Spiral spin-liquid and the emergence of a vortex-like state in MnSc2S4

Spirals and helices are common motifs of long-range order in magnetic solids, and they may also be organized into more complex emergent structures such as magnetic skyrmions and vortices. A new type of spiral state, the spiral spin-liquid, in which spins fluctuate collectively as spirals, has recently been predicted to exist. Here, using neutron scattering techniques, we experimentally prove the existence of a spiral spin-liquid in MnSc2S4 by directly observing the 'spiral surface' a continuous surface of spiral propagation vectors in reciprocal space. We elucidate the multi-step ordering behaviour of the spiral spin liquid, and discover a vortex-like triple-q phase on application of a magnetic field. Our results prove the effectiveness of the J(1)-J(2) Hamiltonian on the diamond lattice as a model for the spiral spin-liquid state in MnSc2S4, and also demonstrate a new way to realize a magnetic vortex lattice through frustrated interactions.

Control of Topological Magnetic Textures by Magnetic and Electric Fields in Magnetoelectric Helimagnet Cu₂OSeO₃

Ping Huang

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

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.

EPFL2017

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

Oleksandr Kriuchkov

EPFL2017