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@PHDTHESIS{Adiga:681987,
author = {Adiga, Shilpa},
othercontributors = {Angst, Manuel and Klemradt, Uwe},
title = {{C}rystal growth and scattering studies on two ferrites},
volume = {135},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Jülich},
publisher = {Forschungszentrum Jülich GmbH, Zentralbibliothek},
reportid = {RWTH-2017-00547},
isbn = {978-3-95806-183-5},
series = {Schriften des Forschungszentrums Jülich. Reihe
Schlüsseltechnologien / Key Technologies},
pages = {iv, 150 Seiten : Illustrationen, Diagramme},
year = {2016},
note = {Druckausgabe: 2016. - Onlineausgabe: 2016. - Auch
veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2017; Dissertation, RWTH Aachen University, 2015},
abstract = {In this thesis I will describe investigations of two
ferrite systems:(1) A detailed ferroelectric study of
magnetite (Fe3O4) Multiferroics, consisting of both
ferroelectric and ferromagnetic phases, have attracted
scientific and technological interest due possible
magnetoelectric coupling between the phases. Such materials
are very rare though, as conventional ferroelectricity
requires an empty d-shell, preventing the presence of
magnetism. Among unconventional mechanisms leading to
ferroelectricity, multiferroicity due to charge ordering
(CO) is a strong candidate for practical applications.
However, proven examples are very rare as of yet. The 120~K
Verwey transition (TV ) in magnetite, reported in 1939, is
the classical example for charge ordering. Despite
controversies regarding the existence of CO, magnetite has
been proposed as one of the CO-based multiferroics. Although
early experiments already indicated for example a
magnetoelectric effect, those studies were mainly focused on
complex low temperature structure rather than possible
multiferroicity. In order to study the ferroelectric
properties of magnetite by dielectric spectroscopy, a new
dielectric measurement set-up was built at the institute.
After an introduction and the description of experimental
techniques, this thesis begins with the presentation of our
newly built dielectric set-up and of the performed test
experiments to standardize measurements of the dielectric
constant. The Verwey transition is very sensitive to oxygen
stoichiometry. The oxygen stoichiometry was tuned by
appropriate gas mixtures of CO2 and CO or $Ar(H2)4\%.$ I
first investigated appropriate ratios of $CO2/Ar(H2)4\%$ at
high temperature on polycrystalline samples and confirmed
the phase purity by x-ray diffraction. Verwey transition was
characterized primarily by thermo-remanent magnetization and
specific heat. The results obtained from the basic
macroscopic analysis were used for the growth of high
quality crystals by optical floating zone method. Proposed
low temperature relaxor ferroelectric property of magnetite
was studied by neutron and high energy X-ray diffuse
scattering experiments. The observed weak diffuse scattering
by neutron diffraction, which was absent in high energy
X-ray studies, indicated that it is magnetic in origin. For
the first time, a time resolved X-ray diffraction technique
has been implemented to test the switchabilty of the polar
structure by application of an electric field in magnetite.
The observed change in the intensity of the Bragg reflection
to its Friedel mate (reflection related by inversion
symmetry) constitutes to the first microscopic proof of
ferroic behavior of classical magnetite. (2) Study of
various physical properties of oxygen deficient strontium
ferrite (SrFeO3-delta).Colossal magnetoresistance effect,
i.e., the huge change in the electrical resistance by the
application of magnetic field is a key to the next
generation of magnetic memory devices. The oxygen deficient
strontium ferrite (SrFeO{3-delta}, delta=0--0.5) system
exhibits various types of magnetoresistance effect depending
on the presence of different magnetic phases. Oxygen
deficient SrFeO3-delta crystals with delta = 0.27 and delta
= 0.35 (as determined by infrared absorption) were grown by
optical floating zone method using different growth
conditions. This oxide system contains a mixture of Fe ions
in tetravalent and trivalent states. Anomalies around ~70 K,
~230 K and ~130 K observed by magnetization measurements
indicated the presence of a tetragonal, orthorhombic and
cubic phase respectively. Presence of these phases were
confirmed by further microscopic measurements by neuron
scattering. Two new magnetic phases at the propagation
vector k = (0.25 0.25 0.25)c and (0.25 0 0.15)c were
observed by our detailed neutron diffraction experiments
with polarization analysis. Results of xyz- polarization
indicated that majority of the spins lies in the ab-plane.
For the first time CO superstructure reflection was observed
at (2 2 3/2)t position, which indicates the doubling of the
c-axis. The observation of diffuse scattering around the
magnetic Bragg reflection indicated the presence of short
range spin correlations in the system. Observed frequency
dependent ac-susceptibility and the presence of memory
effect from magnetization indicated the presence of glassy
state below ~60 K in the system.},
cin = {138730 / 130000},
ddc = {530},
cid = {$I:(DE-82)138730_20140620$ / $I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-rwth-2017-005477},
doi = {10.18154/RWTH-2017-00547},
url = {https://publications.rwth-aachen.de/record/681987},
}
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