Magnétisme, vortex et écho de spin neutronique : une combinaison gagnante

Along with parameters such as charge and mass, the so-called spin – an angular momentum – is an intrinsic characteristic of subatomic particles, atoms and nuclei. Multiple spins can form atomic vortices which are known as skyrmions.

In addition to being an interesting phenomenon in fundamental academic research, skyrmions are becoming increasingly attractive candidates for so-called spintronic devices. These exploit the spin of electrons and are able to store larger amounts of information in a much more efficient manner than current storage technology.

Theoretical predictions imply that waves with different energies can propagate in skyrmion crystals depending on whether a parallel or an antiparallel magnetic field is applied. A very promising magnetic spintron candidate, namely MnSi (Mn = manganese, Si = silicium). An international research team studied the dynamics of the skyrmion dynamics of this material using neutron techniques, notably spin echo.

"The dynamics of such magnetic skyrmions are found in the very low energy range, only a few µeV. Therefore, the only technique that could help us conduct experiments to confirm this hypothesis was neutron spin echo", explains Hazuki Furukawa, PI of the research project. The team recently published their results in the prestigious journal Nature Physics.

In fact, neutron spin echo (NSE) exploits neutrons’ own spin. Briefly, in a NSE experiment, polarised neutrons – which precess in this field according to their own spin - are guided towards the sample through a magnetic field. Having being scattering by the sample, the neutrons are guided through a second magnetic field and precess in the opposite direction to the first one. If the neutrons detect a small molecular motion – such as that of the skyrmions - within the sample, their polarisation will change slightly after passing the sample. ILL’s IN15, the highest resolution NSE spectrometer in the world, is ideally suited for the experiments performed by Furukawa and her team.

The NSE experiments confirmed that asymmetric dynamics were indeed present in the skyrmion phase of MnSi, implying that it is an exceptionally promising candidate for spintronic devices and sustainable technology. The key role of NSE furthermore underlines the importance of neutron techniques in the development of the next generation of sustainable technologies.