Metadynamics Calculations of Magnetic Skyrmion Stabilities

Interfacial magnetic skyrmions are topological spin textures that have been proposed for use as information carriers in spintronic devices. Hence, it is vital that their creation, annihilation, and motion can be accurately controlled. Still, their thermal stability and interactions are not fully understood. Currently, athermal methods such as the geodesic nudged elastic band method (GNEBM) are used to quantify the skyrmion creation and annihilation energy barriers, including only the skyrmion’s internal energy. Furthermore, GNEBM is a low-temperature formalism that samples only saddle points along the energy landscape without providing any information about the transition from a skyrmionic to a ferromagnetic (FM) state and vice versa. Similarly, the skyrmion lifetimes are often estimated using energy barrier calculations and an arbitrary attempt frequency.
In contrast with GNEBM calculations, this thesis successfully demonstrated a novel use of metadynamics combined with atomic-scale magnetic simulations performed to reconstruct the free energy landscape (FEL) of macroscopic skyrmion observables. The reconstructed FELs are then used to quantify the skyrmions creation and annihilation energy barriers as a function of temperature, including both the effects of the internal energy and entropy, which is a major factor in skyrmion stability. The reconstructed FEL shows every possible transition path from a skyrmionic to a ferromagnetic path. Additionally, it is demonstrated that the skyrmions’ attempt frequency in finite temperature can be estimated with metadynamics and not assumed. Nonetheless, with the use of metadynamics, the effects of magnetisation reversal in thin films with DMI are explored, revealing the generation of a chiral domain that expands to reverse the magnetisation. Finally, the procedures to explore the effect of lattice defects and exploration of all possible topological spin textures in any thin film heterostructure in finite temperatures with metadynamics are reported. Ultimately, the results of this thesis demonstrate new procedures which can be used to design and realise future skyrmionic devices by identifying possible spin textures and assessing their thermal stability and lifetimes accurately while fully accounting for temperature effects.