The data storage capacity of multi-terabyte hard drives is several million megabytes, but their data transfer rates are only a few hundred megabytes per second, due to their reliance on tiny magnetic structures. The development of memory devices that operate at picosecond timescales could speed data transfer and improve access to digital information. However, ultrafast control of magnetization states in magnetically ordered systems, like hard drives, is a challenge. One promising new strategy to remove speed restrictions in hard drives, pursued by researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and TU Dortmund University, is to use short current pulses and spintronic effects to readout magnetic states. Instead of electrical pulses, the team used ultrashort terahertz (THz) light pulses to enable the readout of magnetic structures in just picoseconds. The researchers generated THz pulses at the ELBE (Electron Linac for beams with high Brilliance and low Emittance) radiation source at HZDR. Using the THz radiation, they analyzed the magnetization of wafer-thin material samples consisting of two extremely thin, superimposed layers. The lower layer of the sample was made from a magnetic material such as cobalt (Co) or an iron-nickel (Fe-Ni) alloy. The upper layer was composed of metals like platinum (Pt), tantalum (Ta), or tungsten (W). Each layer was no more than three nm thick. The electric field of the incoming terahertz (THz) pulse generates extremely fast oscillating currents in the metal film. The spin Hall effect sorts the electrons according to their spin orientation. It thus changes the electrical properties of the sample, depending on the magnetization of the film, represented by the compass needles. This process leaves a clear fingerprint on the frequency spectrum of the emerging THz beam. Courtesy of B. Schröder/HZDR. “The material can only be penetrated by part of the terahertz radiation when the layers are this thin,” researcher Jan-Christoph Deinert said. This state of partial transparency is necessary to read the magnetization of the lower layer. The researchers used the THz laser with other short-pulse optical lasers to visualize and decode the very fast, relativistic quantum effects in the wafer-thin layers. “In our experiments, the terahertz flashes generate a variety of interactions between light and matter,” researcher Ruslan Salikhov said. The team found that the THz pulses generated extremely short-lived electric currents in the upper metal layer through their electric fields. The electrons arranged themselves according to the orientation of their intrinsic angular momentum, creating a spin current that flowed perpendicular to the layers. Electrons with a specific spin orientation accumulated rapidly at the interface between the layers. Depending on the alignment of these spins, and the magnetization direction of the lower layer, the electrical resistance of the interface changed, exhibiting an effect, previously discovered by an ETH Zurich team, called unidirectional spin Hall magnetoresistance (USMR). USMR was found to provide a straightforward, two-terminal geometry for the electrical detection of magnetization states in the magnetic heterostructures. It enabled the researchers to readout the magnetization direction very fast, using extremely short THz pulses. The THz pulses ensured that the spin current changed direction one trillion times per second. The electrical resistance of the interface also exhibited ultrafast variations. The quantum effect caused a reaction in the THz radiation, altering the THz pulses in a specific way. After penetrating the sample, the THz pulses oscillated at twice the frequency of the original THz radiation, demonstrating second harmonic frequency (SHF). The THz-light-field-driven, ultrafast USMR effect enabled magnetization states to be detected through the direct measurement of the electrical field from the second harmonic generation (SHG) signal. At THz frequencies, USMR thus enabled readouts in picoseconds, initiated by the light fields. The researchers observed ultrafast USMR in the thin film heterostructures via THz SHG. “Depending on the direction of magnetization, we generate fast fluctuations in the transparency of the sample,” professor Sergey Kovalev said. “We can detect this oscillation precisely and thus determine the magnetization of the lower layer within picoseconds.” The team’s findings highlight the potential for all-electrical detection of THz magnon modes. The researchers have begun work to enable the writing, as well as the readout, of magnetically stored data using THz radiation. They believe that transforming the results of their basic research into an ultrafast hard drive will take time, requiring more compact sources for short THz pulses and efficient sensors for analyzing the pulses. Nonetheless, the work shows that THz spintronics has the potential to extend communication bandwidth to THz frequencies and achieve ultrafast writing and reading of magnetic states. “We now can determine the magnetic orientation of a material much quicker with light-induced current pulses,” Deinert said. The research was published in Nature Communications (www.doi.org/10.1038/s41467-025-57432-2).