Raman scattering studies of spinels CoV2O4 and MnV2O4
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.
Spinels, strongly correlated, pressure, phonon, magnon, phase transition, electron itinerancy, magnetism, spin-lattice coupling, Raman, inelastic light scattering
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign