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Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Condensed Matter Physics
Nano-scale magnetic patterned structures and thin films have attracted attention over the past few years due to their unique magnetization distribution and promising application in nonvolatile data storage, magnetic field sensors and spintronic devices. Magnetization states and their switching processes are fundamental investigation subjects of such magnetic systems. This dissertation discusses experimental work and theoretical analysis on investigating the magnetization of these nano-scale magnetic structures and how we can characterize and manipulate the magnetization in order to realize industrial application. The efforts done in this dissertation can be grouped into three topics:
First, we focus on whether we can reverse the vortex circulation of the ferromagnetic nanodisk without breaking its symmetry so as to control and manipulate the chirality of the nanodisk. The vortex state of ferromagnetic nanodisk is characterized with and without in-plane magnetic field. The vortex circulation switching is achieved by applying a local circular magnetic field which is created by passing current through an AFM tip that is in contact with the nanodisk. The experimental results indicate the success of the reversal when a dielectric breakdown happens during the current application and the breakdown voltage is above a threshold value. This implies that the concentration of the current in the center of the disk leads to a stronger magnetic field and lower switching current. The analysis is supported by the results of micromagnetic simulations conducted in this work. Second, a magnetometer based on Magneto-optical Kerr effect is constructed. It is then used to record the hysteresis loop of permalloy and cobalt amorphous thin films. At the same time the affect of skew angles in Magneto-optical Kerr effect is also investigated so that the skew angle that leads to highest Kerr signal can be determined. The setup lays a foundation which enables us to investigate the magnetic properties of various magnetic structures and develop other techniques for related study, so some future work related to interlayer exchange coupling of trilayer structure and Magneto-optical Kerr imaging are proposed. Last, the thermal diffusivity measurement of Co/Cu multilayer by laser flash technique is conducted with and without in-plane magnetic field. The experimental result shows that the application of magnetic field does not lead to a significant change in the thermal diffusivity of this structure. This phenomenon is interesting as it is different from the behavior of the electrical conductivity of this structure when magnetic field is applied (giant magnetoresistance effect). Some exploratory discussion is presented in the end.
Ju, Wenming, "CHARACTERIZATION AND MANIPULATION OF THE MAGNETIZATION OF NANO-SCALE MAGNETIC STRUCTURES" (2019). Doctoral Dissertations. 1501.
Available for download on Saturday, February 01, 2020