Magnetic skyrmion | EPFL Graph Search

In physics, magnetic skyrmions (occasionally described as 'vortices,' or 'vortex-like' configurations) are statically stable soliton which have been predicted theoretically and observed experimentally in condensed matter systems. Magnetic skyrmions can be formed in magnetic materials in their 'bulk' such as in manganese monosilicide (MnSi), or in magnetic thin films. They can be achiral, or chiral (Fig. 1 a and b are both chiral skyrmions) in nature, and may exist both as dynamic excitations or stable or metastable states. Although the broad lines defining magnetic skyrmions have been established de facto, there exist a variety of interpretations with subtle differences. Most descriptions include the notion of topology – a categorization of shapes and the way in which an object is laid out in space – using a continuous-field approximation as defined in micromagnetics. Descriptions

Real space and reciprocal space investigations of the spin dynamics in skyrmion-hosting materials

Le Yu

Magnetic skyrmions are whirl-like spin configurations with particle-like properties protected by non-trivial topology. Due to their unique spin structures and dynamical properties, they have attracted tremendous interests over the past decade, from fundamental science to technical applications. However, experimental methods to study skyrmion dynamics in real space are limited until present. The goal of this thesis is to both apply and develop novel methodologies to firstly discover new skyrmion lattice hosting systems through neutron scattering experiments, and then directly observe the dynamics in these materials in real space by time-resolved imaging. The main content of this thesis is divided into two parts: firstly, I will demonstrate by means of inelastic neutron scattering how the magnon spectrum was measured in VOSe2O5, one of the few known systems hosting a Néel-type skyrmion lattice. By comparing the measured spectra with SpinW simulations, the exchange interactions and magnetic anisotropy were extracted so that the spin Hamiltonian in this system could be constructed. The results provide positive feedback to the recently developed quantum chemistry calculations. Further, the reasonable consistency among experiments, simulations and calculations may enable the community to use quantum chemistry as a novel tool to predict skyrmion hosting materials and understand their formation. The second part of this thesis will introduce my work in integrating a microwave system into a time-resolved transmission electron microscope (TEM). In this project, a pump-probe technique using microwave to coherently excite spin wave dynamics in skyrmion lattice hosting materials in the GHz frequency range was developed and implemented. The time-resolved TEM will enable the direct observation of excited spin waves and skyrmion dynamics in real space. This instrumental development will also allow the study of the spatial homogeneity of resonant modes, and their behavior around individual impurities or grain boundaries. The insights gained from such experiments would help the community to understand microscopic aspects of skyrmion dynamics and tailor spintronic devices using skyrmions in the future. The chapter layout is presented as follows: in Chapter One, I introduce fundamental concepts about skyrmions, and then discuss their general magnon spectrum and how they can be measured. Chapter Two describes methodologies adopted during this thesis work, including experimental techniques and modelling methods. The scattering experiment details and results obtained at various beamlines are presented in Chapter Three. The progress concerning the development of the new technique in time-resolved real space imaging starts from Chapter Four. Here, the implementation of a microwave system in ultrafast TEM is demonstrated systematically. The last chapter contains the conclusion and outlook.

EPFL2022

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

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