Ph.D. Physicshttp://hdl.handle.net/10464/108082024-03-18T19:13:19Z2024-03-18T19:13:19ZFabrication and Characterization of CoFe2O4-BiFeO3 Core Shell Nanocomposite and SrFe(12−2x)CoxRuxO19 HexaferritesMonfared, Sarahttp://hdl.handle.net/10464/163842022-10-06T15:13:45ZFabrication and Characterization of CoFe2O4-BiFeO3 Core Shell Nanocomposite and SrFe(12−2x)CoxRuxO19 Hexaferrites
Monfared, Sara
This thesis consists of three parts. The first section is about CoFe2O4-BiFeO3 core-shell
multiferroic nanocomposite synthesized via a two-step wet-chemical process. The presence
of both spinel and perovskite constituents as well as the core-shell structure of nanocomposite
have been identified using x-ray diffraction and high-resolution transmission electron
microscopy, respectively. Low temperature (5 K) magnetic measurement exhibited a significant
exchange bias in the core-shell nanocomposite which confirms promising connectivity
of the constituents in the interface. An enhancement in magneto-dielectric of the core-shell
nanocomposite over the CoFe2O4-BiFeO3 (0-3)-type nanocomposites has been seen. Further
study on the magneto-loss demonstrated the contribution of the magneto-electric and
Maxwell-Wagner effects in magneto-dielectric of the core-shell nanocomposite.
The second part of this thesis has focused on the bulk of SrFe(12−2x)CoxRuxO19 (x =
0, 0.1, 0.2, 0.3, 0.4) which have been synthesized through the solid-state reaction process.
DC magnetic measurement at room temperature exhibited a significant reduction in the
coercive field of samples as the concentration of dopants increased. Contrary, an increase in
the saturation magnetization was observed in the doped samples. Furthermore, a transition
from conical to uniaxial anisotropy has been seen for the doped samples above x=0.3.
Thin films of SrFe(12−2x)CoxRuxO19 / (111) SrTiO3 fabricated by a pulsed laser deposition
technique, have been studied in the last part of this work. High-resolution parallel
beam x-ray diffraction results have shown single orientations for all thin films except one.
The thickness of thin films has been determined by the x-ray reflectivity measurement. A
certain level of mosaicity has been detected in the prepared films using the rocking curve
measurement. The strain and the epitaxial growth of thin films have been investigated
utilizing the reciprocal space map technique. Finally, in-plane pole figure measurements
revealed high textured films with three-fold and six-fold hexagonal symmetry. The formation
of a perpendicular anisotropy in Co-Ru doped Sr-M thin films has been detected
from room temperature magnetic measurement. The distribution of Co2+ and Ru4+ ions
in different interstitial spaces, the thickness of films, and change in the magneto-anisotropy
with a concentration of dopants have a significant effect on the magnetic characteristic of
thin films.
Structural, Magnetic, Dielectric, and Optical Properties of DyCrO3 and GeNi2O4 MaterialsIndovski, Biljanahttp://hdl.handle.net/10464/156212022-10-06T15:14:08ZStructural, Magnetic, Dielectric, and Optical Properties of DyCrO3 and GeNi2O4 Materials
Indovski, Biljana
GeNi2O4 is a cubic spinel with two antiferromagnetic transitions at low temperatures,
while DyCrO3 has an orthorhombic perovskite structure with an antiferromagnetic transition
at a higher temperature. Thin films of these compounds are widely researched for
their applications in spintronics. The investigation of the structural, magnetic, dielectric,
and optical properties of deposited thin films of these two materials can contribute to a
better understanding of their physical characteristics which may lead to possible applications.
In this research, DyCrO3 and GeNi2-xMgxO4 (x=0, 0.03) epitaxial thin films
were deposited on SrTiO3 substrates using the pulsed laser deposition technique.
As the c-lattice parameter of the orthorhombic DyCrO3 and a-lattice parameter of the cubic
GeNi2O4 are almost having the same value, this is giving a rise to a small strain in their
epitaxial thin films. Therefore, composite thin films of the DyCrO3 and GeNi2-xMgxO4
(x=0, 0.03) materials were also deposited. The structural properties of these thin films
were examined using different X-ray techniques such as an X-ray diffraction, X-ray reflectivity,
reciprocal space mapping, and in-plane pole figures. The results were analyzed to
determine the preferred orientation, the thickness of the thin films, the presence of strain
and defects, and the degree of epitaxy. The reciprocal space mapping results reveal a
dependence between the thickness of the thin films and the presence of defects in the
epitaxial films. The in-plane pole figure results show the presence of domains with different
orientation in several thin films, although the results from the high-resolution X-ray
diffraction indicate a single orientation in those films. The temperature dependence of
magnetization in epitaxial thin films was measured in order to examine the magnetic
anisotropy for the magnetic field normal and parallel to the surface of the films. These
results indicate the presence of magnetic anisotropy in several thin films.
GeNi2O4 is an interesting antiferromagnet as it has two closely spaced antiferromagnetic
transitions that are the result of a spin reorientation in two types of {111} planes.
Therefore, to better understand the ordering of the spins with respect to the {111} planes,
additional measurements and analysis of the results were done on GeNi2O4 single crystals.
The temperature dependence of magnetization was measured for the magnetic field
applied parallel and perpendicular to the {111} planes. In addition, the temperature
dependence of Raman spectra of a GeNi2O4 single crystal was measured and analyzed.
These single crystal results of GeNi2O4 provide a better understanding of spin-phonon
coupling and spin ordering and reorientation in this spinel compound.
Structural, Magnetic and Vibrational Studies of Entropy Stabilized OxidesAfsharvosoughi, Taherehhttp://hdl.handle.net/10464/151282022-10-06T15:13:52ZStructural, Magnetic and Vibrational Studies of Entropy Stabilized Oxides
Afsharvosoughi, Tahereh
The structural, magnetic and vibrational properties of high entropy oxide (HEO) and medium entropy oxides (MEOs) made from mixtures of five and four metal oxides were studied in this work. The HEO and MEOs obtained from solid state reactions in quinary
and quaternary equimolar mixtures of oxides including ZnO, CuO, CoO, MgO and NiO which are called 5-HEO, 4-MEO(-Cu), 4-MEO(-Mg), 4-MEO(-Co), 4-MEO(-Ni) and 4-MEO(-Zn) in this thesis. The samples were sintered at high temperatures followed by air quenching. The x-ray diffraction measurements results show the formation of single rocksalt phase in all of them except 4MEO(-Ni).
The magnetization measurements in zero field cooled (ZFC) and field cooled (FC) regimes under H=5000 Oe have provided strong evidence for long range magnetic ordering in 5-HEO and 4-MEOs. Two samples including 4-MEO(-Co) and 4-MEO(-Ni) exhibit
high frustration factor which is a major component for spin glass phase. The AC susceptibility measurements for 4-MEO(-Co) and 4-MEO(-Ni) with the highest frustration factors have carried out in order to investigate the presence of spin glass. The segregation
of Cu from the rocksalt phase in 5-HEO by resintering at intermediate temperature (700oC) was confirmed by x-ray diffraction measurements, SEM-EDS elemental mapping and magnetization measurements.
Infrared refectance spectroscopy measurements (100-15000 cm-1) for 5-HEO and 4-MEOs with single rocksalt structures have revealed one strong mode and a weaker mode at lower frequency in the far infrared spectra. The Lorentz model and Kramers-Kronig (KK) analysis were implemented for the samples and the results were used in effective charge calculations which reveal that 4-MEO(-Cu) has the greatest ionic character whereas 4-MEO(-Mg) showed the least ionic character. The Raman spectroscopy which complements the infrared spectroscopy was carried out for 5-HEO and 4-MEOs. The results have indicated first and second order Raman modes in addition to a two-magnon mode at around 1600 cm-1 which was confirmed by studying the temperature dependence. While 4-MEO(-Co) also shows a peak near 1600cm-1 it is likely of different origin. In 4-MEO(-Cu) the two magnon scattering peak was not observed and it rather causes a high background which increases with decreasing temperature.
Infrared Spectroscopy of Ge:Mn Thick Films Prepared by Ion Implantation and Post-annealingObied, Laila Hassanhttp://hdl.handle.net/10464/137932022-10-06T15:13:47ZInfrared Spectroscopy of Ge:Mn Thick Films Prepared by Ion Implantation and Post-annealing
Obied, Laila Hassan
An infrared transmission study of Ge:Mn systems is presented in this work. Various
Ge:Mn samples have been prepared by both single (Mn2+ ion energy 4.76
MeV, dose 2x10^16 /cm^2) or multiple (dose 1x10^16 /cm^2) into high resistivity
(100) Ge substrates. Both conventional and flash lamp annealing procedures
have been used to prepare a set of samples that were characterized by X-ray
Diffraction (XRD), Secondary Ion Mass Spectrometry (SIMS) and magnetometry
as well as infrared transmission spectroscopy (60-8000 cm^-1). After implantation,
the Ge:Mn layers are about 3 mm thick and amorphous as revealed by XRD.
Samples conventionally annealed at temperatures below 250C maintained their
amorphous structure. Samples annealed at temperature above 330C recrystallized
into a polycrystalline structure. SIMS measurements show that the diffusion
of Mn is greatly affected by the starting distribution and that diffusion is
much higher in samples prepared with the lower total Mn dose. The presence
of multiple magnetic phases in all the annealed samples was indicated by field and
temperature dependent measurements of the total moment. Our diverse
preparations suggest that the formation of secondary phases in Ge:Mn system in
unavoidable. Magnetic measurements suggest that the maximum volume fraction
of metallic Mn5Ge3 inclusions is of the order of 1x10^-3, much lower than
the percolation threshold of metallic inclusions in an insulating matrix. Infrared
transmission measurements on the same samples show the presence of a low
frequency Drude like absorption with high hole concentration (p~10^ 18 /cm^3)
which is greater than the critical density for metal-insulator (MI) transition in
Ge. This observation suggests Mn has crystallized in substitutional sites to a far
greater degree than was achieved with traditional equilibrium crystal growth.
The MI transition has been investigated in the Ge:Mn system by studying the
temperature dependent transmission between 4K and 300K and comparing to
similar measurements on a sample of Ga-doped Ge with p = 1.5x10^16 /cm^3
which is on the insulating side of the MI transition. Finally, absorption was
observed that may possibly be attributed to Mn5Ge3 inclusions in the Ge:Mn
matrix.