Browsing by Author "Mankey, Gary J."
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Item Ambient-pressure CVD of graphene on low-index Ni surfaces using methane: A combined experimental and first-principles study(American Physical Society, 2018-07-23) Mafra, Daniela L.; Olmos-Asar, Jimena A.; Negreiros, Fabio R.; Reina, Alfonso; Kim, Ki Kang; Dresselhaus, Mildred S.; Kong, Jing; Mankey, Gary J.; Araujo, Paulo T.; Massachusetts Institute of Technology (MIT); Universidade Federal do ABC (UFABC); Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET); National University of Cordoba; Dongguk University; University of Alabama Tuscaloosa; Universidade Federal do ParaThe growth of large area single-layer graphene (1-LG) is studied using ambient pressure chemical vapor deposition on single-crystal Ni(111), Ni(110), and Ni(100). By varying both the furnace temperature in the range of 800-1100 degrees C and the gas flow through the growth chamber, uniform, high-quality 1-LG is obtained for Ni(111) and Ni(110) single crystals and for Ni(100) thin films. Surprisingly, only multilayer graphene growth could be obtained for single-crystal Ni(100). The experimental results are analyzed to determine the optimum combination of temperature and gas flow. Characterization with optical microscopy, Raman spectroscopy, and optical transmission support our findings. Density-functional theory calculations are performed to determine the energy barriers for diffusion, segregation, and adsorption, and model the kinetic pathways for formation of different carbon structures on the low-index surfaces of Ni.Item Development of a highly sensitive magnetometer with a wide temperature range for the study of heterostructured transition metal-gadolinium alloys(University of Alabama Libraries, 2016) Chahil, Narpinder Singh; Mankey, Gary J.; University of Alabama TuscaloosaThe design, construction, and testing of a highly sensitive magnetometer with a wide temperature range for the study of magnetic phase transitions and the effect of thermal processing on heterostructured magnetic films is reported. The magnetometer is based on the magneto-optic Kerr effect which produces a change in polarization of light reflected from a magnetic surface. A laser with a wavelength of 632 nm is used, and the magnetometer measures the magnetic orientation of layers within the skin depth of the incident radiation. For the systems studied in this work, the probed magnetization is assumed to follow the magnetic behavior of the topmost magnetic layer. The apparatus has a temperature range from 65 K to 760 K, enabling the study of a wide range of heterostructured thin films. Alloys of Gd with transition metals have a strong temperature dependence since pure Gd has a ferromagnetic Curie temperature around 300 K. Single-layers of Co, Gd and Co-Gd alloy were used as prototype magnetic system for commissioning the apparatus. The phase transition behaviors as determined by the magnetometer are reported. For testing the thermal processing, bilayers incorporating antiferromagnetic NiMn and NiMn-Gd with the above ferromagnetic layers were studied. These bilayers exhibited magnetic behaviors that were dependent on the thermal processing history, as annealing at 573K and cooling in a static magnetic field produced a change in the exchange bias, coercivity, and shape of the magnetic hysteresis loops.Item Effect of nickel content on the crystallization behavior in nanocrystallin (Co_(1-x)Ni_x)_88Zr_7B_4Cu1 soft magnetic alloys(University of Alabama Libraries, 2012) Hornbuckle, Billy Chad; Thompson, Gregory B.; University of Alabama TuscaloosaA series of (Co_(1-x)Ni_x)_88Zr_7B_4Cu1 soft magnetic alloys, where X was varied from 0 to 1, were fabricated by a melt spinning process into thin ribbons of the material. This process was followed by an isothermal anneal to produce a nanocomposite alloy, i.e. nanocrystalline grains in a residual amorphous matrix. The alloy series was designed to investigate crystallization kinetics and limits to the compositional regime where a nanocomposite could be formed. The primary and secondary crystallization temperatures of each alloy were determined using Differential Scanning Calorimetry (DSC) from which the crystallization activation energies were calculated using the Kissinger Method. When X exceeded 0.75, the as-spun ribbons exhibited partial crystallization, resulting in reduced exothermic crystallization peaks. For lower Ni contents, the ribbons were amorphous in the as-spun state. The activation energy for crystallization decreased with increasing Ni content. Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) revealed fine nanocrystallite and boron segregation to the grain boundaries with increasing Ni content. The previously suspected use of Cu clustering, which can act as heterogeneous nucleation sites, showed no clear correlation with observed spatial location of the crystallites. Chemical partitioning between species in the as-spun and primary crystallization heat treatments were correlated to the resulting changes in magnetic properties. As Ni content increased, the saturation magnetization and normalized magnetization for these samples decreased accordingly.Item Exploring the magnetic phases in dysprosium by neutron scattering techniques(University of Alabama Libraries, 2014) Yu, Jian; Mankey, Gary J.; University of Alabama TuscaloosaWith one of the highest intrinsic magnetic moments (10.6 μ_B) among the heavy rare-earth elements, dysprosium (Dy) exhibits a rich magnetic phase diagram, including a few modulated magnetic phases. Aided by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the magnetic modulations propagate coherently over a long range, even with intervening non-magnetic layers. Neutron diffraction experiments were performed to determine the microscopic magnetic origin of the field induced phases in bulk Dy as a function of temperature, covering regions of the well-known ferromagnetic, helical antiferromagnetic, fan phases and several possible new phases suggested by previous studies. A short range ordered (SRO) fan phase was identified as the intermediate state between ferromagnetism and long range ordered (LRO) fan. The temperature range of a coexisting helix/fan phase was also determined. The magnetic phase diagram of Dy was thus refined to include the newly determined magnetic structures and the associated phase boundaries. Based on the period of the magnetic modulation and the average magnetization, the evolution of the spin arrangement upon heating was derived quantitatively for the modulated magnetic phases. To gauge the effect of nanostructuring on the magnetic phases with nonmagnetic Y layers, epitaxial Dy/Y superlattices of various layer thicknesses and repeats were fabricated by magnetron sputtering under carefully controlled conditions. X-ray characterizations confirmed that the crystallographic and interfacial qualities of the superlattices are comparable to those grown by MBE in previous studies. The macroscopic magnetization was characterized by magnetometry, whereas the microscopic magnetic structures were extracted from neutron diffraction and polarized neutron reflectometry (PNR) measurements. The ordering of helical modulation is sensitive to the interfacial roughness of the multilayer as well as the cooling histories. Off-specular PNR was applied the first time to characterize the helical domain structures in Dy/Y multilayers. The lateral correlation length in the helical magnetic structure was in the order of 100 nm.Item Half-metallic CrO_2 thin films for spintronic applications(University of Alabama Libraries, 2011) Pathak, Manjit; LeClair, Patrick R.; University of Alabama TuscaloosaCrO_2 is a well-established half-metallic oxide with near perfect spin polarization - known to have the highest spin polarization among all known materials theoretically as well as experimentally. This means that the conduction electrons in CrO_2 have only one kind of spin i.e. conduction is due only to the majority spin electrons. Because of its high spin polarization, CrO_2 stands as an ideal and one of the most attractive candidates for spin-electronic applications as well as of fundamental interests. The enormous potential of CrO_2 is still untapped since thin film growth modes, interface/surface properties and various factors affecting them are not very well understood or, relatively unknown. Reported works confirm strained growth of (100) CrO_2 films and strain free growth of (110) CrO_2 films on iso-structural TiO_2 substrates investigated using X$ - $ray diffraction. Superconducting quantum interference device (SQUID) and element specific X-ray magnetic circular dichroism (XMCD) techniques were employed to investigate the effect of this substrate-induced strain on the magnetic properties of the films. Magnetic tunnel junctions (MTJ) were fabricated with CrO_2 , Cr_2 O_3 [natural oxide of Cr] as the thin insulating barrier and Co as the other ferromagnetic electrode using photolithography. I-V characteristics of this spin-electronic device are reported. Also, results on the low pressure chemical vapor deposition (CVD) growth of CrO_2 and its comparison with standard growth technique under atmospheric pressure are reported.Item Implantable ferrite antenna for biomedical applications(University of Alabama Libraries, 2016) Fazeli, Maxwell; Hong, Yang-Ki; University of Alabama TuscaloosaWe have developed an implantable microstrip patch antenna with dimensions of 10×10×1.28 mm, operating around the Industrial, Scientific and Medical (ISM) band (2.4 – 2.5 GHz). The antenna is characterized in skin-mimicking gels and compared with simulation results. The experimental measurements are in good agreement with simulations, having a -16 dB reflection coefficient and -18 dBi realized gain at resonance, with a 185 MHz -10 dB bandwidth. The simulated effects of ferrite film loading on antenna performance are investigated, with comparisons made for 5 and 10 µm thick films, as well as for 10 µm thick films with varying magnetic loss (tan δµ = 0.05, 0.1 and 0.3). Our simulations reveal that the addition of 10 µm thick magnetic layers has effectively lowered the resonant frequency by 70 MHz, while improving return loss and -10 dB bandwidth by 3 dB and 40 MHz, respectively, over the uncoated antenna. Ferrite film coating also improved realized gain within the ISM band, with largest gain increases at resonance found for films having lower magnetic loss. Additionally, the gain (G) variance at ISM band limits, ΔGf(2.5GHz)-f(2.4GHz), decreased from 1.97 to 0.44 dBi for the antenna with 10 µm films over the non-ferrite antenna. The measured dip-coated NiCo ferrite films effectively reduces the antenna resonance by 110 MHz, with a 4.2 dB reflection coefficient improvement as compared to an antenna without ferrite. The measured ferrite antenna also reveals a 6 dBi and 35 MHz improvement in realized gain and -10 dB bandwidth, respectively, at resonance. Additionally, the ferrite-coated antenna shows improved directivity, with wave propagation attenuated at the direction facing the body internal. These results indicate that implantable antenna miniaturization and reliable wireless communication in the operating frequency band can be realized with ferrite loading.Item In situ growth stresses in iron-platinum and iron-copper alloy(University of Alabama Libraries, 2010) Fu, Bianzhu; Thompson, Gregory B.; University of Alabama TuscaloosaIntrinsic thin film stress is evitable with the thin film deposition process and plays an important role in tuning the physical properties of thin films. In this thesis, the in situ and post growth stress evolution of the Fe-Pt and Fe-Cu alloy system was studied and correlated to the microstructure evolutions. At ambient temperature and constant deposition pressure, the growth stresses of both the Fe-Pt and Fe-Cu alloy were found to be dependent on the compositions and affected by their growth rates. The final intrinsic stress states after growth could be tuned to be either tensile, zero or compressive depending upon composition and deposition rate for similar grain sizes. This is due to the preferential segregation of one species (the more mobile element) to the grain boundaries. At elevated growth temperatures, the Fe-Pt alloy forms ordered phase while the Fe-Cu alloy forms phase separation. The magnitude of the compressive stress state is reduced as the Fe54Pt46 thin film orders in situ during growth. The compressive stress relaxation rate is increased with increasing substrate temperature or order parameter. This compressive stress reduction has been rationalized as a reduction of adatom mobility on the surface as Fe and Pt occupy specific lattice sites for L10 on each grain. The ordered nature of the grains contributes to additional chemical energy at the boundary which, upon ceasing deposition, significantly increases the stress relaxation rate. In contrary, the growth compressive stress of the Fe51Cu49 alloys in the continuous growth regime is increased with substrate temperature. This has been rationalized as the migration of adatoms to thermodynamically preferred surfaces during growth.Item Influence of solute segregation on stress and structure evolution in nanocrystalline thin films(University of Alabama Libraries, 2018) Zhou, Xuyang; Thompson, Gregory B.; University of Alabama TuscaloosaAlloy thin films offer several tantalizing opportunities to engineer the growth and subsequent stress conditions in thin films. This can be achieved by the mixed interaction of two or more species. For example, grain size is a known quantity in regulating the stress and is most often controlled through processing via pressure, temperature, deposition rate, or a combination of each of these variables. In an alloy, where one species is highly segregating, the solute can also control the grain size and therefore the stress. Based on this hypothesis, this work has elucidated how alloying effects in the residual growth stresses for a series of W-Cr, W-Fe, and Fe-Cr thin films. In addition, a molecular dynamics (MD) model for the growth of elemental W and Fe films was constructed to understand how different microstructures and processing variables control stress.Item Interface orientation dependent field evaporation behavior in multilayer thin films(University of Alabama Libraries, 2011) Brons, Justin Glen; Thompson, Gregory B.; University of Alabama TuscaloosaIn general, atom probe reconstruction algorithms assume a constant evaporation field across the surface of the specimen. In reality, chemical inhomogeneity modulates the evaporation field at the specimen surface, which introduces reconstruction artifacts and degrades the spatial resolution of the atom probe tomography (APT) technique. Multilayer thin films provide ideal specimen geometries to measure and quantify these artifacts. Thin films can be deposited with near atomic layer precision and can exhibit large planar surfaces with degrees of intermixing across the interfaces. Quantifying and rectifying interfacial compositional differences in atom probe data sets is critical, as such information can be used to understand the growth and intermixing of species in nanolaminate devices, such as giant magneto-resistance multilayers. A series of Fe/Ni and Ti/Nb multilayers featuring a bilayer repeat unit of equal thickness and repeat distance of approximately 4 nm have been sputter-deposited onto n-doped Si [001] substrates. The multilayers were focus ion beam (FIB) milled with an annular milling geometry into the required needle-shaped geometry for the APT analysis. Specimens were prepared with the film interfaces oriented with the chemical modulations for a given bilayer spacing parallel and perpendicular to the specimen apex to compare field evaporation behavior at these limiting geometries. For the Fe/Ni multilayers, the 4 nm bilayer films exhibited Fe intermixing within the Ni layers. The Electron Energy Loss Spectroscopy (EELS) based compositional profiles were used in order to cross-correlate across the two techniques using a chemical comparison. These profiles were acquired using aberration-corrected scanning transmission electron microscopy (STEM) with an approximate 100 pm electron probe. The slope of the compositional gradients between the layered interfaces showed very little differences between the EELS and atom probe data sets for Fe/Ni. In addition, little difference was observed between the parallel and perpendicular field evaporation limiting geometries. This has been contributed to Fe and Ni having a similar elemental evaporation field strength of 33 and 35 V/nm. The Fe/Ni results were compared to data obtained from a Ti/Nb multilayered thin film with a bilayer spacing of approximately 4 nm, prepared in similar parallel and perpendicular evaporation orientations as those discussed above. Compositional EELS profiles were collected using an aberration-corrected scanning transmission electron microscopy (STEM) with an approximate 100 pm electron probe. The slope of the compositional gradients between the layered interfaces showed very little differences between the EELS and atom probe data sets for Ti/Nb. In the perpendicular evaporation orientation, it was found that the layer thicknesses for both the Ti and Nb elemental layers were measured at values closer to the actual specimen, while the perpendicular orientation contained reconstruction artifacts that compressed the layer thicknesses. The EELS based compositional profiles were used in order to cross correlate across the two techniques using a chemical comparison. The compositional gradient across the interfaces was also closer to the true value in the parallel orientation, while the perpendicular orientation contained artifacts that altered the composition across the compressed interfaces. This study showed in the atom probe data there was upwards of 20 atomic % Nb was intermixed in the Ti layers. This finding was verified by the EELS compositional profiles.Item Interfacial interactions of FeCo/Pd and FeCo/Ru multilayers(University of Alabama Libraries, 2009) Walock, Michael James; Mankey, Gary J.; University of Alabama TuscaloosaThe FeCo system of alloys is perched atop the Slater-Pauling curve. As a result of this and its relatively low cost and ease of deposition, it is heavily used within the magnetic recording industry. However, new technology requires an advance past this system with respect to high moment, high magnetization materials. One possible step is the enhancement of the FeCo system. One approach is the lamination of thin, nonmagnetic Pd spacer layers with FeCo. Prior published results have shown an increase in not only the moment, but also the magnetization of the samples. We have developed a FeCo/Pd superlattice samples to test this hypothesis. Our results, using a combination of conventional magnetometry, x-ray magnetic circular dichroism, and polarized neutron reflectivity, show a different result from prior reports. The magnetic dichroism experiments show a definite decrease in the magnetic moments of both the Fe and Co with the introduction of Pd. The preliminary analysis of the neutron reflectivity data shows no increase in the magnetic moment of the alloy, nor an induced moment in the Pd. For comparison with the FeCo/Pd superlattice, we studied an antiferromagnetically coupled FeCo/Ru superlattice.Item Interplay of magnetic anisotropy and magnetization reversal in ferromagnetic thin films for spintronics applications(University of Alabama Libraries, 2016) Paul, Soumalya; Mewes, Tim; University of Alabama TuscaloosaHigh spin-polarization in a magnetic material is essential for excellent performance of future spintronic devices and in that regard, half-metallic materials are promising candidates when incorporated into magnetoresistive devices such as magnetic tunnel junctions (MTJs) for spin transfer torque magnetic random access memory (STT-MRAM). As there is an increasing thrust toward device miniaturization and achieving faster switching times, it is likely that magnetic recording materials will be operating at higher frequencies and hence understanding the interplay between the magnetic anisotropy and the magnetization reversal process is of crucial importance both from technological and fundamental perspectives. Broadband ferromagnetic resonance (FMR) spectroscopy is an excellent tool to probe the dynamic magnetic properties of these half-metallic materials. Our investigation suggests that these low damping materials exhibit ‘anisotropic magnetization relaxation’ due to misfit dislocation (in case of Heusler CoxFe3-xSi thin films) as well as the presence of ‘magnetostatic spin waves’ due to the long-range dipolar interaction (in case of rutile CrO2 thin films). Furthermore, vector magneto-optic Kerr effect (MOKE) magnetometry reveals that single crystal CrO2 thin films are magneto-optically anisotropic with two different refractive indices. The structural anisotropy of the tetragonal CrO2 induces the magneto-optical anisotropy. On the other hand, changing the stoichiometry in epitaxial CoxFe3-xSi thin films results in the co-existence of the uniaxial magnetic anisotropy and the cubic magnetic anisotropy. The magnetization reversal processes are associated with the one-jump and two-jump reversal steps that depend critically on the competition between the uniaxial and cubic anisotropies present in these samples.Item Investigation of magnetic relaxation mechanisms and dynamic magnetic properties in thin films using ferromagnetic resonance (fmr) technique(University of Alabama Libraries, 2016) Khodadadi, Behrouz; Mewes, Tim; University of Alabama TuscaloosaInvestigating the damping processes and the behavior of dynamic magnetic properties in ferromagnetic thin films has been an important key towards design and fabrication of different microwave and magnetic recording devices. This thesis discusses the dynamic magnetic properties and also the physics behind different relaxation mechanisms in ferromagnetic thin films using comprehensive experimental investigations by means of broadband ferromagnetic resonance (FMR) technique. In chapter one the basics of ferromagnetic resonance technique and the experimental features of the FMR setup used in this study are discussed, also the FMR data analysis is explained. Chapter two is devoted to the study of the interfacial perpendicular magnetic anisotropy (PMA) and damping parameter in Co2FeAl thin films. In chapter three broadband temperature dependent FMR measurements were carried out on Ni80Fe20/Gd thin films to investigate the behavior of ferromagnetic relaxation, and gyromagnetic ratio as the system goes through the Curie temperature of Gd. In chapter four, the ferromagnetic relaxation mechanisms in ferrites are discussed. The low loss Nikel Ferrite and Lithium ferrite single crystal, thin and ultra-thin films were characterized by detailed FMR measurements to investigate the effect of microstructural defects on the magnetization relaxation. A comprehensive study on the interlayer exchange coupling strength in Co90Fe10/Ru/ Co90Fe10 multilayers is the subject of chapter five, in which the mutual spin pumping is discussed as a recently discovered channel for relaxation in exchange coupled multilayers.Item The magnetic and chemical structural property of the epitaxially-grown multilayered thin film(University of Alabama Libraries, 2012) Lee, Hwachol; Mankey, Gary J.; University of Alabama TuscaloosaL10 FePt- and Fe-related alloys such as FePtRh, FeRh and FeRhPd have been studied for the high magnetocrystalline anisotropy and magnetic phase transition property for the future application. In this work, the thin film structural and magnetic property is investigated for the selected FePtRh and FeRhPd alloys. The compositionally-modulated L10 FePtRh multilayered structure is grown epitaxially on a-plane Al2O3 with Cr and Pt buffer layer at 600degC growth temperature by DC sputtering technique and examined for the structural, interfacial and magnetic property. For the epitaxially grown L10 [Fe50Pt45Rh5 (FM) (10nm) / Fe50Pt25Rh25 (AFM) (20nm)]×8 superlattice, the magnetically and chemically sharp interface formation between layers was observed in X-ray diffraction, transmission electron microscopy and polarized neutron reflectivity measurements with the negligible exchange bias at room and a slight coupling effect at lower temperature regime. For FeRhPd, the magnetic phase transition of epitaxially-grown 111-oriented Fe46Rh48Pd6 thin film is studied. The applied Rhodium buffer layer on a-plane Al2O3 (1120) at 600degC shows the extraordinarily high quality of epitaxial film in (111) orientation, where two broad and coherent peak in rocking curve, and Laue oscillations are observed. The epitaxially-grown Pd-doped FeRh on Pt (111) grown at 600degC, 700degC exhibits the co-existing stable L10 (111) and B2 (110) structures and magnetic phase transition around 300degC. On the other hand, the partially-ordered FeRhPd structure grown at 400degC, 500degC shows background high ferromagnetic state over 5K~350K temperature. For the reduced thickness of Fe46Rh48Pd6, the ferromagnetic state becomes dominant with a reduced portion of the film undergoing a magnetic phase transition. For some epitaxial FeRhPd film, the spin-glass-like disordered state is also observed in field dependent SQUID measurement. For the tri-layered FeRhPd with thin Pt spacer, the background ferromagnetic state is significantly reduced and spin-glass-like state also disappears. In polarized neutron reflectivity, magnetic depth profiles of tri-layered FeRhPd reveals the asymmetric magnetization between two FeRhPd layers. The asymmetric magnetic profile of FeRhPd tri-layered structure is closely related to the thickness dependent epitaxial film growth of B2 structure.Item Magnetic anisotropies and dynamic magnetic properties in multilayered thin films(University of Alabama Libraries, 2017) Beik Mohammadi, Jamileh; Mewes, Tim; Mewes, Claudia K. A.; University of Alabama TuscaloosaTheoretical and experimental research on magnetic materials and magnetic devices intend to investigate novel materials and structures which can be used for the next generation spintronic devices. Moreover, it is essential to conduct fundamental research on new phenomena aiming for new generation of devices that can be faster, smaller, cheaper, and more reliable. Magnetic anisotropies have been widely used in spintronic devices. From the unidirectional exchange bias anisotropy that is used in magnetic read heads and giant magnetic resonance (GMR) sensors, to the interfacial perpendicular magnetic anisotropy (PMA) that is essential for magnetic tunnel junctions (MTJs). In the first chapter of this dissertation a short introduction to magnetization dynamics including experimental techniques is given. In the second chapter, the exchange bias anisotropy and the interfacial origin of relaxation in Ru/IrMn/CoFe/Ru exchange biased systems is discussed and investigated. The interfacial perpendicular anisotropy is observed in these systems and can be quantified using FMR technique. Such anisotropy can exist in a thin ferromagnetic film (such as NiFe) that is in proximity to a metallic (e.g.Ru) or insulating (e.g. SiO2) non-magnetic layer, which is the topic of the third chapter of this dissertation. In addition, experimental results confirm that spatial fluctuations of the uniaxial perpendicular anisotropy can push the easy axis of the magnetization in an orientation that is neither perpendicular to the film nor normal to it. The effect of the lateral fluctuation of the uniaxial anisotropy on the magnetic energy landscape and the magnetization dynamics of thin magnetic layers is reported in chapter four using micromagnetic simulations.Item Magnetic anisotropy graded media and Fe-Pt alloy thin films(University of Alabama Libraries, 2009) Lu, Zhihong; Butler, W. H.; University of Alabama TuscaloosaAnisotropy graded media is promising to overcome the writability problem in achieving ultrahigh areal density for magnetic recording media. To more conveniently study and compare various media with regard to a particular figure of merit, a new energy landscape method of analysis is suggested. Using this method, the theoretical limit of the figure of merit for a graded medium is found to be 4. This limit can be approached by a graded medium with anisotropy quadratically increasing from zero to its maximum value. In order to characterize the anisotropy distribution of a graded medium, hard axis loops of graded media with various anisotropy profiles are simulated and analyzed. It is found that the second derivative of the hard axis loop can give useful information on the anisotropy distribution in a graded medium. Fe₅₀Pt₅₀ with the L1₀ structure, as one of the magnetically hardest materials, has great potential for media application. By using a first-principles calculation method, the magnetic and electronic structures of L1₀ structured Fe₅₀Pt₅₀ have been studied. These calculations show that although the ferromagnetic phase is the most stable phase for Fe₅₀Pt₅₀ with the L1₀ structure, there is a competition between the antiferromagnetic and the ferromagnetic phases when the ratio of lattice constants, c/a, decreases. Experimental investigations of Fe₅₀Pt₅₀ films with graded order parameter fabricated by varying the growth temperature during deposition demonstrate that these films have much smaller switching field than fully ordered Fe₅₀Pt₅₀, which suggests it is possible to make graded media by using this kind of films. Fe₁₀₋ₓPtₓ films with compositional gradient were also studied; however, the large easy axis dispersion in these films makes them unsuitable for the fabrication of graded media. Films with [FePt₃ordered)/FePt₃ (disordered)]n superlattices were deposited on MgO substrates and sapphire substrates. It was found that the exchange bias in superlattices deposited on MgO substrate show higher exchange bias field. Polarized neutron reflectivity results show that ferromagnetic layers on MgO substrates contain more antiferromagnetic component than those on sapphire substrates. The larger exchange bias of the superlattice on MgO substrate is hypothesized to be due to larger exchange bias in its ferromagnetic layers.Item Magnetic order and phase transitions in Fe50Pt50-xRhx(International Union of Crystallography) Fenske, Jochen; Lott, Dieter; Tartakovskaya, Elena V.; Lee, Hwachol; LeClair, Patrick R.; Mankey, Gary J.; Schmidt, Wolfgang; Schmalzl, Karin; Klose, Frank; Schreyer, Andreas; Helmholtz Association; Helmholtz-Zentrum Geesthacht - Zentrum fur Material- und Kustenforschung; National Academy of Sciences Ukraine; Institute of Magnetism NAS of Ukraine & MES of Ukraine; University of Alabama Tuscaloosa; Australian Nuclear Science & Technology Organisation; City University of Hong KongPolarized and unpolarized neutron diffraction techniques have been applied to study the temperature-dependent magnetic and structural properties of four 200 nm-thick Fe50Pt50-xRhx films with x = 5, x = 10, x = 17.5 and x = 25. Similar to the bulk system, an antiferromagnetic to ferromagnetic transition can be found in the films with decreasing Rh concentration. The application of structure factor calculations enables one to determine the microscopic magnetic configuration of the different films as a function of temperature and Rh concentration. The developed models indicate a magnetic transition from a dominant antiferromagnetic order in the out-of-plane direction to a dominant ferromagnetic order in the in-plane direction with decreasing Rh concentration. The different magnetic configurations can theoretically be described by a phenomenological model which includes a two-ion and a one-ion interaction Hamiltonian term with different temperature dependencies of the anisotropy constants.Item Magnetic properties and structure of Mn-x alloy thin films(University of Alabama Libraries, 2018) Zhao, Siqian; Suzuki, Takao; University of Alabama TuscaloosaAmong the rare earth free/less permanent magnet candidates, the L10 MnAl, L10 MnGa and D022 Mn3-δGa (δ=0~1) exhibit high magnetic anisotropy energy, which is of the order of 10^7 erg/cm3, despite the spin orbit coupling (SOC) of Mn atom is small. The present project aims to understand the magnetic anisotropy Ku and coercivity Hc mechanism of Mn-X alloy thin films in conjunction with structure. The polycrystalline samples of L10 Mn52Al48, L10 Mn52Ga48 and D022 Mn71Ga29 were sputter-deposited onto silica glass substrates, and the epitaxial L10 Mn53Ga47 and L10 Mn53Ga47Alx (x=0~6) of island structure were deposited onto MgO (100) and SrTiO3 (100). The initial curves and “apparent Hc” were measured, and the relation between Hc(T)/Ms(T) and 2Ku(T)/Ms^2(T) were fitted linearly to Kronmuller’s empirical equation to estimate the pinning size r0 and effective demagnetizing factor Neff, in order to understand the Hc mechanism. The Hc in out-of-plane directions for polycrystalline L10 Mn52Al48, L10 Mn52Ga48, and both in-plane and out-of-plane directions for polycrystalline D022 Mn71Ga29 and epitaxial L10 Mn53Ga47 are governed by domain wall pinning mechanism, while the in-plane directions for polycrystalline L10 Mn52Al48 and L10 Mn52Ga48 by a mixture of domain wall pinning and nucleation modes. The Neff for polycrystalline L10 Mn52Al48, polycrystalline L10 Mn52Ga48, and epitaxial L10 Mn53Ga47 are 0.1, -2, and 12, respectively, while the r0 are 1.1~1.9 (non-magnetic), 2.2~3.3 (non-magnetic), and 2.0~2.8 nm (magnetic), respectively. The normalized Ku(T) as a function of Ms(T) was fitted to Ku(T) ≈ Ms(T)^n. The power law exponent n for the epitaxial L10 Mn53Ga47 and L10 Mn53Ga47Alx is found to be n ≈ 1.6~3.9, which is temperature dependent and larger at lower temperatures. The results show discrepancy with the single-ion (n=3) or two-ion mechanism (n=2) predictions for Ku1 based on a localized model. At the current stage, the theory for the temperature dependence of SOC and Ku is not well established, and the Ku for transition metal systems is based on itinerant models. Therefore, it is not understood which model (either single- or two-ion mechanism) is responsible for the magnetic anisotropy. A more detailed theory of Ku(T) for transition metal systems is needed.Item Magnetic resonance coupled wireless power transfer systems(University of Alabama Libraries, 2013) Dang, Zhigang; Abu Qahouq, Jaber A.; University of Alabama TuscaloosaWireless power transfer (WPT) technology has many potential applications such as consumer electronics and electric vehicles (EV). High transmission efficiency with long transmission distance and with large lateral misalignment is desired in WPT systems. Magnetic resonance coupled (MRC) WPT systems are suitable for midrange high efficiency wireless power transfer (WPT). In chapter 2, commonly used four-loop and two-loop MRC-WPT system configurations are analyzed and compared in terms of transmission efficiency and transmission distance first based on the simplified circuit model. An example symmetrical system simulation shows that with the same Tx, Rx, source and load, the four-loop system has longer transmission distance but with relatively lower transmission efficiency compare to the two-loop system. Then, A 3-D physical model of 5-turn, 400mm outer diameter spiral shape four-loop WPT system is developed and simulated by using ANSYS® HFSS® software package. Operation distance of 550mm with nearly constant maximum transmission efficiency of 92.3% is achieved. Laterally misaligned MRC-WPT system is investigated in chapter 3. The TEVD, a region on the transmission efficiency versus Rx lateral misalignment amount curve where the transmission efficiency first sharply drops from high efficiency down to zero and then recovers to a low efficiency value, is identified in this work. The identification of TEVD is verified by simulation results obtained from a developed ANSYS® HFSS® 3-D physical model. Simulation results of the ANSYS® HFSS® 3-D physical model with 5-turn, 60cm outer diameter spiral shape MRC-WPT system show that when the Rx is 30cm vertically away from the Tx, TEVD exists when the lateral misalignment value ranges from 50cm to 70cm. An elimination method for TEVD is proposed in chapter 4. The proposed method utilizes angular rotation of the Rx (or Tx) to eliminate the zero-coupling point which causes the TEVD and boosts the coupling coefficient such that the TEVD is eliminated and the high efficiency region is extended. ANSYS® HFSS® 3-D physical model simulation results show that the proposed method eliminates the TEVD and extends the high efficiency region from 50cm lateral misalignment (83.3% of the Rx diameter) to 70cm lateral misalignment (117% of the Rx diameter). Chapter 5 summarizes the thesis conclusions and sheds the light on future work.Item Magnetics-based antennas(University of Alabama Libraries, 2017) Lee, Woncheol; Hong, Yang-Ki; University of Alabama TuscaloosaRecently, compact and multi-functional wireless mobile devices have been highly demanded. Therefore, miniaturization of radio-frequency (RF) components, including antennas, is essential. Regarding wireless charging networks for mobile devices, performance degradation of near-field communication (NFC) and wireless power charging (WPC) systems is often observed. This is because various components are integrated into a compact space. Moreover, active components, such as amplifiers and oscillators, cause wireless communication devices to experience unwanted or noise signals. The unwanted signals are transmitted and received by an antenna, thereby degrading the quality of wireless communication. Thus, the frequency filtering device is required to suppress the radiation of unwanted signals. However, this increases the volume of a wireless device, which is not desired. The aforementioned demands and issues can be addressed by employing magnetic materials and exploiting the unique characteristics of frequency dependent complex permeability. It is noted that magnetic loss is a concern because the loss degrades device performance. The objective of the dissertation is to develop low-loss ferrites and design magnetics based antennas to meet the aforementioned demands without sacrificing antenna performance. Low loss hexaferrites, including Ba3Co2Fe24O41, BaCo1.4Zn0.6Fe16O27, and Ba2Co2Fe28O46, have been developed with a conventional ceramic process and compared with other reported low-loss ferrites for the figure of merit. The figure of merit is defined as a ratio of the real part of complex permeability to the magnetic loss tangent. Design and fabrication of miniature antennas, such as the dual-polarized hexaferrite antenna for an unmanned aerial vehicle and low-profile multiband antenna for telematics applications, are based on the low-loss ferrites, which was developed in this dissertation. Furthermore, effects of magnetic materials loading on antenna miniaturization and performance were investigated. Excellent antenna performance was demonstrated with ferrite loading. Spinel ferrite, Ni0.38Zn0.47Cu0.15Fe2O4, offers enough magnetic isolation for WPC design, resulting in a high power transfer efficiency for the WPC system. Accordingly, a simple wireless power charging system was designed with the spinel ferrite loading and simulated for its power charging performance. Then, the loaded ferrite was evaluated for its applicability to a WPC system. Lastly, loading an antenna with multiple ferrite cores significantly suppressed harmonic radiation from the antenna by dissipating unwanted signals. On the other hand, the conventional harmonic suppressed antennas suppress the harmonic radiation by reflecting or redirecting unwanted signals, which is not desired. The simulated and experimental data from ferrite loaded antennas suggest that the magnetic materials can play various roles in antenna design and performance.Item Matilda: a mass filtered nanocluster source(University of Alabama Libraries, 2009) Kwon, GiHan; Klein, Tonya M.; University of Alabama TuscaloosaCluster science provides a good model system for the study of the size dependence of electronic properties, chemical reactivity, as well as magnetic properties of materials. One of the main interests in cluster science is the nanoscale understanding of chemical reactions and selectivity in catalysis. Therefore, a new cluster system was constructed to study catalysts for applications in renewable energy. Matilda, a nanocluster source, consists of a cluster source and a Retarding Field Analyzer (RFA). A moveable AJA A310 Series 1"-diameter magnetron sputtering gun enclosed in a water cooled aggregation tube served as the cluster source. A silver coin was used for the sputtering target. The sputtering pressure in the aggregation tube was controlled, ranging from 0.07 to 1torr, using a mass flow controller. The mean cluster size was found to be a function of relative partial pressure (He/Ar), sputtering power, and aggregation length. The kinetic energy distribution of ionized clusters was measured with the RFA. The maximum ion energy distribution was 2.9 eV/atom at a zero pressure ratio. At high Ar flow rates, the mean cluster size was 20 ~ 80nm, and at a 9.5 partial pressure ratio, the mean cluster size was reduced to 1.6nm. Our results showed that the He gas pressure can be optimized to reduce the cluster size variations. Results from SIMION, which is an electron optics simulation package, supported the basic function of an RFA, a three-element lens and the magnetic sector mass filter. These simulated results agreed with experimental data. For the size selection experiment, the channeltron electron multiplier collected ionized cluster signal at different positions during Ag deposition on a TEM grid for four and half hours. The cluster signal was high at the position for neutral clusters, which was not bent by a magnetic field, and the signal decreased rapidly far away from the neutral cluster region. For cluster separation according to mass to charge ratio in a magnetic sector mass filter, the ion energy of the cluster and its distribution must be precisely controlled by acceleration or deceleration. To verify the size separation, a high resolution microscope was required. Matilda provided narrow particle sized distribution from atomic scale to 4nm in size with different pressure ratio without additional mass filter. It is very economical way to produce relatively narrow particle size distribution.