Theses and Dissertations at Montana State University (MSU)
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Item Magnetic and thermal properties of low-dimensional single-crystalline transition-metal antimonates and tantalates(Montana State University - Bozeman, College of Letters & Science, 2017) Christian, Aaron Brandon; Chairperson, Graduate Committee: John J. NeumeierThis work contributes to the study of magnetic interactions in the low-dimensional antiferromagnets M(Sb,Ta) 2O 6, where M is a transition metal. By virtue of the trirutile structure, M-O-O-M chains propagate along [110] at z = 0 and [11overline0] at z = 1=2 of the unit cell. These chains are separated along [001] by sheets of weakly-interacting diamagnetic ions. The spin-exchange coupling perpendicular to the chains is weak, permitting the low-dimensional classification. Single crystals have been grown using chemical vapor deposition and the floating zone method. Magnetization, in-field heat capacity, and high-resolution thermal expansion measurements have been performed along various axes, revealing significant anisotropy due to the peculiar magnetic structures and low dimensionality. The Neel temperature, TN, at which long-range order occurs is found to be unstable against the application of magnetic field above 2 T. Large fields tend to lower TN of the set of moments with projections along the applied field. Moments which are aligned perpendicular to the field are significantly less affected. This can lead to the formation of a secondary peak in heat capacity when magnetic field is along either [110] or [11overline0]. The change in heat capacity at the location of the newly formed peak means there is a change in entropy, which depends upon the direction of applied field with respect to the magnetic moments. Consequently, an anisotropic magnetocaloric effect arises due to the unique magnetic structure. The anisotropic nature of this effect has potential applications in magnetic refrigeration.Item Experimental study of bound magnons and spin cluster resonances in one-dimensional S=1/2, Ising-Heisenberg ferromagnets(Montana State University - Bozeman, College of Letters & Science, 1991) Ravindran, KathirgamathambyItem Bound magnon dominance of the magnetic susceptibility of the one-dimensional Heisenberg spin one-half ferromagnet cyclohexylammonium trichlorocuprate(II)(Montana State University - Bozeman, College of Letters & Science, 1987) Haines, Donald NobleItem Effect of the Morin transition upon electron transport in magnesium doped single crystal hematite(Montana State University - Bozeman, College of Engineering, 1968) Ransom, Calvin LeeItem A phenomonological calculation of the effect of external magnetic fields on the Morin transition in hematite(Montana State University - Bozeman, College of Engineering, 1968) Gable, Robert WilliamItem Effect of the Morin transition on the Seebeck coefficient in single crystals of doped hematite(Montana State University - Bozeman, College of Engineering, 1971) Staab, Roger IrvinItem Numerical calculation of two-magnon-states in lower-dimensional Heisenberg ferromagnets(Montana State University - Bozeman, College of Letters & Science, 1973) Reklis, Robert PeterItem Spin-polarized self-consistent local orbital method and its application to ferromagnetic Ni(001) slabs(Montana State University - Bozeman, College of Letters & Science, 1983) Zhu, XueyuanItem Ferromagnetism in cobalt-doped titanium dioxide(Montana State University - Bozeman, College of Letters & Science, 2005) Lussier, Alexandre Francois; Chairperson, Graduate Committee: Yves U. IdzerdaSemiconductor spintronics is a promising new field of study in the ongoing quest to make electronic devices faster, cheaper, and more efficient. While current spintronics utilizes the spin property of electrons to achieve greater functionality, the integration of spintronics into conventional semiconductor electronics will lead to advances in opto-electronics, quantum computing, and other emerging fields of technology. This integration relies on effective generation, injection, transport, and detection of spin polarized electron currents. To these ends, the successful synthesis of room temperature ferromagnetic semiconductors is mandatory. In this work, we study the properties of cobalt-doped titanium dioxide, a room temperature dilute ferromagnetic semiconductor discovered in 2001. We characterize the Pulsed Laser Deposition (PLD) of Co-doped TiO2 thin films, including the substrate-induced stabilization of the anatase structure of TiO2. We also confirm the substitutional nature of cobalt on titanium sites by X-ray Absorption Spectroscopy (XAS) techniques. The ferromagnetic interaction mechanism remains controversial. Yet, we provide experimental evidence for the polaron mediated ferromagnetic coupling mechanism recently suggested to mediate ferromagnetic interactions in this, and other magnetically doped oxides, in the dilute regime (approximately 0 to 3%). Our evidence is related to a previously unobserved and unreported XAS spectral feature. Finally, we demonstrate the surprising absence of an X-ray Magnetic Circular Dichroism (XMCD) signature at the cobalt L edge.