Dynamics of Metastable Magnetic Skyrmions

Skyrmions, vortex-like objects composed of magnetic moments, have seen a recent surge of research interest due to their unique transport and topological properties. With an ever-increasing demand for more efficient memory and computation, skyrmionic devices have been conceived as an ultra-low power, high density data storage solution. While they are found in a range of materials, in this thesis we will primarily be concerned with skyrmions found in bulk chiral magnets. In such systems, skyrmions are typically only at equilibrium in a small range of temperature and applied magnetic field. However, they can exist in a metastable state over a much wider range of the magnetic phase diagram, formed by cooling the system under an applied magnetic field. Metastable skyrmions therefore have technological application by enabling the existence of skyrmions at room temperature and zero applied magnetic field. However, they also posses a finite, temperature-dependent lifetime, which places limitations on the stability of metastable skyrmions, and also restricts the population remaining after the cooling process. This lifetime is realised in nanoscopic mechanisms which are governed by topological defects, known as Bloch points. Due to the locality of these structures, the development of real-space imaging techniques are vital for gaining true understanding of skyrmion formation and annihilation. In this thesis, the dynamics of metastable skyrmions are thoroughly investigated through the use of magnetometry, and a range of neutron and x-ray scattering techniques. The effect of chemical substitution, or doping, on the magnetic phase transitions in Zn-doped Cu$_2$OSeO$_3$ is explored, and found to introduce pinning effects which dramatically increases the lifetime of metastable skyrmions. Furthermore, by adapting x-ray imaging methods for cryogenic sample environments, the first real-space observation of the vertical, tube-like, structure of skyrmions is demonstrated. The results open the door to a variety of experiments capable of further investigation into the dynamics of the skyrmion spin texture