{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/92693"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/92693","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Raman scattering studies of spinels CoV2O4 and MnV2O4","abstract":"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.","abstract_html":"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&#x27;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 &lt; T &lt; 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.","abstract_has_math":false,"creators":["Byrum, Taylor M"],"institution":"University of Illinois at Urbana-Champaign","degree_name":"Ph.D.","degree_level":"Dissertation","degree_discipline":"Physics","degree_department":null,"school":null,"contributors":["Cooper, Lance","Shoemaker, Daniel","Mason, Nadya","Clark, Bryan"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2016,"date_issued":"2016-11-10T17:49:23Z","date_published":"2016-11-10T17:49:23Z","updated_at":"2026-07-22T22:26:35Z","subjects":["Spinels","strongly correlated","pressure","phonon","magnon","phase transition","electron itinerancy","magnetism","spin-lattice coupling","Raman","inelastic light scattering"],"languages":["en"],"rights":["© 2016 by Taylor Byrum. 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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'. 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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). 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