PRECISE CONTROL OF 360-DEGREE MAGNETIC DOMAIN-WALL FORMATION AND THEIR PROPERTIES IN GEOMETRICALLY CONFINED NANOWIRES

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