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Dissertation angenommen durch: Universität Duisburg-Essen, Standort Duisburg,
Fakultät für Naturwissenschaften, Institut für Physik, 2003-05-12
BetreuerIn: Prof. Dr. Wolfgang Kleemann , Universität Duisburg-Essen,
Standort Duisburg, Fakultät für Naturwissenschaften, Institut für Physik
GutachterIn: Prof. Dr. Wolfgang Kleemann , Universität Duisburg-Essen,
Standort Duisburg, Fakultät für Naturwissenschaften, Institut für Physik
GutachterIn: Prof. Dr. Michael Farle , Universität Duisburg-Essen,
Standort Duisburg, Fakultät für Naturwissenschaften, Institut für Physik
Schlüsselwörter in Englisch: magnetic nanoparticles, superspin
glass, thin films, dipolar interactions |
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Abstrakt in Englisch
Nanometer
scale magnetic materials have gained widespread interest both technologically
and scientifically because of the novel effects arising in connection with
the reduction of their spatial extension. New experimental techniques have
made it possible to prepare and investigate magnetic systems on a nanometer
scale. This leads to a growing theoretical interest to understand the properties
of nanoscale magnetic systems. Especially, the dynamic behavior of an assembly
of magnetic nanoparticles is a subject of considerable current investigation.
The aims of this experimental work can be divided into two parts. First,
we investigate the magnetic properties of an ensemble of interacting nanoparticles
embedded in an insulating matrix. The system is prepared as a discontinuous-metal-insulator
multilayer [Co80Fe20(tn)/Al2O3(3nm)]10, where tn corresponds to the nominal
thickness of CoFe layer. The CoFe forms well-separated and quasi-spherical
nanoparticles in the Al2O3 matrix. The magnetic properties are investigated
by means of ac-susceptibility, dc-magnetization and relaxation experiments.
Dynamic and static criticality studies evidence spin glasslike cooperative
freezing of magnetic moments m = 1000mB
('superspins') at low temperatures in the nanoparticle system with tn =
0.9 nm. Non-equilibrium collective phenomena such as aging, memory, and
rejuvenation are observed in the superspin glass phase. On the other hand,
nanoparticle sytems with tn = 1.3 and 1.4 nm reveal pertinent features
of a superferromagnetic state. This is evidenced by field dependence of
ac-susceptibility in the tn = 1.3 nm system and by a Cole-Cole analysis
of the ac-scusceptibilty in the tn = 1.4 nm system. Second, we investigate
the properties of a granular system consisting of ferromagnetic nanometric
Fe particles in an antiferromagnetic FeCl2 matrix. In this system the nanoparticle-matrix
interaction is effective. Apart from direct exchange coupling at the interface
between the Fe granules and the Fe2 -ions of FeCl2 matrix, the dipolar
stray-fields of the granules play a key role in the magnetic properties
of the system. Giant metamagntic moments containing Fe granules as nucleation
cores are observed under the combined effects of these two mechanisms. |
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