Doctor of Philosophy, The Ohio State University, 2021, Physics

Time domain terahertz spectroscopy has allowed for a new way to analyze the properties of antiferromagnets. Since many materials have been explored using this technique, we took a different route for evaluating their properties. We evaluated how two different antiferromagnets (CaFe2O4 and TbMn2O5) interacted with metamaterials. CaFe2O4 was coupled to split ring resonators and TbMn2O5 was coupled to gammadion crosses. From the experiment performed on the CaFe2O4/split ring resonator sample, we did not find sufficient evidence indicating coupling between the sample and the metamaterial. For the TbMn2O5/gammadion sample, we observed an improvement in the efficiency of the electromagnon excitation compared to the bare sample. To understand why the expected anticrossing, an effect observed in coupled oscillator systems, was absent from either measurement, coupling effects between split ring resonators and a hypothetical antiferromagnet were analyzed more deeply utilizing numerical methods. From here we found that an anticrossing will occur when the spins in the crystal are parallel with the interface of the sample. This would allow for improved coupling between the magnetic moment of the split ring resonators and the antiferromagnet. From the data we were able to confirm the presence of an anticrossing. Following the metamaterial project, we began the development of an additional time domain terahertz technique, on chip terahertz, which allowed us to perform measurements on antiferromagnets that were not easily probed. This technique was applied to two different antiferromagnets, CaFe2O4 and MnPS3. For CaFe2O4, we observed a possible absorption in the spectrum that could be connected to on the magnon modes. For MnPS3, we detected three possible modes, one of which could be a low frequency magnon.

Committee: Rolando Valdes Aguilar (Advisor); Marc Bockrath (Committee Member); Ilya Gruzberg (Committee Member); Louis DiMauro (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)

The behavior of ferromagnetic alloys of manganese and iron with gallium when coupled with different magnetic and/or non-magnetic systems is investigated. The studies explore how the structural and electronic/magnetic properties vary with thickness and composition, probing systems in the sub-monolayer, ultra-thin, and thin film regimes. Molecular beam epitaxy is used to prepare clean sample surfaces that are subsequently investigated in-situ down to atomic level using scanning tunneling microscopy and Auger electron spectroscopy. A variety of ex-situ methods are also utilized to obtain information about the overall system properties, with additional theoretical calculations accompanying the experimental findings for two of the investigated systems. The first study refers to L10-structured ferromagnetic MnGa(111) ultra-thin films grown on semiconducting GaN(0001) substrates under lightly Mn-rich conditions. Room temperature scanning tunneling microscopy investigations reveal smooth and reconstructed terraces, with the surface structure consisting primarily of a hexagonal-like 2 x 2 reconstruction. Theoretical calculations are carried out using density functional theory, revealing that a Mn-rich 2 x 2 surface structure gives the best agreement with the observed experimental images and Auger electron spectroscopy surface composition investigations. It is found that under such growth conditions, the Mn atoms incorporate at different rates: surfaces become highly Mn-rich, while the bulk remains stoichiometric, making the MnGa system very sensitive to the ratio of elements in its structure. Such behavior reveals a potential recipe for tuning, for example, magnetic properties by carefully controlling the surface reconstruction during growth. With the aim of understanding how the properties change as the growth conditions are varied, we also investigate the structure, surface, and magnetism of ferromagnetic Ga-rich MnGa thin and ultra-thin films grown again on GaN(00 (open full item for complete abstract)

Committee: Arthur Smith (Advisor) Subjects: Condensed Matter Physics; Physics