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 LixMg1-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 diffraction, electrical resistivity, thermoelectric power, magnetization, and nuclear magnetic resonance. These compounds range from being antiferromagnetic and insulating for MgV2O4 (Mott type) accompanied with a structural 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 of hole carriers in the variable-range-hopping regime has a critical exponent -1.3 against \x-x(c)\. The metallic phase above x(c) may have two 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 other hand, insulators have short-range ordered spin correlation and/or superparamagnetic effects. At low temperatures, an antiferromagnetic phase is realized for x less than or equal to 0.05 and a spin-glass phase originating from the frustration inherent in the spinel B lattice appears in the region of 0.07 less than or equal to x less than or equal to 0.7. The latter phase is enhanced for concentrations slightly less than x(c).