SPIN FLUCTUATION AND THE TRANSPORT MECHANISM IN VANADIUM-OXIDE SPINELS WITH A METAL-INSULATOR-TRANSITION

Spin fluctuation and the transport mechanism in the spinel systems Lix Mg1-xV2O4 and LixZn1-xV2O4 with 0 less than or equal to x less than or equal to 1 have been studied through measurements of x-ray diffractio n, electrical resistivity, thermoelectric power, magnetization, and nu clear magnetic resonance. These compounds range from being antiferroma gnetic and insulating for MgV2O4 (Mott type) accompanied with a struct ural transition to the metallic state of LiV2O4 with no magnetic order . The metal-insulator transition may be of Anderson type and occurs in the vicinity of x(c)=0.4. The coherence length of the wave function o f hole carriers in the variable-range-hopping regime has a critical ex ponent -1.3 against \x-x(c)\. The metallic phase above x(c) may have t wo kinds of carriers from dynamic mixed valence state of V3+ and V4+. Based on the magnetic susceptibility and relaxation analyses, metallic compounds may be considered to be highly correlated electron systems with a low degeneracy temperature or large mass enhancement. On the ot her hand, insulators have short-range ordered spin correlation and/or superparamagnetic effects. At low temperatures, an antiferromagnetic p hase is realized for x less than or equal to 0.05 and a spin-glass pha se originating from the frustration inherent in the spinel B lattice a ppears in the region of 0.07 less than or equal to x less than or equa l to 0.7. The latter phase is enhanced for concentrations slightly les s than x(c).