BESSY II: Insight into ultrafast spin processes with femtoslicing
The scheme shows (from left to right): Hot electrons generated by a laser in platinum (light blue), the copper (yellow) is used to block the laser pulse so that only the hot electrons propagate and transport a spin current through the magnetic spin valve structure of cobalt platinum (blue-brown) and iron gadolinium (green). © D. Gupta /HZB
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.’
- Spintronics at BESSY II: Real-time analysis of magnetic bilayer systemsSpintronic devices enable data processing with significantly lower energy consumption. They are based on the interaction between ferromagnetic and antiferromagnetic layers. Now, a team from Freie Universität Berlin, HZB and Uppsala University has succeeded in tracking, for each layer separately, how the magnetic order changes after a short laser pulse has excited the system. They were also able to identify the main cause of the loss of antiferromagnetic order in the oxide layer: the excitation is transported from the hot electrons in the ferromagnetic metal to the spins in the antiferromagnet.
- Electrocatalysts: New model for charge separation at the solid-liquid interfaceHydrogen is at the heart of the transition to carbon neutrality, as both an energy carrier and a reagent for green chemistry. However, large-scale production of hydrogen via electrolysis, as well as the production of many other chemical products, requires significantly cheaper and more efficient catalysts. A precise understanding of the electrochemical processes that take place at the interface between the solid catalyst and the liquid medium is highly useful for developing better electrocatalysts. In the journal Nature Communications, an European team has now presented a powerful model that determines charge separation at the interface, the formation of the electric double layer and local electric potential variations, and the resulting influence on the catalytic activity.
- Environmental Chemistry at BESSY II: Radicals in waterwaysHow do radicals form in aqueous solutions when exposed to UV light? This question is important for health research and environmental protection, for example with regard to the overfertilisation of water bodies by intensive agriculture. A team at BESSY II has now developed a new method of investigating hydroxyl radicals in solution. By using a clever trick, the scientists gained surprising insights into the reaction pathway.