Dielectric and Conducting Properties of the Spinel Structures FeV₂O₂, MnV₂O₂ and CoV₂O₂ in High Magnetic Field and under Very High Pressure

It is always very nice when a new discovery found in an old material. The spinel vanadate has become one of the hot topics in the condensed matter physics both experimentally and theoretically. It shows many interesting behaviors due to the interaction among spin, orbital and lattice degrees of freedom. The AV2O4 structure with A is a transition metal ion (Fe2+, Mn2+ and Co2+) exhibits several structural transitions and a magnetic ordering from paramagnetic to ferrimagnetic. Some theoretical approaches have been made in order to explain the physics of spinels. There are four main factors that contribute to the complexity of spinel system, e.g, spin, orbital, crystal structure and charge. Kugel and Khomskii first proposed a physical model combining spin and orbit degrees of freedom followed by Tsunetsugu and Motome who combined the super exchange and Jahn-Teller effect[2,3]. On the other hand, Tchernyshyov proposed a physical model based on the interplay of spin, orbital and Jahn-Teller effect[4]. Many discoveries from the experiments were also found in this type of material; memory effect in the polycrystal FeV2O4[5], the magnetic switching of the crystal structure in MnV2O4[7] and the magnetic structure of MnV2O4 using neutron study[7]. The modern techniques of x-ray and neutron scattering have brought important knowledge of the crystal and its magnetic structure. However, these techniques require a very good apparatus and a very high accuracy. I used much simpler technique to examine the physical properties of single crystals FeV2O4, MnV2O4 and CoV2O4. Firstly, I did the capacitance and dissipation measurements in high magnetic field on single crystals FeV2O4, MnV2O4 and CoV2O4 in our laboratory (NHMFL) at Tallahassee, Florida and secondly, I did the electrical transport measurements under very high pressure on single crystals FeV2O4 and CoV2O4 in Institute for Solid State Physics (ISSP), the University of Tokyo, Japan. Single crystals MnV2O4 and FeV2O4 are insulators and their resistivity can be higher than 200 Ω cm at room temperature but single crystal CoV2O4 is a semiconductor with the resistivity is around 60 x 10-3 Ω cm at room temperature. However, these single crystals become more insulating at low temperatures so the capacitance measurement is a good tool to investigate their electrical properties. It turned out that the capacitance and dissipation measurements in high magnetic field were very interesting measurements in order to study the interaction between the spin, orbital and lattice in these spinels. The capacitance of a material is related to the dielectric constant and also the geometrical factors of the material, i.e, the distance between the electrodes and the area of the electrodes. In addition, the magnetic field creates magnetostriction effect that changes the dimension of crystal. This situation can be very interesting in order to investigate dielectric properties of these compounds in high magnetic field. FeV2O4, MnV2O4 and CoV2O4 have a magnetic ordering from paramagnetic to ferrimagnetic at 110 K, 56 K and 152 K respectively. Furthermore, the interaction between the tetrahedral site and the octahedral site in the spinel structure also creates structural distortions. I observed the changes of the crystal structure in single crystal FeV2O4 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 CoV2O4. 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 MnV2O4, 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, ct/ac . For single crystal FeV2O4 ct/ac, > 1, but ct/ac < 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 CoV2O4 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 FeV2O4 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 CoV2O4. 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. J.B Goodenough predicted a critical distance between vanadium ions in the spinel vanadate system, 2.94 Å. 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 CoV2O4. I found that the energy barrier parameter T0 and the activation energy Ea decreased with increasing pressure.