Improvements on Magnetic Nanoparticle Characterization Theories

• Main Author: Chalifour, Artek Russell
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2022
• Subjects: Applied physics; Nanoscience
• ISBN: 9798438746324

The topic of magnetic nanoparticles (MNPs) has received a lot of interest lately due to their proposed use in a diverse assortment of applications ranging from biomedical treatments to material engineering. To better improve their integration into these technologies it is necessary to determine the fundamental magnetic properties of nanoparticles.One such property is the magnetic relaxation time, which is the characteristic time it takes for a system of magnetic nanoparticles to settle into a thermal equilibrium for the magnetization. An analytic expression for the magnetic relaxation time has existed for some decades, but is only valid for magneto crystalline anisotropy easy axes aligned with the applied magnetic field. Here, a calculation is presented for the magnetic relaxation time for an ensemble of nanoparticles with randomly oriented easy axes. The applied field strength is explicit in the developed expression, allowing a simple way to predict relaxation times in applications such as magnetic hyperthermia where nanoparticles are placed in oscillating external fields.Another property of magnetic nanoparticle systems is the net magnetization at high temperatures. When the nanoparticles are superparamagnetic and their easy axes are randomly oriented, then the net magnetization scales with the applied field strength and the size of the magnetic moment. However, when the easy axes have some ordering with respect to the field, then magneto crystalline anisotropy also affects the net magnetization. This influence is missing in the commonly-used Langevin function to predict the net magnetization. Here, a calculation is presented that includes the effect of anisotropy and modifies the Langevin function.Finally, this dissertation discusses theoretical methods of fitting zero field cooled (ZFC) magnetization versus temperature data. This data is important as the ZFC peak can be correlated with the magnetic aniostropy energy, which is necessary to know for the design of materials with specific applications. The usual method to do this is only valid for vanishingly small applied field strengths. We relax that assumption and test our calculation method on experimental data provided by colleagues at the University of South Carolina. The results show it is possible to fit ZFC data for various applied field values. Only three fit parameters are needed. It is hoped that this provides a useful tool to those making ZFC measurements, as the fitting methods are not more complicated than those currently employed in the literature.