Micromagnetic simulation of magnetic nanowires with different configurations for bio applications

Abstract

Nanostructuring materials has long been an attractive topic. The unique magnetic properties of magnetic nanowires (MNW), such as high aspect ratios, tunable magnetic anisotropy, and controllable domain structures, make them intriguing candidates for diverse technological applications such as bio heating, bio labeling and high frequency identification. This dissertation focuses on the simulation and characterization of MNW with different configurations to study the dynamic behavior of MNW during the magnetic reversal process via finite difference micromagnetic simulation tools. First, previous simulation and results were gathered to show different types of simulation that can be performed for MNW and the intriguing results for the application of MNW in bio-heating, biolabeling and high frequency identification applications. (Chapter 2). Then the micromagnetic theory (especially for MNW related cases) supporting the study of this dissertation is introduced and key parameters are defined and explained. (Chapter 3). Next, to mimic the experimentally fabricated MNW via electrodeposition process, arrays of MNW were simulated. The key magnetic properties (e.g.: coercivity) were obtained as a function of the increasing size of MNW array. To further study the interaction between neighboring MNW in array, a novel parameter EI (effective interaction) was defined and showed a good prediction of switching order for MNW in arrays using the layout of the MNW array directly converted from SEM images of MNW arrays with different sizes (Chapter 4). To further study the micromagnetic dynamics of MNW during the magnetic reversal process, single MNW with different sizes and material were simulated. The formation and propagation of different types of domain wall (DW) depending on the size and shapes of MNW were illustrated (Chapter 5) and using the simulation results, predictions of heating ability were made for MNW when used in bio-heating process. It was estimated that a theoretical optimal heating ability of 2730 W/g can be provided by isolated Co MNW with 50 nm diameters and 3 μm lengths using typical AMF heaters alternating magnetic fields (AMF) of 72 kA/m and 50kHz and a more generalized chart is provided for design guidance when using MNW as bio-heater under different situations (Chapter 6). Ferromagnetic resonance (FMR) of MNW was simulated using a ring-down method and the results showed that Co MNW can generate preferred signal that is within commercial 5G band. (Chapter 7). Finally, the FORC method was introduced and performed for MNW. A special phenomenon was observed for MNW with large saturation magnetization and small diameter which were then explained with thermal fluctuation and relaxation time theory. (Chapter 8) MNW have garnered significant attention in a variety of applications, particularly in the biomedical fields. This study provides an understanding of the intricate magnetization dynamics of MNW and how they can affect the magnetic properties and thus how MNW can be used in different applications.