Graduation Year

2009

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Physics

Major Professor

Hariharan Srikanth, Ph.D.

Co-Major Professor

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

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.

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