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
Fe
3O4 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 Fe
3O4
- and
CoFe
2O4
-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
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.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-5109 |
| Date | 09 June 2009 |
| Creators | Morales, Marienette B. |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
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