Controlling magnetic domain wall pinning using atomic force microscope tip-based nanomachining

In recent decades, atomic force microscope (AFM) tip-based nanomachining has gained the increased attention of researchers as a technique capable of creating nanoscale features on the surfaces of materials. In principle, it represents a potentially low-cost alternative to other more expensive methods for nanoscale fabrication, particularly at the prototyping stage of device development. However, currently the numbers of practical applications that take advantage of this technique are limited. At the same time, magnetic domains at nanoscales have also shown to be of interest in recent decades with the potential for use in various applications. An example of which includes racetrack memory, a possible future non-volatile data storage system that would have higher densities and faster read/write speeds [1]. A key issue in successfully developing these systems relates to the ability to accurately control the motion of the magnetic domain boundaries known as domain walls (DWs). In this context, this thesis presents a study attempting to combine these issues. AFM tip-based nanomachining is used to create vertical nanotrenches cut along the top surface across the width of magnetic nanowires with the intention of pinning DWs at them. A computational study was conducted focusing on the effects of the shape and geometry of vertical nanotrenches on their pinning strength. This was inspired by the fact that nanotrenches created by AFM tip-based nanomaching have a tendency to be triangular in shape due to the pyramidal tips. Simulations were carried out using the Object-Oriented MicroMagnetic Framework (OOMMF). It was found that triangular nanotrenches pin both transverse and vortex DWs more weakly than their square counterparts. The depth of both shape nanotrenches appear to have approximately linear relationships with the pinning strength. It was found that whilst square nanotrenches retain their pinning strength as their length is increased, triangular nanotrenches reduce in pinning strength beyond a significant length relative to its depth. This was shown to be related to the angle between the triangular nanotrench wall and the x-y plane. It is found that the depinning fields drastically reduce when the angle is reduced below 10 ° corresponding to relatively shallow and long nanotrenches. Experiments are carried out to test the viability of AFM tip-based nanomachining for pinning DWs. Anisotropic Magnetoresistance measurements were used to detect the IV presence of DWs and measure the resulting depinning fields. In addition to this, magnetic full-field transmission soft x-ray microscopy was utilised to directly image DWs pinned at the created sites. It was found to be a viable option as multiple ferromagnetic nanowires are machined and DWs are pinned at the resulting nanotrenches. Further work was conducted to verify the computational results regarding nanotrench depth and pinning strength. Experimental data gathered agrees with the computational simulations and show an approximately linear increase in depinning fields with an increase in nanotrench depth