Browsing by Subject "magnetism"

Now showing 1 - 8 of 8
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    4d and 5d compounds as the new frontier of the anisotropic spin physics
    (2017-07) Sizyuk, Yuriy
    I perform a series of studies of the magnetism of 4d and 5d transition metal compounds. In particular I concentrate on the realization of anisotropic magnetic Hamiltonians by use of the spin-orbit coupling to tie together the real space geometry and spin space magnetism. In the first part, I derive the magnetic Hamiltonians of Sr2IrO4 and Na2IrO3 from microscopic parameters. The difficulty of these calculation arises from the fact that many microscopic parameters, such as Hund's coupling, spin-orbit coupling, and crystal field distortions are all of the same order and thus have to be treated on an equal footing. The competition and cooperation of these interactions leads to a rich magnetic Hamiltonians with many different anisotropic interactions. My calculations provide a clear dependence of these interactions on the microscopic parameters. This in turn can be used experimentally to single out and enhance given anisotropies by changing the microscopic parameters. In the second part I propose experimental measurements for the anisotropic interactions. In particular I study how different anisotropic interactions contribute to the anisotropy in the Curie-Weiss temperatures of these compounds. I show that the difference of Curie-Weiss temperatures along particular axes gives a way to measure the strength of the anisotropic interactions in the compounds. In the last part, I study how the multitude of the magnetic anisotropies determine the magnetic ground state in 4d and 5d compounds. We have developed a new method to calculate the fluctuational contribution to the free energy in anisotropic Hamiltonians at any temperature within the magnetically ordered phase. The calculation can be done for both classical (which includes only thermal fluctuations) and quantum (quantum and thermal fluctuations) systems. I also study the effects of external magnetic field applied to the nearest neighbor Kitaev-Heisenberg model, a model of particular interest for alpha-RuCl3.
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    Computationally Driven Characterization of Magnetism, Adsorption, and Reactivity in Metal-Organic Frameworks
    (2016-06) Borycz, Joshua
    Metal-organic frameworks (MOFs) are a class of nanoporous materials that are composed of metal-containing nodes connected by organic linkers. The study of MOFs has grown in importance due to the wide range of possible node and linker combinations, which allow tailoring towards specific applications. This work demonstrates that theory can complement experiment in a way that advances the chemical understanding of MOFs. This thesis contains the results of several investigations on three different areas of MOF research: 1) magnetism, 2) CO2 adsorption, and 3) catalysis. The calculation of magnetic properties within MOFs is quite problematic due to the weak nature of the interactions between the metal centers. The metal atoms in MOFs can be far apart due to the organic linkers and are often in unique chemical environments that are diffcult to characterize. These weak interactions mean that the computational methods must be carefully selected and tested to attain adequate precision. The objective of the work in this thesis was to determine the single-ion anisotropy and magnetic ordering of Fe-MOF-74 before and after oxidation. MOFs have desirable properties for CO2 adsorption such as large pores and high surface areas. Accurate force fields are required in order to make predictions for adsorption interactions with the internal surface of MOFs. Therefore it is important to have computational protocols that enable the derivation of reliable interaction parameters in order to study the trends of adsorption for different metal centers. In the research herein we used ab initio calculations to compute parameters for classical force fields for members of the IRMOF-10 and the MOF-74 series. MOFs have been considered for catalysis due to their thermal stability, reactive metal sites, and large diameter pores. In this thesis we report a series of studies that advance the understanding of the reactivity of MOFs containing Zr6 and Hf6 polyoxometalate nodes. In the first study the proton topology of the nodes within NU-1000 was determined. Several other studies that make use of these MOFs as supports for single-site metal catalysts are also reported. Finally, research where NU-1000 serves as a template for a thermally stable nanocasted material used for high temperature Lewis acid catalysis is also discussed.
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    M-121 Geologic map of pre-Cretaceous bedrock in southwest Minnesota
    (Minnesota Geological Survey, 2002) Southwick, D.L.
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    M-132 Maps of bedrock geology and superimposed magnetic on gravity (SMOG) anomaly for East-central Minnesota
    (Minnesota Geological Survey, 2003) Jirsa, M.A.; Chandler, V.W.; Lively, R.S.; Boerboom, T.J.
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    Magnetic Properties of Transition Metal Oxides from First-principles
    (2022-08) Gautreau, Dominique
    Due to the strong coupling between the spin, lattice, and orbital degrees of freedom, transition metal oxides exhibit a wealth of exotic phases, such as ferroelectricity, superconductivity, and magnetic ordering. In this thesis, I focus on the magnetic properties of three transition metal oxides. The first study I present in this thesis is on the botallackite cuprate Cu2(OH)3Br. I present the results for the excitation spectrum of the material, obtained through a combination of first-principles methods, linear spin wave theory and exact diagonalization. Our calculations of the dynamical structure factor highlight the coexistence of magnon and spinon excitations in the system, and our results qualitatively agree with experimental results obtained through inelastic neutron scattering.I then turn to the rare-earth titanate (RTiO3) compounds, which are well-known to transition from a predominantly ferromagnetic state to a predominantly G-type antiferromagnetic state with increasing rare earth radius. This extraordinary behavior arises from the high sensitivity of the exchange interactions to the crystal structure of RTiO3. As such, the rare-earth titanates are natural candidates for exploring the possibility of controlling a system’s magnetic behavior through the application of uniaxial or biaxial strain. I discuss the results of our comprehensive study of the rare-earth titanates, in which we used a combination of first-principles and analytical methods to show that the application of uniaxial or epitaxial strain in RTiO3 should lead to a host of magnetic and structural phase transitions. This study is then followed by a description of the collaborative works I have participated in, in which I provided first-principles and analytical calculations to complement experimental and theoretical analyses of RTiO3. I then discuss my contribution to the joint experimental and theoretical investigation of PYCCO. In this work, my coauthors demonstrate that simultaneous first-order spin-state/valence-state/metal-insulator transitions can be experimentally induced in PYCCO with applied epitaxial strain. Studying this system from first-principles, I provide evidence that the strain-tunable phase transitions in PYCCO are directly analogous to the first-order thermal phase transitions observed in PCCO.
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    Magnetization Dynamics in Thin Film and Multilayer Structures
    (2022-06) Peria, William
    Spintronics is a field of research that seeks to exploit the spin rather than the charge ofthe electron for information-technology applications, with the promise of computational devices that use less energy while being faster and more powerful. A major challenge in this field has been the understanding and control of how the energy contained in a system of electron spins is transferred, and ultimately lost, to the rest of the material. This thesis presents experimental measurements of magnetization damping using ferromagnetic resonance in a variety of different thin films and multilayer structures, along with unique ways of understanding the physical mechanisms that cause damping. First, the effect of an extrinsic two-magnon scattering mechanism on the magnetization damping is demonstrated in a series of Heusler alloy thin films. A model of two-magnon scattering is developed to fit the data, and particular emphasis is placed on the mechanisms which cause the effect to be stronger in the Heusler films. It is then shown how two-magnon scattering can shift the resonance frequency, an effect that is almost always neglected, which is important due to the ubiquity of using ferromagnetic resonance measurements to extract magnetic anisotropy energies. The following portion of the thesis deals primarily with magnon-phonon coupling and its effect on damping. A mechanism of magnetization damping due to magnon-phonon coupling is shown to dominate the overall damping in a series of Fe0.7Ga0.3 alloy thin films. The mechanism causes a giant anisotropy of the damping, with the damping coefficient varying by as much as a factor of 10 depending on the orientation of the magnetization. This mechanism is extrinsic, and so it is important to account for when measuring the intrinsic damping of a material. Finally, a phonon pumping mechanism is demonstrated in a series of Co/Pd multilayers. Phonon pumping causes a resonant damping of the magnetization dynamics, at a frequency that is determined by the total thickness of the multilayer. The temperature dependence is much stronger than expected, which underscores the importance of magnetic boundary conditions in the problem. There is also a resonance frequency shift that accompanies the resonant damping, which can be predicted accurately using linear response theory.
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    OFR07-06, Upgrade of Aeromagnetic Databases and Processing Systems at the Minnesota Geological Survey
    (Minnesota Geological Survey, 2007) Chandler, Val W
    During 2005-2007 a program to upgrade the aeromagnetic database for Minnesota was conducted by the Minnesota Geological Survey (MGS). Most of the Minnesota aeromagnetic data were acquired during a 1979-1991 state-wide survey program, and compilation and processing were limited to the computer capabilities of the time. Since that time significant improvements have been made in the development of computers and software that can efficiently handle massive database operations, such as line leveling and gridding. This project was consequently initiated to upgrade the MGS database using up-to-date software and hardware. Access to the data was also to be improved by making all upgraded data available via the web.
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    Spin Relaxation and Size Effects in Cu and Al Nanowires
    (2018-12) Watts, Justin
    This dissertation focuses on the quantification of dominant spin relaxation sources in Cu and Al. In light metals, the Elliott-Yafet (EY) theory is widely acknowledged to describe the proportionality between the spin relaxation rate and the momentum scattering rate for a single scattering source. However, the quantitative impact on spin relaxation due to the presence of multiple scattering sources has remained poorly understood. By integrating Cu and Al nanowires into non-local spin valves (NLSVs), spin and charge transport were separately characterized. We test a proposed generalization of the EY theory, where each scattering source is assigned a unique EY proportionality constant. Verification of the generalized EY theory and quantification of the EY constants for specific scattering sources (e.g., phonons, surfaces, grain boundaries, non-magnetic impurities, and local moments), then enables predictive spin relaxation models and improves understanding of specific spin relaxation sources in these model metals.