Dielectric and conducting properties of the spinel structures Iron Vanadium Oxide, Manganese Vanadium Oxide and Cobalt Vanadium Oxide in high magnetic field and under very high pressure.

摘要

I observed the changes of the crystal structure in single crystal FeV 2O4 by measuring the capacitance and dissipation under high magnetic field. I discovered that there was a significant amount of heat released at low temperature as the magnetic moment changes its orientation. This effect was observed from a sharp peak in the temperature, the capacitance and the dissipation versus magnetic field data at the field where the magnetic moment changes its orientation. This was not observed in the previous measurements on polycrystal FeV2O4 done by Takei, et al. [5] and in the field dependence of capacitance of the other spinels, MnV2O4 and CoV2O 4. Moreover, from the magnetization and capacitance measurements on single crystal FeV2O4, a small plateau at low temperature was observed in the vicinity of 0 Tesla. This is the evidence that there are two magnetic moments exist in single crystal FeV2O4.;In single crystal MnV2O4, I also observed the changes of the crystal structure and the magnetic ordering. I applied the magnetic field at different temperatures and measured the capacitance and dissipation of single crystal MnV2O4. From the field dependence of the capacitance and dissipation of single crystal MnV2O 4, I confirmed that there is a structural transition at 52 K and a magnetic ordering at 56 K. However, at low temperature, the field dependence of the capacitance of single crystal MnV2O4 behaves differently compared to FeV2O4. This is most likely due to the ratio between lattice constant c in tetragonal phase and lattice constant a in cubic phase, ctac . For single crystal FeV2O4, ctac > 1, but ctac 1 for single crystal MnV2O4.;The third one, single crystal CoV2O4, is the most conducting among three spinels and I could not measure the capacitance from room temperature down to 30 K. However, below 30 K this crystal becomes more insulating and I could measure its capacitance. As I swept the field below 10 K, the field dependence of the dielectric constant showed a time dependent behavior. Moreover, I observed a dipole like behavior in single crystal CoV 2O4 although it was not very pronounced.;Another good tool to probe the electrical properties of spinels is the resistivity measurement under very high pressure. I did resistivity measurements on single crystals CoV2O4 and FeV2O 4 under very high pressure up to 8 GPa using cubic anvil system in ISSP. In general, the resistivity of these compounds decreased with increasing pressure. I could not observe the magnetic ordering of FeV2O4 under ambient pressure because it was very insulating. The magnetic ordering could be observed above 2 GPa and the magnetic ordering temperature increased linearly with increasing pressure. The effect was similar with CoV2O 4. More interestingly, CoV2O4 showed a metallic behavior and a metal to insulator transition under high pressure. This is a new observation in this type of material. The interesting aspect of spinel vanadate is that the system approaches the itinerant electron limit with decreasing distance between vanadium ions[8,9]. J.B Goodenough predicted a critical distance between vanadium ions in the spinel vanadate system, 2.94 A[10]. If the distance between vanadium ions (V-V distance) is smaller than this critical distance, the system becomes metallic. The V-V distance of single crystal CoV2O4 is close to this critical value. The resistivity data of single crystal CoV2O4 under very high pressure confirmed that CoV2O4 is sitting at the boundary between the insulator and the metal regime. Finally, Variable Range Hopping (VRH) model and Arrhenius model were used to fit the resistivity data of single crystals FeV2O4 and CoV2O 4. I found that the energy barrier parameter T0 and the activation energy Ea decreased with increasing pressure. (Abstract shortened by UMI.)