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 based devices such as hard disk drives (HDDs) are widely used in the computer industry because of their high memory capacity, non-volatility and low cost compared to semiconductor-based solid state disk drives (SSDs). However, they also suffer from low energy efficiency and low speed, due to the requirement for mechanical motion in order to access the data. In my thesis, I will first give a brief introduction to the motivation and background in the study of magnetic domain walls (DWs), which have attracted great attention due to their ability to be moved by field and/or current and corresponding potential applications in high speed memory or logic devices. I will then discuss how to geometrically control the behaviors of DWs in a ferromagnetic nanowire. I will first discuss how natural geometry distortions such as edge tapering from sputtering on an undercut resist profile and wire width variation from the patterning process would affect DW behavior, including static configurations, stability and dynamics under current pulsing. I will then discuss how similar geometrical effects will affect the properties of materials with perpendicular magnetic anisotropy (PMA). The same geometry modulation will have different effects depending on the origin of the PMA. Such results are confirmed by observing the magnetic reversal process. Besides the study on 180DWs, we will then discuss the field and current effects on 360 degree DWs (360DWs), which have many unique properties compared to 180DWs and are an alternative candidate for DW based devices. I will then discuss control of 360DW behavior by designing a geometrical heterostructure. We have found that by utilizing the asymmetric Oersted field originated from the heterostructure, we are able to control the 360DWs depending on their chirality. The structure can function as a 360DW chirality filter, which provides extra freedom in DW-based applications. These studies were conducted by a combination of micromagnetic simulations and experimental implementations. Techniques being used including OOMMF micromagnetic simulations, Comsolfinite element simulations, electrical measurements, magnetic force microscopy and other characterization techniques.

Recent advances in thin film growth technology to create complex oxide heterostructures with atomic-level precision have enabled the discovery of a wide range of novel physical phenomena at engineered interfaces. These phenomena arise from the complex interactions between the lattice, charge, spin, and orbital degrees of freedom that are highly sensitive to external stimuli such as strain, chemical doping, and electric and magnetic fields. Among these complex oxide systems, heterostructures consisting of layers with competing magnetic characteristics have attracted great attention from a fundamental perspective as well as for their potential applications in magnetic sensors, magnetic random access memory, and future spintronics devices. One of the fundamental building blocks of such devices is the exchange-bias (EB) effect which is typically associated with interfacial exchange interactions between a ferromagnetic (FM) and an antiferromagnetic (AFM) material. A similar effect has also been observed at interfaces between hard and soft FM layers, where the hard (soft) layer possesses high (low) coercivity and low (high) saturation magnetization. In analogy to AFM/FM interfaces, the biasing effect at FM/FM interfaces originates from the magnetic unidirectional anisotropy induced by the exchange interactions between the hard and soft FM layers. The exchange interactions in complex oxide heterostructures consisting of La0.7Sr0.3MnO3 (LSMO) and La0.7Sr0.3CoO3 (LSCO) layers were systematically studied. LSMO is a soft FM metal that shows coincident FM-to-paramagnetic (PM) and metal-to-insulator transitions at ~ 360 K in its bulk form. LSCO is a hard FM material and is known to show magneto-electronic phase separation (MEPS), where FM/metallic clusters are embedded in a non-magnetic/insulating matrix. Synchrotron radiation based resonant x-ray reflectivity, soft x-ray magnetic spectroscopy, and bulk magnetometry were used to investigate the magnetic and electronic structure of the LSMO/LSCO heterostructures. It was found that a 6 nm LSMO/ 6 nm LSCO heterostructure displayed unconventional magnetic switching behavior, which deviated from conventional metallic FM/FM systems in that reversible switching occurred not only within the soft LSMO layer but was also accompanied by the switching of a thin interfacial LSCO layer. This unique magnetic switching behavior was strongly dependent on the thickness of the LSCO layer. Soft x-ray magnetic spectroscopy allowed us to develop a physical picture where a form of MEPS occurred vertically through the LSCO film thickness and was driven by the competition between two different interfacial effects at the LSMO/LSCO and the LSCO/substrate interfaces. These findings provide further evidence of the high tunability of magnetic properties in complex oxide heterostructures through interface engineering. In addition, domain wall injection and propagation in LSMO nanowires was investigated to ascertain its potential for magnetic memory device applications. A nanofabrication process combining e-beam lithography and ion implantation was used to pattern LSMO thin films. With the help of state-of-the-art x-ray photoemission electron microscopy, the magnetic domain patterns in various nanowire structures were directly imaged and magnetic field-assisted domain wall injection and propagation processes were monitored. Detailed domain wall structures were identified and the range of magnetic fields needed to move the domain walls were determined. It was found that the domain wall structures in LSMO nanostructures differed from the ones found in permalloy (Ni81Fe19) and were dependent on the crystallographic orientation of the nanowires. Furthermore, electrical transport studies on LSMO nanowires were performed. Pd metal was identified as the ideal contact metal that showed Ohmic behavior and low contact resistance. Resistance measurements as a function of temperature and magnetic field indicated that the LSMO nanowires preserved the electrical properties of the LSMO thin film. These results provide insight on the effect of nanostructuring on the magnetic and electrical properties of complex oxide nanowires, and illustrate the possibility of their application in magnetic memory devices.

We study current-induced domain wall dynamics in a thin ferromagnetic nanowire. We derive the effective equations of domain wall motion, which depend on the wire geometry and material parameters. We describe the procedure to determine these parameters by all-electric measurements of the time-dependent voltage induced by the domain wall motion. We provide an analytical expression for the time variation of this voltage. Furthermore, we show that the measurement of the proposed effects is within reach with current experimental techniques. We also show that a certain resonant time-dependent current moving a domain wall can significantly reduce the Joule heating in the wire, and thus it can lead to a novel proposal for the most energy efficient memory devices. We discuss how Gilbert damping, non-adiabatic spin transfer torque, and the presence of Dzyaloshinskii-Moriya interaction can effect this power optimization. Furthermore, we propose a new nanodot magnetic device. We derive a specific time-dependent current that is needed to switch the magnetization of the nanodot the most efficiently.