Future information technologies will rely not only on the charge of electrons but also on their spin. Electron spin, the source of an electron's magnetic moment, can be oriented by a magnetic field – similar to how a compass needle aligns with Earth’s magnetic field. This property is used as an information carrier in so-called spintronic devices, such as magnetic random-access memories (MRAM).

A key spintronic process is the conversion of a charge current into an accumulation of spin. It happens, in particular, at the interface of a layer of cobalt, which is a ferromagnet with a magnetic moment, and a platinum layer. This phenomenon is highly important to set the direction of the magnetic moment to and, thus, write a magnetic bit (1 or 0). So far, spin accumulation was achieved by voltages applied through electrodes which strongly limits the available current rate and speed. However, other information carriers, such as light in optical fibers, routinely work at extremely high rates at one terahertz, i.e., one million times one million cycles per second.

To push the speed of spin accumulation to the terahertz range, Dr. Alexander Chekhov of the Terahertz Physics Group at the Department of Physics of the Free University Berlin made use of cutting-edge ultrashort electric-field pulses at terahertz (THz) frequencies to drive the ultrafast charge current in the cobalt-platinum stack. To monitor the temporal evolution of the resulting spin accumulation, he made use of an additional optical pulse with controllable time delay.

This experiment led to three discoveries.

First, and very surprisingly, the measured signal turned out to look exactly like the electric field of the driving terahertz pulse. “At first, I couldn’t believe my eyes: It was directly the driving pulse, which I was never able to detect without distortion. This observation immediately meant that such process can serve as a broadband terahertz detector,” says Alexander Chekhov. Indeed, the detection of such short and, thus, broadband pulses is challenging, since the detection process is almost always frequency-dependent.

“Second, the extremely fast response of the sample also allowed us to identify the mechanism behind the observed ultrafast spin accumulation: the so-called Rashba-Edelstein effect. Such identification is hard with other methods," explaines the physicist.

Third, the spin accumulation is localized at the very interface of the cobalt and platinum layers. Therefore, the technique developed here may enable a novel way to probe the terahertz dynamics of electrons, spins and ions at interfaces, which are ubiquitous in physics, chemistry and even biology, but notoriously hard to observe.

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Science Advances, 21 Mar 2025, Vol 11, Issue 12; DOI: 10.1126/sciadv.adq7741