Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and TU Dortmund University are providing proof of the feasibility of such fast data sources. Instead of electric current, they use ultrashort terahertz light pulses to read out magnetic structures in picoseconds.
Determining magnetic alignment with light
"The magnetic alignment of a material can be determined much faster with light than with current pulses," says Dr. Jan-Christoph Deinert from the Institute of Radiation Physics at the HZDR, according to a statement from the research center. The physicist used the light of terahertz radiation with a wavelength of just under 1 mm. Using the Elbe radiation source at the HZDR as a light source, scientists generated short and intense terahertz pulses. This enabled them to analyse the magnetization of thin material samples.
The samples consisted of at least two superimposed layers. The bottom layer consisted of a magnetic material, such as cobalt or an iron-nickel alloy. The top layer was made of platinum, tantalum or tungsten. None of these metallic layers was thicker than 3 nm. "Only when the layers are this thin can the material be penetrated by some of the terahertz radiation," explains Deinert. This is a prerequisite for being able to read the magnetization of the bottom layer with light.
Simple material, complex mechanism
"In our experiments, the terahertz flashes generate interactions between light and matter," says Dr. Ruslan Salikhov from the Institute of Ion Beam Physics and Materials Research at the HZDR. First, the terahertz pulses generate currents in the upper metal layer with their electric field. The electrons sort themselves according to the orientation of their spin, and a spin current is generated perpendicular to the layers. Electrons with one spin orientation accumulate at the interface between the layers. Depending on the alignment between these spins and the magnetization direction of the lower layer, the electrical resistance of the interface changes. This effect is called unidirectional spin Hall magnetoresistance (USMR).
The USMR effect was discovered a few years ago at ETH Zurich. But the HZDR team went further. Thanks to this effect, the researchers can read out the direction of magnetization extremely quickly. The terahertz pulses ensure that the spin current changes direction around one trillion times per second. The electrical resistance of the boundary layer thus also varies 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. This changes the terahertz pulses: after penetrating the sample, they receive a harmonic with twice the frequency of the original terahertz radiation.
Researchers are already working on using terahertz radiation not only to read the magnetically stored data, but also to write it. But the team also knows that there is still a long way to go before an ultra-fast hard disk is possible.