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Browsing by Author "Patton, Carl E., committee member"

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    A study of magnetostructural parameters related to spin crossover and single molecule magnetism
    (Colorado State University. Libraries, 2013) Fiedler, Stephanie R., author; Shores, Matthew P., advisor; Kennan, Alan J., committee member; Anderson, Oren P., committee member; Crans, Debbie C., committee member; Patton, Carl E., committee member
    Herein are described several methods to probe transition metal complexes that were designed by systematic structural modifications to allow for comparison of the resultant magnetic properties. In Chapter 1, a brief introduction is presented to introduce the broader goal of our research: controlling spin on the synthetic level. The introduction provides background regarding spin crossover and single molecule magnetism as well as some previous research to put our projects in context relative to endeavors by other researchers. In Chapter 2, heteroleptic complexes of the form [Fe(H2bip)2(pizR)]Br2 and [Fe(H2bip)2(pizR)](BPh4)2 are described, which have the opportunity to chelate an anion via hydrogen bonding to the H2bip ligand. The third ligand, pizR, is varied between two ligands that we predict will have similar ligand field strengths: pizH and pizMe. Because pizH has an additional hydrogen-bonding site, while pizMe does not, we selected these ligands in order to understand the effect of hydrogen bonding on the anion-binding/spin-state switching event independent from ligand field strength. From these studies, the pizH anion hydrogen bond is observed in crystallographic studies, but does not affect the anion-binding or spin-state switching properties in solution. In Chapter 3, we further investigate the geometry of the pizR ligand in Fe(II) complexes. What began as attempts to study hydrogen bonding in solution revealed unexpected structural distortions of the ligand that are correlated to the spin state of the complexes. The R-substituted nitrogen atom on the imidazoline moiety of the pizR ligand switches between a planar geometry, which is observed for high-spin species, and a pyramidalized geometry, which is observed for low-spin species. We reason that this occurs as a result of the weak-field, non-pizR ligands that influence the ligand field in the high-spin species. Chapters 4 and 5 delve deeper into understanding the relationship between structural parameters and magnetic properties in complexes with non-covalent interactions. In Chapter 4, a series of complexes with metallophilic Pt-Pt interactions show antiferromagnetic magnetic coupling of non-bonded transition metals through a Pt-Pt bond. By comparing complexes with Pt-Pt interactions to those without Pt-Pt interactions, we are able to determine that the Pt-Pt bond is a unique superexchange pathway for the transition metal coupling. Off-set complexes, exhibiting two Pt S interactions instead of one Pt-Pt interaction, do not show evidence of magnetic coupling between transition metals. Furthermore, by comparing magnetic properties of complexes where the apical ligand varies, we determine that the presence or absence of intermolecular interactions is largely independent from the strength of coupling through the Pt-Pt bond. In Chapter 5, an asymmetric trinuclear manganese complex with unique magnetic exchange properties and two high-spin square planar complexes of iron and cobalt, are investigated. The trinuclear manganese complex consists of a central octahedral Mn(II) ion that is coupled antiferromagnetically to another octahedral Mn(II) ion and ferromagnetically to a terminal tetrahedral Mn(II) ion. The different coupling is rationalized as a result of the change in geometry, which affects the orbital overlap that is predicted for each pair of ions. The high-spin square-planar Fe(II) and Co(II) complexes illustrate an unusual pairing of spin-state with square-planar geometry. Moreover, the Fe(II) complex exhibits signs of easy-axis molecular anisotropy and slow-relaxation of magnetization, albeit in the presence of a magnetic field. Lastly, in Chapter 6, we investigate a trinuclear Fe(III) complex bridged by a triethynylmesitylene ligand. The magnetic properties of the complex are compared to a previous Fe(III) complex bridged by a triethynylbenzene ligand. Steric interactions between the aromatic core of the ethynylmesitylene ligand and the auxiliary dimethylphosphinoethane ligands on Fe(III) are predicted to engender a ligand conformation to promote strong orbital overlap. Magnetic susceptibility data for the two complexes both exhibit ferromagnetic coupling between metal centers as expected. Further studies are necessary to confirm the observed behavior, but the new triethynylmesitylene complex appears to have slightly stronger coupling than the previous triethynylbenzene complex.
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    Damping mechanisms in magnetic recording materials & microwave-assisted magnetization reversal
    (Colorado State University. Libraries, 2014) Lu, Lei, author; Wu, Mingzhong, advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Marconi, Mario C., committee member; Patton, Carl E., committee member
    Understanding the damping of magnetization precession in magnetic recording materials is of both fundamental and practical significance. From the practical perspective, the relaxation processes not only set a natural limit to the time of magnetization switching which determines recording data rates, but also play critical roles in advanced magnetic recording techniques such as microwave-assisted magnetic recording and two-dimensional magnetic recording. Experimental and theoretical studies of magnon-electron scattering and two-magnon scattering (TMS) contributions to magnetization relaxations in magnetic recording head and media materials were conducted for the first time in this dissertation. The accuracy of ferromagnetic resonance (FMR) measurements was increased by the use of vector network analyzer (VNA) FMR techniques. Working equations of the grain-to-grain TMS and grain boundary TMS processes were developed based on the TMS models of Krivosik and Mo, and were applied to understand the relaxation mechanisms in various recording-related thin film materials. The dependences of the FMR behavior and relaxation rates on the external field orientation, the microwave frequency, and the temperature were investigated experimentally in the following three domains: the exchange-coupled composite media, the free layers of tunnel magneto-resistance readers, and FeCo alloy films for future writers. The theoretical models were used to analyze the experimental data and to understand the relaxation mechanisms. Microwave-assisted magnetization reversal (MAMR) is considered as a promising mechanism for further increasing the recording area density and pushing it beyond the super-paramagnetic limit. The MAMR operation was demonstrated with a 700-Gbit/in2 perpendicular media sample in this thesis study. For microwaves with frequencies close to the FMR frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.
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    Part 1: Synthesis and characterization of magnetic Cr5Te8 nanoparticles. Part 2: Local atomic structure studies using theory to simulate polarons in superconducting cuprates and experiment to analyze alternative energy nanomaterials
    (Colorado State University. Libraries, 2012) Martucci, Mary B., author; Prieto, Amy L., advisor; Elliott, C. Michael, committee member; Fisher, Ellen R., committee member; Rickey, Dawn, committee member; Patton, Carl E., committee member
    The field of spintronics, the development of spin-based devices that utilize the spin degree of freedom to increase memory capacity, has emerged as a solution to faster more efficient memory storage for electronic devices. One class of materials that has been extensively studied is the half-metallic ferromagnets, compounds that are 100% spin-polarized at the Fermi level. One material in this group that has been investigated is chromium telluride (Cr1-xTe), whose family of compounds is known to exhibit a wide range of interesting magnetic and electronic properties. We have developed a hot injection solution synthesis of Cr5Te8 nanoplatlets which show similar magnetic behavior to the bulk material. It has also been shown that selenium and sulfur analogues can be obtained without changing the reaction conditions, making progress toward a better understanding of the reaction as well as an interesting family of compounds. Using real-space simulations, the effect of polarons in the high-Tc superconducting cuprates has been studied. The simulations demonstrate energetically favorable sites for the defects and show evidence of longer-range pairing interactions. Variations of the stripe show similar energetic results. X-ray absorption fine structure spectroscopy and neutron scattering have been utilized to examine the local structure of Ni-doped Mg nanoparticles, a hydrogen storage material as well as Cu2ZnSnS4 (CZTS) nanoparticles, a photovoltaic material. The Mg-Ni material shows much local disorder upon hydrogen cycling. The CZTS data demonstrate a loss of sulfur from around the copper sites upon annealing, helping to explain the changes observed in the optical absorption properties resulting from the annealing process.