The magnetic state of a ferromagnet can be determined from the strong interactions between a terahertz beam and an ultrathin structure that combines a metallic film with the magnetic material. [Image: B Schröder/HZDR]
Researchers in Germany have shown that intense flashes of terahertz radiation can read out the information stored in a magnetic memory within just a few picoseconds (Nat. Commun., doi: 10.1038/s41467-025-57432-2). The demonstration could inform the development of magnetic storage devices that exploit terahertz technologies to read and write data at ultrafast speeds.
Boosting data rates
While today's hard disk drives can store huge amounts of information, the speed of data transfer is typically limited to a few hundred megabytes per second. One promising strategy to boost data rates is to determine the magnetic state through the behavior of electron spins, with previous studies showing that femtosecond laser pulses can be used to induce short pulses of spin current within a material. Extending those spintronic techniques to the terahertz regime would enable ultrafast data exchange while also making use of efficient and compact electronic components.
To investigate the feasibility of this approach, the researchers used extremely short and intense pulses of terahertz radiation generated by the ELBE (electron linac for beams with high brilliance and low emittance) source at the Helmholtz-Zentrum Dresden-Rossendorf. They focused these flashes of terahertz light onto ultrathin samples consisting of a layer of a heavy metal, such as platinum or tungsten, stacked on top of a ferromagnetic material.
A second-harmonic signal
Strong interactions between the terahertz beam and the electron spins within these materials generate a second-harmonic signal that can reveal the magnetic state of the ferromagnet. More specifically, the incident light field generates a rapidly oscillating electrical current in the metallic film, which in turn produces a spin current oscillating parallel to the interface. Depending on the direction of the spin current, electrons with a specific spin orientation then accumulate at the interface with the ferromagnet.
The presence of these spins increases the electrical resistivity at the interface when they are aligned with the magnetic field and reduces it when they are pointing in opposite directions.
The presence of these spins increases the electrical resistivity at the interface when they are aligned with the magnetic field and reduces it when they are pointing in opposite directions. This effect, called unidirectional spin Hall magnetoresistance (USMR), produces a second-harmonic signal that is modulated by the ultrafast variation in the resistance. “We can detect this oscillation precisely and thus determine the magnetization of the lower layer within picoseconds,” says Sergey Kovalev from TU Dortmund University.
Other spin effects can contribute to the second-harmonic response, but the researchers showed that these can be filtered out to isolate the USMR component. Testing with different samples revealed that the USMR effect is most pronounced when a metallic layer of platinum is combined with a magnetic alloy of nickel and iron, and the response can be further enhanced by adding a third layer of tantalum below the magnetic alloy.
While this research result demonstrates the potential of terahertz technologies for reading and writing magnetic information, much more technical innovation will be needed to develop practical storage devices. In the meantime, the experimental technique developed by the team could offer valuable insights into complex spin interactions that could be harnessed in future devices.