Authors: Shilpa Adiga
Source: DESY (CERN, DESY, Fermilab, IHEP, and SLAC) (article, 2019-12-02)
Field: Materials Science — Materials Chemistry

Crystal growth and scattering studies on two ferrites 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 attractedscientic and technological interest due possible magnetoelectric coupling between thephases. Such materials are very rare though, as conventional ferroelectricity requires anempty d-shell, preventing the presence of magnetism. Among unconventional mechanismsleading to ferroelectricity, multiferroicity due to charge ordering (CO) is a strongcandidate 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 examplefor charge ordering. Despite controversies regarding the existence of CO, magnetite hasbeen proposed as one of the CO-based multiferroics. Although early experiments alreadyindicated for example a magnetoelectric eect, those studies were mainly focused oncomplex low temperature structure rather than possible multiferroicity.In order to study the ferroelectric properties of magnetite by dielectric spectroscopy, anew dielectric measurement set-up was built at the institute. After an introduction andthe description of experimental techniques, this thesis begins with the presentation ofour newly built dielectric set-up and of the performed test experiments to standardizemeasurements of the dielectric constant.The Verwey transition is very sensitive to oxygen stoichiometry. The oxygen stoichiometrywas tuned by appropriate gas mixtures of CO2 and CO or Ar(H2)4%. I rst investigatedappropriate ratios of CO2/Ar(H2)4% at high temperature on polycrystalline samples andconrmed the phase purity by x-ray diraction. Verwey transition was characterizedprimarily by thermo-remanent magnetization and specic heat. The results obtainedfrom the basic macroscopic analysis were used for the growth of high quality crystals byoptical oating zone method. Proposed low temperature relaxor ferroelectric property ofmagnetite was studied by neutron and high energy X-ray diuse scattering experiments.The observed weak diuse scattering by neutron diraction, which was absent in highenergy X-ray studies, indicated that it is magnetic in origin.For the rst time, a time resolved X-ray diraction technique has been implemented totest the switchabilty of the polar structure by application of an electric eld in magnetite.The observed change in the intensity of the Bragg reection to its Friedel mate (reectionrelated by inversion symmetry) constitutes to the rst microscopic proof of ferroicbehavior of classical magnetite.(2) Study of various physical properties of oxygen decient strontiumferrite (SrFeO3􀀀).Colossal magnetoresistance eect, i.e., the huge change in the electrical resistance bythe application of magnetic eld is a key to the next generation of magnetic memorydevices. The oxygen decient strontium ferrite (SrFeO3􀀀, =0{0.5) system exhibitsvarious types of magnetoresistance eect depending on the presence of dierent magneticphases. Oxygen decient SrFeO3􀀀 crystals with = 0.27 and = 0.35 (as determinedby infrared absorption) were grown by optical oating zone method using dierentgrowth conditions. This oxide system contains a mixture of Fe ions in tetravalentand trivalent states. Anomalies around 70 K, 230 K and 130 K observed bymagnetization measurements indicated the presence of a tetragonal, orthorhombic andcubic phase respectively. Presence of these phases were conrmed by further microscopicmeasurements by neuron scattering. Two new magnetic phases at the propagationvector k = (0:25 0:25 0:25)c and (0:25 0 0:15)c were observed by our detailed neutrondiraction experiments with polarization analysis. Results of xyz- polarization indicatedthat majority of the spins lies in the ab-plane. For the rst time CO superstructurereection was observed at (2 2 32 )t position, which indicates the doubling of the c-axis.The observation of diuse scattering around the magnetic Bragg reection indicated thepresence of short range spin correlations in the system. Observed frequency dependentac-susceptibility and the presence of memory eect from magnetization indicated thepresence of glassy state below 60 K in the system. Crystal (programming language) ; Materials science ; Crystallography ; Chemistry ; Computer science Magnetic Properties and Synthesis of Ferrites