Theses and Dissertations at Montana State University (MSU)
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Item Broken-symmetry phases of matter and their effects on electronic and magnetic properties(Montana State University - Bozeman, College of Letters & Science, 2023) Peterson, Sean Fahlman; Chairperson, Graduate Committee: Yves U. Idzerda; This is a manuscript style paper that includes co-authored chapters.Physical symmetries inherent to a material are often reflected in its electronic and magnetic properties. The in-plane four-fold rotational symmetry of thin-film ferromagnets inherent to their tetragonal lattice is also exhibited by their cubic anisotropy. The magnetization as a function of applied magnetic field can be calculated via the Stoner- Wohlfarth model. These calculated hysteresis loops were fit to measured hysteresis loops to determine anisotropy constants consistent with known values. An electronic nematic state reduces the in-plane four-fold rotational symmetry of materials by inducing a structural transition from tetragonal to orthorhombic/monoclinic, with two-fold symmetry. This reduced symmetry persists in the electronic thermal transport. Nematicity enhances nearest-neighbor hopping along one axis and reduces it along the other. This results in a deformed Fermi surface compressed (elongated) along the axis of stronger (weaker) electron hopping. This drags van Hove singularities through the Fermi level, affecting quasiparticle lifetimes. Calculating conductivity from the Boltzmann kinetic equation, nematicity enhances thermal transport along one axis and diminishes it along the other. Additionally, s-wave superconductivity coexisting with nematicity creates a feedback on the superconducting gap with a d-wave instability, which can lead to gapless excitations. In the case of weak feedback, nematic superconductors behave like fully-gapped superconductors along both axes, where transport decreases exponentially with temperature. Once gapless excitations form, transport along both axes becomes T -linear at low-T . Similarly, striped antiferromagnetism (AFM2 and AFM3) reduces the rotational symmetry of a square unit cell to a larger two-fold symmetric magnetic cell. Modeling the band structure with a tight- binding model and considering a smaller periodicity in momentum-space, gaps the Fermi surface along one axis. Calculating conductivity reveals diminished transport along one axis and enhanced thermal transport along the other. Considering d-wave superconductivity in this model results in two cases. One has highly anisotropic transport with greatly enhanced T -linear transport along one axis and diminished transport decreasing exponentially with temperature along the other. The second has weakly anisotropic transport with diminished T -linear conductivity along both axes. The symmetry of a material's properties, such as magnetic anisotropy and thermal transport, are intrinsically linked to their crystalline, electronic, and magnetic symmetries.Item Size dependence of the magnetic properties of cobalt oxide nanoparticles mineralized in protein cages(Montana State University - Bozeman, College of Letters & Science, 2005) Resnick, Damon Aaron; Chairperson, Graduate Committee: Yves U. IdzerdaA major question in the physics of magnetic nanoparticles is how the size affects the magnetic properties in magnetic nanoparticle systems. In particular, the magnetic properties can be affected by finite-size effects or surface effects. It is this author's belief that surface effects and not finite-size effects play the dominate role. This study is a specific example of how to try to answer this question by looking at different sizes of Co 3O 4 nanoparticles. In order to answer this question as well as better understand this system, different antiferromagnetic Co 3O 4 nanoparticles of 4.35 nm and 6.3 nm in diameter were synthesized. These particles were determined to be relatively uniform and monodispersed. In this study, Transmission Electron Microscopy (TEM), electron diffraction (ED) and X-ray Absorption Spectroscopy (XAS) were used to study the physical and electronic structure of the nanoparticles. Alternating Current Magnetic Susceptibility (ACMS) was used to measure the magnetic anisotropy energy density of the different size nanoparticles. It was found that the anisotropy energy density increases with decreasing size, from 1.09 x10 4 J/m 3 for the 6.3 nm particles to 7.53x10 4 J/m 3 for the 4.35 nm particles, consistent with the importance of surface anisotropy with decreasing particle size. Vibrating Sample Magnetometry (VSM) was used to measure the Neel temperature and coercive field of the different particles. It was found that the Neel temperature decreases with decreasing size, from 40 K to 15 K, consistent with a simple surface approximation of the finite-size scaling theory, while the coercive field increased with decreasing particle size consistent with a surface model. The main conclusion of this work is that surface effects and not finite-size effects play a major role in the change of the magnetic properties with size in Co 3O 4 nanoparticles. The evidence also suggests that the increase in the anisotropy energy density is due to the creation of a surface anisotropy component normal to the surface of the particles.