BESSY II: Insight into ultrafast spin processes with femtoslicing

An international team has succeeded at BESSY II for the first time to elucidate how ultrafast spin-polarised current pulses can be characterised by measuring the ultrafast demagnetisation in a magnetic layer system within the first hundreds of femtoseconds. The findings are useful for the development of spintronic devices that enable faster and more energy-efficient information processing and storage. The collaboration involved teams from the University of Strasbourg, HZB, Uppsala University and several other universities.

Spintronic components are not based on moving charges, but on changes in the orientation of magnetic moments, such as electron spins. Spin-current-based devices can therefore operate extremely quickly, currently on time scales of up to one hundred picoseconds (one picosecond is 10^-12 s). However, the microscopic processes themselves run much faster, in the range of a few hundred femtoseconds (1 fs = 10^-15 s).

Magnetic layers form a spin valve

Now, an international team led by Prof. Christine Boeglin, University of Strasbourg, has been able to experimentally observe some of these particularly interesting dynamic processes in a magnetic layer system for the first time. They investigated a so-called spin valve consisting of alternating layers of platinum-cobalt and an iron-gadolinium alloy layer. In this system, interactions between excited (hot) electrons and magnetic layers are particularly strong. First author Deeksha Gupta and her colleagues conducted the experiments at the femtoslicing station at BESSY II together with the HZB team that is operating this worldwide unique infrastructure.

With a femtosecond infrared laser (IR), they generated hot electrons (HE) in a platinum (Pt) top layer. A thick copper layer (Cu, 60 nm) ensures that only HE pulses reach the Co/Pt layer at the front of the spin valve, which acted as a spin polariser, generating spin-polarised HE pulses (SPHE).

Femtoslicing beamline offers unique options

The team was able to characterise these SPHE pulses by analysing the demagnetisation dynamics within the Fe₇₄Gd₂₆ ferrimagnetic layer at the end of the spin valve. To do this, they used methods that are only available in this combination at BESSY II: ‘Thanks to the unique capabilities of the femtoslicing beamline at BESSY II, we can separately probe the ultrafast spin dynamics for each component of a complex sample system,’ says HZB scientist Christian Schüßler-Langeheine. The team used ultrashort (~100 fs) soft X-ray pulses tuned to resonances of iron and gadolinium atoms recorded their respective dynamic reactions to SPHE pulses.

With the help of theoretical models developed by a team led by O. Eriksson at Uppsala University, it was possible to determine the crucial parameters of the SPHE current pulses, in particular the pulse duration, the spin polarisation direction and the current densities required to reproduce the experimental results.

Deeksha Gupta, who carried out the experiments as part of her PhD, has now joined HZB as a postdoctoral researcher, where she will continue to explore magnetic materials. She says: ‘This is a rapidly developing field. For the first time, we have been able to really shed light on the behaviour of spin currents in complex magnetic materials. This could pave the way for faster technological developments.’