The main goal of this project is to control the DW dynamics in three-dimensional systems through the effects of the 3D geometry, including curvature, topology, and chirality. To achieve this aim our initial focus was to elucidate the influence of these three-dimensional effects on the domain wall dynamics through systematic investigations of carefully designed 3D systems.
The project has addressed the following goals:
1. Stabilization of spin textures in curved nanostructures: Two types of domain walls are commonly observed in cylindrical nanowires: transverse vortex domain walls (TDWs) and Bloch point domain walls (BPDWs). TDWs involve a rotation of magnetisation perpendicular to the wire axis, while BPDWs feature a curvature of magnetic moments within a plane perpendicular to the wire axis, with a Bloch point at its centre. While the effect of curvature-induced symmetry breaking has been explored for TDWs, allowing the selection of the chirality of TDWs, the stabilisation of BPDWs presents a new opportunity to investigate their fundamental properties. Bloch point singularities, known for their topological nature, are of interest for both fundamental research and technological applications, as they represent one of the smallest magnetic textures suitable for information storage, with is expected to show ultrafast velocities. However, our understanding of these singularities and our ability to control them experimentally have been limited. In this project we succeeded in stabilising BPDW by introducing curvature into the system. By fabricating a model nanowire system with curvature-induced variable DMI regions separated by straight chiral segments, we demonstrate the stabilisation of the domain walls within the straight regions using high X-ray magnetic microscopy.
2. Pinning in undulating conduits: Through experimental observations, we have identified that BPDWs tend to pin in straight sections of nanostructures between curved regions. Micromagnetic finite element simulations shed light on the influence of curvature-induced symmetry breaking on the energy of BPDWs. The achiral nature of BPDWs makes them susceptible to being pin in straight and achiral regions, which are stable energy minima for this kind of domain walls.
3. Propagation of Bloch point domain walls in undulating conduits: Exploiting the energy dependence of BPDWs, we introduce regions of varying curvature to control the energy landscape, creating well-defined pinning points. By patterning the curvature, we engineer asymmetric potential wells that induce non-reciprocal motion of BPDWs. This control of the energy landscape enables the realization of a robust Bloch point shift-register with tuneable depinning fields and non-reciprocal behaviour.
4. Integrated DW conduits: To translate these fundamental discoveries into practical applications, we need to integrate nanostructures into devices. By directly depositing FEBID Co nanostructures on chips, we enable the application of electric currents to control the magnetic state and the domain wall motion. These integrated nanostructures demonstrate suitable conductivity for electrical measurements, paving the way for the development of spintronic devices based on current control.
