” says Dr Sergey Kovalev from TU Dortmund University. And this changes the terahertz pulses in a very specific way. After penetrating the sample, they receive a harmonic, a so-called “second harmonic” with twice the frequency of the original terahertz radiation. “We can detect precisely this harmonic and thus determine the magnetization of the lower layer within picoseconds,” Kovalev explains.
Research is already underway to not only read the magnetically stored data with terahertz radiation, but even to write it. But the team also knows that there is still a very long way to go from this success in basic research to an ultra-fast hard disk. This would require both much more compact sources for short terahertz pulses and efficient sensors for analyzing them. However, the USMR effect shows which complex mechanisms in comparatively simple material systems can play an important role here.
Publication
R. Salikhov, I. Ilyakov, A. Reinold, J.-C. Deinert, T. V. A. G. de Oliveira, A. Ponomaryov, G. L. Prajapati, P. Pilch, A. Ghalgaoui, M. Koch, J. Fassbender, J. Lindner, Z. Wang, S. Kovalev: Ultrafast unidirectional spin Hall magnetoresistance driven by terahertz light field, in Nature Communications, 2025 (DOI: 10.1038/s41467-025-57432-2)
Contact
Dr. Ruslan Salikhov
Institute of Ion Beam Physics and Materials Research at the HZDR
Tel.: +49 351 260 3758 | E-Mail: r.salikhov@hzdr.de
Jan-Christoph Deinert
Institute of Radiation Physics at the HZDR
Tel: +49 351 260 3626 | E-Mail: j.deinert@hzdr.de
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Further links
👉 www.hzdr.de
Photo: B. Schröder/HZDR
