Graduation Year
2009
Document Type
Thesis
Degree
M.S.
Degree Granting Department
Physics
Major Professor
Hariharan Srikanth, Ph.D.
Committee Member
Dale Johnson, Ph.D.
Committee Member
Martin Mu˜noz, Ph.D.
Committee Member
Garrett Matthews, Ph.D.
Keywords
magnetic colloids, nanoparticles, transverse susceptibility, relaxation phenomena, superparamagnetism
Abstract
Nanoparticle assemblies are of current interest as they are used in a wide variety of industrial and biomedical applications. This work presents two studies aimed at understanding the magnetization dynamics and interparticle interactions in nanoparticle assemblies and various types of ferrofluids. First, we studied the influence of varying strengths of dipolar interaction on the static and dynamic magnetic properties of surfactant-coated monodispersed manganese-zinc ferrite nanoparticles using reversible transverse susceptibility. We tracked the evolution of the anisotropy peaks with varying magnetic field, temperature, and interaction strength. The anisotropy peaks of weakly interacting particles appears as non-symmetric peaks and at lower fields in a unipolar transverse susceptibility scan. On the other hand, a strongly interacting particle system exhibits symmetric anisotropy peaks situated at higher field values. In the second study, we successfully synthesized stable ferrofluids out of high quality Fe3O4 and CoFe2O4 nanoparticles. Such ferrofluids are excellent systems for the investigation of physics of relaxation phenomena in magnetic nanoparticles. Motivated by the need to understand their peculiar magnetic response, a comparative study on Fe3O4- and CoFe2O4-based ferrofluids was performed. We investigated cases in which particle blocking and carrier fluid freezing temperatures were close and far apart from each other. Our experimental results reveal the true origin of the glass-like relaxation peaks that have been viii widely observed in ferrofluids by many groups but remained largely unexplained. Contrary to the speculation of previous literature, we argue that the formation of the magnetic anomaly is due not only to the particle blocking but also to its correlation with the the carrier fluid freezing effects. It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher scaling law clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time.
Scholar Commons Citation
Morales, Marienette B., "Magnetization Dynamics and Interparticle Interactions in Ferrofluids and Nanostructures" (2009). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/3913