For modern memory and data processing technologies based on ferromagnetic materials, it is essential to understand the dynamics of magnetic phenomena on time scales of a thousandth of a billionth of a second (terahertz range). This applies, for example, to applications in MRAMs (Magnetic Random Access Memories) or the classic and still relevant hard disks in data centers. Until now, these have operated in the gigahertz range for data transmission (one gigahertz corresponds to an oscillation with a period of one billionth of a second).

The results are now available from an international research team’s basic research, which included the participation of the Professorship of Magnetic Functional Materials (https://www.tu-chemnitz.de/physik/MAGFUN/ ) (Head: Prof. Dr. Olav Hellwig) at Chemnitz University of Technology, open up possible applications for faster and more power-efficient data transfers in the terahertz range. One terahertz corresponds to an oscillation of one-thousandth of a billionth of a second. 

Ultrafast nutation observed for the first time in ferromagnetic thin film systems

The core of the team's observations were so-called thin-film systems. All modern memory and data processing technologies are based on thin-film systems. This usually refers to layers from one atomic layer down to the micrometer range. Researchers here use layers that are typically in the thickness range of 1 to 50 nm. What happens in these ferromagnetic layers on such a short time scale was previously unclear due to a lack of experimental techniques and corresponding data on such a short time scale.

The research team has now succeeded for the first time in observing ultra-fast nutation in ferromagnetic thin film systems. In simplified terms, nutation is the spinning motion of a force-free gyroscope’s figure axis (see figure).

The team included physicists from Chemnitz University of Technology, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), University of Duisburg-Essen, German Aerospace Center, TU Berlin, École Polytechnique (France), University of Naples Federico II, Parthenope University of Naples, Ca' Foscari University of Venice and Stockholm University. The lead was taken by Kumar Neeraj and Stefano Bonetti, scientists and experts in ultrafast experiments from Sweden and Italy.

The TELBE facility at HZDR was used for the studies, which were supported by Prof. Hellwig and his team from Chemnitz and Dresden. The TELBE facility is part of the ELBE electron accelerator and uniquely allows the generation of phase-stable high-field terahertz pulses with extremely flexible parameters such as repetition rate, pulse shape, and polarization. The samples required for the experiments were produced at the Professorship of Functional Magnetic Materials at Chemnitz University of Technology. The so-called "magnetron sputter deposition technique" was used. 

Chemnitz and Dresden expertise in magneto-dynamic properties

"My group produced the samples for these measurements and optimized them accordingly for these measurements together with our collaboration partners," explains Prof. Olav Hellwig. This involved optimizing the layer sequence, layer thickness, and lateral microstructure, as well as the magneto-dynamic properties, such as magnetic damping, he said. "This process is part of the special expertise of my working group for magnetic functional materials in Chemnitz and Dresden," says Hellwig.

The method used was the common "pump-probe experiment." For this, the researchers irradiated the thin film samples with ultrashort pulsed radiation in the terahertz wavelength range. These were in turn detected with an ultrashort, variably time-delayed 800 nm femtosecond laser pulse. In this way, the team tested how the magnetic moments in the sample responded to the terahertz pulse.

"These super-short terahertz pulses can be used to target magnetic systems on ultrashort time scales and then, hopefully, soon control them. In doing so, in addition to the already known precession motion, we have observed in this publication for the first time a nutation motion of the ferromagnetic moments, which takes place on an even faster time scale," says Olav Hellwig.