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Cooper, Lance Shoemaker, Daniel Mason, Nadya Clark, Bryan Byrum, Taylor M 2016-11-10T17:49:23Z 2016-11-10T17:49:23Z 2016-05-04 2016-08 In this thesis, I present Raman scattering studies of the strongly correlated spinels CoV2O4 and MnV2O4. In CoV2O4, exclusive attention is given to a triply degenerate (T2g) phonon in order to determine the material's structural properties and explore coupling between the lattice and other degrees of freedom. Temperature-dependent studies confirm that the cubic symmetry of CoV2O4 is retained down to low temperatures (T = 7 K), unlike other spinel vanadates. The absence of a structural distortion in orbitally degenerate CoV2O4 supports the previous speculation that the t2g valence electrons in CoV2O4 are not localized. In our pressure-dependent studies, we discover a pressure-induced symmetry-lowering structural transition for P ~ 40 kbar and 50 < T < 150 K. A preliminary P-T structural phase diagram is mapped out, and the close proximity of the structural transition to a previously reported pressure-induced semiconductor-to-metal transition indicates these two phase transitions are correlated. In MnV2O4, we identify the q = 0 Raman-active magnetic excitation spectrum and investigate the interplay between spin, orbital, and lattice degrees of freedom. We find a q = 0 magnon spectrum that differs significantly from spin-wave calculations and analysis of previous inelastic neutron scattering data for MnV2O4. Our high-resolution q = 0 spin-wave excitation results put constraints on spin-wave calculations that should provide improved estimates of magnetic exchange parameters and a more accurate test of proposed orbital ordering schemes for MnV2O4. Below TN, we also observe an anomalous temperature dependence for a two-magnon excitation in MnV2O4, which we attribute to strong magnon-phonon coupling at the Brillouin zone boundary. Our pressure-dependent studies of the one-magnon excitations reveal increased magnon damping with increasing pressure, which supports an increasing itinerant electronic character as the itinerant electron limit is approached from the insulating side. We also find a positive pressure dependence of the one-magnon energies, consistent with an increase of TN with decreasing V-V bond distance (RV-V), which may contradict previous assumptions concerning the effects of decreasing RV-V on magnetic ordering in MnV2O4. Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2016-11-09 without embargo terms The student, Taylor Byrum, accepted the attached license on 2016-05-02 at 16:54. The student, Taylor Byrum, submitted this Dissertation for approval on 2016-05-02 at 16:57. This Dissertation was approved for publication on 2016-05-04 at 13:14. DSpace SAF Submission Ingestion Package generated from Vireo submission #9597 on 2016-11-09 at 10:19:20 Made available in DSpace on 2016-11-10T17:49:23Z (GMT). No. of bitstreams: 8 BYRUM-DISSERTATION-2016.pdf: 6548033 bytes, checksum: d836456623519c79a48970e4f68266d1 (MD5) Blanco-CanosaPermission.pdf: 213929 bytes, checksum: 8e01ba442907b9bf89619d5e956183be (MD5) ChungPermission.pdf: 245925 bytes, checksum: 8adfcf3440b08b703b3da5f623bcef43 (MD5) KiswandhiPermission.pdf: 245775 bytes, checksum: b9fbda14416f0f9ee41c313070d79a39 (MD5) LICENSE.txt: 4209 bytes, checksum: dec882e1f6a7409a0d385464e3f3a109 (MD5) PROQUEST_LICENSE.txt: 4555 bytes, checksum: 1e118f1a09db29e8f09acba4798da97a (MD5) SnowPermission.pdf: 207835 bytes, checksum: cdbf5fd307f8f1d41cfd2bfb0693d61e (MD5) ZhouPermission.pdf: 186299 bytes, checksum: be0aca09e4935687cd2f9a4c95e0da00 (MD5) Previous issue date: 2016-05-04 application/pdf http://hdl.handle.net/2142/92693 en © 2016 by Taylor Byrum. All rights reserved. Spinels strongly correlated pressure phonon magnon phase transition electron itinerancy magnetism spin-lattice coupling Raman inelastic light scattering Raman scattering studies of spinels CoV2O4 and MnV2O4 text text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.