The magnetic behaviour in nanoscale structures is of great interest for the fundamental understanding of magnetisation processes and also has importance for wide ranging technological applications. This thesis examines mechanisms for the enhanced control of domain walls in these structures via focussed ion beam modifications to magnetic nanowires and through the inclusion of periodic geometrical modifications to the nanowires geometry. A detailed investigation into the effect of focussed ion beam irradiation on the structure of NiFe/Au bilayers was performed through x-ray reflectivity and fluorescence techniques. This analysis revealed the development of interfacial intermixing with low dose irradiation. This is associated with complex changes of the magnetic behaviour including a rapid decrease, followed by a recovery of the saturation magnetisation with low dose irradiation. This behaviour is attributed to changes in the local environment of the atoms at the interface; resulting in modifications to the magnetic moment on Ni and Fe. The development of an induced moment on Au and a change in the spin-orbit interaction is also suggested. Localised control of the magnetic properties in nanowires demonstrates the ability to manipulate domain walls in these structures. Here, irradiated regions provide pinning sites where the width and dose of the irradiated region give control over the pinning potential. The inclusion edge modulation to nanowires geometry provides additional control over their magnetic behaviour. The direct magnetisation reversal field of these structures is explained by an analytical model based on the torque on the spins following the modulated wire geometry. This model is scalable for different modulation parameters and combines with the effect of localised regions of orthogonal anisotropy along the wire; explaining the reversal behaviour over the entire parameter space. Domain wall mediated reversal in modulated wires was also investigated in these structures. The inclusion of modulation shows an improvement in dynamic properties by the suppression of Walker breakdown. This is due to the relationship between geometrical modulations and the periodicity of micromagnetic domain wall structural changes during the Walker breakdown process. The combination of this work shows a route to the optimisation of the dynamic properties whilst minimising the detrimental increase in the de-pinning field from the modulation.

Magnetic transmission X-ray microscopy (M-TXM) is used to image domain walls in magnetic ring structures formed by a 300 nm wide, 24 nm thick Ni1Fe19 nanowire. Both transverse and vortex type domain walls are observed after application of different field sequences. Domain walls can be observed by comparing images obtained from opposite field sequences, or else domain wall propagation observed by comparing successive images in a particular field sequence. This demonstrates the potential use of M-TXM in developing and understanding planar magnetic nanowire behavior.

For decades, magnetism is widely applied in the industry as technologies such as sensors, memories, motors, generators, and others. Since the invention of the giant magnetoresistance effect (GMR) effect and the resulting magnetic read head, which was awarded the 2007 Nobel Prize in Physics to Albert Fert and Peter Grunberg, the study of magnetic-based technology has developed rapidly. There are many advantages to using magnetic-based devices such as high storage capacity, high reliability, cheaper cost, and non-volatility. Thanks to those advantages, magnetic-based devices for example hard disk drives (HDDs) is now widely used in computer memories even compared with solid state disk drives (SSDs) [1]. However, different from SSDs which store data in microchips, HDDs use a fixed read/write head to read information from the mechanically moved magnetic disk, which is slow and energetically inefficient. Such kind of low speed and high power needed consumption is preventing magnetic based devices from further applications. In my thesis, I will illustrate my study towards resolving these disadvantages, using a newly discovered phenomenon called spintronics. Due to the spin transfer torque between electron spins and lattice in materials such as ferromagnets, the magnetic domains can be driven by injecting a current, via a domain wall (DW) motion. Such property enables the potential applications of DWs in high-speed memory or logic devices. I will first give a summary of the magnetic energy terms which relevant to understanding thin film domain wall behavior. Next, I will give a brief introduction to magnetic energy terms and the motivation and background of my study on magnetic domain walls (DWs). There are two types of transverse DWs, a 180° domain wall (180DW) and a 360° domain wall (360DW). My research will mainly focus on the study of fast and in-situ formation of these two types of DWs, especially 360DWs which have not been well understood previously. In my method, these two types of DWs will be generated by using an external Oersted field, then injecting a current pulse in the transverse current line, and the chirality of DWs is based on the design and control of nanowire geometry. By using this method, not only the reliability is high for application purposes, but also the chirality of the formed 180DW and 360 DW can be well controlled, which is critical in applications as devices. After discussing the results of 180/360DWs formation, I will then talk about their dynamics property under the magnetic field or spin current, and further on how the chirality of 180/360DWs will response to geometry effects of the nanowire. Finally, with a combination of DW chirality and topological effects, I have discovered that the trajectory of the DWs can be controlled by the DWs chirality in a well-controlled Y-shape nanowire, which allows us to design a chirality sorter of 180/360DWs using such devices. My research is implemented mainly by micromagnetic simulations using finite element differentiation methods. The dynamics of magnetization is based on the one-dimensional Landau-Lifshitz-Gilbert (LLG) equations where both magnetic field and spin current will exert torques to magnetic moments. Two different tool kits are used for my simulations, OOMMF and Mumax3. Both of the two tools have their respective advantages and disadvantages and are more appropriate in respective studies, which will be discussed in further detail. I have also compared the results of the two tools. In the last, I will talk about the experimental study of DW behaviors. I have built a magnetoresistance system that can apply a magnetic field and spin current pulses into the samples and detect the change of sample magnetization by measuring the change of sample resistance. I will show the preliminary results for experimental measurements in the thesis and present my plans for future work.

Nonvolatile memory and logic devices rely on the manipulation of domain walls in magnetic nanowires, and scaling of these devices requires an understanding of domain wall behavior as a function of the wire width. Due to the increased importance of edge roughness and microstructure in narrow lines, domain wall pinning increases dramatically as the wire dimensions decrease and stochastic behavior is expected depending on the distribution of pinning sites. This work reports on the field driven domain wall statistics in sub-100 nm wide nanowires made from Co films of 8 nm thickness made by an electron beam lithography and etching process that minimizes edge roughness.

Over two volumes and 1500 pages, the Handbook of Spintronics will cover all aspects of spintronics science and technology, including fundamental physics, materials properties and processing, established and emerging device technology and applications. Comprising 60 chapters from a large international team of leading researchers across academia and industry, the Handbook provides readers with an up-to-date and comprehensive review of this dynamic field of research. The opening chapters focus on the fundamental physical principles of spintronics in metals and semiconductors, including an introduction to spin quantum computing. Materials systems are then considered, with sections on metallic thin films and multilayers, magnetic tunnelling structures, hybrids, magnetic semiconductors and molecular spintronic materials. A separate section reviews the various characterisation methods appropriate to spintronics materials, including STM, spin-polarised photoemission, x-ray diffraction techniques and spin-polarised SEM. The third part of the Handbook contains chapters on the state of the art in device technology and applications, including spin valves, GMR and MTJ devices, MRAM technology, spin transistors and spin logic devices, spin torque devices, spin pumping and spin dynamics and other topics such as spin caloritronics. Each chapter considers the challenges faced by researchers in that area and contains some indications of the direction that future work in the field is likely to take. This reference work will be an essential and long-standing resource for the spintronics community.