Broadband, variable, and random excitations are often suppressed using active vibration absorbers (AVAs). While AVAs can be effective, they also are expensive and subject to instability when the disturbance is ill defined. A state-switched absorber (SSA) can be used for these same vibration classes while reducing the expense and instability because an SSA is only allowed to be active at discrete instances. SSAs are spring-mass-damper devices in which at least one element is controllably variable. The work presented in this dissertation evaluates the properties of magnetorheological elastomers (MREs) to assess their use in SSAs as variable springs. MREs are elastomers doped with magnetically permeable material, generally iron. They are modeled as lossy springs, and have stiffness and loss factor components. Natural frequency and stiffness behavior, and their relationships to static displacement, iron content, and forcing frequency and amplitude were determined. Loss factors were found to be independent of MRE content, configuration, and static displacement. This was confirmation that MREs are in fact controllable springs. Natural frequencies changed in the presence of magnetic fields by as much as 360%. The corresponding change in static displacement could not account for this frequency change. Transient data was found by determining the length of time it took for an MRE to achieve quasi-steady state oscillation behavior when subjected to a harmonic excitation. This time was referred to as the characteristic response time. The characteristic response time correlated to the ratio of the forcing frequency to the zero-field natural frequency. When a magnetic field was turned on, the characteristic response time on average was found to be consistently longer than when the magnetic field was turned off, regardless of iron content or configuration. The difference between these two characteristic response times is caused by the particles' mechanics. To form a chain, a magnetic field must both be set up, and particles must move to join together. When a chain is broken, the magnetic field must merely be removed. However, this difference gives opportunities for future research to be conducted on controlling MREs' transient responses.

This thesis presents the results from Monte Carlo calculations on classical vector spins in face-centered-cubic (FCC) lattices. The goal of the study was to understand the effect of interlayer coupling, dilution of magnetic atoms in the lattice, and symmetry-changing strain. Experimental work by T. M. Giebultowicz et al. and J. A. Borchers et al. greatly inspired my work [1, 2]. J. A. Borchers's group studied NiO/CoO superlattices and observed that the magnetic order of CoO persisted above its Neel temperature due to the effect of interlayer coupling with NiO, which has a higher Neel temperature than CoO [1]. Simulating on a model of NiO/CoO bilayer reproduced the experimental results from Borchers et al. [1]. I concluded that exchange pinning on the NiO/CoO interface preserves the magnetic order of CoO above its Neel temperature significantly. Building on this initial result, a ferromagnet/antiferromagnet/ferromagnet (FM/AFM/FM) trilayer model was studied, where the ferromagnetic (FM) layers were antiferromagnetically coupled. First, I calculated the strength of the AF coupling as a function of the number of antiferromagnetic (AFM) spacer monolayers and concluded that the strength of AFM coupling decreases as the number of AFM spacer monolayers increases. Secondly, I added a uniaxial anisotropy to the model and obtained magnetization curves which exhibited hysteresis-like features with an external field and a first order magnetic transition. Lastly, I diluted the AFM spacer layer in the FM/AFM/FM trilayer by replacing magnetic spins with zero spins in the model. The dilution of AFM spacer layer caused fluctuations in the magnetization curves with external field but the strength of AFM coupling decreases as the number of AFM monolayers increases as in the nondiluted cases. The experimental results from T.M. Giebultowicz's group on MnSe/ZnTe superlattices by neutron scattering showed incommensurate helical spin order in MnSe, where MnSe layers were under tensile strain due to a small mismatching in the lattice parameter [2]. In addition, they observed that the pitch of the spin helix increased as the temperature increased [2]. I modeled the MnSe/ ZnTe system with Monte Carlo method and found that the pitch of the spin helix increased with temperature. In fact, the dependence of helix pitch on temperature was present regardless of the thickness of the sample, so I concluded that this pitch increase is not from the weakening of coupling of surface spins / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Jan. 18, 2013 - July 18, 2013

Molecule-based magnets (molecular magnets) have attracted much interest in recent decades both from an experimental and from a theoretical point of view, not only because of their interesting physical effects, but also because of their potential applications: e.g., molecular spintronics, quantum computing, high density information storage, and nanomedicine. Molecular magnets are at the very bottom of the possible size of nanomagnets. On reducing the size of objects down to the nanoscale, the coexistence of classical properties and quantum properties in these systems may be observed. In additional, molecular magnets exist with structural variability and permit selective substitution of the ligands in order to alter their magnetic properties. Therefore, these characteristics make such molecules suitable candidates for studying molecular magnetism. They can be used as model systems for a detailed understanding of interplay between structural and magnetic properties of them in order to optimize desired magnetic properties. This thesis considers the investigation of magnetic properties of several new transition metal molecular compounds via different experimental techniques (continuous wave electron spin resonance (CW ESR), pulse ESR, high-field/high-frequency ESR (HF-ESR) and static magnetization techniques). The first studied compounds were mono- and trinuclear Cu(II)-(oxamato, oxamidato)/bis(oxamidato) type compounds. First, all components of the g-tensor and the tensors of onsite ACu and transferred AN HF interactions of mononuclear Cu(II)- bis(oxamidato) compounds have been determined from CW ESR measurements at 10 GHz and at room temperature and pulse ELDOR detected NMR measurements at 35 GHz and at 20 K. The spin density distributions of the mononuclear compounds have been calculated from the experimentally obtained HF tensors. The magnetic exchange constants J of their corresponding trinuclear compounds were determined from susceptibility measurements versus temperature. Our discussion of the spin density distribution of the mononuclear compounds together with the results of the magnetic characterization of their corresponding trinuclear compounds show that the spin population of the mononuclear compounds is in interplay with the J values of their corresponding trinuclear compounds. The second studied compounds were polynuclear Cu(II)-(bis)oxamato compounds with ferrocene and ferrocenium ligands. The magnetic properties of these compounds were studied by susceptibility measurements versus temperature to determine J values. In addition, the ESR technique is used to investigate the magnetic properties of the studied compounds because they contain two different magnetic ions and because only the ESR technique can selectively excite different electron spin species. These studies together with geometries of the ferrocenium ligands determined by crystallographic studies indicate that the magnetic interaction between a central Cu(II) and a Fe(III) ions changed from the antiferromagnetic coupling to the ferromagnetic coupling when a stronger distortion of the axial symmetry in the feroccenium cation exists. Therefore, the degree of the distortion of the feroccenium cation is a control parameter for the sign of the interaction between the central Cu(II) ion and the Fe(III) spins of the studied compounds. The last two studied molecular magnets were a binuclear Ni(II) compound (Ni(II)-dimer) and a cube-like tetranuclear compound with a [Fe4O4]-cube core (Fe4-cube). HF-ESR measurements enabled us to determine the g-factor, the sign, and the absolute value of the magnetic anisotropy parameters. Using this information together with static magnetization measurements, the J value and the magnetic ground state of the studied compounds have been determined. In Ni(II)-dimer, two Ni(II) ions, each having a spin S = 1, are coupled antiferromagnetically that leads to a ground state with total spin Stot = 0. An easy plane magnetic anisotropy with a preferable direction for each Ni(II) ion is found. For Fe4-cube, a ground state with total spin Stot = 8 has been determined. The analysis of the frequency dependence and temperature dependence of HF-ESR lines reveals an easy axis magnetic anisotropy (Dcube = -22 GHz (-1 K)) corresponding to an energy barrier of U = 64 K for the thermal relaxation of the magnetization. These results indicate that Fe4-cube is favorable to show single molecular magnet (SMM) behavior.

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Ultrathin films of Cobalt, Iron and Manganese have been thennally evaporated onto an fcc Copper (111) single crystal substrate and investigated using a variety of surface structural teclmiques. The small lattice mismatch between these metals and the Cu (111) substrate make them an ideal candidate for the study of the phenomena of pseudomorphic film growth. This is important for the understanding of the close relationship between film structure and magnetic properties. Growing films with the structure of their substrate rather than their bulk phase may provide an opportunity to grow materials with novel physical and magnetic properties, and hence new technological applications. Both Cobalt and Iron have been found to initially maintain a registry with the fee Cu (111) surface in a manner consistent with pseudomorphic growth. This growth is complicated by island rather than layer-by-layer growth in the initials stages of the film. In both cases a change in the structure of the film seems to occur at a point where the coalescence of islands in the film may be expected to occur. When the film does change structure they do not form a perfect over-layer with the structure of their bulk counterpart. The films do contain a number of features representative of the bulk phase but also contain considerable disorder and possibly remnants of fcc (111) structure. The order present in these films can be greatly improved by annealing. Manganese appears to grow with an fee Mn (111) lattice spacing and there is no sign of a change in structure in films of up to 4.61 ML thick. The gradual deposition and annealing of a film to 300°C, with a total deposition time the same as that for a 1 ML thick film, causes a surface reconstruction to occur that is apparent in a R30° (√3×√3) LEED pattern. This is attributed to the formation of a surface alloy, which is also supported by the local expansion of the Cu lattice in the (111) direction.

Este trabalho teve como objetivo o desenvolvimento de um mini-motor síncrono trifásico, sem escovas e com ímãs permanentes no rotor, utilizado em aeromodelos. O núcleo do estator, usualmente construído com chapas laminadas, foi obtido a partir do processo de Metalurgia do Pó (M/P). Confeccionaram-se corpos de prova de ferro puro, Fe1%P, Fe2%P, Fe3%P, Fe1%Si, Fe3%Si e Fe50%Ni. Foram analisadas as características físicas dos materiais sinterizados que compreendem as grandezas magnéticas: curvas de histerese, resistividade elétrica. Analisaram-se as grandezas mecânicas: dureza e tensão de escoamento. Com os dados magnéticos obtidos, realizaram-se simulações computacionais de funcionamento, onde se observou o fluxo de entreferro e torque. Efetuaram-se ensaios em núcleos de transformadores para analisar as perdas por correntes parasitas. Os resultados foram comparados também com os do estator de chapas. Foi realizada a construção do núcleo do estator a partir de Fe1%P. O estator do mini-motor foi bobinado e instalado em uma carcaça de um mini-motor existente no comércio, para efetuar testes em bancada de acionamento em correntes variadas, onde pôde ser analisada sua eficiência em comparação com o mini-motor com núcleo do estator de chapas. Foi mostrado que é possível a construção de um motor elétrico com núcleo do estator em bloco único de Fe1%P por processo de Metalurgia do Pó, atingindo 74% da eficiência em relação ao mini-motor com núcleo do estator de chapas. / This study aimed to develop three-phase synchronous motor small, brushless and permanent magnets in the rotor, used in model airplanes. The stator core usually constructed of rolled sheets, was obtained from the powder metallurgy process (M / P). Crafted to pure iron specimens, Fe1% P, Fe 2% P, Fe3% P, Fe1% Si, Fe3% Si and Fe50% Ni. They analyzed the physical characteristics of the sintered materials which comprise the magnetic quantities: hysteresis curves, electrical resistivity. Hardness and yield stress: mechanical magnitudes were analyzed. With the magnetic data, computer simulations were carried out of operation where the flux of airgap observed and torque. Tests were affected in cores of transformers for analyzing eddy current losses. The results were also compared with those of the stator plates. Construction of the stator core from Fe1% P was performed. The mini-motor stator was wound and installed in an enclosure of an existing mini-motor trade, for testing in drive bench in varying currents, which could be analyzed efficiency compared to the mini-motor with stator core plates. It has been shown that the construction of an electric motor with the stator core in a single block P Fe1% by Powder Metallurgy process is possible, reaching 74% efficiency relative to the mini-core motor with stator plates.

The controlled and targeted synthesis of new solid materials is still a challenge difficult to overcome. Slow diffusion rates and long diffusion lengths require long reaction times and high synthesis temperatures, resulting in limited control over the reaction pathway. The Modulated Elemental Reactants (MER) method uses compositionally modulated precursors with atomically thin elemental layers that form amorphous alloys upon annealing while maintaining composition modulation. In this amorphous intermediate, nucleation, not diffusion, control the formation of the product, enabling kinetic control of the reaction, and the synthesis of new metastable compounds, heterostructures with designed nanoarchitecture, and thin films with a high degree of texturing. This dissertation uses experimental and computational methods to investigate compounds synthesized by the MER method. Firth, the MER method is used to synthesize ferromagnetic CuCr2Se4 films that show a large degree of crystallographic alignment and interesting magnetic properties such as temperature-dependent easy axes and negative magnetoresistivity. The second part investigates ferecrystals, rotationally disordered members of the misfit layer compounds family. The MER method’s ability to control the nanoarchitecture of the products is used to synthesize a new type of structural isomers, allowing for the synthesis of thousands of ternary compounds using the same elements. Experimental methods are also used to monitor the formation of ferecrystalline compounds using [(SnSe)1+δ][VSe2] as a model system. Despite the vast number of compounds available, however, explaining the properties and stability of ferecrystals is still in its infancy. In the last part of this dissertation, ab initio methods are employed to investigate the components in our ferecrystals. Specifically, isolated layers of VSe2 with its structural distortions due to a charge density wave, SnSe with its thickness-dependent structures, and BiSe with its flexible lattice and anti-phase boundaries are investigated to complement experimental results. Some properties, such as the structural distortion in VSe2 and the different stabilities of BiSe layers, can be explained very well using this simplified model, but others, such as the structure of SnSe layers, are not exclusively determined by their dimensionality, underlining the complex nature of the interactions in ferecrystals. This dissertation includes previously published and unpublished co-authored material.

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