XFEL: Controlling tiny magnetic swirls in the sea of spins

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2020/10/07
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Controlling tiny magnetic swirls in the sea of spins

Research on skyrmions may lead to more effective data storage

  

 

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A laser pulse transforms a uniform magnetization (magnetization down everywhere) to a skyrmion swirl where the magnetization in the center points up. This transformation changes the so-called topology of the system. Image credit: B. Pfau, Max Born Institute
Skyrmions, commonly imagined as tiny magnetic swirls, are nanoscale magnetic quasi-particles that have recently become a hot topic because of their potential in the development of faster and more effective data storage devices. For the first time, an international group of scientists, with lead scientists from the Massachusetts Institute of Technology, US, and the Max-Born-Institut in Berlin  have successfully been able to observe the formation of skyrmions in a magnetic material by using ultrashort laser pulses in a magnetic material, shedding light into the microscopic process and its time period. The X-ray pulses of the European XFEL’s revealed the creation of tiny skyrmion structures on nanometer length scales at a speed which is faster than previously thought possible. The results have been published in Nature Materials.

At the atomic level, magnetic materials resemble a sea of magnetic spins in either an ‘up’ or ‘down’ orientation. These spins are linked to each other so that a single spin change will affect the orientation of other spins. Skyrmions are tiny swirl-like structures where the center spin is oppositely aligned to the spins located at its boundary with a twisted spin configuration in between. These complex spin structures are very stable and small, making them interesting candidates for future spintronic devices. Spintronics exploits both the spin and the charge of electrons that could lower energy consumption in future memory devices and data storages.

Scientists have already shown how to control these structures with electric currents. However, the skyrmions in the experiment performed at European XFEL’s instrument for Spectroscopy and Coherent Scattering (SCS) were generated and controlled all-optically by a single light pulse. “By using intense optical laser pulses we are able to induce just the right level of excitement in the materials for skyrmion generation. In the future, this knowledge could help with the development of new energy efficient data storage devices,” says lead author Felix Büttner. The scientists used the extremely short X-ray pulses generated by the European XFEL to study the topological phase transition from a uniform magnetic phase (all spins point in one direction) to the skyrmion phase. They followed the skyrmion nucleation and condensation on ultrafast time scales, a process that takes about 300 picoseconds. The team could show that a laser-induced highly fluctuating spin phase at elevated temperature is key to lift the energy barrier between the two phases. When the system cools down, the energy barrier protects the nucleated skyrmions and makes them stable against thermal fluctuations, a property that is important for data storage.

Lead scientist at the SCS instrument, Andreas Scherz, explains: “It is exciting to see what types of experiments are now possible thanks to the unique characteristics of the European XFEL. We can now really begin to explore the phenomenon of magnetism in detail.”