Putting a leash on the domain wall: a TEM investigation into the controlled behaviour of domain walls in ferromagnetic nanostructures

Abstract

Domain walls may be treated as single entities that can be used to convey bits of information in potential magnetic memory devices such as magnetic logic and racetrack memory, which use controlled domain wall movement in complex magnetic nanowire networks. Greater understanding and control over certain aspects of their behaviour is required, however, before such devices can be realised. The structure of magnetic domain walls in ferromagnetic nanowires is both material and geometry dependent, thus providing an extremely large parameter space to explore. The structure and control of magnetic domain walls in permalloy nanostructures is investigated throughout this thesis using static and pulsed magnetic fields in combination with deliberately fabricated pinning features using Lorentz microscopy. The effect of an abrupt corner on the structure of domain walls is investigated with the use of a castellated wire geometry. Two different domain wall structures are observed at the corners of the wire, and a completely different domain wall is observed in a straight segment of the wire. Additionally, the reversal behaviour observed is entirely different depending on the direction of the applied field. Reproducibility experiments are also performed to asses the suitability of this geometry for potential use in magnetic memory applications. The ability to control the behaviour of both vortex and transverse domain walls in nanowires with deliberately fabricated defects is also explored, using a range of domain wall nucleation techniques. The magnetic spin structure of a vortex domain wall is completely different from a transverse domain wall and as a result their interaction with deliberately fabricated pinning sites differs greatly. Both domain wall types possess a chirality or sense of rotation. In wider wires however, transverse domain walls also possess an asymmetry which acts as an additional degree of freedom in the interaction with a trap. The result of both increasing and decreasing the wire width on a domain wall is investigated by patterning a single or double notch or anti-notch along a permalloy wire. The propagation of a magnetic domain wall along both a gently tapered and straight wire under the application of a pulsed magnetic field is also investigated, where a distorted vortex domain wall structure is observed to form. The distance a domain wall travels following the application of a short field pulse is measured and a lower limit of the velocity is calculated. Additionally, a domain wall is observed to undergo a number of changes in structure and chirality as it is moved along a wire under pulsed fields. The wire edge roughness also has a significant effect on the domain wall propagation velocity and focused ion beam irradiation is utilised to smooth the wire edges