The Study Of Physical Properties And Phase Transitions In Spin-Orbital Coupling Vanadium Oxides

Phase transition is an ancient and significant topic in Condensed Matter Theory. In recent dacedes, transition-metal oxides attract board researching attention due to their ordering behavior during the transitions. Wherein, the orbital ordering and spin ordering are usually involved in the transitions of the vanadium oxides with spinel structure (with general formula AV2O4). Meanwhile, their geometrical frustration and spin-orbital coupling would also bring some fascinating physical phenomena. In this dissertation, physical properties and phase transitions of vanadium oxide (CdV2O4, ZnV2O4 and V2O3) have been studied, to further deepen the understanding of ordering behavior and relative phenomena during the phase transitions of transition-metal oxides. The primary coverage of this dissertation include:1. The high quality samples of CdV2O4, ZnV2O4 and V2O3 were synthesized by solid phase reaction and sintering method, after which the lattice structure of samples were characterized by X-Ray diffraction spectra. Via the measurement of temperature dependent susceptibility and specific heat, Curie temperature, effective magneton number, electron and phonon coefficient to specific heat, Debye temperature is evaluated, as well as analyzed and compared among the CdV2O4, ZnV2O4 and V2O3. On the other hand, according to whether the hysteresis exists in specific heat curves measured with cooling and warming process, the structural transition of CdV2O4 and ZnV2O4 is determined to be the first-order transition while the magnetic transition belong to the second-order type. As for the structural transition, the excess specific heat, enthalpy change, entropy change and the Clausius-Clapeyron relation are given, respectively.2. The excess specific heat curves of magnetic transition for CdV2O4 and ZnV2O4 are also plotted. By comparing the specific heat and the Gaussian fluctuation theory, the critical region of CdV2O4 and ZnV2O4 is estimated to be 0.5K<|T-TM|<1.5K and|T-TM|<0.5K, respectively. In critical region, by fitting the excess specific heat data to the relative formula in the renormalization theory of the second order transition, critical exponent and critical amplitude ration were calculated, which indicate that the critical behavior of both CdV2O4 and ZnV2O4 deviate from 3D-Heisenberg Model and incline to 3D-XY Model. It should be attributed to the degree of freedom of spin vector component confined by spin-orbital coupling with ordered orbits. Using the Ginzburg criterion and related theory, the correlation length of CdV2O4 is found to be one or twe order of magnitude larger that that of ZnV2O4.3. The isothermal magnetization curves of CdV2O4, ZnV2O4 and V2O3 were measured at various temperatures, and the curves of temperature dependent magnetic entropy change are given. With regard to CdV2O4 and ZnV2O4, when the temperature is above the structural transition, the magnetic order of whole system is from the paramagnetism in the external field; when temperature is approaching to the structural transition, as well as in the range between the magnetic transition and structural transition, the magnetic order comes from the competition between the nonlinear isolated spin chains and the paramagnetism in the field; when temperature is below the magnetic transition, the magnetic order is from the competition between the paramagnetic contribution from the impurities at low temperature and linear antiferromagnetism in the field. The contribution from nonlinear isolated spin chains around structural transition and the paramagnetism of impurities at low temperature to magnetic order in ZnV2O4 is obviously greater than those in CdV2O4. In the case of V2O3, when when the temperature is above the transition, the magnetic order is from the paramagnetism in the field; when temperature is in the range between 90K and the transition, magnetic order mainly comes from the antiferromagnetism of system; when the temperature is below 90K, the magnetic order is mainly from the paramagnetic contribution from the impurities at low temperature. Besides, the transitions type of CdV2O4 and ZnV2O4 are reconfirmed with Arrott plots.