Square and rhombic lattices of magnetic skyrmions in a centrosymmetric binary compound

Jonathan White, Le Yu
2022
Article

Résumé

Magnetic skyrmions are topologically stable swirling spin textures with particle-like character, and have been intensively studied as a candidate of high-density information bit. While magnetic skyrmions were originally discovered in noncentrosymmetric systems with Dzyaloshinskii-Moriya interaction, recently a nanometric skyrmion lattice has also been reported for centrosymmetric rare-earth compounds, such as Gd2PdSi3 and GdRu2Si2. For the latter systems, a distinct skyrmion formation mechanism mediated by itinerant electrons has been proposed, and the search of a simpler model system allowing for a better understanding of their intricate magnetic phase diagram is highly demanded. Here, we report the discovery of square and rhombic lattices of nanometric skyrmions in a centrosymmetric binary compound EuAl4, by performing small-angle neutron and resonant elastic X-ray scattering experiments. Unlike previously reported centrosymmetric skyrmion-hosting materials, EuAl4 shows multiple-step reorientation of the fundamental magnetic modulation vector as a function of magnetic field, probably reflecting a delicate balance of associated itinerant-electron-mediated interactions. The present results demonstrate that a variety of distinctive skyrmion orders can be derived even in a simple centrosymmetric binary compound, which highlights rare-earth intermetallic systems as a promising platform to realize/control the competition of multiple topological magnetic phases in a single material. Typically, skyrmions appear in magnet systems which are non-centrosymmetric. Here, using neutron and X-ray scattering, Takagi et al show the emergence of a skyrmion phase in the centrosymmetric material EuAl4. This expands the range of materials potential hosting skyrmions.

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https://infoscience.epfl.ch/record/293648?ln=fr

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Jonathan White, Le Yu
2022
Article

Résumé

Official source

https://infoscience.epfl.ch/record/293648?ln=fr

À propos de ce résultat

Concepts associés (15)

Concepts associés

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Publications associées (13)

Publications associées

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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.

EPFL2017

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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

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Magnetic skyrmions, topologically non-trivial chiral whirl-like spin configurations, have been under massive investigations in both fundamental and application aspects. In some special compounds, these quasi-particles can formtriangular skrymion lattice (SkL) exhibiting abundant unique properties. This thesis presents my comprehensive studies on the SkL in the magnetoelectric (ME) coupling helimagnet Cu2OSeO3. Starting from measurements on single crystal bulk Cu2OSeO3 samples, including dc magnetization, magnetoelectric susceptibility and neutron scattering, so as to get the general understanding of this system, the main work of this thesis focused on the investigation of quasi-two dimensional thin slab samples by the state-of-the-art Lorentz electron transmission electron microscopy (LTEM). Variousmagnetic textures including the spin helices and skyrmions were observed and analyzed. What separates the work presented here from literatures is the special emphasis on the dynamics of the SkL under various excitations in real space and real time, such as thermal fluctuation, magnetic field and electric field. Specifically, the magnetic field induced melting of the SkL in Cu2OSeO3 was, for the first time, observed and analyzed quantitatively. Two-step melting process was observed and demonstrated to follow the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. The application of electric field can not only rotate the SkL, but also create skyrmions fromthe helical phase. My studies, especially those focused on the emergent properties induced by external fields, are intended to provide deeper insight into the fundamental knowledge of the SkL in Cu2OSeO3 and to promote the the development of skyrmion based spintronic applications.

EPFL2017

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EPFL2017