Skyrmion | EPFL Graph Search

Le skyrmion est une particule théorisée en 1962 par le physicien britannique Tony Skyrme et dont la découverte a été annoncée en 2009 par des physiciens de l'Université technique de Munich. Son antiparticule est l'antiskyrmion. Description Un skyrmion est une superposition quantique de baryons et d'états de résonance, ou plus simplement un vortex ou tourbillon de spin sur une surface, qui peut être créé par la pointe d'un microscope à effet tunnel. C'est sous la forme du vortex de spin que les physiciens allemands ont fait leur découverte. Un skyrmion peut être approché par un soliton de l'équation Sine-Gordon ; après quantification par la méthode de l'ansatz de Bethe, il devient un fermion interagissant selon le modèle de Thirring. Localisation Des skyrmions ont été rapportés comme étant présents dans des condensats de Bose-Einstein, des supraconducteurs, des couches minces aux propriétés magnétiques et des cristaux liquides chiraux néma

Skyrmion Control and Phase Transitions in Cu2OSeO3

Thomas Schönenberger

Magnetic skyrmions are nanometric and non-trivial spin textures with non-zero topological charge. Their robustness against perturbations and the possibility to control them using external stimuli make them ideal candidates for future spintronic applications. In particular the magnetoelectric skyrmion host Cu2OSeO3 holds a lot of promise for low power devices since skyrmions in this compound can be controlled by electric fields alone. Using Lorentz transmission electron microscopy to perform real space and real time biasing experiments on thin lamellas of Cu2OSeO3 in a geometry that is most suitable for technological applications, we observe reproducible creation and annihilation of skyrmions. For a more quantitative analysis, we develop new feature detection algorithms to reliably extract skyrmion positions even in noisy images. We further produce Due to its low pinning, Cu2OSeO3 allows for the formation of large and well-arranged triangular skyrmion lattices. This makes this compound a perfect testbed to study the evolution of skyrmion configurations under external stimuli. Experiments are carried out again using Lorentz transmission electron microscopy on thin lamellas of Cu2OSeO3. We investigate how defects in skyrmion lattices are arranged at grain boundaries and develop algorithms to extract them and to directly visualize their alignment. These defects are at the core of the melting of skyrmion lattices in this system. We show that a controlled magnetic field ramp can induce skyrmion ensembles in Cu2OSeO3 to transition from a two-dimensional solid through a thus far unknown ordered liquid phase called the hexatic phase, to a liquid. We find that this transition is a topological defect-induced two-step process as predicted by the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. Finally, we go beyond equilibrium phenomena to explore the effect of quenching the system from its liquid phase to its solid phase using different quench rates and find first evidence that our system belongs to the Kibble-Zurek universality class.

EPFL2022

Electric Field Effects in the Skyrmion Lattice Hosting Magnet Cu2OSeO3

Saumya Shivam

Magnetic skyrmions have garnered much attention in recent years because of their non trivial topology and potential to be used in next generation memory devices. They may exist individually or as a lattice of skyrmions(SkL). A number of investigations have been done on SkL in metallic samples, but recently a few magnetoelectric insulators have also begun to attract a lot of attention. The magnetoelectric nature of these materials presents a possibility to make prototype memory devices where skyrmions form a storage bit and can be controlled with only electric fields without the losses caused by Joule heating. We focus here on the material Cu2OSeO3, for which interesting electric field effects have been seen like the control of size of the region in the phase diagram where SkL is stable, creation of metastable skyrmions and the use of magnetolectric susceptibility to map the phase diagram. We use the magnetoelectric susceptibility to explore the above mentioned effects of electric fields, attempting to confirm previous reports of such effects using different techniques like neutron scattering and AC susceptibility. We also propose a new design to create metastable SkL state at low temperatures and confirm its existence using magnetoelectric susceptibility. We also attempt to measure the change in polarization between the SkL phase and the trivial phase on application of DC electric fields, which in principle could lead to electrical readout of skyrmions in a prototype skyrmion based memory device.

2018

Theories of topologically-induced phenomena in skyrmion-hosting magnetoelectric insulators

Oleksandr Kriuchkov

There once was a harsh competition between different computer memory technologies, and now we cheer triumph for the Random Access-Memory (RAM) devices, -- cheap, fast, tiny, stable. The competing Magnetic Bubble Memory had faded away as magnetic bubbles are undesirably large and pretty much not robust, manifesting both low data density and high operational costs. However, what we do possess now are nanosize objects of topological nature, magnetic skyrmions, which are protected from continuous field variations and take very little energy cost to be moved. Thus, skyrmions are considered as promising information carriers for future memory devices and ultradense data storage, while skyrmion phases in bulk materials are interesting from fundamental point of view in exploring topological states of matter. In this thesis, I develop and advance several effective theoretical approaches, diverse both in their methods and use, which were of appeal for several skyrmionic experiments in our lab (LQM/EPFL). We were and are primarily interested in the open issues in the applied field of skyrmionics, which may be taken under the umbrella of creation, stabilization and control of magnetic skyrmions under electric fields, mechanical strains, thermal gradients, etc. For the goals achieved and yet to be achieved, the magnetoelectric insulator Cu2OSeO3, which uniquely responses to all the above mentioned fields, is a highly advantageous candidate. Magnetoelectric here means that the spins of a magnetic material are coupled to external electric fields, while insulating properties are very advantageous to preserve both the state and the very existence of skyrmions by eliminating the Joule heating. The novel results in this thesis are calculations for both individual and arrayed skyrmions under electric fields, mechanical strains and uniform pressures, and thermal gradients. Furthermore, several fundamental questions were addressed by developing an extended formalism for calculation skyrmion-pocket phase diagrams, studying the topologically-governed crossover between skyrmions and magnetic bubbles, and discussing the possible role of merons (half-skyrmions) in skyrmion phase formation. The result with the most immediate appeal is probably the theoretical and experimental study of skyrmion lattices in electric fields, with a direct demonstration of writing and erasing of the full skyrmion phase under electric fields of few Volts per micrometer, as compatible with modern microelectronic devices.