Study of the dynamic properties of magnetic skyrmions using diffractive ferromagnetic resonance

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


Magnetic skyrmions, i.e., vortex-like swirling spin textures characterized by a quantized topological invariant, have been found in chiral-lattice magnets and engineered heterostructures. Their dynamics under external fields is an issue of vital importance for both fundamental science, owing to their emergent quantum properties, and technological applications in spintronic devices owing to their topological protection.
The aim of the Project is to use ferromagnetic resonance (FMR) and resonant elastic soft x-ray scattering (REXS) to study long-range ordered magnetic systems and their dynamic properties. One example are the three different dynamical modes of the skyrmion lattice in the chiral-lattice magnet Cu2OSeO3. Apart from being a well-characterised model system, this insulating skyrmion material has also multiferroic properties leading to spin-induced ferroelectricity with an application potential as, e.g., microwave diodes.
The Project is focused on gaining understanding of the magnetisation dynamics of skyrmion systems in great detail, which paves the way for the practical use of skyrmions in fast-switching devices. An individual skyrmion vortex has the diameter of around 60 nm. Such a small length scale requires high spatial resolution, therefore most of the magnetic microscopy techniques are unsuitable. REXS can be used to characterise the magnetic ordering by using polarized soft x-rays with the photon energy tuned to the Cu L edge. Due to the polarized nature of the incidence x-rays, REXS has different sensitivities to the three components of the magnetization vector. REXS has unique advantages for the study of skyrmions: it is element-selective, has a very high sensitivity for magnetic order, provides a variable sampling area, and allows for fast and efficient data acquisition. FMR, on the other hand, is a powerful tool for the study of dynamic magnetic properties such as the resonance modes and damping characteristics, as well as anisotropies. These complementary techniques will be applied to study the spin dynamics of various spin structures.
This project is a joint effort with the Diamond Light Source, and in particular with the group of Prof Gerrit van der Laan. Industrial contacts exist via our EPSRC skyrmion grant with Western Digital, Hitachi, IBM, Samsung, and Seagate Technology.
This project aligns with EPSRC's research areas "Condensed Matter: Magnetism and Magnetic Materials" and "Spintronics".


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1949805 Studentship EP/N509711/1 01/10/2017 30/09/2021 Richard Brearton
Description Magnetic skyrmions are small vortex-like swirls of magnetization, which are of great interest for their potential for technological application. This project has focused on understanding the motion of magnetic skyrmions under the influence of various forces, both theoretically and experimentally. A popular technique often used to study magnetic skyrmions en masse is resonant elastic x-ray scattering; from the angle at which x-rays are scattered from a skyrmion hosting material in these experiments, it is possible to deduce many of their properties. One phenomenon which had puzzled researchers for some time was the irregular broadening of the scattered signal, an indicator that the skyrmions were in some sense strained in these experiments. We showed theoretically that skyrmions were necessarily strained by the nature of the experimental setup, linking the theoretical finding with some of our recent experimental work which emphasises this peak broadening. Digging deeper, we showed that the broadening of these peaks was intrinsically linked to the dynamical generation of topological defects in the skyrmion lattice, which is of great interest to those interested in the stability of the skyrmion lattice.

In fact, in some recently performed further work, we exploited the existence of these topological defects to shear the skyrmion lattice, leveraging the force which acts on a skyrmion in the presence of a non-uniform magnetic field. This allowed us to perform the first ever measurement of the so-called skyrmion Hall angle in a large class of skyrmion hosting materials. The skyrmion Hall angle is one of the topological properties of a skyrmion: under the influence of an external force, skyrmions move not in the direction of the applied force but instead at an angle to the force; this angle is defined as the skyrmion Hall angle. After performing the first measurement of this angle in an insulating material, we found that when there are no conduction electrons to dampen the skyrmion Hall angle, skyrmions are driven at nearly 90 degrees to the direction of an applied force! A manuscript is in preparation for these exciting findings.

Skyrmions can be manipulated very efficiently by a current, which was one of the main drives behind their surge of popularity in recent years. However, as skyrmions are also driven by magnetic fields, it is important to quantify the effect of the field generated by the current on the motion of current-driven skyrmions. During this project this force was quantified, it was found that the force due to the magnetic field negligibly deflects skyrmions from their path as the force is many orders of magnitude smaller than the spin-transfer torque delivered by the current.

Finally, a theoretical project investigating the interaction potential between skyrmions and their surroundings has been undertaken. The interaction between a skyrmion and the magnetic surface-twists which are present at the boundaries of their host materials was calculated, providing a significant correction to previously published results. This work is at the time of writing under review, but a preprint is available on the arXiv.
Exploitation Route A more full understanding of the nature of the skyrmion lattice under the strain provided by the experimental setup (involving magnetic fields generated by rare earth magnets) has already been found useful by several groups, and has contributed to an explanation of the scattering pattern obtained by another experimental group. Additionally, bounding the influence of the magnetic field generated by currents which are used to manipulate skyrmions will be useful for all device designers in the future. The theoretical work on skyrmion interactions changes the way that particle model skyrmion simulations are performed, vastly increasing the number of different geometries which can be explored with these techniques (involving the numerical integration of Thiele's famous equation of motion). Finally, the experimental determination of the skyrmion Hall angle paves the way for material engineers to design devices which exploit precisely the topological properties of the skyrmion.
Sectors Digital/Communication/Information Technologies (including Software),Other