Today, a single modern hard drive can store several million megabytes — providing enough storage for hundreds of thousands of photos. These multi-terabyte hard drives rely on tiny magnetic structures. However, with data rates of only a few hundred megabytes per second, access to this digital information remains relatively slow. Initial experiments have already shown a promising new strategy: Magnetic states can be read out by short current pulses, whereby recently discovered spintronic effects in purpose-built material systems could remove previous speed restrictions. Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and TU Dortmund University are now providing proof of the feasibility of such ultrafast data sources. Instead of electrical pulses, they use ultrashort terahertz light pulses, thereby enabling the read-out of magnetic structures within picoseconds. The team has presented its results in the journal Nature Communications.
“We now can determine the magnetic orientation of a material much quicker with light-induced current pulses,” explains Dr. Jan-Christoph Deinert of HZDR’s Institute of Radiation Physics. For their experiments, the physicist and his team employed light that is invisible to the human eye — so-called terahertz radiation. With a wavelength of just under a millimeter, this light is located between infrared radiation and microwave radiation on the electromagnetic spectrum. The source of this light was the ELBE radiation source at HZDR, where researchers can generate extremely short and intensive terahertz pulses. These proved to be ideal for analyzing the magnetization of wafer-thin material samples.
The samples consisted of two extremely thin superimposed layers. For the lower layer, the researchers selected a magnetic material made from elements such as cobalt or an iron-nickel alloy. The upper layer was composed of metals like platinum, tantalum or tungsten. None of these metal layers exceeded three nanometers in thickness. “The material can only be penetrated by part of the terahertz radiation when the layers are this thin,” Deinert explains. Such partial transparency is a crucial prerequisite for being able to read the magnetization of the lower layer.
Simple material, complex mechanism
“In our experiments, the terahertz flashes generate a variety of interactions between light and matter,” describes Dr. Ruslan Salikhov from HZDR’s Institute of Ion Beam Physics and Materials Research, who was responsible for growing the sample films. In combination with other short-pulse optical lasers, the team managed to visualize and decode the very fast relativistic quantum effects in the wafer-thin layers. First, the terahertz pulses, through their electric field, generate extremely short-lived electric currents in the upper metal layer. Remarkably, the electrons arrange themselves according to the orientation of their intrinsic angular momentum, the spin, creating a spin current that flows perpendicular to the layers. At the interface between the layers, electrons with a specific spin orientation accumulate in quick succession. Depending on the alignment of 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 or USMR, for short.
The USMR effect was discovered some years ago by researchers at ETH Zurich, but the team at HZDR and TU Dortmund University has now made significant advancements. Thanks to this effect, the researchers can read out the magnetization direction very fast using extremely short terahertz pulses. They ensure that the spin current changes direction a trillion times per second. In addition, thanks to the USMR effect, the electrical resistance of the interface varies ultrafast, as well. Consequently, the quantum effect causes a reaction in the terahertz radiation itself.
“Depending on the direction of magnetization, we generate fast fluctuations in the transparency of the sample,” says Dr. Sergey Kovalev from TU Dortmund University. This alters the terahertz pulses in a very specific way. After penetrating the sample, they oscillate at twice the frequency of the original terahertz radiation — known as the “second harmonic” frequency. “We can detect this oscillation precisely and thus determine the magnetization of the lower layer within picoseconds,” Kovalev summarizes.
Work is already underway not only for reading out but also for writing the magnetically stored data using terahertz radiation. However, the team recognizes that transforming this basic research achievement into an ultrafast hard drive may take considerable time. It would require far more compact sources for short terahertz pulses as well as efficient sensors for analyzing them. Nonetheless, the ultrafast USMR effect highlights the complex mechanisms within relatively simple material systems that can play a significant role in the future development of ultrafast magnetic memory applications.
