engAdiga, ShilpaAngst, ManuelKlemradt, UweCrystal growth and scattering studies on two ferritesinfo:eu-repo/classification/ddc/530charge order ; colossal magnetoresistance ; antiferromagnetic ; ferromagnetic ; field-cooled ; zero field-cooled ; exchange bias ; spin glass ; neutron powder diffraction ; x-ray diffractionIn 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.Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / Key Technologies 135, iv, 150 Seiten : Illustrationen, Diagramme (2016). doi:10.18154/RWTH-2017-00547 = Dissertation, RWTH Aachen University, 2015info:eu-repo/semantics/doctoralThesisinfo:eu-repo/semantics/publishedVersionForschungszentrum Jülich GmbH, Zentralbibliothek2016info:eu-repo/semantics/openAccessDEhttps://publications.rwth-aachen.de/record/681987https://publications.rwth-aachen.de/search?p=id:%22RWTH-2017-00547%22StudentsStudent Financial Aid ProvidersTeachersResearchersinfo:eu-repo/semantics/altIdentifier/doi/10.18154/RWTH-2017-00547info:eu-repo/semantics/altIdentifier/issn/1866-1807info:eu-repo/semantics/altIdentifier/urn/urn:nbn:de:hbz:82-rwth-2017-005477info:eu-repo/semantics/altIdentifier/isbn/978-3-95806-183-5